Advances in Marine Biology, Volume 18

Advances in

MARINE BIOLOGY VOLUME 18

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Advances in

MARINE BIOLOGY VOLUME 18 Edited by

J. H. S. BLAXTER Dunstaffnage Marine Research Laboratory, Oban, Scotland

SIR FREDERICK S. RUSSELL Ptymouth, England

and

SIR MAURICE YONGE Edinburgh, Scotland

Academic Press

1980

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British Library Cataloguing in Publication Data Advances in marine biology. Vol. 18 1. Marine biology I. Blaxter, J. H. S. 11. Russell, Sir Frederick Stratten 111. Yonge, Sir Maurice 574.92

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63-14040

ISBN 0-12-026118-9

Printed in Great Britain by John Wright & Sons Ltd, The Stonebridge Press, Bristol

THE BIOLOGY OF MYSIDS AND EUPHAUSIIDS

JOHNMAUCHLINE The Dunstaffnage Marine Research Laboratory P.O. Box No. 3, Oban, Argyll, Scotland

Dedicated to my wife Isobel and to John, Rowena and Jenqer

PREFACE A N D ACKNOWLEDGEMENTS Euphausiids and mysids are linked together historically because of mperficial similarities-they were originally known as the Schizopoda. The euphausiids, however, are now recognized as decapod and the mysids as peracarid crustaceans. This early linkage of the two Orders has resulted in a continuing interest in both of them by modern workers. This volume should encourage the dual interest which has benefited earlier work and will be of considerable advantage in the future. Many colleagues throughout the world have encouraged and helped in the production of this work. I wish to acknowledge the helpful letters from and/or discussions with Dr M. S. de Almeida Prado, Dr M. Bacescu, Mr A. de C. Baker, Dr E. Brinton, Dr I. Everson, Dr R. R. Makarov, Dr M. Murano, Dr T. Nemoto and Dr J. R. Sargent. It is not possible to mention everybody but I would like to thank them all most sincerely. Mr R. I. Currie and my colleagues within the Dunstaffnage Marine Research Laboratory have in many cases been patient listeners and I thank them for their advice and criticism. Our Librarians, Miss E. Walton and Miss R. A. Gow, have enthusiastically sought papers from obscure sources and I am very grateful indeed to them for their perseverance. I owe much to Miss A. E. Leitch, Mrs E. MacDougall and Miss C. A. Keenan who have done much of the typing of the manuscripts. My greatest debt of gratitude is to my assistant Mrs Catherine M. Webster who has been involved in most aspects of this work, both practical and secretarial. Finally, it is once again a pleasure to acknowledge the helpful comments and criticisms of the Editors, Sir Frederick S. Russell, Sir Maurice Yonge and Dr J. H. S. Blaxter.

JOHN MAUCHLINE

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CONTENTS PREFACE AND ACKNOWLEDGEMENTS . .

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2. The species of mysids and key to genera. . . .. 3. The larvae and reproduction

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81 117

8. Physiology and responses to physical-chemical para.. .. . . . . meters of the environment

140

Part 1

The Biology of Mysids 1. Introduction.

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4. Vertical distribution and migration

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5. The gut, food and feeding . 6. Chemical composition

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7. Internal anatomy. .

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11. Geographical distribution .

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9. Behaviour

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10. Population dynamics 12. Predators and parasites

13. Mysids in the marine economy

References .

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Appendix I. Taxonomic list of lysidacea

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220 258

Appendix 11. Classified list of literature for each genus

343

Appendix 111. Classified list of literature for geographical . . .. . . . . . . regions . . . .

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ix

Part I t T h e Biology of Euphausiids

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3. Distribution and synonymy

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1.

Introduction.

2.

The species of krill .

4. The larvae. .

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373

420

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596

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624

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679

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681

5. Vertical distribution and migration . . . . 6. Food and feeding . .

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8. Vision and bioluminescence

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9.

Chemical composition

Internal anatomy and physiology

10. Growth, maturity and mortality .

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Ecology of distribution

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13. Euphausiids in the marine economy

References. .

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Addendum to Biology of Euphausiids Addendum t o Biology of Mysids Taxonomic Index. .

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Cumulative Index of Titles

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Subject Index

Cumulative Index of Authors

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375 384 430 443 459 478 484 490

554

PART ONE

THE BIOLOGY OF MYSIDS

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CHAPTER 1

INTRODUCTION Species of the Order Mysidacea are shrimp-like animals that occur in vast numbers in coastal regions of the world. They are often overlooked during sampling programmes because they are not sampled efficiently by either conventional benthic or pelagic samplers. A great many species inhabit water of a few centimetres to a few metres in depth, in the environment that lies between the tidal and strictly sublittoral. Many species swim in the water layer immediately above the sediment, others rest on the surface of the sediment, and a few species actually burrow into the sediment. I n consequence, a variety of sampling methods are required, the most important of which is the towing of plankton nets mounted in frames which will slide over the surface of the sediment. This is an equally valuable method for catching many offshore species living on the continental shelves of the world at depths as great as 200 m. I n addition to the species associated with the sea bed, there are many strictly pelagic species. Some of these live in coastal regions and others are oceanic, a few living in the meso- and bathypelagic environments. The meso- and bathypelagic species are well known, especially species of the genus Gnathophausia, some of which attain a large body size and are easily seen in samples of deep plankton. The shallower living epipelagic species are all smaller, less than about 30 mm in body length, are relatively scarce in samples and not so easily noticed among the mass of euphausiids, decapods, copepods and other organisms that occur in such samples. Mysids are a highly adaptive group. They inhabit all regions of the oceans to depths as great as 7210 m, where Belyaev (1966) records the capture of Amblyops magna in the Kuril-Kamchatka Trench. There are many brackish water species and a few species that occur in freshwater environments. Further species have become adapted to the specialized environments of caves and wells, while yet others live in commensal association with animals such as sea anemones. Superficial similarities exist between the mysids and the euphausiids and resulted in the Mysidacea and Euphausiacea being grouped

4

THE BIOLOUY OF MYSIDS

together in the early literature as the Schizopoda (Mauchline and Fisher, 1969). The euphausiids are now recognized as Decapoda. The Mysidacea are placed in the Class Eumalacostraca, Subclass Peracarida which contains the Orders Cumacea, Tanaidacea, Isopoda, Amphipoda, Mysidacea and a small Order Spelaeogriphacea created by Gordon (1957, 1958, 1960) to contain some peculiar subterranean shrimps. A full historical account of the problems of classification of mysids and euphausiids is given in Mauchline and Fisher (1969) and discussed in a broader context by Raw (1955). The earliest published record of a mysid is in 1808 and Tattersall and Tattersall (1951) provide an excellent historical account of the nineteenth century literature describing these animals. Their monograph emphasizes British species although it does refer to certain aspects of other species, especially bionomics. Gordan (1957) produced a bibliography of the Order listing over 1100 references up to the end of 1955. Gordan’s bibliography is divided into three parts. The first contains an alphabetically arranged list of publications. The second part lists the genera and species of mysids and details the relevant literature. The third part contains classified lists of references under the following subjects : general references ; taxonomy, morphology, embryology, life history and growth ;physiology ; behaviour ; symbiosis, including commensalism and parasitism. She refers, in the last sections of this part, to literature pertinent to the following geographical regions : Freshwater and brackish water distribution : Europe, America, Asia, Australia and New Zealand, Japan, Africa and the Arctic. Oceanic distribution : Atlantic Ocean, Mediterranean, Indian Ocean, Pacific Ocean, Arctic Ocean and Antarctic Ocean. Coastal distribution : Europe (Atlantic coasts), the Mediterranean (including the Adriatic and Black Seas), east coast of North America, Caribbean Sea (including Gulf of Mexico and Bermuda), east coast of South America, the coasts of Africa (including Arabia, Iran, India and Burma), Australasia (including Malay Peninsula, Indo-China, Indonesia, the Philippines Australia and New Zealand), east coast of Asia (including China, Formosa, Korea, Japan and the Soviet Union), Oceania (including Micronesia, Polynesia, Fiji Islands and the Hawaiian Islands), west coast of North America, west coast of Central America, west coast of South America, Arctic Ocean (including Greenland, Iceland, Spitzbergen and northern coasts of North America, Europe and Asia) and the Antarctic Ocean. This detailed classification of the literature has made Gordan’s bibliography extremely valuable. The late Dr W. D. Clarke, through

1.

INTRODUCTION

5

correspondence with specialists in the group, identified omissions from and additions to the original bibliography and produced supplementary lists of new references up to 1969 (Holthuis, 1967). Over 700 such references are combined with Gordan’s original reference list and published by Beeton and Clarke (1973), an alphabetical list now consisting of 1852 publications. They do not reproduce Gordan’s subject indices and give no ancillary information on the subject content of the 723 additional references. More than 300 of these are used in the present work and are entered in the classified bibliographies in Appendices I1 and 111. Of the remaining 400 references, some 150 are either general textbooks, fauna lists etc. or are quoted within other references used here. Another 75 publications, issued before 1900, are also in this category. Some 70 papers refer to studies on freshwater environments and most have been superseded by literature quoted here. About 30 references are to theses and internal reports, data from which have usually been published in a more formal way elsewhere. This leaves about 75 papers of the original 723, published predominantly between 1900 and 1950, that have not been obtained during the course of the present work; the titles of the great majority of them suggest that they are concerned with the distribution of individual species rather than with other aspects of their biology. The distributions of the majority of species of mysids are not well known and the production of distributional maps for most of the species is considered to be of little value at the present time. Mauchline and Murano (1977) list broad geographical regions of occurrence of all species and a list of pertinent literature for these regions is given in Appendix 111. Supplementary information is given in Chapter 10. This work does not contain a treatise on the taxonomy of mysids and consequently the following publications are required by specialists as companions to this volume: Tattersall and Tattersall (1951), for British and North Atlantic species ; Tattersall (1951), for American species ; Ii (1964a) and the papers of Murano, for Japanese and western Pacific species; Pillai and some papers of Nouvel, for Indian Ocean species; the papers of Bacescu, Brattegard and 0. S. Tattersall, for species in regions specified in the titles of the papers. Access to other relevant literature is easily obtained through the references quoted by Mauchline and Murano (1977).

CHAPTER 2

THE SPECIES OF MYSIDS AND KEY TO GENERA There are 780 known species of mysids distributed between about 120 genera. Each species is listed by Mauchline and Murano (1977) in the World List with brief notes on their occurrence and a reference to a good description in the literature. There are several corrections and additions to this list and these are set out in Table 11. Mauchline and Murano do not list synonymy of the various species. Informatioh on this aspect is available in Gordan (1957), the taxonomic papers listed at the end of Chapter 1 and the references quoted by Mauchline and Murano. An abbreviated classification and world list of Mysidacea is given in Appendix I. Individual species are identified through differences in their external morphology, especially the structure of such appendages as the thoracic legs, antenna1 scales, telson and uropods. A key to the genera of mysids is g-iven a t the end of this chapter and amply illustrates the characters used. Identification of many species is difficult because of variation in the characters used and their ontogenetic development. Information on the amount and type of variation encountered within some species is available. Much of this has been published in the early taxonomic and morphological literature quoted by Gordan (1957). The Order is sub-divided into the sub-orders Lophogastrida and Mysida. Many species in the Lophogastrida live in the deep oceanic environment and are large in body size relative to species in the Mysida. The largest mysid known, an individual of Gnathophausia ingens of 351 mm total body length (Clarke, 1961a), belongs to this sub-order. Species of the Lophogastrida, with the exception of those in the genera Paralophogaster, have adult body lengths ranging from 17 to 350 mm; species of Paralophogaster are smaller, having a range of body length of 6-20 mm. The majority of species of the Mysida have adult body lengths of less than 15 mm (Mauchline, 1972). Many morphological characters of lophogastrid mysids vary (Fage, 1940, 1941, 1942, 1952; Banner, 1954b), and to such an extent that further investigations of Eucopia species may result in a’ revision of the genus. The variation of some of the taxonomic characters in a

2. THE

SPECIES O F MYSIDS

7

few species of Mysida has been examined. The form of the antennal scale and its relative proportions is frequently an important character used to distinguish species; variation is present within species as shown by Mauchline (1971~)for the closely related P. JEezuosus and P. neglectus and by Williams et al. (1974) for Neomysis americana. The telson of the species in the genus Mysis has a cleft a t its distal end (Fig. 8.17) and such forms of telson occur in other genera. The depth, width and armature of the cleft not only varies between species but also within species, as demonstrated by Holmquist (1949, 1959b). The number of lateral spines on such telsons increases with increasing body size of the animal as shown in Fig. 1 for the two Praunus species and as described in Gastrosaccus vulgaris by Matsudaira et al. (1952). Similar correlations between numbers of spines on the margins of telsons and of both the endopod and exopod of the uropods on the one hand, and increasing body size on the other, have been found in Anchialina m-dagascariensis and A . sanzoi by Nouvel (1969), in Pseudomma afine by Wigley and Burns (1972), in Tenagomysis chiltoni by Hodge (1964), in Neomysis americana by Williams et al. (1974) and in Xiriella clausii by Genovese (1957). Geographi.ca1 variation in the ranges in numbers of spines on the lateral margins of the telsons of both the Praunus species are evident when the Danish and Scottish populations are compared (Mauchline, 197lc). Such variations in several morphometric relationships in Neomysis americana are demonstrated by Williams et al. (1974). Casanova (1977) showed that the eyes of Eucopia hanseni contain different numbers of ommatidia in Atlantic as compared with Mediterranean individuals. Juveniles of closely related species are often indistinguishable. This is true in the two species of Praunus discussed previously; the antennal scales have the same proportions (Mauchline, 1971c) and the lateral margins of the telsons are armed with the same range in numbers of spines (Fig. 1). The patterns of groupings of spines on the margins of the telson or uropods can be diagnostic of some species but the distinctive groupings are often not present in the juveniles. This is instanced by the study of Genovese (1956) on the outer margins of the exopod of the uropod of Siriella clausii (Fig. 2). Juvenile Lophogaster typicus have pectinate edges to the anterior region of the carapace and ventral margins of the abdominal pleura; this ornamentation is absent in the adult (Sars, 1916; Tattersall and Tattersall, 1951). The changing proportions of the body of Gnathophuusia ingens were examined by Clarke (1961a). The rostrum of juveniles, in common with that of many other crustaceans, is longer relative to body length

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FIG.1. Number of lateral spines on the telson of Praunwflexwsus (solid triangles) and P . negleetw (open triangles)related to total body length.

2. THE

SPECIES OF MYSIDS

9

FIG.2. Development of spines on the outer edge of the right exopod of the uropod of Skiella clausii. Body lengths of individuals are from left to right: 9, 8.8 mm; 9,8.4 mm; $2, 8.3mm; 3, 8.1mm; d, 7.9mm; 8 , 7.9mm; 9, 7.4mm; 9, 6.6mm; d, 5.6 mm (after Genovese, 1956).

then in fully mature animals. Various biometrical relationships between measurements of the body, rostrum, eye, appendages, telson and uropods are used by Holmquist (1956)in her study of seven species in the genus Boreomysis, The occurrence of secondary sexual characteristics in the adults provides further characters of taxonomic value, especially the detailed form of modified pleopods of males. These are used in the generic key at the end of this chapter. Chemical methods of distinguishing species and populations of mysids have not been investigated in any detail. Populations of the freshwater Mysis relicta exhibit isoenzyme polymorphism (Fiirst and Nyman, 1969). Homogenates of whole M . relicta were subjected to horizontal zone-electrophoresis on starch gel, stained for esterases, and found to contain two zones of activity, A range of patterns was obtained that suggested that five allelic genes producing 15 possible phenotypic combinations were involved. The populations could be separated on the basis of these electrophoretic patterns. One molecular form of esterase was found to occur in M . mixta and Praunus inermis and two in P. Jlexuosus and Neomysis integer by Nyman and Westin (1969). The different patterns obtained allowed electrophoretic separation of the species. Holmquist (1957a) and Nouvel (1964~)found aberrant individuals of Boreomysis rostrata, Mysis oculata, Praunus Jexuosus, P. neglectus, P. inermis and Neomysis integer. Holmquist suggested that in one

10

THE BIOLOGY O F MYSIDS

locality investigated by her, an industrial chemical effluent might cause abnormalities, although she also considered a general source to be genetical. Nouvel considered such abnormalities to be the result of regeneration after injury. Mysids are capable of regenerating damaged regions as originally demonstrated by Przibram (1901) in Hemimysis Zamornae. Mauchline (unpublished) noticed healed but malformed abdomens in Praunus species in Scotland, as well as the more minor abnormalities of telsons and antenna1 scales described by Holmquist and Nouvel. It seems most likely, as Nouvel suggests, that these are a result of damage inflicted, presumably by predators such as fish and amphipods, and the resultant regeneration. These aberrant individuals are relatively rare in a population and the species to which they belong can usually be identified. Sexual intermediates (intersexes) were first noted by Kinne (1955) in Neomysis integer. Three individuals had feminine characteristics in addition to the fourth pair of pleopods being modified as in the males. Two of these animals had a normally developed marsupium containing, developing embryos. Holmquist (1957a) also recorded intersexes of the same form in this species. I n addition, however, she observed one individual, badly damaged, with the male genital organs a t the bases of the fourth thoracic legs and with a t least one pair of brood lamellae. Another form of variation between populations of the same species is in the overall body size. The maximal or modal body size of a species can vary between different localities within one geographical region, as shown by the data of Williams et al. (1974) on Neomysis americana. Body size also varies within a species when its latitudinal range is considered. Casanova (1977) shows that Eucopia hanseni is of a larger body size in the Bay of Biscay than in the Mediterranean. Mauchline (1970b), however, found two distinct forms of Mysidopsis gibbosa in the Clyde Sea Area, Scotland, which differ from each other in body size, general form and the depth at which they occur. The normal smaller form, with a range of adult body length of P 6 m m , lives in bays usually over sand in depths ranging from 1 to 5 m. The body is as illustrated by Tattersall and Tattersall (1951) with the abdomen sigmoid in shape in lateral view. The larger form, measuring 5-8mm when adult, occurs a t depths of approximately 50m over mud or gravel. Its abdomen is straight and it closely resembles young M . didelphys from which it can be distinguished by the form of the telson. Analogous large-sized races of Holmsiella afinis and H . anomala from the Pacific Ocean are discussed by Murano (1970c, 1976). Again, the large-sized race occurs deeper than the small-sized race. Murano

2. THE

11

SPECIES OF MYSIDS

describes minor morphological differences between the two races of H . afinis. So far, the variations that have been discussed are considered to be of a minor nature and to be consistent with the normal patterns of variation expected within species. There are, however, documented variations within other species that have been judged significant enough to require the establishment of sub-species. A list of such sub-species is given in Table I. There is aome inconsistency between the description of forms or races of a species on the one hand and the establishment of sub-species on the other. For instance, Murano (1970~) states that the large form of Holmsiella afinis has larger eyes relative to its body size than the smaller form and there are also other morphological differences. Two of the sub-species listed in Table I also differ from their respective species in minor morphological characters and the size of their eyes : they are Boreomysis rostrata var. Illig and Mysidetes posthm microphthulma. TABLEI. LIST OF SUB-GENERA AND SUB-SPECIES OF MYSIDS Acanthomysis lonqicornis &bra Bacescu, 1972 Acanthomysis sculpta huda Banner, 1948 Anchialina typica orientalis Nouvel, 1971 Anisomysis (Anisomysis) kunduchiana Bacescu, 1975 A n k m y s i s ijimi estafricana Bacescu, 1973c Anisomysis mixta aiwrtralis Zimmer, 1918; Bacescu, 1973b Anisomysis (Paranisomysis) mark rubri Bacescu, 1973d Antromysis (Anophelina) anophelinae Bacescu and Orghidan, 197 1 Antromysis (Parvimysis) almyra Brattegard, 1969 ; Bowman, 197713 A n t r m y s i s (Parvimysis) bahamensis Brattegard, 1969 ; Bowman, 1977b Antromysis (Surinamysis) americana Bowman, 1977b Boreomysis californica longirostris Birstein and Tchindonova, 1958 Boreomysis rostrata japonica Ii, 1964a Boreomysk rostrata orienta1l;e Ii, 1964a Boreomysis rostrata var. Illig, 0. S. Tattersall, 1955 Boreomysis tridens lobata Nouvel, 1942 Diamysis bahirensis mecnikowi (Czerniavsky, 1882); Bacescu, 1954 Cfastrosaccw lobatw var. armata Nouvel, 1951b Cfastrosaccw sanctw widhalmi (Czerniavsky, 1882); Bacescu, 1970a Hemimysis lamorme mediterranea Bacescu, 1941 Hemimysis lamornae pontica (Czerniavsky, 1882); Bacescu, 1954 Heteromysis bermudensis ceaari Bacescu, 1968b Leptomysis apiops banyubnsie Bacescu, 1966 Leptomysis sardiccl pontica Bacescu, 1938 Mesopodopsis africana mmkgascariensis Nouvel, 1978 Metamysidopsis elongata atlantim Bacescu, 1968c mtd.

12

THE BIOLOGY OF M Y S I D S

TABLEI.-(cont.)

Mysidella t y p b mauritanica Bacescu (in press) Myaidetea poathon microphthalma Rustad, 1930 Mysidopst mortenaeni c u b a n b Bacescu, 1968c M y a t (Auricomyak)atenolepis Holmquist, 1959b Nouvelia natalemis mombaaae Bacescu and Vasilescu, 1973 Paramysia baeri biapinoaa Martinov, 1924; Bacescu, 1954 Paramysir, keaaleri sarsi (Derjavin, 1925) ; Bacescu, 1938, 1954 Paramyah lucwtri8 tamitica (Martinov, 1924) ; Bacescu, 1954 Paramysis lacuatrk turcia Bacescu, 1949; Bacescu, 1954 Siriella jaltensia brooki Norman, 1886; Tattersall and Tattersall, 1951 Sirielh jaltewk craasipes (G. 0. Sars, 1877); Bacescu, 1938 Siriella jaltemk grm'lipes Nouvel, 1942 Sirielh japonica izuensis Ii, 1964a Siriella japonica sagamiemis Ii, 1964a Siriella vulgaris rostrata W . M . Tattersall, 1951 Siriella watasei koreana Ii, 1964a Siriella wataaei macropsis Ii, 19648 Tenagomysis (Nouvelia) tanzaniana Bacescu, 1975

Additions .and corrections to the World List published by Mauchline and Murano (1977) are shown in Table 11. The genus Antromysis has been revised by Bowman (197713) and now includes the four subgenera Antromysis Creaser, 1936, Anophelina Bacescu and Orghidan, 1971, Surinamysis Bowman, 1977 and Parvimysis Brattegard, 1969. The full list of species now in this genus is given in Appendix I. It includes the original three species listed by Mauchline and Murano (1977), three new species shown in Table 11, the two species listed by Mauchline and Murano in the genus Parvimysis ( P . almyra and P . bahumensis), and a species in the genus Diamysis, D. americana. Bowman transfers this last species to the genus Antromysis, creating the new sub-genus Surinamysis to receive it. A re-examination of species in the genus Eucopia is required because of the intraspecific variation that has been found in this genus. Likewise, there are probably outstanding taxonomic problems k the gonus Boreomysis. Bacescu (personal communication) questions whether B. richardi Nouvel, 1942, is synonymous with B. vanhoeflenir, Zimmer, 1914. Murano (personal communication) considers that B. tanakai Ii, 1964 is certainly a juvenile form of B. rostrata Ii, 1964 or of B. rostrata orientalis Ii, 1964. Bacescu (personal communication) points out that Potamomysis assimilis Tattersall, 1908 should not be transferred to the genus Diamysis as shown by Mauchline and Murano (1977) since the fourth pleo-

2. THE SPECIES

OF MYSIDS

13

pods of the male differ from those of males in that genus; nor does this species conform to the genus Potamomysis Czerniavsky, 1882. Bacescu suggests that it should be known as Gangemysis assimilis (Tattersall, 1908), this name being used for it by Tattersall and Tattersall (1951). Hatzakis (1977) describes a new species in the genus Haplostylus Kossman, 1880 (Table 11); Mauchline and Murano (1977) wrongly ) the authority for this genus. Species in the quote Bacescu ( 1 9 7 3 ~ as genera Haplostylus and Gastrosaccus are closely similar but can be distinguished by the form of the endopod of the third pair of pleopods of the male (see key to genera). Hatzakis suggests that, on the basis of this character, G. lobatm Nouvel, 1951, G. magnilobatus Bacescu and Schiecke, 1974 and G. normani G. 0. Sars, 1877 should be transferred to the genus Haplostylus. I n the genus Leptomysis, Bacescu (1938), and in a recent personal communication, disagrees with the re-naming of L. sardica G. 0. Sars, 1877 as L. lingvura, f i s t suggested by Illig (1930) and continued by Tattersall and Tattersall (1951). Mauchline and Murano (1977) list three species in the genus Mesomysis. According to Bacescu (personal communication) M . incerta should be transferred to the genus Paramysis as P. incerta (G. 0. Sars, 1895). The other two species, Mesomysis czerniavsky and M . kowatewski have been synonymized with Paramysis lacustris. The genus Nouvelia Bacescu and Vasilescu, 1973 has been re-examined by Bacescu (1975). It now contains three species, all transferred from other genera (Appendix I). These species were previously named (Mauchline and Murano, 1977) as Dozomysis valdiviae (Illig, 1906), Tenagomysis natalensis 0. S . Tattersall (1952) and T. nigeriensis 0. S . Tattersall, 1957. Bacescu considers that T. similis W. M. Tattersall, 1923 may also belong to the genus Nouvelia. No comprehensive world key to the genera of Mysidacea has been published. There are various keys to species within several genera but with the current rate of description of new species most of the keys become quickly out of date. Ii (1964a) provides generic and species keys t o the Japanese mysid fauna, Tattersall and Tattersall (1951) and Nouvel (1950) provide keys for the north-east Atlantic species, and Brunel (1960) to species of the Canadian Atlantic Shelf. Many authors give keys to species within genera and these are among the references listed in the generic bibliography (Appendix 11).The World List of Mauchline and Murano (1977) serves an important function. The genera are listed alphabetically and each species is shown along with its geographical region of occurrence and a t least one source

TABLE

11. ADDITIONSAND CORRECTIONS TO WORLD LIST O F M A U C H L W E AND

Genera and species Antromysis Creaser, 1936 A . juberthiei Bacescu and Orghidan, 1977 A. peckorum Bowman, 1977 A. reddelli Bowman, 1977

Body length (mm)

Zones of occurrence (see Fig. 62)

22N

2.6-3.2

2

caves

Cuba

2 2

caves caves

Jamaica Mexico

Bacescu and Orghidan (1977) Bowman (1977b) Bowman (1977b)

9 9-1 1 10-11

2 2 2

coastal coastal coastal

Brazil Brazil Brazil

Silva (1970a, 1971a) Silva (1972, 1977) Silva (1971b, 1977)

24

1l c

50-585

Kerguelen I

Ledoyer (1977)

3

50-100

S. Africa

Nouvel(1973d)

3a

30-100

Mediterranean Hatzakis (1977)

23s 5-7s

8s

3 4

Ceratomysis Faxon, 1893 C . ericula Ledoyer, 1977

47s

Eryth,rops G. 0. Sam, 1869 E. bidentata Nouvel, 1973

33s

5.5

Haplostylus Kossmann, 1880 H . bacescui Hatzakis, 1977

38N

7-11

Heteromysis S. I. Smith, 1874 H . cotti

(1977)

Latitude (deg)

18N 17N

Bouwmniella Bacescu, 1968 B . atlantica Silva, 1971 B . inarticulata Silva, 1972 B . recifensis Silva, 1971

MURAN0

Habitat or depth (m)

Notes on occurrence

Sources of description

Transfer this species to the genus Heteromysoides Bacescu, 1968

Idiornysis W. M. Tattersall, 1922 I. japonim Murano, 1978

33N

3.3-3.9

7

1-5

Japan

Murano (1978)

Katerythrops Holt and

Tattersall, 1905 R. t k n g u h t a Panampunnayil, 1977 Mesomysis Czerniavsky, 1882 M . incerta G. 0. Sars, 1895

9N

3.5-4.0

8

'coastal

India

Panampunnayil (1977)

Cancel this genus Transfer this species to the genus .Paramy& Czerniavsky, 1882

Metamysidop&, W. M. Tattersall, 1951 M . mamen& Silva, 1970

24s

-

2

coastal

Brazil

Silva (1970b)

Paramy& Czerniavsky, 1882 P . grimmi (G. 0 . Sars, 1895) P . infEata (G. 0 . Sars, 1907)

-

30-40 40

3b 3b

100 8-36

Caspian Caspian

G. 0. Sars (1895) G. 0. Sars (1907)

Pburerythrops Ii, 1964 P . constricta Panampunnayil, 1977 Pseudomysidetes W. Tattersall, 1936 P . cochinensis Panampunnayil, 1977

Sirielh Dana, 1850. S. castellabatensis Ariani and Spagnuola, 1976 S. melloi Silva, 1974

9N

3.5

8

coastal

India

Panampunnayil (1977)

10N

> 3.5

8

coastal

India

Panampunnayil (1977)

40N

10.5-1 1.5

3a

coastal

Italy

9

2

coastal

Brazil

Ariani and Spagnuola (1976) Silva (1974, 1977)

5N-5S

16

THE BIOLOGY O F MYSIDS

of adequate description. Many coastal species occur in restricted localities so that the site of sampling is frequently of help in identifying the species. The key to the genera, appended to the end of this chapter, should be used in conjunction with the World List. The unknown species can be identified to the generic level from the key and descriptions of species within the genus can be found by reference to the World List, taking into account the additions and corrections listed in Table I1 and discussed above. There are two areas of the Key to Genera that are difficult. The four closely related genera Doxomysis, Afromysis, Iimysis and Tenagomysis within the tribe Leptomysini will probably require revision once males of all species have been described (Bacescu, 1968a). Identification of genera within the tribe Mysini is not easy because the principal character used is the form of the maIe pleopods, especially the fourth pair. These are generically distinct and in practice the females can be identified by comparison with the males because large sexual differences in features such as the antennal scale, telson and uropods are rare. In addition, the distributional patterns of chromatophores on the body of live or freshly preserved mysids are often e

FIQ.3. Families Lepidomysidae and Stygiomysidae.A, Spelaeomysis body form showing B, the antennal scale and C, the telson. D, Stygimysis body form showing E, the antennal scale and F, the telson (after Ingle (1972) and Bowman (1976)). e, eye; s, antennal scale.

2. THE SPECIES

17

OF MYSIDS

specific. Identification of juveniles can sometimes be made in this way when other adult morphological characters are undeveloped. This key is a first attempt a t including all genera but areas of it will no doubt require revision when more information becomes available. There are two families of Mysida not included in the generic key. They are the Lepidomysidae and Stygiomysidae, the species of which inhabit caves and wells. Gordon (1958, 1960) and Nath and Pillai (1971) discuss their systematic positions. Ingle (1972) re-examines the Lepidomysidae and groups the species within the single genus Spelaeomysis. These mysids have the body form illustrated in Fig. 3 ; there is no statocyst in the uropods, the eyes are reduced, but the antennal scale is normally developed although small. The Stygiomysidae is represented by a single genus, Stygiomysis. The carapace does not cover the posterior four thoracic segments (Fig. 3), no statocysts are present in the uropods, and both the eyes and antennal scales are reduced. The marsupium consists of seven pairs of lamellae in the Lepidomysidae but the number of pairs of lamellae present in Stygiomysidae has not been determined with certainty; there are at least four and possibly seven pairs.

OF MYSIDS KEYTO GENERA

1. Branchiae present on at Ieast some of the thoracic legs. Pleo-

pods well developed and unmodified in both sexes. No statocyst in endopod of uropod. Marsupium with seven pairs of lamellae (Fig. 4.2) (Sub-order LOPHOQASTRIDA) .. ..

2

Branchiae absent. Pleopods of female reduced, rudimentary (Fig. 4.1); of male variable. Marsupium usually with less than seven pairs of lamellae. Statocyst usually present in endopod of uropod (Sub-order MYSIDA) .. .. ..

8

2. Pleural plates on abdominal segments distinct (Family LOPHOQASTRIDAE)

..

..

..

..

..

..

3

No pleural plates on abdominal segments. Outer margin of antennal scale naked (Fig. 4.3); telson entire (Figs 4.4 and 4.6) (Family EUCOPIIDAE) .. .. .. .. Eucopia 3. Exopod of uropod with suture (Fig. 4.6)

..

Exopod of uropod without suture (Fig. 4.7)

..

..

4

..

..

5

A1

as

u

I

A1

3 su

P ux

0

5

6

FIG.4. Families Eucopiidae, Lophogastridae and Petalophthahnidae. A l , first abdominal segment; as, antenna1 scale; c, carapace; e, eye; 1, lamellee of marsupium; p, rudimentary pleopod; su, suture; t, telson; u, uropod; un, endopod of uropod; ux, exopod of uropod.

2. T H E

4. Long spear-like rostrum present

..

Very small triangular rostrum present 5. Telson of form shown in Fig. 4.8

Telson entire

..

19

SPECIES OF MYSIDS

..

..

.. ..

.. ..

.. ..

Gnathophawia Paralophogaster

..

Ceratolepis

..

..

6

6. Eyes developed; telson and uropods of form shown in Fig. 4.7 .. .. .. .. . . Lophogmter

..

..

Eyes reduced; telson and uropods as in Fig. 4.9 7. Outer margin of antennal scale with 2-4 spines

..

..

7

Chalarmpidum

Outer margin of antennal scale with 9-12 spines Pseudochalaraspidum 8. Statocyst absent (Family

PETALOPHTHALMIDAE)

Statocyst present (Family

MYSIDAE)

I.

..

..

9

..

..

11

9. Body with very extensive spiny armature (Fig. 4.10)

Body without very extensive spiny armature

Ceratomysis

..

..

10

10. Eyes small but stalked and normally shaped (Fig. 4.12)

Petalophthalmus Eyes plate-like or extremely reduced (Figs 4.11, 4.13) Hansenomysis 11. Exopod of uropod with distinct distal suture (Fig. 5.1)

12

Exopod of uropod with the proximal part of the outer margin naked and maxked distally with one or two spines and a transverse articulation (Fig. 5.2); telaon cleft as in Fig. 5.2 Marsupium consists of seven pairs of lamellae (SubFamily BOREOMYSINAE) .. - . .. . . Boreomysis Exopod of uropod undivided

..

..

..

..

..

14

12. Endopod of uropod with distinct distal suture and a strong

spine present on the ventral 'surface close to the statocyst (Fig. 5.3). Marsupium consists of three pairs of lamellae (Sub-Family RHOPALOPHTHALMINAE) .. Rhopulophthalmus Endopod of uropod undivided (Sub-Family SIRIELLINAE)

..

13

20

TEE BIOLOGY OF MYSIDS

17

no. 6. Sub-FamiliesBoreomysinm,Siriellinae,Rhopalophthalminae, Gestrosaccinae and Mysinm. a, precoxa; b, coxa; c, basis; ca, carpus; cp, fused carpopropodus subdivided into 6 perts; d, preischium; da, dectylus; en, endopod; ex, exopod; fm, flexor muscle; is, ischium; k, knee; me, merus; n, nail; 08, oblique articulation; pr. propodus; pr2, propodus composed of two sub-segments;8, statocyst; BU, Butwe; te, transverse articulation.

13. 'Third pair of thoracic legs normal and similar to the more

posterior legs

..

..

..

*.

..

* .

Siriella

Third pair of thoracic legs extremely elongated, almost twice as long as more posterior legs . . .. . . Hemisiriella

2.

21

THE SPECIIES OF MYSIDS

14. One to many spines but no setae on the outer margin of the

exopod of the uropod (Figs 5.4, 5.5, 5.6). Marsupium consists of two pairs of lamellae (Sub-Family QASTROSACCINAE)

15

Outer margin of exopod of uropod with setae but no spines

19

16. Outer margin of exopod of uropod with one or two spines

16

Outer margin of exopod of uropod with more than ten spines

17

16. Outer margin of exopod of uropod naked with one terminal spine (Fig. 6.5) . . .. . . Pseudanchialina

..

,.

Proximal half of outer margin of exopod of uropod naked, marked distally by two spines (Fig. 5.6) .. Paranchialina 17. In male, exopod of pleopod I11 slightly elongated, endopod with many segments (Fig. 5.7) Anchialina

..

..

..

In male, exopod of pleopod I11 greatly elongated and forming a copulatory organ; endopod unsegmented (Fig. 5.8) Bouvmaniella In male, exopod of pleopod I11 greatly elongated, without copulatory organ, and with the cndopod unsegmented (Fig. 6.9) .. .. .. .. .. . . Haplostylus

..

In male, exopod of pleopod I11 greatly elongated and endopod of four segments (Fig. 5.10) . . .. A rchaeomysis In male, exopod of pleopod I11 greatly elongated, endopod with many segments (Fig. 5.11) .. .. . Qaatrosmus Iidla

.

Pleopod of other form

..

..

18. In females, pleopods I-V biramous

In females, pleopods I-V uniramous

..

..

..

..

..

..

..

A rchaeomysis Bowmaniella .. Anchialina Iiella

..

18

I n females, pleopod I biramous, pleopods 11-V uniramous Qaatrosaccus Haplostylus 19. Labrum normal and symmetrical. Mandible with the cutting

lobe not expanded and its edge with spines. Maxillules

22

THE BIOLOGY OF MYSIDS

normal. First thoracic leg with distal segment of endopod normal and without spines on a free distal margin. Marsupium consists of two or three pairs of lamellae. (Sub-Family

..

MYSINAE)

..

..

..

..

..

..

20

Labrum asymmetrical, produced posteriorly into a large plate with two unequal lobes (Fig. 5.12). Mandibles with cutting lobe expanded and without spines (Fig. 5.13). Maxillules with lobes strongly bent inwards. First thoracic leg with last segment of endopod expanded and with a free distal margin armed with spines (Fig. 5.14). Marsupium consists of three pairs of lamellae (Sub-Family MYSIDELLINAE)

..

..

..

..

..

..

..

..

91

20. Third thoracic leg normal and similar to the more posterior legs, with the propodus or fused carpopropodus (Fig. 5.15)

..

..

..

..

21

Third thoracic leg strongly thickened with the carpus and propodus undivided, thickened and armed with spines (Fig. 5.16) (Tribe Heteromysini) . . .. .. .. ..

90

sub-divided

..

..

..

21. Endopods of third to eighth thoracic legs with undivided

carpus marked off from propodus by an oblique articulation (Fig. 5.17) (transverse in Arachnomysis and some species of Pseudomma). Antennal scale with outer margin non-setose (setose in Nipponerythrops) nearly always with a pronounced external spine (scale sometimes absent or represented by a spine). Pleopods 11-V of male well developed and biramous . . .. .. .. telson entire (Tribe Erythropini)

23

Endopods of third to eighth thoracic legs with carpus and propodus fused and sub-divided (Fig. 5.18); no oblique articulation except in Inusitatomysis .. .. ..

22

22. Pleopods 11-V of male usually well developed and biramous

but are rudimentary in Mysidetes. Antennal scale setose all .. .. around, telson variable (Tribe Leptomysini)

59

At least pleopod I1 of male is rudimentary and uniramous, exopod of pleopod IV elongated and modified except in Inusitatomysis where the endopod is elongated and the exopod rudimentary. Antennal scale and telson very variable (Tribe Mysini) . . .. .. .. .. ..

75

2. THE SPECIES

23

OF MYSIDS

..

.. Antennal scale absent or reduced to a spine . .

23. Antennal scale present

..

..

.. ..

..

..

24. Antennal scale with outer margin naked

Antennal scale setose all round 25. Last thoracic segment not elongated

.. ..

.. ..

..

..

24 56 25

Nipponerythrops Thallasomysis

..

..

26

Last thoracic segment greatly elongated making %hecarapace appear very short (Fig. 6.1) .. .. . . Longithorax 26. Visual elements of eye divided into two quite distinct regions .. .. .. .. .. .. . . (Fig. 6.2)

27

Visual elements of eye, when present, not divided into distinct regions . . .. .. .. .. .. ..

28

27. Antennal scale longer, or a t least only slightly shorter, than

antennular peduncle; proximal region of outer margin of scale naked and marked distally by a spine (Fig. 6.3) Euchaetomera Antennal scale about equal in length to the antennular ped.. . . Euchaetomeropsis uncle, setose all round (Fig. 6.4) Antennal scale about half the length of the antennular peduncle and armed with long plumose setae in the distal region only (Fig. 6.5); telson usually of form shown in Fig. 6.6 .. .. .. ., Caesaromysis 8

..

..

Antennal scale about half the length of the antennular peduncle and armed with plumose setae round the distal half (Fig. 6.7). Telson of form shown in Fig. 6.8 . . . . Echinomysis Antennal scale of peculiar form shown in Fig. 6.9

Echinomysides

28. Eyes well developed with normally functioning visual

..

29

Eyes rudimentary with visual elements reduced or absent

46

elements

..

..

..

..

..

..

..

29. Eyes reniform or markedly flattened dorso-ventrally

Eyes more or less globular

.,

..

..

..

.. ..

30 31

FIG. 6. Sub-Family Mysidae, tribe Erythropini. ep, eye-plate; pp, papillifom process ; sp, spinous process.

2.

25

THE SPECIES OF MYSIDS

30. Telson shorter than broad, its apex armed with four strong spines, the lateral margins unarmed (Fig. 6.10) or their . . Erythrops (in part) distal regions serrulated (Fig. 6.11) . .

Telson longer than broad, its apex truncate, armed with 6 spines, the innermost pair being minute, the outer pairs strong; the lateral margins entirely or partially armed with spines (Fig. 6.12) . . .. .. .. HypererytRrops 31. Lateral margins of telson unarmed

Lateral margins of telson armed

.. ..

..

..

..

32

..

..

..

38

32. Apical region of telson p m e d with two or more pairs of

spines with or without plumose setae

..

..

..

33

Apical region of telson armed with one pair of spines (Fig. 6.13) ; antenna1 scale setose without a spine marking distal Heteroerythrops end of naked outer margin (Figs 6.7, 6.14) . . 33. Apical region of telson armed with two pairs of spines as in Fig. 6.15 . . .. .. .. . . Amuthimysis

..

..

Apical region of telson armed with two pairs of spines of .. .. .. .. .. .. other proportions

34

Apical region of telson armed with three pairs of spines Parerythrops (in part) Apical region of telson armed with four pairs of spines

..

37

34. Eyes sub-globular, oval; telson of form shown in Figs 6.10, 6.11 .. .. .. . . Erythrops (in part)

..

..

..

..

..

35

35. No constriction between thorax and abdomen

..

..

3E

Eyes globular

..

..

..

..

Constriction present between thorax and abdomen (Fig. 6.16) .. .. .. .. .. .. Pleurerythrops 36. Pleopod I of male rudimentary

..

Parerythrops (in part)

Pleopod I of male with normally developed exopod and an .. .. .. . . Meterythrops unsegmented endopod Kuterythrops

26

THE BIOLOGY OF MYSIDS

37. Eyes small and imperfectly developed ; antennal scale shorter than antennular peduncle (Fig. 6.17) Teraterythrops

Eyes of normal size; antennal male much larger than antennular peduncle . . .. .. .. Synerythrops (in part) 38. Only apical region of telson armed, with more than four pairs of spines (Figs 6.18, 6.19) .. .. ..

..

Distal half of telson armed with spines 39. Telson of form shown in Fig. 6.19

Telson of form shown in Fig. 6.18

.. ..

..

..

..

39 41

Metamblyops (in part)

..

..

..

40

40. Antennal scale longer than antennular peduncle

Synerythrops (in part) Gibberythrops

Antennal scale shorter than antennular peduncle 41. Telson elongated, triangular (Figs 6.20-22)

Telson linguiform (Fig. 6.23)

..

..

.. ..

..

43. Antennal scale of form shown in Fig. 6.24

Antennal scale of form shown in Fig. 6.25

.. ..

42

Australerythrops

42. Apical region of telson with two long spines (Fig. 6.22)

Apical region of telson without such long spines

..

..

..

43

..

44

Pseuderythrops

..

Pteromysis

44. Endopod of pleopod IV of male elongated and terminating

..

..

..

Endopod of pleopod IV of male not elongated

.. ..

45. Pair of apical plumose setae present on telson

..

in a stout seta

No apical plumose setae on telson

..

..

..

Holmsiella

..

Eoarythrops A tlanterythrops

Metamblyops (in part)

46. Eyes fused to form a single plate or as two immovable plates in contact with each other at their inner margins (Fig. 6.26)

Eyes separste and distinct

..

..

45

..

..

..

47

50

2.

27

THE SPECIES OF MYSIDS

47. Eye plates without a long sharp spinous process on the outer distal corners (Fig. 6.27) .. .. .. . .

Eye plates with lateral spinous process (Fig. 6.26) 48. Pleopo’ds of male biramous

Pleopods of male uniramous

. .. ,

.. ..

.. ..

48

. . Scolamblyops .. ..

..

49

Michthyops

49. Endopod of pleopod IV of male not longer than exopod Pseudomma

Endopod of pleopod I V of male longer than exopod and armed with an extremely long non-plumose seta Parapseudomma 50. Eyes more or less pyriform with definite stalks

Eyes in form of flat plates without definite stalks

.. ..

..

51

..

54

51 Eyes small, imperfectly developed, no visual elements and

a minute papilliform process on the inner lateral surface (Fig. 6.28) .. .. .. .. .. Pseuclumblyops Eyes of similar form (Fig. 6.29), but without the papilliform process . . .. .. .. .. Paramblyops (in part) Eyes larger and of different form

..

..

..

..

52

52. Eyes large, lens-shaped; antennal scale narrow and shorter

than the antennular peduncle ; apical plumose setae present on the telson .. .. .. .. . . Hyperamblyops Eyes smaller, often imperfectly developed ; antennal scale variable in length ; no apical plumose setae on telson Dactylamblyops 53. Eye plates quadrangular, outer corner rounded (Fig. 6.27)

54

Eye plates triangular, outer corner produced into a pointed process (Fig. 6.30) .. .. .. .. .. ..

55

54. Antennal scale long and usually narrow with the unarmed outer margin terminating in a strong spine beyond which . Amblyops the small apex does not extend (Figs 6.31, 6.32)

.

Antennal scale oval with its apex produced well beyond the spine marking the distal end of the unarmed outer . . .. .. .. Amblyopsoides margin (Fig, 6.33)

28

THE BIOLOGY OF MYSIDS

55. Telson as shown in Fig. 6.34

..

..

..

..

Mysimenzies

Telson as shown in Fig, 6.35; antennal scale of form shown in Fig. 6.32 .. .. .. . . Paramblyops (in part) Telson ranging between the triangular forms shown in Figs 6.36 and 6.37; antennal scale variable, in some like Fig. 6.32, in others like Fig. 6.33 . . .. .. Dactylerythrops 56. Visual elements of eye not divided into separate regions

..

57

Visual elements of eye divided into two quite distinct regions (Fig. 6.2) .. .. .. .. Caesaromysis $2 57. Thorax not elongated between first and second thoracic legs

58

Thorax elongated betiween fmt and second thoracic legs leaving a conspicuous limbless region A rachnomysis

..

58. Posterior margins of abdominal segments armed with spines

Chunomysis Posterior margins of .abdominal segments naked 59. Antennal scale setose all round

..

..

Gymnerythrops

..

..

60

Antennal scale with outer margin naked; eyes in the form . . Calyptomma of Iamellae (Fig. 7.1); telson as in Fig. 7.2 . . 60. Pleopods of male well developed, biramous

..

..

..

61

Pleopods of male rudimentary and like those in female. Carpopropodus of endopods of third to eighth thoracic legs . . Mysidetes composed of many sub-segments (Fig. 7.3) . . 61. Telson without cleft

..

..

..

..

Telson cleft, or at least with an apical incision 62. Endopod of uropod armed with spines

..

.. .. 1 .

..

62

..

65

..

63

Endopod of uropod without spines, or with one minute spine Brmilomysis 63. Telson of form shown in Fig. 7.5; endopod of first thoracic .. .. .. . Metamysidopsis leg as in Fig. 7.6

.

Telson of form shown in Fig. 7.7 ; Endopod of first thoracic leg as in Fig. 7.8; eyes large; endopod of uropod without

FIQ. 7. Sub-Family Mysidee, tribe Leptomysini. e eye; ep eye-plate; truberance.

8s

spiny pro-

30

THE BIOLOGY OF MYSIDS

spines except for short rows of 2-7 close to the statocyst .. .. .. .. .. . . Mysideis (Fig. 7.9) Telson of other form

..

..

..

..

..

..

64

64. Endopod of first thoracic leg as in Fig. 7.10 Mysidopsis (in part)

Endopod of first thoracic leg as in Fig. 7.11 65. Eye small or vestigial

..

..

..

..

..

..

..

Leptomysis

..

66

Eye with accessory eye on dorsal lateral side (Fig. 7.12) Dioptromysis Eyes normal in size

..

..

..

..

..

..

67

66. Eyes conical (Fig. 7.13) or reduced with a finger-like process .. . . Pseudomysis (Fig. 7.14); telson of form in Fig. 7.15

Eyes small, sub-quadrangular with or without definite stalks Bathymysis (in part) (Fig. 7.16); telson of form in Fig. 7.17 67. Telson of form in Fig. 7.18

Telson of other form

..

.. ..

..

..

Pseudomysidetes

..

..

..

..

..

.. ..

68. Telson with unarmed apical incision (Fig. 7.19)

Telson cleft

..

..

..

..

..

..

68 69 70

Mysidopsis (in part)

69, Endopod of first thoracic leg as in Fig. 7.10

Endopod of uropod with spiny protuberance on outer edge and unarmed apical incision in telson (Fig. 7.20) Promysis (in part) 70. Cleft of telson not armed with spines

Cleft of telson armed with spines 71. Distal end of telson as in Fig. 7.21

I.

..

..

..

.. ..

Telson of general form shown in Fig. 7.22

..

..

.. ..

71 72

Afromysis (in part) Promysie (in part) Prionomysis

72. Cleft of telson armed with spines and a pair of plumose setae

73

Cleft of telson armed with spines but without plumose setae

74

2.

31

THE SPECIES OF MYSIDS

73. Distal segment of maxilla not expanded but rectangular (Fig. 7.23, 7.31); telson of form in Figs 7.24 and 8.1 Tenagomysis

Distal segment of maxilla expanded, crescent shaped (Fig. 7.25); telson of form in Fig. 7.26 .. . . Afromysis (in part) Distal segment of maxilla triangular (Fig. 7.27); telson as in Figs 7.26 or 7.28 .. .. .. .. . . Doxomysis Distal segment of maxilla as in Fig. 7.29; telson as in Fig. 7.26 .. .. .. .. .. .. .. Iimysis Distal segment of maxilla as in Fig. 7.30; telson as in Fig. 7.26 . . .. .. .. .. .. . . Hyperiimysis Distal segment of maxilla as in Fig. 7.31 ; telson as in Fig. 8.1 Nouvelia Distal segment of maxilla as in Fig. 8.3; telson as in Fig. 8.2 Pseudoxomysis 74. Telson of form in Fig. 8.4

..

..

..

. . Cubanomysis

Telson of form in Fig: 8.5 with long spines on apices Australomysis Telson of form in Fig. 8.6

..

..

Bathymysis (in part)

76. Antennal scale with outer margin naked, armed or unarmed

with spines or dentate

..

..

Antennal scale setose all round

..

..

..

..

..

..

.. ..

76 81

76. Outer margin of antennal scale armed with more than one spine (Fig. 8.7); telson as in Fig. 8.8 Hemimysis (in part)

Outer margin of antennal scale dentate (Fig. 8.9); telson cleft, armed with spines and two plumose setae (Fig. 8.10) Inusitatomysis Proximal region of outer margin of antennal scale naked, marked or unmarked distally by a spine . . .. ..

77

77. Outer margin of antennal scale without distal spine (Fig. 8.11) . .. .. ..

..

78

Outer margin of antennal scale with distal spine (Fig. 8.12)

79

.

..

..

78. Telson triangular, unarmed (Fig. 8.13)

..

..

..

Idiomysis

Telson varying between forms shown in Figs 8.14 and 8.15 Hemimysis (inpart)

FIG.8. Sub-Family Mysidae, tribes Leptomysini and Mysini.

2. THE SPECIES

33

OF MYSIDS

79. Spine on outer margin of antennal scale articulated (Fig. 8.16); telson cleft (Fig. 8.17) . .. .. Praunus

.

..

Spine on outer margin of antennal scale not articulated 80. Telson entire (Fig. 8.18)

..

Telson of forms as in Figs 8.17-8.19

..

..

..

..

I

..

80

..

Katamysis Caspiomysis

..

Paramysis Schistomysis

81. Eyes of complex form or markedly reduced, with or without

pigment

..

..

..

Eyes normally developed

.. ..

.. ..

..

..

..

82

..

..

..

83

82. Eyes very large, divided into a dorsal and ventral region .. Carnegieomysis (Fig. 8.20); telson as in Fig. 8.21

..

Eyes divided as in Fig. 8.22; telson of forms shown in Figs 8.23 and 8.24 . . .. .. .. Anisomysis (in part) Eyes as in Fig. 7.12; cleft of telson armed with two plumose setae and no spines (Fig. 8.25) .. . . Kainommatomysis Eyes reduced in size, ommatidia absent, pigmented or unpigmented; telson triangular with or without two spines on lateral margins (Fig. 8.26) .. Antromysis (in part) Eyes reduced in size, of form shown in Fig. 6.26. Telson of .. .. .. . . Troglomysis form shown in Fig. 10.7

..

84

Telson without cleft or with cleft without plumose setae .. .. .. .. .. .. .. present . .

85

83. Telson with cleft in which two plumose setae are present

84. Telson as in Fig. 8.27; pleopod IV of male of form in Fig. 8.28 .. .. .. .. .. .. . . Arthromysis

Telson as in Fig. 9.1 ; pleopod IV of male of form in Fig. 9.2 Parastilomysis

The remaining genera of the tribe Mysini are separated on the detailed morphology of the pleopods of the males, especially the form of pleopod I V . Pleopods 111 and V are rudimentary, uniramous and like those of the female unless otherwise stated.

i9 11

I9

16

Bra. 9. Sub-Family Mysidae, tribe Mysini.

2.

35

THE SPECIES OF MYSIDS

85. Exopod of pleopod IV of males with: . .. one or 2 segments

.

three, four or five segments six or more segments . .

.. ..

..

..

..

..

..

..

..

..

..

86 88 89

86. Exopod of pleopod IV unsegmented (Fig. 9.13); telson of form in Fig. 9.4; antennal scale as in Fig. 9.5 . Limnomysis

.

Exopod of pleopod IV unsegmented (Fig. 9.6); telson of form in Fig. 9.7; antennal scale as in Fig. 9.8; pleopod V elongated (Fig. 9.9) .. .. .. .. . . Indomysis Exopod of pleopod IV unsegmented (Fig. 9.10); telson and endopod of uropod as in Fig. 9.11 ; antennal scale of form in Fig. 9.12 .. .. .. .. . . Mesacanthomysis Exopod of pleopod IV unsegmented (Fig. 9.13); telson of form in Fig. 9.14; antennal scale of form in Fig. 9.8 ;pleopod I11 biramous, endopod unsegmented but exopod of four segments .. .. .. .. . . Paracanthomysis Exopod of pleopod IV of two segments (Fig. 9.16); telson without cleft, variable in shape but of general form in Fig. 9.17 87. Distal end of antennal scale pointed (Fig. 9.18)

..

87

Neomysis Acanthomysis

Distal end of antennal scale rounded (Fig. 9.19)

88. Exopod of pleopod IV of three segments (Fig. 9.15); telson with shallow cleft or straight edge (Fig. 8.15); antennal scale .. .. .. .. . Diamysis of form in Fig. 9.12

.

Exopod of pleopod IV of three segments (Fig. 9.26); telson with straight edge (Fig. 8.15) ; antennal scale of form in Fig. 9.12 .. .. .. .. .. .. . . Gangemysis Exopod of pleopod IV of three (four) segments (Fig. 9.20); telson varying between forms shown in Figs 9.21 and 9.22; antennal scale of form in Fig. 9.12 . . .. . . Lycomysis Exopod of pleopod IV of three segments (Fig. 9.23); telson of form in Fig. 9.24; antennal scale of form in Fig. 9.12; pleopod I11 biramous with unsegmented endopod and an .. Mesopodopsis exopod of two segments (Fig. 9.25) . . Exopod of pleopod IV of three segments (Fig. 9.27); telson of variable form, without cleft, and usually like Fig. 9.17;

36

THE BIOLOGY OF MYSIDS

antennal scale of general form in Fig. 9.8; pleopod V may be like that of female or elongated . . .. . . Proneomysis Exopod of pleopod IV of three or four segments (Fig. 10.1); telson varying between forms shown in Figs 10.2 and 10.3; antennal scale of form in Fig. 9.12; pleopod I11 uniramous but elongated . . .. .. .. .. . . Mysidium Exopod of pleopod IV with four segments and with large endopod (Fig. 10.4); telson of forms shown in Fig. 10.5; pleopod I11 uniramous and not elongated Antromysis (in part) Exopod of pleopod IV of four segments (Fig. 10.6); telson of form in Fig. 10.7; antennal scale of general form in Fig. 9.8; pleopod I11 (Fig. 10.8) and pleopod V (Fig. 10.9) segmented and elongated . . .. .. .. . . Taphromysis Exopod of pleopod IV of four segments (F'ig. 10.10); telson of form in Fig. 10.11 ; antennal scale as in Fig. 9.12 ; pleopod I11 biramous, with unsegmented endopod and exopod of three segments (Fig. 10.12) . . .. .. . . Nanomysis Exopod of pleopod IV of four segments (Fig. 10.13); telson variable, examples being shown in Figs 10.14-10.18 antennal scale of general form in Fig. 9.12 . . Anisomysis (in part) Exopod of pleopod IV of four or five segments (Fig. 10.19); telson of form in Fig. 9.17; antennal scale of form in Fig. 10.20 ; pleopod I11 biramous, endopod unsegmented and .. .. .. . . Stilomysis exopod of four segments 89. Exopod of pleopod IV of six or seven segments (Fig. 10.21); telson of form in Fig. 8.17; antenna1 scale varying between forms in Figs 9.8 and 10.20; pleopod I11 biramous, endopod

unsegmented and exopod of four to six segments

..

. . Mysis

Exopod of pleopod IV wibh more than seven segments (Fig. 10.22); telson of form in Fig. 10.23; pleopod I11 (Fig. 10.24) and pleopod V (Fig. 10.25) biramous .. Antarctomysis 90. Eyes more or less quadrangular (Fig. 10.26) with the cornea

in the antero-lateral region of the eye-stalk

..

Heteromysoides

Eyes globular or cylindrical, cornea more or less symmetrical relative to the eyestalk. Telson variable (Figs 10.27-10.29) and antennal scale of form shown in Fig. 10.30 . . Heteromysis Neoheteromysis

14

Fw

15

‘P I

19

16

i

I

17 ,

D

8

6

I

w 18

i

22

FIG. 10. Sub-Family Mysidae, tribe Mysini and Heteromysini; Sub-Family Mysidellinae.

38

THE BIOLOGY OF MYSIDS

91. Terminal segment of endopod of first thoracic leg armed with spines (Fig. 10.31); antenna1 scale of form in Fig. 10.30; telson of form in Fig. 10.28 or with a deeper or shallower cleft .. .. .. .. .. .. . . Mysidella

CHAPTER 3

THE LARVAE AND REPRODUCTION Mysids, like other peracaridan crustaceans, carry their embryos in a marsupium within which the entire, although abbreviated, larval development takes place. Young mysids emerge from the marsupium at an early juvenile stage of development. The marsupium is a chamber formed by lamellae or oostegites present on all or some of the second to eighth pairs of thoracic legs. Each of these posterior seven pairs of legs have a pair of lamellae in species belonging to the Sub-order Lophogastrida (Fig. 4.2). I n the Sub-Order Mysida, there are seven pairs of lamellae in species belonging to the Families Petalophthalmidae, Lepidomysidae and also probably Stygiomysidae, although this requires confirmation. The number of pairs of lamellae comprising the marsupium of species belonging to the Family Mysidae of the Sub-Order Mysida vanes; there are seven pairs in the Sub-Family Boreomysinae, three in the Families Siriellinae, Rhopalophthalminae and Mysidellinae, two in the Gastrosaccinae (Fig. 4.1) and two or three in the Mysinae. When only two or three pairs of lamellae are present, they arise from the seventh and eighth and sixth to eighth thoracic legs respectively. The lamellae are normally thin-walled, transparent concave plates fringed with short strong setae that interlock ventrally to form a closed chamber below the thorax. This chamber is normally single but Heteromysis species have two chambers formed side by side because the lamellae fold inwards and upwards in the mid-ventral line so producing lateral chambers (Tattersall and Tattersall, 1951). The eggs emerge within the marsupium from the external genital openings of the oviducts located near the bases of the sixth pair of thoracic legs. The ripe ovary fills the posterior thorax and extends into the abdomen (Fig. 11). Nair (1939) describes the ovary of Mesopodopsis orientalis, Casanova (1977) that of Eucopia hanseni, Ishikawa and Oshima (1951) that of Neomysisjaponica, and that of PraunusJlexuosus, examined in this investigation, appears to be closely similar. The germinal area is located centrally and ventrally within the ovary and the eggs pass from this region into the ventral lateral areas where they

SP

hD

I

h

I

vd

sv

tc

C

SP

\

a

sv

3 spT$ .:,..... ..,.:.. ...:;,,;.. . ":. ...

'tc

I

D

sv

SSI

E

F

G FIG.11. Reproductive systems of mysids. A, Praunus fEexuosw, female system; B, P. f E e x w m s , male system; C, lateral view of male system; D, structure of male system of species of Archaemysis and Neomysis; E, cross section of testicular structures shown in D, arrow indicates movement of spermatozoa; F, spermatozoon of Mysis oculuta; G , spermatozoon of Praunus Jezuo8u.s (A, B. G from Mauchline, unpublished; C, after Labat, 1961; D, E after Kasaoka, 1974; F, after Fain-Maurel er al., 1975a). a, ejaculatory duct; h, heart; hp, digestive diverticulae; i, intestine; 1, larvae within marsupium; 0,ovary; ov, oviduct; p, penis; sp, spermatidic pouch; ssl, and ss2 first and second spermatocytic sacs; st, stomach; sv, seminal vesicle; tc, testicular cord; vd, vas deferens.

42

THE BIOLOGY OF MYSIDS

grow in size. They later move t o the dorsal lateral regions of the ovary and so posteriorly to the mouths of the oviducts. Nair briefly describes the histology of the ovary and reviews earlier work. The development of the eggs within the ovary of Metamysidopsis elongata was observed by Clutter and Theilacker (1971). They found that a period of about one week was required for eggs in the dorsal lateral regions of the ovary t o become invested with yolk. The testes of the male are located in the same general region of the thorax as the ovary in the female (Fig. 11). The form of the male system has been examined in Praunus jlexuosus by Labat (1961) and in Arch,aeomysis grebnitzkii and Neomysis awatschensis by Kasaoka (1974); both these authors review the earlier work. Holmquist (1959b), for Mysis relicta, and Kasaoka (1974) describe the histological structure of the system in considerable detail and reference should be made to their plates. The terminology adopted in this description is that of Kasaoka. The testicular cords, positioned ventrally, contain the spermatogonial cells. These cells produce the spermatocytes which, during maturation, cause the epithelial layer around the cords t o be formed into pockets, the spermatocytic sacs. These are not so obvious in Praunus jlexuosus as in the species investigated by Kasaoka. The spermatocytes move dorsally from the sacs to the spermatidic pouches. The wall of each pouch has a distinctive histological structure that includes a middle layer of muscle tissue. The spermatids develop within the pouches and mature spermatozoa pass through the relatively narrow duct into the seminal vesicle. Kasaoka describes this as a U-shaped tube while Labat shows it to be two tubes, a left and a right. A re-examination of P . jlexuosw during the present investigation (Fig. 11) confirmed Labat’s description of the system in P. jlexuosus. Kasaoka found spermatocytes in early phases of meiotic prophase I in the testicular sacs in both Archaeomysis grebnitzkii and Neomysis awatschensis. This is contrary to the findings of earlier authors. Holmquist ( 195913) suggested that little, if any, spermatogonial division occurred in adult mysids but that this must take place in young immature animals. Labat (1962) drew the same conclusion from his work with Praunus jlexuosus. The non-motile spermatozoa of the following species have been examined; Mgsis oculata by Retzius (1909), M . reEicta by Holmquist (1959b), Praunus jlexuosus by Labat (1962), Archueomysis grebnitzkii and Neomysis awatschensis by Kasaoka (1 974) and Praunus inermis in considerable detail by Fain-Maurel et al. (1975a, b). They are of t h e general form found in peracaridan crustaceans (Fig. 1l ) , consisting of a long narrow head and a tail or lash which is banded in structure.

3. THE

LARVAE AND REPRODUCTION

43

Reger et al. (1970) and Reger and Fain-Maurel (1973) have examined the origin and distribu.tion of the extracellular tubules surrounding the bundles of spermatozoa in the vas deferens. They are secreted by the Golgi complex in cells surrounding the developing spermatids but their function is unknown. Mature spermatozoa can often be obtained easily for examination from the vasa deferentia because species with well-developed penes frequently have spermatozoa stored there and in the ejaculatory duct (Fig. 11). An androgenic gland is associated with the outer wall of each ejaculatoryduct, occurring laterally. The gland is pyramidal to ovoid in shape, measures 50-200 pm in height, and has been described in Paramysis nouveli, Praunus Jlexuosus and Spelaeomysis longipes (Juchault, 1963 ; Meusy, 1963; Chaxniaux-Cotton et al., 1966; Nath et at., 1972). The chromosomes of several species have been examined by making squash preparations of the testes and early embryonic stages. Nair (1939) counted a haploid number ( n )as 22 in Mesopodopsis orientalis and Suomalainen (1954) recorded n as 18 in Neomysis integer but suggested that might be an under-estimate. Holmquist (1959b) re-examined N . integer and estimated n as 24-29. Anchialina agilis was the only species in which she made an accurate count of n ; it was 8. In all other species, the chromosomes stuck together a t meiosis and counting was usually impossible. Mitotic divisions within the embryos are less difficult to interpret but even here clear enough separation of individual chromosomes, to ensure accurate counting, was not found. Holmquist estimated 2n t o be 43-48 in Gastrosaccus spinifer, about 100 in Mysis relicta, 90-94 in Praunus jlexuosus, and 90-100 in Mysis litoralis, M . mixta and M . oculata. Later, Holmquist (1973, 1975) found n as 32 in Neomysis intermedia and probably 32 in N . mercedis while Archaeomysis grebnitzkii had a value for 2n of 10. This latter species, however, had a further smaller chromosome present in many of the preparations but its significance could not be determined. Labat (1959) determined the numbers of chromosomes in three species. He found n to be 48 in PraunusJlexuosus, so confirming Holmquist’s estimate. His value of n as 30 in Neomysis integer compares with an estimate of 24-29 by Holmquist. Paramysis nouveli has n as 56. Mating usually takes place a t night and lasts for only a few seconds. Observed mating positions are shown in Fig. 12. The possession of penes allows implantation of the sperm mass within the marsupium, but many genera do not have these organs, the vma deferentia opening at small papillae. Nouvel (1937) suggests that Praunus Jlexuosus discharges the spermatozoa into the water outside the marsupium and &at they are transported to the eggs by the water currents generated

44

THE BIOLOGY OF MYSIDS

FIG.12. Mating in mysids. A, Praunusflexuosus. Male approaches female in upper figure and moves into the coupling position in the lower figure (after Nouvel, 1937). B, Paramysis nouweli. Male approaches female in upper figure and moves into the coupling position in the lower figure (after Labat, 1954). C, Heteromysis armoricana coupling position (after Nouvel, 1940). D, Mesopodopsis orientalis coupling position (after Nair, 1939).

by the thoracic legs. Kinne ( 1 955), however, disagrees with Nouvel, discussing various possible uses of the male pleopods of Neomysis integer including the possibility that they actually transfer the sperm mass to the marsupium. The antennae may also be used as shown in Pig. 12A to hold the male against the marsupium; in the position shown in the lower figure, the sperm would be ejected when the male straightens his body. The lamellae of the marsupium are capable of muscular lateral movements, used for irrigation of the embryos. Such movement of an empty marsupium would probably be effective in drawing in the spermatozoa from the surrounding water. According to Clutter (1969) and Clutter and Theilackes (1971), male Metamysidopsis elongata transfer the spermatozoa to the empty marsupium of newly moulted females and the eggs are subsequently laid.

3.

THE LARVAE AND REPRODUCTION

45

Eggs sensu stricto do not occur in the marsupium since they are fertilized immediately they are extruded from the oviducts. The early embryos, however, are spherical or sub-spherical (Fig. 17) and their size is closely similar to that of ripe eggs within the oviducts. The term “early embryo” (Stage I larva) in the following discussion refers to this egg-like embryo, referred to as “egg” in Fig. 14 and Table 111. The number of early embryos contained in the marsupium of a female depends upon her body size, the size of the individual eggs and, in temperate and high latitudes, the season of the year. A relationship between the increasing body size of the female and increasing numbers of embryos in the brood is difficult to demonstrate in species of body length less than 10 mm and with small numbers of embryos, less than 15, in the brood. This relationship, however, has been shown to exist in the following species by the following authors : Anchialina agilis by Macquart-Moulin (1965); Erythrops elegans and E . serrata by Mauchline (1968, 1973b); Gastrosaccw lobatus by Macquart-Moulin (1965); G. psammodytes by Brown and Talbot (1972); G. spinifer by Mauchline (1971e, 197313); G. vulgaris by Matsudaira et al. (1952); Leptomysis gracilis by Mauchline (1969a) and Wittmann (1978); Mesopodopsis slabberi by Macquart-Moulin ( 1965); Metamysidopsis elongata by Clutter and Theilacker (1971); Mysidium columbiae and M . integrum by Brattegard (1973); Mysidopsis almyra by Price and Vodopich (19 79); M . angusta and M . gibbosa by Mauchline (1970b, 1973b); M . bahia by Price (1978); M . taironana, M . tortonesei and M . velifera by Brattegard (1973); Mysis mixta by Apstein (1906) and Mauchline (197313); M . relicta by Hakala (1978); M . stenolepis by Amaratunga and Corey (1975); Neomysis americana by Pezzack and Corey (1979); N . merc edis (awutschensis) by Heubach (1969); N . integer by Kinne (1955), Wi ktor (1961), Mauchline (1971b, 1973b) and Parker and West (1979); N . intermedia by Murano (1964a); N . japonica by Ishikawa and Oshima (1951); Paramysis arenosa by Liao (1951) and Mauchline (1971a, 1973b); P. intermedia and P. ullskyi by Boroditch and Havlena (1973); Praunus Jlexuosus, P. inermis and P. neglectus by Blegvad (1922), Liao (1951) and Mauchline (1965, 1971c, 1973b); Schistomysis kervillei by Mauchline (1971d, 1973b); S. spiritus by Liao (1951) and Mauchline (1967, 1973b); Siriella armata by Liao (1951); Taphromysis bowrnani by Beck (1977). This relationship has been examined in only one bathypelagic species, Eucopia unguiculata ( E . hanseni) ; Casanova (1977) found that this species had 8-23 embryos in the marsupium but the number was not related to the size of the female. Brood size can vary seasonally as illustrated in Fig. 13. The mean numbers of young per brood of ovigerous females of three different

46

THE BIOLOGY OF MYSTDS

size classes of Praunus inermis were determined as frequently as possible throughout the year in the Clyde Sea Area. The largest broods were produced during the early summer months. A similar analysis of the same size group of ovigerous Schistomysis spiritus was made in two populations; the first in the Clyde sea area, the second in Loch Ewe, north-western Scotland. The brood size changed seasonally in both populations but broods tended to be larger in the Clyde than in Loch Ewe. The range of body size, as indicated by the mean body length in Fig. 13, of ovigerous females within a population can also change seasonally and, of course, also influences the brood sizes produced by the total populations. The late summer and autumn breeding females are usually smaller in body size than those breeding in the spring and earij summer. Mauchline (1965-1971) has shown that a regular seasonal change in brood size takes place in Scottish populations of the following species, in addition to Praunus inermis and Schistomysis spiritus : Erythrops serrata, Leptomysis gracilis, Praunus Jexuosus, P. neglectus, Schistomysis kervillei and S . ornata. The work of Blegvad (1922) on Praunus spp. and Wiktor (1961)) Kinne (1955) and Parker and West (1979) on Neomysis integer also demonstrated a seasonal change. Wittmann (1978) found seasonal variation in the brood size of Leptomysis lingvura in the northern Adriatic Sea. Matsudaira et al. (1952) found such a change in populations of Castrosaccus vulgaris in north-eastern Japan but Brown and Talbot (1972) found no such change in South African populations of G. psammodytes. TWO species of Paramysis living in reservoirs of the Volga River, P . intermedia and P . ullskyi, showed marked seasonal changes in brood size (Boroditch and Havlena, 1973) as did populations of Neomysis mercedis (awatschensis) in the Sacramento-San Joaquin River estuary in California (Heubach, 1969), and N . intermedia in Japan (Murano, 1964s). The size of the earliest embryos in the marsupium can also vary seasonally in some species. Mauchline (1973b) found that the winter embryos were larger than the spring and summer ones in Leptomysis gracilis, L. lingvura, Neomysis integer, Paramysis arenosa and Praunus jlexuosus. No measurable difference was found seasonally in the embryos of eight other species examined. The smaller spring and summer embryos appeared to have fewer and smaller oil globules present in them. So far, we have been discussing variations in brood size within a species. Mauchline (1973b), however, has made a comparative study of embryo size and number, brood volume and the respective body sizes of the ovigerous females of many species. More data are now available and are summarized in Table 111. Casanova (1977) examined

Schistomysis spiritus

10

L

I

I

J

F

I

I

M

A

I M

I

I

I

J

J

A

I S

I

I

I

O

N

D

FIG.13. Praunus inermis. The seasonal change in the mean numbers of young present in the marsupium of females of A, 13-14, B, 12-13 and C, 11-12 mm total body length. Schistomysis spiritus. Upper figure: seasonal change in the mean numbers of young present in the marsupium of females nf 12-13 mm total body length in Kames Bay (dots) and Loch Ewe (triangles). Lower figure: seasonal changes in the mean body length of ovigerous females (broken lines) and mean numbers of young per female (solid lines) in Kames Bay (dots) and Loch Ewe (triangles] (after Mauchline, 1965, 1967).

TABLE111. BODYLENGTH, BROODSIZE AND SIZEOF THE “EGG” (STAGE 1 LARVA) IN MYSIDS. The bathymetric habitat of the species is indicated as: FW, freshwater; E, estuarine; S, shore; C, coastal; EP, epipelagic; MP, mesopelagic; BP, bathypelagic.

Species

Body Maximum length brood (mm) size

Range in body length

Acanthomysis sculpta A. strauchi A f r m y s i s guinensia Amathimyais cherados A. gibba

12 14 8.5 2.7 2.8

46 17 8 7 9

10-14 10-18 1.s2.7 24-24

A. polita Amblyops abbreviata Anchialina agilis

2.9 15 8.4 7.7 7 5.6 3.5 33 3.5 3.2 2.8 4 28 17 10.4 9 10

8 29 17 33 9 18 3 23 4 4 4 4 29 3 28 30 25

2.0-2.9 10-15 6-9 4.5-7.7 6.7-7.6 3.9-5.6 3.5-4.0 31-77 3.0-3.5 2.1-2.8 3.5-4.0 20-28 8 ~ 0 . 4 9-10

A. oculccta A. typica Aniaomysis hanseni Antarctomysis maxima Antromysis anophdinae Antromysis bahamensia A. cubanica Boreomysis arcticu B. megalops Bowmaniella bacescui B. bradie& B. johmoni

-

-

Range in brood size

“Egg” diamter

5-46 -

-

2-7 6-9 2-8 11-17 7-33

-

2-18 2-3 7-> 23

-

4 2-4 3-4 3-29 18-28 -

(mm)

-

0.3 0.3 (0.28-0.32) 0.27 0.4 -

1.3 0.25 0.25 0 0.8 -

-

Depth zone

Reference

S E S S S

Green (1970) Bacescu (1935) Bacescu (196th) Brattegard (1974a) Brattegard (1973, 1974a)

C BP C C C C C MP S S S S MP MP S C C

Brattegard (1974a, b) . Wigley and Burns (1971) Mauchline (1973b) Macquart-Moulin (1965) Ariani and Spagnuola (1976) Nouvel (1971) Nouvel (1967) Mauchline (1973b) W. M. Tattersall (1951) Brattegard (1969) Brattegard (1973, 1974a, b) Bacescu and Orghidan ( 1971) Jepsen (1965) Hoenigman (1963) Brattegard (1974b) Bacescu (1968d) W. M. Tattersall (1937)

B. p o r t o ~ c e w k B. sewelti Brasilornysis castroi

Ceratolepis hanzatu Cubanomysis jimenesi

Diamysis bahirens i s

D. frontieri D . pengoi Erythrops elegans E . erythrophtha1rn.a

E . parva E . serrata Eucopia, grimaldii E . sculpticauda Gaatrosaccus lobatus G. m a n g i i G. psammodytes ci. sanctus C . spinifer G. vulgaris Cnathophausia giyus G. gracilis

10 8.2 9 9.1

30 11

74-10

10 16

7-9 5.6-9.1

6-10 4-16

14 4 3.2

22 3 4

13-14 3.5-4.5 2.2-3.4

2-5 2-6

6 11.5 5 13 5.9 7-1 4.5 3.5

7 28 7 20 13 15 8 4

6.5-8.0 S11.5 8-13 4-6 5.7-7.1 3.2-3.5

8.7 32 35 10.4 9.5 15 8 12.7 17 127 76

27 8 1 42 35 69 12 67 100 9 69

-

6.5-10 27-55 30-60 74-10.4 7.5-8.5 13-2 1 (23

10-14 14-18 49- > 100

1-30

-

3-7 15-28 2-7 8-20 1-13 1-15 2-4 2-27 4-8 < 20-42 12-69 6-12 21-67 30-106 -

2-97

0.38 (0.36-0-4) 1.0

-

0.25 (0.24-0.27) 0.31 0.38 0-4 0.29 (0.26-0.30) 0.35 1.5 0.5 0.46 (0.54-0.59) 3.2

C C

C C

EP S S

Wigley and Burns (1971) Brattegard (1974a) Bacescu (1968a) Brattegard (1973, 1974a) Fage (1941) Bacescu (1968~) Brattegard (1973, 1974a, b)

C Genovese (1956) E Bacescu (1954) C Nouvel (1965) E Bacescu (1954) C Mauchline (197313) MP Wigley and Burns (1971) M P Macquart-Moulin (1965) C Brattegard (1974a) C BP BP

C-

s-c

S C S S BP BP

Mauchline (1973b) Fage (1942) Fage (1942) Macquart-MouKn (1965) Bacescu (1975) Brown and Talbot (1972) Hoenigman (1964a) Mauchline (1973b) Matsudaira et al. (1952) Mauchline (197313) Fage (1941);Mauchline (197313) cowt.

TABLEI I I - c o n t .

Species

Body Maximum length brood (mm) size

Range in body length

140

350

G . longispina C. zoea

42 80

1 42

40-64 40-105

Hemimysis anomala H . bmornae

11 8.7 8 6 4 4 8 4.5 6 7 7 7 5 4.5 10 9 7.2 13.9 11.9 11.3

31 35 9 20 4 7 15 5 2 5 5 5 4 8 23 15 3 76 26 51

8.5-1 1 7-1 1 6.5-8 6-8 3.3-4

G . ingens

Heteromysis urmoricana H . bermudensis H . elegans H . formosa H . guitarti H . gymnura H . harpax H . mariani H . rubrocincta H . zeybnica Idiomysis tsumzantali Katamysis wurpaehowskyi Leptomyaia apiops

L. gracilis L. lingvura

130-> 145

-

5-8 4-4.5 4-6 6.5-7.8 6-7.5 6.5-8 -

4’2-4.5 5-10 9-13 6-8 9-16 8.5-12 7-1 1.5

Range in brood size

“Egg” diameter (mm)

4.3-200

8-3 1 5-35 4-9 9-20 4

-

13-15 3-5

-

1-5 3-5 2-5 2-4 5-8 3-23

-

(0-46-0.54)

Reference

Mauchline (1973b); Childress and Price (1978) BP Fage (1941); W. D. Clarke BP Fage (1941); Mauchline (1973b) C Bacescu (1954) Mauchlhe (1973b) C Bacescu (193613) C Nouvel (1940) C Brattegard (1973) C C Brattegard (1974a) C Wigley and Burns (1971) C Bacescu (1968b) S Fage and Monod (1936) S 0. S. Tattersall (1962) C Bacescu (1970b) C Bacescu (1968b) C W. M. Tattersall (1922) S Bacescu (1973a) E Bacescu (1954) C Bacescu (1966) C Macquart-Moulin (1965) C Mauchline (197313) C Macquart-Moulin (1965) C Mauchline (1973b) BP

20-42

3-76 13-26 7-51

Depth zone

L. mditerranea L. pereai Linznmysis benedeii i Lophogaster afinis L. typicw

-

9-27

-

6-8

-

18 8.5 15 24 23

27 8 40 25 34

8-8.5 7-15 17-30 17-23

Mesacanthmysis pygmaea 2.5 Mesopodopsis orientalis 7.6 M. slabbe& 11 M . zeylanica 5.6 Metamy&p&s elongata 6.5

5 9 20 4 30

7-8 7-1 1 6'2-5.6 4-7

3-5 8-10 5-20 1-4 4-30

0.32 0.35

M . insularis M. s ~ f t i

13 20 14

13 13-20 5-14 3-8

0.4 0.25

8

4.5-5.0 5.5-6.5 4-5 3-4-3.9

3.2 23 7 7

6 10 11 9

3.0-3.2 22-23 5-7 4.8-7

5-6 1-11 2-9

M. angwta M . ankeli

5-6 5.9 4-8 6.8 3.4

10 6 13 17 10

3-6.6 5.1-6.5 4.3-5.8 5-7 2.6-3-4

2-10 3-6 3-13 5-17 5-10

M . arenoaa

4.3

4

-

-

Mysidella minuta Mysidetes posthon Mysidium colunzbiaa M . integrum MYsidOpSi8 almyra

5.0

7 5 3.9

-

20-40 22-25 22-34

0.8

-

-

0.25 (0.20-0.26) 0.34

0.40 (0.40-0'44) (0.40-0.42)

-

(0.35-0.39) 0.29 (0.28-0.30) -

C

Liao (1951) Bacescu (1966) E Bacescu ( 1954) MP Fage (1942) MP Fage (1942) ; Hoenigman (1963) S Nouvel (1967) E Nair (1939) S Macquart-Moulin (1965) S Nouvel (1954) S Clutter and Theilacker (1971) S Quintero and Roa (1973) C Bacescu (1969) C Brattegard (1970a) S Brattegard (1973, 1974a)

C

-

C MP S S

Brattegard (19744 0. S. Tattersall (1965) Davis (1966) Brattegard (1973, 1974a, b)

S S S S

Brattegard (1973, 1974a) Price and Vodopich (1979) Price and Vodopich (1979) Mauchline (1973b) Brattegard (1973, 1974a)

S

Brattegard (1974b)

C

cont.

TABLEI I I - c o n t .

Species

M . bahia

Body Maximum length brood (mm) size

Range in brood size

“Egg” diameter

(mm)

Depth zone

-

0.25

4-6 4.8-5-0 2.8-4-4 2.1-3.0 3-6-6.2

3-12 2-4 12-16 9-16 17-60 5-34 8-10 2-4 2-8 3-1 1

11 8 21 97

2.6-4.4 4.2-6.2 29-35 13-23

3-1 1 2-8 12-2 1 9-97

0.28 0.34 (0.32-0’36) 0.34 0.40 1* 7 6-1 a92 0.60-0.64

21

40

13-21

10-40

0.66

Fw

22

32

15-24

5-32

-

Fw

188 245

-

46-1 88 140-245

0.4

-

C C

26

6-16

626

0.4

C

5.0 5.3 3.0 6.5 7.5 4.5 14-5 12.5 5.8

8

4.4 2.8 6.2

14 4 23 16 16 60 10 34 10 4 8 11

M . velifera M . virgulata My& litoralis M . mixta

4.4 6.2 35 22

M . relicta

M . bispinulata M . camelina M . coralicolu M . cultl.ccta M . dide~!phys M . eremita M . gibbasa M . mawhlinei Id.mortemeni M . taironana M . tortonesei

Range i n body length

5.0

M . stenolepis

25.5

Neomyeis amrkxzna

12

4.0-6.6 2.1-3.0

-

7’5-8.5 4.0-4.5 10-15

-

-

-

0.47

-

0.36 0.35

-

E E S S S S C C

C S S S S S C C C

Reference

Nimmo et al. (1978) Price (1978) Brattegard (1974a) 0. S. Tattersall (1955) Bacescu (1975) Brattegard (1973) Mauchline (1973b) 0. S. Tattersall (1962) Mauchline (197313) Brattegard (1974b) Brattegard (1973) Brattegard (1973, 1974a, b ) Brettegard (1973, 1974a) Brattegard (1973, 1974a, b) Brattegard (1974a) Mauchline (197313) Apstein (1906); Mauchline (197313) Tattersall and Tattersall (1951) Reynolds and De Graeve (1972) Wigley and Burns (1971) Amaratunga and Corey (1975) Wigley and Burns (1971); Mauchline (1973b)

N. rnercedia (awatschensis) 17 N. integer 16.6 16 N. intermedia 16 N. japonica 13 Paramysis arenosa 9.2 10 P. bacescoi 9 P. i n t e d i a 15 14

57 72 55 46 30 33 34 15 42 36

P. kroyeri P. nouveli P. pontica P. ullskyi

16 10 19 18 21

Praunua jlexuosus

25.3 23 24 25 13.9 16 15 21.3 21 4.6 10.9 11.8 11.5 15

P. inermis P . neglectua Prornysis atlccntica P s e u d m afine P. craa&entatum P. ear& P. scholhrtenais

7-17

-

10-17

5-57 15-72

10-16 12-16

8-55 1-46

5-9.5 5-10 5.5-8.5 8-15 6-14

2-33 4-34 4-15 7-42 7-36

(0'45-0.60)

30 28 43 30 62

10-16 6.5-10 10.5-1 9 13-18 12-2 1

6-30 6-30 6-43 6-30 17-62

(0.6-0.8)

63 76 40 44 56 50 > 28 81 > 42 5 11 23 14 18

13-24 12-23 17-24 16-28 10-16 9-16 9-15 14-23 13-2 1 3.6-4.6 8-11 10.9-11.8 -

4-63 17-76 14-40

(0.78-0.8 1) 0.67 -

-

-

15-56 4-50 5->28 13-81 5-> 42 2-5 1-1 1 15-23 -

(0.5-0.54)

-

0.50 (0.47-0.49)

-

0.80

-

(0.32-0.36) 0.41

-

E E E FW E S S S C

C

E S C C C S S S S S S S S S C C BP BP E

Heubach (1969) Mauchline (1973b) Kinne (1955) Murano (1964a, b ) Nakazawa (1910) Mauchline (1973b) Liao (1951) Labat (1957) Bacescu (1954) Boroditch and Havlena (1973) Bacescu (1954) Labat (1957) Bacescu (1954) Bacescu (1954) Boroditch and Havlena (1973) Mauchline (1973b) Liao (1951) Blegvad (1922) Wigley and Burns (1971) Mauchline (1973b) Liao (1951) Blegvad (1922) Mauchline (197310) Blegvad (1922) Brattegard (1973, 1974a) Wigley and Burns (1971) Murano (1974b) Murano (197413) 0. S. Tattersall (1955)

TABLE111-cont. ~~

Species

Body Maximum length brood (mm) size 8

12

14.8 14.3 14.5 13.5 23 24 11 9.5

47 23 29 30 48 99 15 13

S. clausii

10 8

S. jaltensis

Range in body length

Range in brood size

“Egg” diameter (mm)

Depth zone

Reference

2-12

S

8-16 11-16 9.5-1595 9-14.5 19-23 16-24 10.5-1 1.5 7.8-9.5

4-47 2-23 4-29 3-30 31-48 30-99 6-13

S C S S S S C C

Mauchline (197313) Mauchline (197310) Mauchline (197313) Liao (1951) Mauchline (197313) Liao (1951) Ariani and Spagnuolo (1976) Brattegard (1974a)

23 12

5-9 6-8

10-23 5-12

C C

8-1 11.9 8.5

16 22 13

5-8.5 10-11.9 8-13

8-16 -

C C C

Speluewnysis longipes

18.5 9 8.6 5

46 7 6 9

14-18.5 5-9 5-8.6 4.4-5.1

2-7 2-6 7-9

Taphromyeis b o m n i Tenagomy& macropsis

10 9

17 25

7-10 -

6-17 -

Mauchline (197313) Genovese (1956);Ariani end Spagnuola (1976) Macquart-Moulin (1965) Mauchline (1973b) Macquart-Moulin (1965) ; Ariani and Spagnuola (1976) Hoenigman ( 19634 Bacescu and Mayer (1961) Hoenigman (1963) Pillai and Mariamma (1964); Nath (1973) Beck (1977) Barry (1956)

Rhopalophthalmus longicauda Schistomysis kervillei S . ornata S. spiritus Siriella armata

S. castellabatensis S. chierchiae

S. norvegica S. thompsonii

7.6-8

-

C

E E S S C

0. S. Tattersall (1957)

3.

55

THE LARVAE AND REPRODUCTION

the brood sizes of Eucopia unguiculata ( E . hanseni) but measured carapace length as’an index of total body length; his data, are not included in Table 111. Larger sized early embryos tend to be produced by larger species but there is considerable variation (Fig. 14). There appears t o be a minimum size of embryo, about 0.25-0.30 mm in diameter. These embryos are produced by species of less than 5 mm total body length with an exception, on present information, of Metamysidopsis swifti (Table 111). Another species that has early embryos

10

102

103

104 Body length3

105

106

Fio. 14. The relationship of ‘‘egg” (Stage I larva) volume to total body length (raised to the third power) of the parent mysid. Dots are coastal and epipelagic species, triangles are meso- and ‘bathypelagicspecies. The regressi6n equations for the lines and their respective correlation coefficients are : coastal, epipelagic: log “egg” volume = 0.4510 log L8--2*6117,T = 0.8197*** meso-, bathypelagic: log “egg” volume = 0.9367 log Ls-4.2395, T = 0.9577***

A 100

I-

I

ee

/

A

'

ee

A

1 10

I

I

I

I

1

lo2

lo3

lo4

lo5

106

Body length3 FIG.15. The relationship between maximum numbers of young per brood and total body length (raised to the third power) of the parent mysid. Dots are coaatal and epipelagic species, triangles are meso- and bethypelagic species. T h e regression equations for the lines and their respective correlation coefficients are: Coastal, epipelagic: log number of young = 0.4264log L3+0.0281,r = 0.8107*** Mew-. bethypelagic: log number of young = 0.1547 log Ls+0.5667, r = 0.3631

3.

THE LARVAE AND REPRODUCTION

57

that are markedly smaller relative tc its body size is Mysis stenolepis. Meso- and bathypelagic species tend t o produce eggs that are larger relative to their body size than epipelagic and coastal species (Fig. 14). The numbers of young per brood varies greatly between species and sometimes between closely related species. There is, nevertheless, a tendency for larger species t o have larger numbers of young per brood (Fig. 15). The species, however, are divided into two natural groups. The first consists of epipelagic and coastal species while the second consists of the meso- and bathypelagic species. These latter species (Fig. 15) produce smaller numbers of young per brood relative to their body size than do the epipelagic and coastal species. Brood volume is directly related to body size (Fig. 16). The volume of the brood is obtained by calculating the volume of the spherical or near spherical early embryos and multiplying by the number of embryos present. No distinction exists between the deep living species and the others ; the few data suggest the possibility that the largest mesoand bathypelagic species may produce slightly larger broods relative to their body size than the other species (Fig. 16). The embryology of Hemimysis lamornae, Mesopodopsis orientalis, Neomysis integer and Praunus jlexuosus has been studied in considerable detail (Nusbaum, 1887 ; Wagner, 1896; Nusbaum and Schreiber, 1898; Manton, 192813; Needham, 1937; Nair, 1939). None of these papers has been superseded, nor is it possible to summarize them adequately here. Consequently, direct reference should be made to these descriptions which include many plates. The larval development of mysids takes place entirely within the marsupium. Detailed descriptions of the histology of successive larval stages are given by Nusbaum (1887) for Praunus jlexuosus, by Wagner (1896) for Neomysis integer, by Manton (1928b) for Hemimysis lamornae and by Nair (1939) for Mesopodopsis orientalis. The external morphology of the larvae of the following species has been described : Acanthopysis sculpta by Green ( 1 970) ; Boreomysis arctica by Jepsen (1965) ; Gastrosaccus psammodytes by Brown and Talbot (1972) ; G. vulgaris by Marsudaira et al. (1952); Gnathophausia species by Fage (1941); Metamysidopsis insularis by Quintero md itoa (1973) ; Mysidium columbiae by Davis (1966, 1968) ; Mysis relicta by Berrill (1969) ; Neomysis integer by Kinne (1955); N . intermedia by Murano (1964b); Schistomysis spiritus by Almeida Prado (1966). The development within the marsupium can be divided into three stages (Nair, 1939; Matsudaira et al., 1952; Jepsen, 1965; Davis, 1966, 1968) which correspond with the stages described by Mauchline (1972, 1973b) as “eggs”, eyeless larvae and eyed larvae.

A

/

/* 10

. I

I

1o2

1o3

1 1o4 Body length3

I

1

1o5

106

FIG.16. The relationship between volume of the brood and total body length (raised to the third power) of the parent mysid. Dots are coastal and epipelagic species, triangles are meso- and bathypelagic species. The equation for this relationship and it8 correlation coffioient are : log brood volume = 0.9147 log L3--2.6280, r = 0.9583***.

3.

THE LARVAE AND REPRODUCTION

59

Stage I . The early embryo, at first egg-like (Fig. 17A) but later with rudiments of antennae and abdomen developing (Fig. 17B). It is still within the egg membrane which is shed at the end of this stage. Stage I I . The larva has hatched from the egg membrane by puncturing it with the abdomen (Fig. 17GF). The antennae and thoracic appendages develop during this stage and the eyes become.pigmented. This stage terminates in a moult.

A

0

C

Fro. 17. Larvae of a mysid. A, egg-shaped embryo (“egg”);B, embryo with developing abdomen; C-F, stage I1 larvae at successive stages of development; G , stage I11 larva after the first larval moult and with the eyes stalked; H, stage I1 larva of a Cfnathophausiaspecies. Oil or yolk is represented by areas of darker stippling.

Stage I I I . The moulted larva now has the eyes on stalks (Fig. 17G). No lith is present in the developing vesicle of the statocyst in the uropods. This stage also terminates in a moult that takes place as, or shortly after, the larvae are released from the marsupium. Davis (1966) found that the early Stage I embryos (Fig. 17A) of Mysidiurn columbiae swelled within the marsupium and burst a thin

60

THE BIOLOGY OF MYSIDS

external membrane from which they emerged. This membrane is additional to the one shed a t the end of Stage I but no other authors have reported its presence. The embryos of most species have globules of oil (yolk) present within their tissues. These are usually in the form of many small globules but Schistomysis spiritus also has a large single globule present in the central region of the Stage I larva and in the anterior region of the Stage I1 larva, These globules are spread throughout most of the body of Stage I and I1 larvae but, as they decrease in volume relative to the body volume of the larva, they become concentrated in the anterior dorsal regions by the end of Stage 11 (Fig. 17F). The quantity of oil varies from species to species and its presence is very marked in larvae of the bathypelagic Gnathophausia species (Fig. 17H). Herring (1974) found that the large eggs of pelagic decapods have relatively high lipid contents and a low density. Their range of density was 1~091-1.025g/ml which compared to the value of 1.04 g/ml for the Gnsity of the Stage I larvae of Eucopia unguiculata. All the p v a e within a single marsupium are at the same stage of developmeht. They are regularIy orientated, their heads pointing posteriorly (Fig. 11A, p. 40) and closely packed together. Occasionally a Stage I1 larva occurs among Stage 111 larvae but this is rare and probably results from adoption (p. 62). Mauchline (1973b) found indications of a rate of mortality of about 10% during marsupial development. Amaratunga and Corey (1975) found fewer Stage I11 larvae in marsupia than Stage I indicating a loss of larvae in Mysis stenolepsis. The Stage I larva of Praunus jlexuosus, Mysis relicta and Neomysis americana is inactive and only morphological differentiation takes place. The Stage I1 larva exhibits a slow rhythmic beating of the heart and contractions of the gut; the gut contractions cause movement of the oil globules (yolk) (Berrill, 1969, 1971). The entire body of the late Stage I1 larva of Praunus jlexuosus and Neomysis americana flinches within its integument which has stretched and now serves as an enveloping membrane. Once this integument is shed the abdomen is free t o flex in the manner of the adult “escape reaction”. The appendages of these, now Stage 111, larvae have periods of active rhythmic movement appropriately described by Berrill as fluttering. The larvae have become increasingly active throughout Stage 111. They are markedly larger in size than the comparable Stage I embryos (Table IV and V) and the marsupium is distended to contain them. The brood lamellae are moved laterally in a rhythmic manner by the female to irrigate the larvae and consequently it would be surprising if some accidental loss of larvae from the marsupium did not occur.

3.

61

THE LARVAE AND REPRODUCTION

TABLEIV. LENGTH(MM) OF LARVAEWITHIN THE MARSUPIUMAND OF JUVENILES ON EMERGENCE OF EPIPELAUIC AND COASTALSPECIES( A f t e r Mauchline, 1972) Species Acanthomysis sculpta Anchialina agilis Eythrops elegans E . serrata Gaatrosaccua psammodytes G. spinifer G. vulgaris Hemimysis lam& Heteromysis formosa Leptomysis gracilis L. lingvura Mesopodopais orientalis Metamymdopsis elongata Mysidiurn columbiae Mysidopsis angusta M . didelphys M . gibboaa Myais mixta M . oculata M . stenolepis Neomyais americana N . integer N . intermedia Paramysia arenosa Praunus jexuosua P. inermk P. negbctua Paeudomma affgne Schistomysis kervillei S. ornata S. spiritua Siriellu armata S. clauaii S. jaltensis Taphromysis bowmani

Stage I

Stage 11

Stage 111

-

1-P1.6 0.9-1.1 0.8-0.85 0.8-0'9 1.6-1.8 1.1-1.3 1*o-1*2 1.0 1.25 1*3-1.4 0.7-0.9 0.5-0.7 0.8-1.1 0.8-0.9 1.3-1.5 0.9-1.1 1.3- 1.7 4.4-5.7 1.3- 1-6 0.6-1.2 1.3-1.5 1'0-1 '4 1.2 1.7-2.2 1 3- 1.7 1.7-2.1 1.3-1.5 1.5-1.7 1.1-1'3 1.3-1.4 1'2-1.3 0.9-1.0

1.8 1.3 1.4 2.0 1.7-2.1

-

0.38 0.35 0.5 0.45-0.50 0'54-0.59 0.5 0.4 0.38-0.48 0.40-0-55 0.35 0.4 0.4 0.35-0'42 0.4-0.5 0'35-0.40 0.6-0.64 1.76-1'92 0'49-0.55 0.41 0.5-0.6 0.5 0.4-0.5 0.7-0'9 0.6-0.8 0.7-0.9 0.4 0.6-0.7 0.6-0.7 0.5-0.6 0'5-0.6 0.45-0.5 0.5-0.6

O n emergence c. 2.0 2.0 2.5

-

-

1.P1.6 1.7 1.7-1.9 1.7- 1*8 1-3 1.2

1.6-1.9 1.2-1 '3 1.7-2.2 6.6-7.0 1.6-1.8 1.3 1-9-2.1 1.4-1.7 1-4-1.6 2.5-3.0 2.2-2.6 2.7

1.6-1.9 1.8-2.2 1.7-2.0 2.2-2.3 1.6

2.0

-

C. C.

2-5 2.5 1-5 -

2.5 1.5 2.0 2.0 1-5 2.5-3.0 2.5 2.5-3.0 C. 3.0 2.0 2.5 2.0 2.5 -

Female mysids not only protect the larvae by carrying them within a marsupium but have also evolved behavioural patterns that enable them to care for the brood in a much wider sense. Wittmann (1978) found that ovigerous Leptomysis lingvura are capable of catching larvae that have escaped from the marsupium. The process of catching them appears to be identical with that for catching larger particles of

62

THE BIOLOGY OF MYSIDS

TABLEV. LENGTH(M M ) OF LARVAEWITHIN THE MARSUPIUMAND OF JUVENILES ON EMERGENCE OF MESO- AND BATHYPELAGIC SPECIES (After Mauchline, 1972) Species Antarctomysis maxima Boreomyais arctica Erythrops erythrop-a Eucopia grimaldii E. sculpticauda Gnathophawia gigas G. gracilis G. ingens G. longispina G. zoea Lophogaster typicus

Stage I

Stage II

Stage III

1.3 0.3 0.4 1.5 -

3.8 1.0-1-2

5 1.4 4.0

3.2 4.0 3.4 0.8

7 5’0-9.1 13 6 10-12 2

O n emergence

22.4 13.3-16.0

13 5

food. The female, however, when the larva reaches the mouthparts usually does not eat it but’ moves it posteriorly by movements of the thoracic legs so that it enters the marsupium via the anterior or, occasionally, the ventral space between the brood lamellae. Adult males and immature males and females will also catch escaped larvae but they either release them immediately or eat them. Larvae that are at a later stage of development than those of the female’s own brood are adopted more frequently than younger larvae. Leptomysis lingvura adopts larvae of L. burgii, but less frequently than larvae of its own species. It can also distinguish the relative age and species of parts of larvae introduced to it and freely adopts parts of larvae of its own species. Adopted parts of larvae remain in the marsupium along with introduced sand grains so indicating that no active cleansing of the larvae takes place within the marsupium. Wittmann shows experimentally that adopted larvae survive and are liberated from the marsupium in the usual manner. Their liberation is usually before that of the female’s own brood because the adopted larvae are usually older. The female moults soon after liberating her own brood and so younger larvae that have been adopted and are not yet ready for release are lost a t this moult. Wittmann found that 0.8% of females of L. lingvura and 0.25% of females of L. burgii in natural populations in the northern Adriatic Sea had adopted one or more larvae; this was determined by the presence of one or more older larvae among broods. These adopted larvae represented 0*082%and 0.034% respectively of the total numbers of larvae within the broods of all the females of these two species examined.

3.

THE LARVAE AND REPRODUCTION

63

Wittmann’s observations have been confirmed by Mauchline and Webster (unpublished). The following species were found to be able to adopt escaped larvae : Mysidopsis gibbosa, Neomysis integer, Paramysis arenosa, Praunus JEexuosus, P. inermis, P . neglectus and Xchistomysis spiritus. Larvae that were older than those of the female’s own brood were ado ted more frequently. Dead Iarvae were usually rejected completely or ten but were not adopted. Pruunus jkxuosus and P. neglectus w o d adopt each other’s larvae and those of P. inermis; P. inermis, however, would not adopt the larvae of the other two species. Consequently, this adoption of free larvae decreases the potential mortality of larvae arising from their accidental loss from the marsupium. The duration of marsupial development is related to environmental temperature (Blegvad, 1922; Ishikawa and Oshima, 1951 ; Murano, 1964a). The duration in Praunus species ranges from 14 to 44 days depending upon the seasonal environmental temperatures in Denmark and in Scotland (Blegvad, 1922; Mauchline, 1965); the seasonal range of temperature is approximately 7-18°C. The duration of the development in Neomysis intermedia in Japan ranges from 6 to 9 days over a range of temperature of 21-3OoC, while that of AT.japonica is almost linearly related to temperature over the range 12.2-29.5OC (Table VI). Some examples of development times are given in Table VI. The cold water species, Boreornysis arctica, Mysis relicta and the bathypelagic Gnathophausia ingens have the longest reported development times, while the shortest is reported for Mesopodopsis orientalis at 25-29°C. Salinity affects the duration of the larval development of Neomysis integer. Vlasblom and Elgershuizen (1977) found that the range of salinity within which the larvae develop is more restricted than the total range in which the female can survive. Larvae adapted to a salinity of 7 x 0 had a development time of 15-18 days while those adapted to a salinity of 23% had a comparable development time of 17-22 days. A pair of tubes extend ventrally from the female’s thorax into the marsupium of species in the genus Neomysis. They are illustrated for N . integer by Vorstman (1951) but their function is unknown. McLusky and Heard (1971) found that Praunus jlexuosus can regulate the composition of the marsupial fluid hyper-/hypo-osmotically, with a notable degree of control. There was a linear correlation between the osmotic concentration of the blood and that of the marsupial fluid but the degree of regulation was much strider in the case of the blood. Consequently, larvae within the marsupium of species living in fluctuating salinities may be protected from the extreme ends of the range.

i

TABLEVI. THE DURATION (DAYS)OF

THE

DEVELOPMENT OF THE LARVAL STAGES WITHIN PERIOD O F M A R S U P I A L RESIDENCE

THE

MARSUPIUM AND

THE

TOTAL

Duration of Temperature

species Acanthmysis sculpta Boremysis arctica Gaatrosaccus vulgaris Gnathophawria ingens Hemimysis speluncola Mesopodopsis orientalis MetanZyaidopsis elongata Mysidium columbiae Mysis relicta Neomysis americana N . integer

N . japonica Praunwr jexuosus 0

("(3 12 V7) 10.9 3.5 12 14 23 25-29 17-19 3 2-10 0.2-3'8 12-15 10 16 15 12.2-29.5 12-15

Stage I

Stage I I

Stage I I I

Total

1

3

2

c. 28

c. 40

c. 30

c. 100

11 163-202

8 105-154 9-14 -

25 313-530 26-31} 17-19

6

6-7

-

2-3 2-3

-

10-11

-

I

5-6

11-13

4 10 6-7 c. 150 c. 270 c. 21 23-25 12-14 15-22 6-17* c. 21

Reference Green (1970) Jepsen (1965) Matsudaira et al. (1952) Childress and Price (1978) Macquart-Moulin and Patriti (1966); Gaudy and Guerh (1979) Nair (1939) Clutter and Theilacker (1971) Davis (1966) Lssenby and Langford (1972) Berrill (1971) Pezzack and Corey (1979) Pezzack and Corey (1979) Vlasblom and Elgershuizen (1977) Ishikawa and Oshima (1951) Berrill (1971)

Log incubation period = 1.9313-0.0559T where T is environmental temperature in 'C (Ishikawa and Oshima, 1951).

3.

THE LARVAE AND REPRODUCTION

65

Emergence of larvae from the marsupium generally takes place a t night and over a short period of time, a few minutes t o a n hour (Green, 1970; Berrill, 1971). The late Stage I11 larva within the marsupium is active and shows rhythmic twitching of the body (Berrill, 1969, 1971). The larvae are released from the marsupium when the female spreads the lamellae laterally and the larvae fall out, one by one (Nair, 1939). The larvae are unable t o swim until after the moult that terminates Stage I11 of development. Berrill (1971) found that this moult took place as the larvae were emerging from the marsupium. Nair (1939) suggests t h a t it takes place in Mesopodopsis orientalis within 10-20 min. of emergence, while Amaratunga and Corey (1975) found that the larvae of Mysis stenolepis moulted up to 3 h after emergence. This postmarsupial moult is synchronized in broods of Neomysis arnericana; Pezzack and Corey (1979) found that a t 4, 10, 16, 22 and 25°C it took place 312, 144, 72, 48 and 24 h respectively after emergence. It is likely, however, that this moult takes place irl the natural environment in most species very close t o the time of emergence. Immobile larvae are helpless and would be subject t o heavy predation. One possible source of such predation would be cannibalism by the female but Amaratunga and Corey (1975) did not observe any such behaviour in Mysis stenolepis. One notable feature of this post-emergence moult is the development of the lith in the vesicle of the statocyst, a feature absent in Stage I11 larvae. The young mysids are now miniature adults, usually 1.5-3mm in total body length, except in the large G n u € ~ o ~ ~ a u s i a species where they may attain lengths of' over 20 mm (Tables IV and V).

CHAPTER 4

VERTICAL DISTRIBUTION AND MIGRATION Mysids are predominantly marine organisms, a few species living in fresh water and in specialized environments such as caves and wells. Some 36 species live in non-marine habitats, about 160 are more or less restricted t o the littoral environment, about 300 t o the neritic region, 30 are epipelagic and over 200 live in the meso- and bathypelagic regions. Information on the vertical distribution of many species is not available. Some species, reported as meso- or bathypelagic, are probably not markedly pelagic in habit but usually restricted t o the hyperbenthic region-the water layer adjacent to the sea bottom. Nets mounted on sledges (Fig. 18) are especially useful in sampling mysids as found by Bossanyi (1957), Beyer (1958), Hesthagen (1973) and several others. Comprehensive sampling of mysids and investigation of their vertical distribution within a coastal area is difficult because various types of sampling equipment have to be employed. The pelagic species or hyperbenthic species that have migrated upwards can be sampled with the conventional plankton nets. These can range from the conical tow-nets to the Isaacs-Kidd Midwater Trawl which has been used by Pequegnat (1965) and Fried1 (1971b), or as modified by Pearcy et al. (1977) to sample offshore species. The Rectangular Midwater Trawl (RMT) and other offshore samplers used to capture euphausiids (Mauchline, 1980) are equally suitable for pelagic mysids. Littoral and estuarine species can often be taken with hand dipnets, push-nets, or very simply with the hand-pulled D-net shown in Fig. 18. A modification of the push-net that quantitatively samples contiguous parts of a transect is described by Miller (1973) and could be adapted for shallow water sampling of littoral mysids. The Ocklemann detritus sledge (Fig. 18) is heavier to use by hand than the D-net but is favoured by Brattegard. A further multilevel net system to that shown in Fig. 18 is described by Ellertsen (1977) and is potentially useful in littoral and sub-littoral regions. Heavier and more robust equipment is required to sample the neritic species. Various forms of beam trawl incorporating runners of 10-20 cm diameter are

4.

VERTICAL DISTRIBUTION AND MIORATION

67

A

FIG.18. Various towed samplers used to catch mysids. A, the D-net used by Mauchlhe; B, bottom plankton sampler designed by Colman end Segrove (1955) ;C, the modified Beyer epibsnthic sledge as described by Oug (1977); D, the Ockelmann detritussledge described by Brattegard (1973); E, the multilevel net described by Fager et al. (1966).

used to tow over soft substrates. One or several nets, in addition to the main trawl, can be attached to the wooden beams of such trawls to provide samples at heights of 1-2 m above the surface of the sea bottom. Several more specialized samplers are available (Fig. 18). Wickstead (1953), Clutter (1965), Macer (1967) and Omori (1969b) have designed closing epibenthic sledges that are all excellent samplers for mysids. Poirier et al. (1969) have modified the Macer sledge,

68

THE BIOLOGY O F MYSIDS

and the larger Beyer system (Fig. 18) is used by Hesthegan and Gjermundsen (1978) and Schnack (1978). Light lures can be incorporated in many of these nets as first used by Herdman (1890) and later by Brattegard (1970a) and Bacescu (1975). Mysids can avoid nets and can be observed escaping from samplers used in the littoral region. Fleminger and Clutter (1965) studied the escape of Metamysidopsis elongata and found that sampling was more efficient in darkness. They also discovered that less avoidance by individuals took place in dense populations than in populations that were dispersed. Quantitative sampling of mysids, many littoral species of which form aggregations, is extremely difficult. The difficulties are increased in the case of some Gastrosaccus and other species that partially or totally bury themselves in the sediment. during the daylight hours. The capture of shallow living species of mysids in good condition for experimental work is relatively easy and many can be maintained in the laboratory for considerable periods, of time. Greve (1975) has reared Praunus Jlexuosus and Mesopodopsis slabberi through a t least one generation in his “Meteor Planktonkuvette”. Deeper living species especially the meso- and bathypelagic species, are more difficult t o obtain in excellent condition. Childress et al. (1978) have solved the problem by designing a thermally protected cod-end for their nets. The terminal region of the net is contained within a heavy-walled (1-15 cm thick) polyvinylchloride tube with valves that can be closed. The tube is open during the shooting and towing of the net but it is closed for retrieval. The animals within the tube are then thermally insulated from the outside environment as the net is raised through the warmer surface layers of the ocean. Gnathophausia species caught in this manner have been in excellent condition for experimental investigations (Childress and co-workers). The depth of occurrence of the different species is indicated by Mauchline and Murano (1977) in the World List and, for new species, in Table I1 of this review (p. 14). The information is often incomplete because the entire bathymetric range of very few species has been investigated. Mysids, unlike euphausiids, occur in small numbers in oceanic samples but can often occur in vast numbers in neritic and littoral samples. Vinogradov (1970a, b), Vinogradov and Parin (1973), Vinogradov et al. (1970) and Murano et al. (1976) have examined the vertical distribution of the oceanic biomass of mysids and euphausiids. That of mysids tends to be reciprocal to that of euphausiids; mysids are dominant in the littoral and shallow shelf environments, euphausiids are dominant in the epipelagic and upper mesopelagic

4.

VERTICAL DISTRIBUTION AND MIGRATION

69

regions, while mysids are again dominant in the lower mesopelagic and bathypelagic environmenbs. The number of papers defining the vertical distributions of neritic and oceanic mysids are few: Colosi (1930), Taniguchi (1969), Wigley and Burns (1971), Nikolaev et al. (1973), Childress and Nygaard (1974), Murano (1975, 1976, 1977a). Other papers are referred to later in the discussion of the die1 vertical migrations performed by some species. As mentioned above, mysids occur in small numbers in oceanic samples and there is little evidence to suggest that they are important constituents of the deep sonic scattering layers. Populations of shallow water species, however, do occur a t densities such that they can constitute scattering layers. McNaught (1968) discusses the possibilities of observing sonic scattering by populations of the freshwater Mysis relicta in the Great Lakes of North America. Friedl (1971a) concluded that Acanthomysis macropsis and Neomysis kadiakensis appeared to contribute to the records of sound scattering at 38.2 kHz in Puget Sound. Mysids occur to depths of a t least 7210m where Belyaev (1966) recorded Amblyops magna. Bacescu (1971a) found Mysimenzies hadalis at a depth of 6200 m in the Peru Trench and Amblyops aequispina was taken at 5760 m by Birstein and Tchindonova (1958). Bacescu has suggested that further sampling of the hadal region will discover more species of mysids to be present. Wigley and Burns (1971) have presented a bathymetric classification of mysids based upon their studies of the fauna off the eastern coasts of the United States, extending from the littoral zone to the continental slope at depths of 700m. The classification given in Table VII incorporates their findings but modifies the ranges of depths ascribed to their regions and introduces regions outside the scope of their investigations. The first three groups of species, the freshwater, brackish water and littoral are relatively easy to define and recognize in the environment. Shallow shelf species are numerous in number and appear t o form a natural bathymetric group; they are predominantly hyperbenthic in habit. Eurybathic shelf species are probably fewer in number but again form a natural group, and are also predominantly hyperbenthic in habit. Members of the remaining groups are more difficult to recognize in samples from the environment. The deep shelf and upper slope species and the slope species are principally hyperbenthic in habit although the mesopelagic regime impinges on the slope and species can be mis-classified. The catching of a mysid in pelagic as opposed to a hyperbenthic sampler does not necessady define its normal habitat. There are pelagic species occurring over

70

THE BIOLOGY O F M Y S I D S

the continental shelf but they are considered to be relatively few in number and are grouped with the more strictly oceanic epipelagic species. The meso- and bathypelagic species are self-explanatory except in the region of the continental slopes as mentioned above. There is also a possibility that some of the rare deep sea species, occasionally caught in pelagic nets, may be representative of deep hyperbenthic populations. TABLEVII. BATHYMETRIC CLASSIFICATIONOF SPECIES OF MYSIDS

Title of species

Normal habitat

Total range of depth ( m ) ~~

Freshwater Brackish water Littoral Shallow shelf Eurybenthic shelf Deep shelf and upper slope Slope 8. Epipelagic 9. Mesopelagic 10. Bathypelagic 1. 2. 3. 4. 5. 6. 7.

0-50 m depth 0-10 m depth Intertidal or subtidal Less than 50 m depth Broad range of depth Outer shelf and on slope On the slope Oceanic surface layer Oceanic, 200-700 m depth Oceanic, c. 500 m to very deep (6000 m)

~~

0-200 0-20 0-10 2-100 2-400 100-400 200-700 0-300

As already stated, not enough information is available to classify all known mysids into one or other of the classes shown in Table VII. They can, however, be divided between several broad regions (Fig. 19). Region A, brackish, caves and freshwater, contains species in classes 1 and 2 in Table VII. Region B and class 3 are synonymous. Region C includes all shallow shelf, eurybathic shelf, deep shelf and upper slope species ; it probably also includes a few species t,hat should be classed as epipelagic. Region D and class 8 are synonymous. Region E and classes 9 and 10 are synonymous but region E also contains the slope species of class 7. The hyperbenthic habit of many species is reflected by the distribution of stippling in Fig. 19. Mauchline (1972) suggested that deeper living species of mysids tended to be larger in adult body size than shallower living species. This thesis is illustrated in Fig. 20 where the frequency distributions of the maximum body lengths of the various species inhabiting the regions illustrated in Fig. 19 are shown. Data on either body length or depth of occurrence of 24 species are not available and these species are therefore not included in Fig. 19. The majority of species of less

4.

VERTICAL DISTRIBUTION AND MIGRATION

. . ' E

,

.

. .. .

. . ..

71

.

A. Brackish,caves,freshwater

C. Coastal .neritic

D. Epipelagic

E. Meso- and bathypelagic

FIQ. 19. The distribution, shown by the degree of stippling, of the species of mysids in the different regions of the marine environment.

than 5 mm total length occur in the littoral and neritic regions and few species in these regions are greater than 25mm in length. No species less than 5 mm in length is recorded from the meso- and bathypelagic region (including the slope) and an appreciable proportion of these species are more than 25 mm in length; the largest known mysids occur in this region. Many species perform a die1 vertical migration, rising towards or t o the surface layers during the night and returning to the deeper layers at daylight and remaining there throughout the day. The amplitude and form of the vertical migration is limited in the case of littoral and other shallow living species by the depth of water available to them. Mysids generally avoid bright illumination although Praunus $extiasus, for example, can be observed resting on the sand surface facing into the water current in bright sunlight a t a depth of less than 0.5 m; the related species P . neglectus is found resting in these conditions among macroalgae. Macquart-Moulin (1972, 1977a) demonstrated experimentally that Anchialina agilis, Qastrosaccus amatus, Q. lobatus and Siriella jaltensis avoid bright light but may be attracted to weak light. He further showed (1973b, 1977c) that some of these littoral

72

THE BIOLOQY OF YYSIDS

A . Brackish. caves, freshwater

n = 32

lo

----

n = 164

---E. Meso-and bathypelogic

n

=234

45 55 65 75 >80 Body length ( m m ) FIG. 20. The frequency of species of differing maximum body length in the various regions of the marine environment. 5

15

25

35

and shelf species have an endogenous rhythm of swimming activity. Anchialina agilis, frequently living pelagically, showed no clear rhythm and appeared to swim more or less continuously throughout the 24 hours. The hyperbenthic shelf species, Leptomysis gracilis and Siriella jaltensis also showed no clear die1 rhythm of activity. The littoral species S . clausii tended to be more active at night than during the day. The four species, Gastrosaccus armatus, G . lobatus, G. mediterraneus

4.

VERTICAL DISTRIBUTION AND MIGRATION

73

and G. spinifer burrow into the sand during the day and emerge a t night into the pelagic environment; these exhibited the most pronounced diel rhythm of activity. A sexual difference of behaviour was observed in Anchialina agilis where no diel rhythm was present in females but a pronounced rhythm recorded from males. The only deep living species investigated, the upper slope Lophogaster typicus, rested continuously half-submerged in the sediment and showed no Eign of a rhythm of activity. Macquart-Moulin’s experimental observations confirm the conclusions of other investigators sampling wild populations. The following species investigated by the following authors live on or in close association with the surface of the sediment during the day but migrate to or towards the water surface a t night, a distance of up to 50 m : Acanthomysis sculpta by Hobson and Chess (1976) ; Anchialina agilis by Russell (1925), Macquart-Moulin (1965), Champelbert and Macquart-Moulin (1970) ; A. typica by Wigley and Burns (1971) ; Gastrosaccus lobatus by Macquart-Moulin ( 1965), Champelbert and MacquartMoulin ( 1 970) ; G. normani by Russell (1925)) Macquart-Moulin (1965) ; G. psammodytes by McLachlan et al. (1979); G. spinifer by Arntz (1971), Hesthagen and Gjermundsen (1979)) Moller (1979) ; Heteromysis formosa by 0. S. Tattersall (1967); Leptomysis gracilis by Russell (1925), Macquart-Moulin (1965), Champelbert and Macquart-Moulin (1970) ; L. lingvura by Colman and Segrove (1955) ; Metamysidopsis elongata by Clutter (1969); Mysis mixta by Wigley and Burns (1971); M . relicta by Mundie (1959), Beeton (1960), Wells (1960), McNaught and Hasler (1966), Lasenby and Langford (1972), Teraguchi et al. (1972, 1975), Wilson and Roff (1973), Grossnickle and Morgan (1979) ; Neomysis americana by Whiteley (1948), Hulburt (1957), Herman (1963), Hopkins (1965), Wigley and Burns (197 1); N . mercedis (awatschensis) by Heubach (1969) ; Paramysis intermedia and P. ullskyi by Boroditch and Havlena (1973) ; Schistomysis spiritus by Colman and Segrove (1955) ; Siriella chierchiae by Brattegard (1970a) ; S. clausii by Champelbert and Macquart-Moulin (1970) ; S. jaltensis by Macquart-Moulin (1965) ; S. norvegica by Champelbert and Macquart-Moulin (1970) ; S. pacijica by Hobson and Chess (1976). These migrations are probably not performed by the whole population. The investigations are not usually detailed enough to demonstrate this but where several depth horizons were sampled the mysids occurred at all depths during the night. This is a common form of migration in shelf and littoral species, part of the population rising all the way to the surface but a significant portion remaining a t depth. The exact proportion remaining in the hyperben thic habitat is extremely difficult to determine because it

74

THE BIOLOGY OF MYSIDS

involves comparison of the sampling efficiencies of pelagic nets and hyper benthic sledges. Some of the investigations quoted above, for example on Neowysis americana, were made in water columns of less Qhan 15 m depth. These shallow living species are often so closely associated with the sediment during the day that a diel vertical migration can be clearly demonstrated a t night. It can be interpreted as a spreading of the vertical range of the species rather than a vertical migration in which there is an appreciable vertical change in the depth of modal occurrence of the species. There is no evidence suggesting that some individuals migrate to the surface a t sunset and remain there until sunrise while others perform no migration at all. There may be a continuous interchange of individuals, upwards and downwards, within the vertical structure of the population. This would give the impression of a vertical migration when a time series of net samples was analysed but is really an extension of the vertical range of foraging of the population as a whole. A few species do not seem to perform a diel vertical migration and future work may show that many of these species do not migrate. Mysis mixta and Mesopodopsis slabberi are two examples according t o Hesthagen (1973). Mauchline (unpublished) obtained no evidence of a diel migration in Mysidopsis gibbosa or Erythrops elegans in the Clyde Sea area nor in Schistomysis ornata in the Clyde or Loch Etive. Mysidopsis didelphys migrated to a small extent but on many occasions appeared to remain hyperbenthic a t night. Not all populations of a species need necessarily perform a vertical migration. Whiteley (1948) found no evidence of a regular diel migration in Neomysis americana on Georges Bank where the depth at which they were living, as deep as 75 m, was greater than in the coastal regions where this species is known to migrate fairly regularly. The sexual difference in the rhythms of activity of male and female Anchialina agilis noted by Macquart-Moulin ( 1973b) has been observed in the environment. Tattersall (1951) found that male A . typica outnumbered females in samples taken a t night and Brattegard (1970a) made a similar observation and suggested that migration may be associated with sexual activity. Zatkutskiy (1970) and Herman (1963) examining Gastrosaccus sanctus and Neomysis americana found that females carrying embryos or larvae appeared to migrate more regularly than other components of the population. Bacescu (1975), on the other hand, noted that female Gastrosaccus msangii carrying young remained buried in the sand. No consistent differences between the behaviour of male, female and juvenile Mysis relicta were observed by Beeton (1960) although Grossnickle and Morgan (1979) found that

4.

75

VERTICAL DISTRIBUTION AND MIGfCATION

the largest males and females in a population in Lake Michigan occurred deeper than smaller individuals. Studies of the vertical migrations of the meso- and bathypelagic species are relatively few. Waterman et al. (1939) found that Boreomysis microps and Eucopia unguiculata performed a die1 migration (Table VIII) near Bermuda but Pearcy et al. (1977) could demonstrate no such migration in populations of the latter species in the eastern Pacific off Oregon. Pequegnat (1965) found evidence of a migration in the shallowest living Gnathophausia species, G . ingens, that he investigated while Murano (1977a) concluded that a similar migration is TABLEVIII. THEVERTICALMIGRATION OF OCEANIC MYS~DS Average day depth (m)

Average night depth (m)

Total vertical range (m)

Arachnomysis megalops Boreomysis californica B . microps

200-500

200-500

200-500

500-900

500-900

800

B . rostrata

600

Species

Diatance migrated (m)

Reference

0

Murano (19774

0-1000

0

400

200-1200

400

600

0-1000

0

100-700 200-700 0-300 0-500 200-700

100-3000 200-700 0-300 0-500 100-700

P 0

Pearcy et aE. (1977) Waterman et al. (1939) Pearcy et al. (1977) Murano (1977a) Murano (1977a) Murano (1977a) Murano (1977a) Murano (1977a)

400

200-1200

400

700

700

100-1 000

0

Gnathophausia gigas 2100

2100

600-4000

0

G . gracilis

1600

1600

700-3500

0

G . ingens

1100

500

250-2500

400

500-700 500

200-1200 0-2000

400

400-1500

0

Caesaromysis h,iapida 300-3000 Euchaetomera tenuis 200-500 E . typica 0-300 E. glyphidophthalmica 9 Euchaetomeropsis 100-300 merolepis Eucopia unguiculata 800

Katerythrops oceanae 500-700 Meterythrops 900 microphthalma Teraterythrops robUSta -

-

0 0 0

0

Waterman et al. (1939) Pearcy et al. (1977) Pequegnat (1965) Pequegnat (1965) Pequegnat (1965) Murano (1976) Murano (1977a) Murano (1975)

76

THE BIOLOGY OF MYSIDS

performed by Meterythrops microphthalma. Boreomysis curtirostris, B. incisa, B. plebeja and B. semicoeca performed vertical migrations through 4000-5000 m according to Tchindonova (1 959) who found that they had fed on diatoms and surface living crustaceans. Such extensive vertical migrations may not be regular in occurrence but only be performed irregularly. Nemoto (1959) recorded a juvenile Gnathophausia gigas among the stomach contents of a surface feeding fin whale, while Holthuis and Sivertsen (1967) found G. ingem, G . zoea and Chalaraspidum alatum among the stomach contents of an albatross. These latter authors suggest that upwelling of deep water may have been responsible for the presence of these species at the surface and, without further information, this seems plausible. It would appear, therefore, that the majority of deep living species do not perform a diurnal vertical migration to any marked extent. The cost to an animal of a die1 vertical migration in terms of energy expended may be less than previously considered (Vlymen, 1970; Klyashtorin and Yarzhombeck, 1973). Barker (1973) and Foulds and Roff (1976), examining energy expended by Mysis relicta, found no evidence of extra energy expenditure. Teraguchi et al. (1972), however, working with male M . relicta found that they lost about 8% of the calorific content between the start of the evening ascent and the end of the morning descent. Factors that modify the vertical distribution and migrations of mysids are not well documented but they are undoubtedly essentially the same as those that influence the behaviour of other crustaceans. Light intensity appears to be the dominant factor controlling the vertical distribution of mysids (Beeton, 1960 ; McNaught and Hasler, 1966; Heubach, 1969; Wilson and Roff, 1973; Teraguchi et al., 1975). The presence of moonlight or fog influences the extent of the migration. Mysis relicta appears to have increased its depth of occurrence in response to increased penetration of light following the diversion of sewage effluents in Lake Washington (Eggers et al., 1978). The vertical distribution of temperature can have a severe modifying effect upon migration. A sharp thermocline with a gradient of 2'C is an effective barrier to the upward migration of Mysis relicta; migration takes place through weaker thermoclines (Beeton, 1960 ; McNaught and Hasler, 1966). Young mysids were found to be tolerant to vertical stratification of the water, under experimental conditions (Harder, 1968). The vertical distribution of M . relicta in Lake PaajBrvi, Finland is controlled by the position of the 7°C isotherm (Hakala, 1978). Boroditch and Havlena (1973) found that Paramysis intermedia and P. ullskyi, two species that have been introduced t o the freshwater

4.

VERTICAL DISTRIBUTION AND MIGRATION

77

reservoirs of the Volga River, avoid wave action in the litt.ora1 region by moving temporarily offshore to a depth of 2 m. This is true of other littoral species such as Schistomysis spiritus and S. kervillei which move to depths of 2-3 m in exposed situations on the Scottish coast. Flood and ebb tides may also affect the vertical distributions and migrations of some species. Heubach (1969) found that Neomysis mercedis (awatschensis) in the Sacramento-San Joaquin river estuary occurred higher in the water column during both the day and night when the tide was flooding; the flood tides had lower velocities than the ebb tides. Finally, Wilson and Roff (1973) found that My& relicta appears to migrate higher in the water column when the values of surface chlorophyll are high in Lake Ontario. Ontogenetic migrations have been investigated in few deep living species. Casanova (1970, 1977) found that younger Eucopia unguiculata ( E . hanseni) live higher in the water column than older ones. Page (1 941, 1942) presents evidence of ontogenetic migrations in Ceratolepis hamata, Gnathophuusia zoea and Lophogaster spinosus. A similar situation exists in populations of the freshwafier Mysis relicta ; Lasenby and Langford (1972) and Reynolds and De Graeve (1972) found that in vertical water columns of 30 and 80 m respectively adult M . relicta tend to have a modal depth horizon deeper than that of the juveniles. Almeida Prado (1972, 1973) noted that young Bowmaniella brasiliensis and Mysidopsis tortonesei migrate to the surface while the adults remain a t 8 m depth close to the surface of the sediment. A horizontal migration is combined with an ontogenetic vertical migration in a population of Schistomysis ornata studied within a sea loch in Scotland (Mauchline, 1970a). This is illustrated in Fig. 21. The young are concentrated in the innermost region of the loch at depths of approximately 35 m while the mature adults are concentrated towards the seaward end of the loch at depths of approximately 65 m. The region between, which has depths of over 100m present, is sparsely populated by immature juveniles. Hulburt (1957) found a gradient of body size in samples of Neomysis americana over a 20 km length of the Delaware River Estuary; larger individuals tended to occur higher up the estuary than small individuals. Seasonal changes in the vertical distribution of several species have been found. Changes to avoid seasonal extremes of temperature and salinity can be expected in littoral and brackish environments. Kinne (1955) found that, Neomysis integer living in a canal migrated horizontally at first to warmer regions in the autumn. They then moved to deeper but warmer hyperbenthic regions when the warmer shallower areas cooled in the early winter. Mesopodopsis slabberi

-.N Depth (m) 35

lemp. Sol.

("C)

(%o)

10-5 25.0

2km

Depth (m) 110

Body length ( mm ) juveniles;

0immature d a n d ?;

my?with empty marsupium;

* Temp.

Sol.

("C)

(%o)

8.8

=

27.0

??with young;

mature males

4.

VERTICAL DISTRIBUTION A N D MIGRATION

79

appears to exhibit the same behaviour in the Black and Azov Seas (Zatkutskiy, 1970). It is normally regarded as a shallow brackish wa,ter species (Tattersall and Tattersall, 1951) but has a relatively wide bathymetric range of 0-20m during the spring and summer. According to Zatkutskiy, it migrates from the shallow lagoons of the Black and Azov Seas to offshore regions with depths as great as 60 m where it passes the winter. Hesthagen (1973) suggests that Gmtrosaccus spinifer behaves similarly in the western Baltic. It is probable that some littoral and brackish water species will migrate during summer to avoid adversely high environmental temperatures. Many coastal situations subject to freshwater run-off from the land have seasonal periods of low salinity regimes. Praunus jlexuosus, occurring in the littoral region of Loch Etive (Fig. 21), inhabits the inner end of the loch but moves seaward after periods of rainfall have reduced the salinity markedly below 20%,. Teraguchi et al. (1975) examined the changing seasonal response of the freshwater Mysis relicta to illumination. Lasenby and Langford (1972), whom the previous authors do not refer to, found that this species progressively commences its vertical ascent earlier and its descent later during the period spanning the early summer and late autumn. Teraguchi et al. suggest that it has a circannual rhythm in its response to changes in environmental illumination and this modifies the resultant pattern of vertical migration. Pearcy et al. (1977) found that the layer of maximum abundance of tihe bathypelagic Boreomysis californica, within the 500-1000 m depth horizon, occurs about 100 m shallower in winter than in summer. This could be an effect of the changing seasonal altitude of the sun off Oregon resulting in a seasonally changing intensity of ambient light. Mysids live in a wide range of habitats and exhibit a wide range of behaviour. Many species, living in the immediate environs of marine laboratories, are excellent experimental organisms for a variety of purposes. The vertical distribution and migratory behaviour are aspects of the biology of many species bhat require further investigation. Ontogenetic differences in the behaviour of individuals within a population, often reflected in seasonal differences in the behaviour of the population as a whole, compounded with horizontal migrations

FIG.21 (opposite). Population histograms of samples of Schistomysis ornata taken from different regions of Loch Etive, Scotland, in November, 1967. The positions of the samples are shown by the bars; the 25, 50 and 100 m depth contours are indicated by the dotted lines. Depth, salinity and temperature of the water where the samples were taken are shown.

80

THE BIOLOGY OF MYSIDS

of aggregations or swarms that cause difficulties in location of the animals a t subsequent sampling dates, make studies of their vertical distribution difficult. Nevertheless, studies of the behaviour of these animals would aid considerably in an understanding of that of less accessible organisms.

CHAPTER 5

THE GUT, FOOD AND FEEDING Mysids live in a wide range of habitats with a diversity of food resources that they exploit very successfully. This is reflected in the different forms of the feeding appendages. These have been modified in a number of ways throughout the Order but the functional significance of many of the modifications is unknown. The feeding appendages of Gnathophausia xoea, Lophogaster typicus and Hemimysis lamornae have been studied in detail by Manton (1928s) and Cannon and Manton (1927a), who review the earlier work. The general arrangement of the appendages is described in mysids by these authors and in euphausiids, in which they are analogous, by Mauchline and Fisher (1969). They consist, in anterior to posterior order, of an unpaired labrum and paired mandibles, paragnaths (labia), maxillules and maxillae. The thoracic legs also frequently function in feeding. The labrum is single, anterior to the mouth, and projects ventrally. The biting surfaces of the mandibles fit against its posterior surface. The paragnaths are relatively simple in form, plate-like and project ventrally, fitting closely against the posterier faces of the mandibles. Next are the maxillules and maxillae followed by the thoracic legs. The labrum and paragnaths form a chamber ventral to the mouth and cutting surfaces of the mandibles, which constitute the lateral walls of this chamber and macerate food within it. The appendages of the different species of mysids have been illustrated many times in the taxonomic literature. No comprehensive, comparative study however, of their morphology relative to their function has been made. The mandible has cutting, grinding and macerating regions and also a forwardly directed mandibular palp (Fig. 22). The general form and proportions of the mandible and its palp can vary considerably. Those of Schistomysis ornata are of a general form found throughout the Order while that of Petalophthalmus armiger is an extreme form with a much enlarged and modified palp. The terminal segment of the palp often has a cleaning mechanism of the form found in Schistomysis ornata (Fig. 22); this is presumably used to clean the cutting

82

THE BIOLOOY OF MYSIDS

FIQ.22. Right mandible of (A) Schistomysis ornatu and (B) Petalophthalmw armiger. cm, cleaning mechanism.

and grinding surfaces of the mandible as well as to remove food from the more posterior mouthparts and push it into the mouth. The morphology of the cutting and grinding surfaces varies between species quite considerably (Fig. 23). The left and right mandibles are asymmetrical, as illustrated in Neomysis rayii (Fig. 23) and Gordon (1964) points out that, in consequence, both mandibles should be drawn. This asymmetrical design allows the cutting surfaces to dovetail into one another. All mandibles illustrated in Fig. 23, with the exception of those of N . ra ii, are those of the right-hand side viewed from the anterior aspect. aking that of Boreomysis microps as an example, the cutting

4

' I

F D

H

G

J FIG.23. Cutting edges of the right mandibles of A, Gnathophawk zoea; B, Lophogaster typicus; C , Eucopia sculpticauda; D, Boreomysis microps; E, Meterythrops p i c k ; F, Antarctomysis maximu; G , Praunua $ e m o m ; H, Longithorax f w c w ; J, Neomysis rayii showing asymmetry of right and left mandibles. c, cusp, synonymous with lacinia mobilis; ip, incisor process;'lm, lacinia mobilis; pm, pars m l a r i a ; 81, spine row.

84

THE BIOLOGY OF MYSIDS

edge includes a dorsal grinding region, the pars molaris, very large in this species. This region is' adjacent to the mouth. An extensive row of prominent spines, the spine row, is present in the lower and central region. Ventral to it is the lacinia mobilis, spines which are movable, and the incisor process. Not all of these elements are present in other species. The left-hand mandibles of Gnathophausia, Lophogaster and Eucopia species have a lacinia mobilis but the right-hand mandibles only have a fixed cusp (Fig. 23). The spine row is simple in the above genera of lophogastrid mysids and almost completely absent in Longithorax fuscus, but highly developed in Meterythrops picta. The grinding area of the pars molaris is reduced in size in Lophogaster typicus and absent in Meterythrops picta and Longithorax fuscus. It is also reduced in size or absent in species of Amuthimysis, Arachnomysis, Echinomysides, Mysidopsis, Heteromysis and Stygiomysis. The commonest forms of mandibles found throughout the Order are similar to those exemplified by Antarctomysis maxima, Praunus jlexuosus and Neomysis rayii (Fig. 23). The size of the pars molaris relative to the rest of the mandible is related to the diet of euphausiids (Mauchline, 1980). A large pars molaris is associated with herbivorous feeding, the hard diatom frustules being ground by this region. This relationship is probably true in many species of mysids, in, for example, the genera Praunus, Antarctomysis, Neomysis and Acanthomysis. The function of a large grinding region in mandibles of deep living species where a diet of diatoms is not obviously available may be different. Tchindonova ( 1 959) examined and figures the mandibles of Gnathophausia gigas, Eucopia australis, Boreomysis plebeja, B. sibogae, Meterythrops microphthalma, Dactylamblyops laticauda and Petalophthalmus armiger. She examined the stomach contents of a wide range of oceanic species and found that the stomachs of Boreomysis curtirostris, B. incisa, B. plebeja and B. semicoeca caught in the 2000-4000 and 4000-7500 m horizons contained slightly digested remains of diatoms and surface living crustaceans that they had eaten during their vertical migration. Consequently, the presence of a well developed pars molaris in a supposedly deep living species may indicate that it obtains phytoplankton in significant quantities either directly or indirectly from the surface layers. The labium or lower lip of mysids is cleft, the ventrally projecting plates being known as a paragnaths (Fig. 24). These lie closely against the posterior faces of the mandibles. Gordon (1960) examined the paragnaths of a variety of species because those of Stygiomysis holthuisi (Fig. 24) are peculiar in that they are spoon-shaped rather than

5.

THE CUT, FOUL) AND FEEDINC:

B

85

B

E

FIG. 24. The paragnaths of the labium of A, Neomysis ‘czerniavskii;B , Gnatlwphausio zoea, anterior view showing asymmetry of paragnaths; C, ct. zoea, posterior view showing how left paragnath fits into the recess in the incisor region of the mandible; D, Pekclophthalmus arrniger ; E , Stygiomysis holthuisi ; F, Boreomysis Tostrata (Figs D-F after Gordon, 1960).

rectangular like the more typical paragnaths of Neomysis czerniavskii (Fig. 24). Clarke (1961a) describes the asymmetrical paragnaths of Gnathophausia species and points out that such paragnaths also occur in at least some species in the genera Chalaraspidum, Pseudochalaraspidurn, Lophogaster, Paralophogaster, and Eucopia. Those of Gnathophuusia zoea are shown in Fig. 24. The left paragnath is more developed and is armed with tubercles and ridges. It fits into the posterior face of the left mandible which is curved in the incisor region to receive it (Fig. 24). The right paragnath is thin, conventional in form and is not coupled with the right mandible. The paragnaths have no muscular system and the mandible induces movement in the left paragnath whose tubercles and ridges then form an integral part of the mandible.

86

THE BIOLOGY OF MYSIDS

The functional significance of variations in the shapes of the symmetrical paragnaths of the mysids is unknown. The form of the maxillule in a variety of species is shown in Fig. 25. It is relatively simple, consisting of a proximal and distal endite. The distal endite invariably has a row of more or less strong spines while the proximal endite is armed with setae and three or four long spines. The posterior face of the distal endite often has a group of three or more strong setae, shown hatched in Fig. 25. The appendage

A

D

C

B

E

F

FIG. 25. The right maxitlule of A, Eucopia sczclpticauda; B, Meterythrops picta; C , Longithorux fuscus ; D, Antarctomysk maxima ; E, Schistomysis ornata; F, Praunus Jexuosus. de, distal endite; pe, proximal endite.

is the most constant in form and, therefore, probably in function. One variation that exists in relatively few genera is a reduction in the number of long spines, as opposed to setae on the proximal endite. Species in the genera Mysidopsis, Metamysidopsis, Promysis and probably some species of Lophogaster and Siriella have only two spines while Pillai (1973) figures this endite of Siriella jonesi with one spine. The only other marked variation in form occurs in Mysidella species where the spines on the distal endite decrease in size dorsally round its curved edge. The maxilla can vary considerably in form, especially in the relative proportions of the exopod and terminal endopodite and in the setulation of the appendage as a whole (Fig. 26). The maxilla of Lophogaster typicus has long weak setae and no spines; the form of maxilla

1

I

e

C

'

\\\

FIQ.26. Right maxilla of A, Lophogaster typicus; B, Meterythrops picta; C, Boreomys~s microps; D, Longithorax fuscus; E, Eucopia sculpticauda ; F, Petalophthalmua armiger; G , Praunus fiexuosus; H, Antarctomysis nutxima. e, exopod; 8 , spine on proximal lobe.

88

THE BIOLOGY OF MYSIDS

found in this species is uncommon among mysids. The maxilla of Meterythrops picta is of a form typical of other species in the tribe Erythropini; it is heavily setose with a few spines present. The maxilla of Boreomysis microps has many fine filtering setae and a row of spines is present on the internal (lateral) edge of the terminal endopodite; such a maxilla occurs in many species of the tribe Erythropini and also in species in the genera Hansenomysis, Siriella, Dioptromysis and Heteromysis. The type of maxilla found in Longithorax fuscus also occurs in some species of Siriella, Anchialina and Gastrosaccus; it is lightly armed with plumose setae and has few, if any, spines. The maxilla of Eucopia sculpticauda is closely similar to those of species in the genera Gnathophawia, Anchialina and Mesopodopsis ; the terminal endopodite is enlarged and the appendage is heavily armed with plumose setae but with few spines. The form of maxilla found in Yetalophthulmus armiger is rare and may be restricted to this genus; it is fragile and armed with weak setae and a long terminal spine on the terminal endopodite. The maxilla of Praunus Jlexuosus is typical of those of many species in the tribe Mysini but also occurs in species in the genera Dactylamblyops, Eoerythrops, Pseudomma, Leptomysis, Mysidetes and Spelaeomysis; it is characterized by a long strong spine on the proximal lobe (Fig. 26) and a row of spines on the inner (lateral) edges of the Oerminal endopodite and the other lobes. The terminal endopodite of the maxilla of Antarctomysis maxima has a more extensive and ventrally situated row of spines which is present in this region of the maxillae of species in the genera Bathymysis, Mysis and Hemimysis. This row of spines is even more accentuated in the maxillae of species in the genera Afromysis, Doxomysis, Hyperiimysis, Iimysis, Nouvelia, and Pseudoxomysis (see Figs 7.27, 7.29-31, 8.3). The only major form of maxilla not illustrated in Fig. 26 is a maxilla of the general form of that of Longithorax fuscus but with relatively few short setae interspersed between relatively many sharp spines, arranged and directed internally towards those of the opposite appendage. This maxilla occurs in species in the genera Cubanomysis, Mysidopsis, Metamysidopsis and Promysis of the tribe Leptomysini and also in species of Stygiomysis and Spelaeomysis. The form of the thoracic appendages varies and their morphology is documented in detail in the taxonomic literature. They have a welldeveloped endopod and exopod. The exopods itre of the general form shown in Fig. 27A, B and may rarely, in the first thoracic leg, be reduced as in Longithorax fuscus (Fig. 27C), or absent. The form of the endopods is more variable. That of the first thoracic leg is usually relatively shorter than those of more posterior legs. It may be of the

5.

8!,

T H E UUT, FOOD AND FEEDING

relatively simple form found in L. fuscws or it may have well-developed lobes on the second, third and fourth segments as in Schistomysis ornata (Fig. 27). The endopods of more posterior thoracic legs are usually of the form illustrated for the second leg of S. ornata. The morphology of the terminal segments of the endopods can vary. The terminal segment of either one or both of the first and second legs may have a cleaning mechanism similar in structure to that on the

en

e

E Fro. 27. Thoracic legs. A, first thoracic leg and B, second thoracic leg of Schistomysia ornata; C, first thoracic leg of Longithorarfwcus; D, terminal segment of the endopod of the third thoracic leg of Eucopia sculpticauda; E , terminal segments of the endopod of the third thoracic leg of Heteromysis nzicrops; F, enlargement of one of the structures indicated in the penultimate segments of the leg of H. microps (E and F after Tattersall and Tattersall, 1951). en, endopod, ex, exopod, 1, lobe.

distal segment of the mandibular palp of S. ornata (Fig. 22, p. 82). These segments may terminate in a nail (spine) or a series of spines as instanced in Longithorax fuscus and Eucopia sculpticauda (Fig. 27), and in Petalophthalmus armiger. The presence of a terminal nail or spine, usually of a less robust form than that of Eucopia sculpticauda, on the terminal segment of the endopods of the thoracic legs is a

00

THE BlOLOCY OF MYSIDS

general feature of mysids. It occurs on the endopods of a t least the third to eighth thoracic legs of most Lophogastrid species, of species of Boreomysis, Siriella, the tribes Erythropini and Leptomysini, some genera of the tribe Mysini, and Heteromysini. They are much less developed in genera such as Gastrosaccus and Anchialina, Parampsis and Schistomysis, and Mysidella. In genera such as Eucopia and Hansenomysis, the terminal nail and one or more setae on some endopods form a pseudochela. Several species of mysids have one or more pairs of the thoracic endopods especially modified. One of the most spectacular modifications is the enlargement of third endopod of Heteromysis microps (Fig. 27) which is larger than the whole cephalothorax. The penultimate segment, the merus, has ten peculiar structures with a barbed seta emerging from a terminal socket (Fig. 27). The nail and two terminal projections (spines) form a pseudochela. Elongation of one or more pairs of thoracic endopods is not a common feature of mysids. The fifth to eighth pairs are elongated relative to the others in Eucopia species while the third pair alone is elongated in Wemisiriella species. The funct;ional aspects of these modifications of the mouthparts and thoracic appendages of mysids are, for the most part, unknown. The only detailed study of how the appendages are used in feeding is that of Cannon and Manton (1927a) on Hemimysis lamornae and some earlier studies referred to by them. This species might be described as a conventional or generalized mysid whose appendages are of a form common throughout the order. It feeds, according to Cannon and Manton, by several methods. One way is by a modification of the generalized filter feeding method that is presumably used by the animal when it is not in direct contact with the surface of the sediment. The modification involves the animal standing on its head on the surface of the sediment and lifting the surface material into suspension by creating a water current with its thoracic exopods. This material is then filtered from the water by the appendages. Another method is used to feed on large particles, a method often described as raptorial. Dead carcasses can be lifted from the surface of the sediment by the thoracic endopods and manipulated against the mouthparts, primarily by the mandibular palps and the endopods of the first two pairs of thoracic legs. The anterior thoracic endopods in conjunction with the mouthparts in many species form a “food basket” analogous with that described in euphausiids (Mauchline and Fisher, 1969). Organic matter from the surface of the sediment is collected by the appendages, especially the mandibular palp, and held in the “basket” prior to maceration by the mouthparts. Gastrosaccus simulans

5.

THE OUT, FOOD AND FEEDING

91

appears to be a strict filter feeder (Nath and Pillai, 1976), feeding on diatoms and dinoflagellates and some algal filaments, although it becomes carnivorous in the laboratory. Mysis relicta is considered by Bowers and Grossnickle (1978) to be a major grazer of phytoplankton in Lake Michigan. Lophogaster typicus according to Manton (1928a) is incapable of filter feeding and the modification of its mouthparts facilitates feeding on large lumps of food material on the surface of the sediment. Molloy (1958) observed Praunus Jtexuosus browsing on the epifauna of seaweeds. Some species of mysids are carnivorous and Lasenby and Langford (1973) observed Mysis relicta attacking and eating live Daphnia. Tchindonova (1959) and Vinogradov (1962) state that Petalophthalmus armiger and Longithorax fuscus catch their prey and suck out the internal contents, a habit known to occur among euphausiids feeding on copepods (Mauchline and Fisher, 1969). Molloy (1958) found that Neomysis integer and Praunus jlexuosus, feeding on dead carcasses of mysids, preferred the thoracic to abdominal region of the carcass. Once the food has been gathered and macerated by the appendages it is passed to the mouth and eaten. Gelderd (1907) describes the histological structure of the alimentary canal of Anchialina agilis, Gastrosaccus spinifer, Mesopodopsis slabberi, Mysidopsis gibbosa, Neomysis integer, Praunus jlexuosus, Schistomysis ornata, S . spiritus and a species of Siriella. Siewing (1953) illustrates the internal armature of the stomach of Eucopia sculpticauda. Molloy (1958) has re-examined some of these species but added others ; she studied Anchialina agilis, Gastrosaccw spinifer, Hemimysis lamornae, Leptomysis mediterranea, Mesopodopsis slabberi, Neomysis integer, Praunus jlexuosus, Siriella armata and 8. jaltensis. More recently, Nath and Pillai (1972, 1976) examined the structure of the canal in Spelaeomysis longipes and Gastrosaccus simulans. The alimentary canal of mysids has the same general structural plan as that of the decapods. The arrangement of the alimentary tract is shown in Fig. 28. The oesophagus is short and muscular, lined with chitin and has spines or setae that tend to point upwards towards tihe stomach. The stomach is divided into an anterior cardiac region and a posterior pyloric region. Both regions are usually armed internally with spines and setae. The detailed form and armature of the stomach varies between species. The stomach of Neomysis integer (Fig. 29) is typical of that of many species. The anterior cardiac region of the stomach of Mesopodopsis slabberi seems to be exceptionally small relative to the pyloric region. The latter species, however, has an unusually long oesophagus. The armature of the stomach of Hemimysis lamornae, Neomysis integer

92

THE BIOLOGY OF MYSIDS

FIQ.28. Generalized diagram of the digestive glands and anterior region of the alimentary canal of a mysid (after Molloy, 1958). cr, cardiac region of stomach; dd, dorsal diverticulum; dg, digestive glands shown stippled; i, intestine; oe, oesophagus; pr, pyloric region of stomach.

and Praunus Jlexuosus is strong but less strong than that found in the stomachs of Leptomysis mediterranea, Siriella armata and S. jaltensis. These armatures contrast strongly with the markedly reduced armature of the stomach of Mesopodopsis slabberi, studied by both Gelderd (1907) and Molloy (1958). Nath and Pillai illustrate a heavy armature of the stomach in Gmtrosaccus simulans but a lighter armature in Spelaeomysis longipes. Mysids have a single or paired dorsal diverticula that arise as extensions of the mid-gut in the posterior dorsal area of the pyloric region of the stomach (Fig. 29). The peritrophic membranes, within which the faecal material is bound, are continuously secreted by them into the intestine. The diverticulum is relatively large, but single, in Hemimysis lamornae, Neomysis integer, Praunus Jlexuosus, Schistomysis ornata and S. spiritus but are smaller and paired in Leptomysis mediterranea, Mysidopsis gibbosa, Mesopodopsis slabberi and Siriella armata (Gelderd, 1907; Molloy, 1958). Both Molloy and Gelderd found that the diverticula are paired in Gmtrosaccus spinifer while Nath and Pillai (1976) found a single diverticulum in G . simulans. These latter authors found that Spelaeomysis longipes also has a single diverticulum and that it is extremely long, reaching the anterior end of the stomach. The digestive gIands consist of two groups of five digitiform processes, one group on each side of the body (Fig. 29). They open by a common duct from the posterior ventral area of the pyloric region of the stomach, the longest pair of elements reaching as far posteriorly as the fourth abdominal segment. The intestine emerges from the posterior end of the pyloric region of the stomach in approximately the third or fourth thoracic segment.

A

Fro. 29. Longitudinal sections of the stomachs of A, Neomysis integer and C, Mesopodopsis slabberi. B, a transverse section of the cardiac region of the stomach of Hem.imysis Zamomae (after Molloy, 1958). cr, cardiac regions of stomach; dd, dorsal divert~uulum;m, mouth; oe, oesophagus; pr, pyloric region of stomach.

94

THE BIOLOGY O F MYSIDS

It extends posteriorly as the mid-gut to the posterior end of the fifth abdominal segment where it becomes the hind-gut lined with chitin. The hind-gut is thicker in diameter and more muscular than the midgut, the detailed histology of the whole intestine being described by Gelderd (1907) and Nath and Pillai (1972, 1976). The mid-gut of the freshwater subterranean species, Spelaeomysis longipes, has posterior caeca that are the site of calcium storage. This calcium is accumulated during premoult stages because it is not readily available in the environment (Nath, 1972). The intestines of mysids exhibit retroperistalsis (antiperistalsis) which is more or less continuous in some species and not in others. Molloy (1958) found that these contractions were present more or less continuously in Praunus Jlexuosus, whether faecal material was presenb in the abdominal region of the intestine or not. Retro-peristalsis in Neomysis integer, on the other hand, seemed to be initiated when faecal material passed into the abdominal region of the intestine. Faecal material, however, was able t o move posteriorly along the intestine even in the absence of any obvious retro-peristaltic contractions (Molloy, 1958). Vigorous retro-peristaltic contractions start when the faeces enter the wider hind-gut region, continue until the faeces are successfully voided, and then cease shortly afterwards. Fox (1952) found that Hemimysis lamornae and Siriella arrnata, in common with many crustaceans, draw water into the gut both through the mouth and anus. The function of such oral and anal intake of water is probably to stretch the gut wall so that the muscles are toned to act rhythmically and effectively. Molloy noted that the food required 30-40 min, occasionally as much as 1.5 h, to pass along the complete length of the alimentary tract. The food was in the stomach and anterior region of the midgut for most of this time. A faecal string, once formed in the thoracic region of the mid-gut, usually passes out of the anus within a few seconds. Part, a t least, of the food passes from the pyloric region of the stomach into the lumina of the digestive glands where it is circulated from one tube to another via the lower pyloric region. The walls of the digestive glands contain circular muscles which power the movement of food and digestive enzymes within the lumina. The digestive glands not only produce digestive enzymes but the finely particulate food is digested and absorbed there. They act as storage tissues for lipids and possibly glycogen; Molloy did not find marked quantities of glycogen but, as Raymont et al. (1968) point out, this is used rapidly and would probably only be detected in freshly caught animals. Molloy found that it was present in the glands when the diet contained excess carbohydrate.

5.

THE GUT, FOOD AND FEEDING

95

The stomach contents of various species of mysids have been examined (Table IX). The food of mysids, like that of many other types of crustaceans, is macerated so that stomach contents primarily consist of unidentifiable, finely particulate matter. The main conclusion that can be drawn from examination of the stomach contents of species is that they eat a great variety of organic materials in their environments. The two Mysidopsis species (Table IX) are rather exceptional i n that their diets would appear to be dominantly carnivorous. On the other hand, Gastrosaccus simubns appears to be a strict filter feeder (Nath and Pillai, 1976) and Mysis relicta can undoubtedly exploit the spring bloom of the diatom Melosira sp. in Lake Michigan although its diet is more catholic a t other times (Bowers and Grossnickle, 1978). Muus (1967) suggests that dense shoals of Neomysis integer in a Danish fjord were feeding on the organic effluent from a slaughter house. Ontogenetic and seasonal changes can be expected in the diet of many species although these have not been investigated, with one exception. Kost and Knight (1975) found that diatoms become progressively less important while detritus becomes progressively more important in the diet of Neomysis mercedis as the youngest juveniles of 2 mm body length grow to adults of 15 mm body length. Brown and Talbot (1972) suggest that Gastrosaccus psammodytes feeds more commonly a t night than during the day. Die1 feeding rhythms are probably present in some species and not in others. Siriella pacijica is quite clearly a nocturnal predator among the kelp beds of California, feeding on copepods and cladocerans, while Acanthomysis sculpta feeds by both day and night on Macrocystis pyrifera and crustaceans (Hobson and Chess, 1976). Assimilation of food by mysids has not been studied in any detail. Blegvad (1922), feeding Praunus species a mixture of mussel flesh and algae, found that 42-113 g dry weight food produced 1 g dry weight mysids. Mysids have been maintained experimentally in the laboratory but on a restricted range of foods. Pechen’-Finenko and Pavlovskaya (1975) fed Neomysis mirabilis on diets of vegetable or animals’ detritus, melanin and dinoflagellates. They fed most voraciously on Gymnodinium kowalewskii and preferred animal to vegetable detritus. Murano ( 1 9 6 6 ~ )found that N . intermedia ate live Artemia and daphnids but would not eat the yolk of chicken eggs or fish meal. They survived better if the bottoms of the culture vessels were covered with mud or sandy mud. Lasenby and Langford (1973) determined the assimilation efficiencies for different foods in Mysis relicta. Daphnia pulex was assimilated with an efficiency of about 85% and chironomid

96

THE BIOLOGY OF MYSIDS

TABLE IX.

Species Acanthomysis aculpta Antarctomyais maxi^ Borwmyais sp. Erythrops elegans Erythrops serrata Gastroaaccua psammodytes Qaatrosaccua apinifer Leptomyais gracilia Mesopodopais slabberi Mysidopais didelphya Mysidopais gibbosa Mysis mixta Myaia relicta Neomyais integer Neomysis intermedia Neomyais mercedis Paramyais arenosa Praunua JEexuosUs Praunua neglectua Praunus spp. Schiatmyaia kervillei Schistomyais ornata Schistomysia spiritua Siriellu paci$ca Spelaeomyaia longipes

DinoDetri&geltus Algae D i a t o m lates

TintinPolynida Rotifers Sponges chaetes

X

x

X X

X

X X

X

X

x

x

X X

x X X

X X

X

X

X

X

x

x

X

X

X

X

X

X

X X

X

x x

x x

X

X X X

X

X

X

X

X

X

X

X X

X

X

X X

X

X

1, Apstein, 1906; 2, Blegvad, 1922; 3, Depdolla, 1923; 4, Lucas, 1936; 5, Tattersall end 1968; 10, Mauchline, 1969a; 11, Mauchline, 1970a; 12, Mauchline, 1970b; 13, Mauchline, 19718; 1972; 18, Neth end Pillai, 1972; 19, Nemoto, 1972; 20, Lesenby and Langford, 1973; 21, Kost and Groesnickle, 1978.

larvae with an'efficiency of 82%. Clutter and Theilacker (1971), using a mysid that they do not name, fed phytoplankton and estimated the assimilation efficiency to be 90%. Mysis stenolepis is able to assimilate sterile raw cellulose with efficiencies of 30-50y0 and sterile hay with efficiencies of 20-35y0 (Foulds and Mann, 1978). The presence of micro-organisms on the cellulose and hay surprisingly decreases the assimilation efficiencies. Foulds and Mann suggest that a gut microflora may be involved in the breakdown of the cellulose. Shushkina (1972) determined K,, the percentage of assimilated food that is converted into new tissue, in Neomysis mirabilis as close to 57%.

5. THE

97

OUT, FOOD AND FEEDINQ

FOODOF MYSIDS ~

Terrigenous Mol- Ostra- Clado- Cope- Amphilwca cods pods pods Crustacea Carrion Material Insects cera X X

Faecal Pellets

X X X X

x

X

X X

X

X X

X X

X

X

X

X

X

X X

X

X X

X

X X

X X

X

X X

X X X

X

X

X

X

x

x

X X

X

X

X

X X X

X

Referencm 22 23 19 9 9 17 2 10 6 12 12 1 5, 20 4, 6, 14 7 21 13 3, 15 15 2 16 11 5, 8 22 18

Tattersall, 1951; 6, Kinne, 1955; 7, Murano, 1963b, 1966c; 8, Mauchline, 1967; 9, Mauchline, 14, Mauchline, 1971b; 15, Mauchline, 1971c; 16, Mauchline, 1971d; 17, Brown and Talbot, and Knight, 1975; 22, Hobson and Chess, 1976; 23, Mauchline, unpublished; 24, Bowers

Pasternak (1977) extended this study and shows that K, changes with. age (body size). It has values of 70-80% .in early juveniles of N . mirabilis but these decrease t o 15-30% in the adults.

CHAPTER 6

CHEMICAL COMPOSITION The chemical composition of mysids has been investigated in recent years but centres on relatively few species. The genus Neomzjsis, which includes many shallow-water, estuarine and so readily accessible species, has received particular attention ; this is especially true of the north-eastern Atlantic species N . integer investigated primarily by Raymont and his collaborators. The results of proximate analysis (Table X ) of mysids are closely similar to analogous data on euphausiids. Dry body weight approximates closely to 20% of the body wet weight with the exception of an adult female Gnathophausia ingens. This is g, bathypelagic species and Childress and Nygaard (1974), in a comparative study of Crustacea, found that shallow living species (0-150 m) and deep living species (900-1200 m) have significantly more water present in their bodies than species living at intermediate depths. This is a broad generalization drawn from moderately variable data. Ash, chitin and the low values for carbohydrate are as expected (Table X) from analyses of comparable organisms. The ash content represents about 1*5-2*5y0 of the wet weight and consequently about 7-5-12.5% of the dry weight. Fukai et al. (1962) determined 8.3% of the body dry weight of Neomysis japonica, Fujika et al. (1969) 5.6% of the dry weight of a Neomysis sp. as ash, and Tracy and Vallentyne (1969) state that the freshwater Mysis relicta contains 21% ash and 15.6% lipid on a dry weight basis. Small sexual differences in body composition were demonstrated in Taphromysis bowmani ; males have significantly more protein and ash and significantly less chitin than females but there are no differences between the carbohydrate and lipid contents (Johnson and Hopkins, 1978). The variation in values for fat and protein are discussed later. Simultaneous seasonal variations in these constituents are described in N . integer by Raymont et al. (1966), in N . awatschensis by Chin (1974), and in Taphromysis bowmani by Johnson and Hopkins (1978); these sets of data are all on a dry weight basis. Elementary analysis of mysids are few and little can be concluded

'TABLE x. PROXIMATE ANALYSESO F

Species Gnathophausia ingens G . ingens (adult 0 ) G . gigas a. gracilia Qnath0pham.a sp. E w p i a sp. Boremy& culijornica Siriella aequiremia Leptomyais lingvura Metamydops-is elongataa Mys-is atenobpis M . relicta Praunua Jexuoaua P . negbctua MesOpodOpd8 slabbe& Neomys-ia integer N.a m h n a N . intermedia N . awatachemk Taphromysis bowmania

MYSIDS

(%

WET WEIGHT)

Water

Ash

Chitin

Carbohydrate

Fat

Protein

72.8 f 0.7 90.7 79.5 f 2.0 81.6f 0.4

3-90 2.65 3.77 3.81

2.54 0.76 2.15 1.18

82-6f 0-9 81.3 70.0

3.31 1.90

0-61

0.14 0.03 0.10 0.13 0.6 0-6 0.1

10.5 2-6 5.4 5.8 4-0 15.0 4.6

8.6 2.3 6.3 5.5 8.0 22.0 4.9

2.50

1.40

0.7-0.9 0.3

2.9-3.5 2.0

20.3-21.6 14.0

79.0 f 3.0 76.3 74.3-7 7.8 88.0 78.3 78.0-85.4 80.2 76-82

1.70-2.2 1'9-3.7

1.6 f 0.2 0.07-0.90

1.6-4.3

1.40

0.1-0.3 0.2-0.5 0.2

2.63 1.40 3-3-4.0

1.40 1.3-1.9

0.3-0.5 0.35-0.50

3.2-5.9

20.7f0.6

2.0 0.2-4.9

13.0-14.0

1.6-2.4 2-7 2-8-3.0

14.9 14.1 11-1-11.6

References 17 17 17 17 10 10 17 9 5 11 15 19,21 8, 18 4 1,4, 6, 7, 13, 18 2 3, 14 12, 16 20

0 Dry weight values transformed to wet weight by assuming dry weight is 20% of wet weight. References: 1, Raymont and Krishnaswamy, 1960; 2, Raymont and Conover, 1961; 3, Hirano et al., 1964a, b ; 4, Raymont et al., 1964; 6,Linford, 1965 and Raymont and Linford, 1966; 6, Raymont et al., 1966; 7, Raymont et al., 1968; 8, Seguin, 1968; 9, Omori, 1969a; 10, Raymont et al., 1969; 11, Clutter and Theilacker, 1971; 12, Lee and Chin, 1971; 13, Srinivasagam et al., 1971; 14, Teshima and Kanazawa, 1971; 15, Tyler, 1973; 16, Chin, 1974; 17, Childress and Nygaard, 1974; 18, Moore, 1976; 19, Salonen et al., 1976; 20, Johnson and Hopkins, 1978; 21, Yaguchi et al., 1974.

100

THE BIOLOGY OF MYSIDS

from them as yet, except that they appear closely similar to those of euphausiida, about wliich more is known. Vinogradova and Petkevich (1967) examined the concentrations of 25 elements in the ashes of Mysis microphthalma and of mixed samples of this species and Paramysis loxolepis (Table X I ) ; their data on these Caspian Sea species have been converted from a yo ash weight basis to an approximated mg% dry weight basis in Table XI by assumjng that the ash TABLEXI. ELEMENTARY COMPOSITION OF MYSIDS (MG/100 0 DRYW E I G H T )

Element

Neomysis integer

Neomysis Mysis Mysis microphthalma intermedia microphthalma Paramysis loxolepis 1140 51

Na Mg

Al Si

K C&

Ti Cr Mn Fe Ni

1050" 1.2b 47.6b

cu

Zn Ga Sr

497 750

26.5 6.1

3"

188 125 113 12.5 313 63 113 2-5 14 25 0.1 125

Ag SZl

Ba Pb

1-3 125 1.1

12-376 62-113 31-150 3-12 187-31 3 2 0.6 3-39 19-25 4-18 12-38 0.1 37-113 1.0

0.3 12-50 0'3-1.2

Data from: Fukai et aE., 1962; Hirano et al., 1964a, b; Raymont et al., 1964, 1966; Vinogradova and Petkevich, 1967; Fujika et al., 1969; Omori, 1969a. a Nemnysis japonica.

bNwmyak sp.

content represents 12.5% of the dry weight. Thommes et al. (1972) measured the mercury content of Mysis reticta in the'Great Lakes of North America in the years 1967-1968 and recorded 0.003 mg Hg/ 100 g wet weight. Mysis relicta, M . mixta, Neomysis integer and Praunus Jexuosus had 0-006-0.008 mg Hg/lOO g wet weight in 1971-1973 in the Baltic Sea (Nuorteva and Hasanen, 1975). The tantalum content of Neomysis integer was 0.23 mg/lOO g dry body weight; other invertebrates had lower concentrations, 0.001-0.024 mg/100 g dry weight

6. CHEMICAL COMPOSITION

101

(Burton and Massie, 1971). Leatherland et al. (1973) found the following concentrations per 100 g dry body weight of the following elements in Eucopia sculpticauda taken from the entrance to the Mediterranean in October, 1970; 3.0 mg arsenic; 0-2 mg cadmium; 0.0011 mg antimony; and 0-02mg mercury. The concentrations of many metals vary considerably within marine organisms in time and space because there are often major contributions of them to the environment from industrial effluents. Analyses of carbon, nitrogen, hydrogen and phosphorus have been made ir, the species shown in Table XII, and are within the expected TABLEXII. CARBON,NITROGEN, HYDROGEN AND PHOSPHORUS IN MYSIDS (yoDRYWEIGHT)

Speciea

C

N ~~

cfnathophazcsia ingens G . ingem (adult) c f . gigas

gracilia Lophogaster sp. Boremy& ealifornka Siriella aequiremia Metmysidop8is elongata whole body cf.

49.2 36.8 41.3 66.2 41.9 46.8 43.4 42.4

cest

36.8 23.5

egg

68.0

larva

Mysis relicta N e m y s i s integer N . internzedia

45.7 50.0 22-35

H

P

~

6.3 6.4 7.1 6.9

8.1 6.1 6.8

7.0

6.9

7.9 11.0

6.5

6.7

11.5

10.9 11.4

2.0-2.5 1.2

Data from: Curl, 1962; Hirano et al., 1964a, b; Tundisi and Krishnaswamy, 1967; Omori, 1969a; Tracy and Vallentyne, 1969; Vinogradov et al., 1970; Clutter and Theilaoker, 1971; Childress and Nygaard, 1974; Salonen et al., 1976.

ranges. Bordovskiy et al. (1976) estimated 65.2% of the dry weight of Gnathophausia gigas as carbon, a value somewhat higher than those given in Table XII. Jawed (1969) found 8-8-11.4% dry weight of Neomysis rayii as nitrogen. Some 1 6 2 3 % of the total nitrogen in Neomysis integer was found to be non-protein nitrogen (NPN), an increasing concentration within this range being associated with increasing environmental salinity ; this NPN is predominantly present &S free amino acids (Raymont et al., 1968). A lower range of NPN,

102

THE BIOLOGY OF MYSIDS

7-10% of the total nitrogen, was recorded in the warm-water species Taphromysis bowmani by Johnson and Hopkins (1978). Mysids, unlike euphausiida, have seldom been used in studies of uptake, retention and dispersion of radioactive isotopes. Fukai et al. (1962) recorded 0.8 p Ci90 Sr/kg wet weight in Neomysis japonica in Tokyo Bay in June, 1959. Marshall et al. (1975)studied the partitioning of plutonium-239, originating from radioactive fallout from weapons’ testing in the Great Lakes, and found that Mysis relicta and the amphipod Pontoporeia afinis Lindstrom had higher levels present in their bodies than other crustaceans. The concentration factors for Plutonium in these two species are 760 and 1600 respectively (Yaguchi et al., 1974). The higher concentrations of plutonium in these animals were assumed to have arisen from their feeding close to the surface of the sediment where the highest environmental accumulations of the isotope occurred. Marshall et al. (1975) also examined the ratios of fallout plutonium-239 to caesium-137 in the animals and environment during this period, 1972-1973. The calorific value of Mysis stenolepis has been estimated and found to range from 4563 to 4824, mean 4714, gcal/g dry weight representing a mean value of 99Ogcal/g wet weight (Tyler, 1973). The calorific values of temperate and high latitude species are likely to show seasonal variation as Tyler has demonstrated in several species of organisms, and Brattelid and Matthews (1978) showed in Boreomysis arctica. The calorific value of some deep-sea species has been calculated by Childress and Nygaard (1974) who found a range of 400-1500 g cal/g wet weight in four species of Gnathophausia and in Boreomysis californica. Johnson and Hopkins (1978) found that female Taphromysis bowmani have calorific contents consistently higher than males; the ranges were 4428-4712 g cal/g dry weight in females and 4178-4227 g cal/g dry weight in males. A similar range of values was found in Neomysis mirabilis by Shushkina et al. (1971). The calorific content of male Mysis relicta showed a loss of about 8% over the period of the die1 vertical migration (Teraguchi et al., 1972), the mean content being about 6200 g cal/g dry weight; such a loss was not detected by Barker (1973) and Foulds and Roff (1976), the former recording a range of 5500-7000gcal/g dry weight in this species. Wissing et al. (1973) measured a calorific value of 7533 g cal/g dry weight in unidentified mysids from the Gulf of Mexico. The calorific content of Boreomysis arctica increases slightly with increasing body weight in maturing and spent females but decreases slightly with increasing body dry weight in males. Males have a lower calorific content than maturing females but small spent females have the least

6.

103

CHEMICAL COMPOSITION

calorific content of all (Brattelid and Matthews, 1978). Maturing females, spent females and males of 15 mg body dry weight have calorific contents of 6500, 5975 and 5830 g cal/g dry weight respectively. Carbohydrates occur a t low concentrations in mysids (Table X, p. 99. Raymont et al. (1968)) in preliminary experiments, concluded that 4&50% of the carbohydrate in N e o m y s i s integer is present as insoluble material, probably as ribose (as nucleic acids). Some soluble pentose, about 40% of the total carbohydrate, is also present. Only traces of glucose and galactose have been detected. The glycogen stores appear to be utilized rapidly under conditions of unsatisfactory feeding, for example when the animals are caught in the wild and transferred to laboratory conditions ; the largest concentrations of glycogen were recorded from freshly caught individuals. The amount of information available about the lipids of mysids is less than that for the lipids of euphausiids. L. R. Fisher determined the gross lipid content of a number of species and his unpublished data are presented in Tsbles XI11 and XIV. Further analyses from TABLEXIII. LIPIDS(yoWET WEIQHT)IN SPECIESOF LOPHOGASTRIDA (L. R. Fisher, unpublished) Average body weight Species Eucopia grimaldii E . sculpticauda

Gnathophawria gigas G . gracilis

G . iagens

G. zoea

Lophogaster spinoswr L. typicwr

Sea area

Date

N. Atlantic N. Pacific N. Atlantic N. Atlantic N. Atlantic N. Pacific N. Pacific N. Atlantic Madeira N. Atlantic Madeira N. Pacific Madeira N. Atlantic Madeira Madeira N. Atlantic N. Atlantic N. Atlantic N. Atlantic

November 1955 April 1956 November 1955 November 1957 April 1958 April 1956 April 1956 November 1955 July 1954 November 1954 May 1955 April 1956 June 1956 October 1959 May 1955 June 1956 April 1958 October 1959 November 1955 September 1955

yo

(ms)

lipid

60 171 146 295 258 786 924 1964 11152 446 11536 3812 6167 3703 1108 733 571 374 70 2.4

8.1 24.0 12.0 12.0 15.0 3.3 12.0 26.0 6.4 2.5 5.2 9.4 2.1

8-9 14-0 1.9 15.0 7.4 9.7 8.3

104

THE BIOLOGY OF MYSIDS

TABLEXIV. LIPIDS(% WET WEIUHT) IN SPECIES OF MYSIDA (L: R. Fisher, unpublished)

Species Acanthomysis longicornis Amblyops kenvpi Boreomysis arctica B. californica Erythrops erythrophthalma Ewhaetomera tenuie Gaatrosaccwr spinifer Hemimyeis lamornae Leptomysis gracilis

L. mediterranea Mysidopsis didelphys

Neomysis americana Paramyeis arenosa Schistomysis ornata Siriella armata s. clawrii S. norvegica

Date

Average body weight fmg)

N. Atlantic

November 1955

5.3

1.9

N. Atlantic N. Atlantic N. Pacific Barents Sea

July 1955 September 1955 April 1956 January 1955

21.0 8.3 35.0 3.8

6.1 4.8 4.8 4.3

N. Atlantic N. Atlantic Plymouth N. Atlantic Scotland Monaco Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Virginia, USA Monaco Scotland Scotland Scotland Monaco N. Atlantic Scotland Scotland

September 1955 September 1955 June 1952 September 1955 November 1955 February 1952 June 1954 September 1954 September 1954 November 1954 December 1954 March 1955 November 1955 November 1959 February 1954 February 1952 March 1955 November 1955 April 1957 February 1952 November 1955 December 1955 March 1955

4.8 1.1 5.0 5.5 4.6 4.9 28.0 20.0 19.0 19.0 18.0 20.0 18.0 21-0 27.0 2.2 9.0 10.0 12.0 22.0 1-1 93.0 7.1

Sea area

yo lipid

3.3 20-0 3.4 0.8 13.0 1.5 6.2 4.1 3.7 2.6 2.2 7.1 1.3 7-8 0.5 8.6 1.8 0.3 0.6 1.9 6.0 4.0 5.6

other sources are given in Table X and XV. The highest values for lipids, with a modal range of 5-12% of body wet weight, occur in species of the genera Gnathophusia, Lophogaster and Eucopia .of the Sub-order Lophogastrida. Lower values, with a modal range of 1-6% of body weight, occur in species belonging to the genera of the SubOrder Mysida. Species of the Lophogastrida are predominantly mesoand bathypelagic, but some of the species of Mysida enalysed are also deep living, for example Boreomysis arctica, B. californica and

6. CHEMICAL COMPOSITION

105

TABLEXV. LIPIDS(yoWET WEIGHT)IN MYSIDS Species Qnathophawia gigas ffnathophawiasp.

Lophogaster sp. Eucopia sculpticauda males females Neomy& integer gravid females females males juveniles N . awatschensis N . intermedia N . nakazawai

N . spinosa

Sea area

Date

yo lipid

Kuril-Kamchatka 31"N 119OW 29"N 23"W 30"N 23OW 37"N 14OW 37"N 14"W 30"N 23"W 25"N 19"W

Summer April March April April March November

63= 42a 1.0 2.3 4.7 1.2 0.7 1.7

37"N 25"W 37"N 25"W

October October

9.7 11.5

England England England England Japan Japan Japan

October October October October -

Japan

-

-

1.7 1.6 0.9 1.1 1.3 3.9" 6*5a 1.34 (10.3a) 2.0

Data from: Yamada, 1961, 1964; Lee et al., 1971; Morris, 1972, 1973; Morris and Sargent, 1973; Bordovskiy et al., 1976. a On dry weight basis.

Amblyops kempi. There would, therefore, appear to be a relationship between increasing depth of occurrence and increasing lipid storage but further data are required to confirm this. Childress and Nygeard (1974), in a general study of the chemical composition and buoyancy of midwater organisms, found that lipid contenb increased with increasing depth but decreased again at the greatest depths. Female Mesopodopsis slabberi and Neomysis integer store significantly greater quantities of lipids than males. Consequently, the sex ratio of mysids in samples will affect the estimations of lipid content, not only in these two species but probably also in most other species. Seasonal variation in the lipid content of euphausiids has been demonstrated in temperate and high latitude species. Moore (1976), however, found relatively constant levels of lipids present in the mysids Neomysis integer and Praunus JEexuosus, although he did not analyse samples in the autumn and early winter. Linford (1965), although finding fluctuating levels of lipids, did not demonstrate a

106

THE BIOLOQY OF MYSIDS

significant seasonal variation in their concentrations in Mesopodopsis slabberi. Such fluctuations are present in Fisher’s data on Mysidopsis didelphys (Table XIV) ; a seasonal trend could be inferred were it not for the high value of 7.8% in November, 1959. A continuing decrease in the lipid content has been shown to occur throughout the summer in Neomysis integer by Morris (1971). Animals that he kept a t different temperatures in the laboratory showed a correlation between higher lipid concentrations and lower environmental temperatures. Bamstedt ( 1 978) examined the seasonal fluctuations in protein, carbohydrate, lipid, chitin and ash in Boreomysis arctica but expressed all his data as percentage dry body weight. He determined the seasonal changes in the dry weight of two standard mysids, one of 19 mm and the other of 13 mm total body length, over a period of 13 months. The dry weight content was highest in the autumn and early winter when the animals were preparing to reproduce. These changes in the dry weight undoubtedly reflect changes in the lipid content. Seasonal fluctuations in the quantities of lipids stored are expected to occur in many species but such fluctuations have not yet been clearly demonstrated. Some general characteristics of the lipids are given in Tables XVI and XVII. Yamada (1964) examined some properties of the lipids of Neomysis mirabilis, N . intermedia, N . spinosa and N . awatschensis. Their density ranged from 0.9407 to 0.9561 and their refractive index from 1.4639 to 1.4866. The lipids can be divided into various fractions such as monoglycerides, diglycerides, triacylglycerols, phospholipids, free fatty acids, wax esters, sterol esters and sterols (Sargent, 1976). The dominant fractions within the lipids are the phospholipids, triacylglycerols, free fatty acids and, in some species, wax esters. The proportions of some of these fractions in the lipids of several species are shown in Table XVII. These data demonstrate the virtual absence of wax esters in shallow living species, represented in the genera Neomysis and Praunus, and their dominance in the lipids of the offshore, deep living lophogastrid species in the genera Gnathophausia, Lophogaster and Eucopia. Sargent et al. (1978), studying several species of mesopelagic decapods and Eucopia sculpticauda, concluded that the wax esters are largely produced by the biosynthetic activities of the animal’s metabolism rather than supplied to it in its diet. The fatty acid composition of the total lipids of several species are shown in Table XVIII. Lewis (1967) examined the lipids of Gnathophuusia sp. and found a high proportion of C-18 acids. The major fatty acid and fatty alcohol constituents of the wax esters are C-18: 1 acid and C-16: 0 alcohol (Table XIX). Morris (1972) found 51-58% of the wax esters to be oleic acid (C-18: 1) and 67-73% of the wax

TABLEXVI. CHARACTERISTICS OF LIPIDSOF Neomysis SPECIES (Yamade,1964; Raymont et al., Acetone soluble

Nonsapon.

(%I

di6

ng

sv

IV

AV

(%I

N . awatachensis N . integer N. intediu N . mirabilis

95.8

0.9407

1.4838 1.4821 1.4639

195.7 137.0 109-8 181.3 200.4 202.6

116.8

0.9561

175.2 143.0 150.0 163.6 186.9 179.5

11.3 14.0 17.89 10.62 11.12 10.10

98.5 96.9

acid

Bromides insol. in ether

IV

(%I

Faty

Species

N . spinosa

1968)

0.9468 1.4866

76.4 133.2 90.4 76.0

NV

69.55 197.6 190.0 193.2 195-2

121.5 207-9 192.0 208.1

dC,density; n$, refractive index; SV, saponification value; IV,iodine value; AV, acid value; N V , neutralization value.

26.75 73.7 79-92 74.95

108

THE BIOLOOY OF MYSIDS

TABLEXVII. PERCENTAGE COMPOSITION OF TOTAL LIPIDS Species Gnathophazcsia sp.

Lophogaster sp. Eucopia sculpticauda males females Praunw Jexuoaua N e m y s i s integer

Phospholipida

Triacylglycerole

11 11 6 18 18

26 ( + F F A " ) 35 11 44 37 (FFAO) 12

Wax esters 60 52 77 32 40 69 5 67 77

24-27.5 50 2 3-2 7.5

34 6gb

0.90

Date from: Raymont el al., 1968; Lee et al., 1971; Morris, 1972; Lee and Hirote, 1973; Morris and Sargent, 1973; Morris et al., 1973; Moore, 19.76. Plus free fatty acids. Plus diglycerides. Plus sterol esters.

alcohols to be (3-16: 0 alcohol in male and female Eucopia sculpticauda. Species of Gnathphawia have been shown by Morris and Sargent (1973) to be able to synthesize the C-18: 1 acids and C-16: 0 alcohols in the wax esters, as well as the higher polyunsaturated fatty acids, from palmitic acid. Similar biosynthesis of these long-chain fatty acids for incorporation in the phospholipid and triacylglycerol fractions of the lipids has been demonstrated in Neomysis integer by Morris et al. (1973). Further, N . integer was able to synthesize a significant portion of its required lipid directly from starch but after a week on a starch diet the levels of long-chain polyunsaturated fatty acids declined (Morris et al., 1977). The proportions of the constituent fatty acids of the lipids fluctuate. Morris (1971) showed, in N . integer, that the polyunsaturated acids increased during the summer and autumn while the short-chain low unsaturated acids decreased. Similarly, he found that an increase in the long-chain polyunsaturated C-22: 6 acids took place at the expense of the short-chain low unsaturated acids when environmental temperatures decreased. In addition to such variations, Morris (1973) demonstrated sexual differences in the fatty acid compositions of male and female N . integer, with juveniles somewhat intermediate in composition. The females had a higher total lipid content associated with a

TABLEXVIII. FAW ACIDCOMPOSITION OF Lmms OF M Y S ~ (%) S

Patty acid 10 : 0 12 : 0 13 : 0 14 : 0 15 : 0 16 : 0 17 : 0 18 : 0 19 : 0 21 : 0

Total

Neomysis integer

Neomysis awatschensis

Neomysis mirabilis

Anchialina agilis

Lophogaster

Tr

Tr

0.4

0.2

0.3

0.5

0.3

0.2

Tr

Tr

Tr

Tr

3.8 0.8 21.3 1.9 5-5 0-7 0.4 34-6

4.6 18.2 21.2 1.6 3.0

6.9 0.8 20.1 0.9 3.3 0.4 0.5 33.3

4.7 1.2 20.9 1.4 4.4 0.5

5.3 1.6 24.5 1.1 3.8 0.4

33-4

36-9

7.4

0.5

0.8 0.4 10.5 1.5 15.7

5-1 0.3 13.2

7.0 0.9 13.4

11.5 1.3 50-6

0.4

0.7

7.5

19.0

22-0

70.9

Tr 3.9 0.7 25.2

0.8 4.0 0.5 0.5 36.0

Tr 31.9

Neomysis spinosa

8P.

Gnathophausia SP.

0.8 0.3 6.3

Tr

Mono-unsaturated 14: 15: 16: 17: 18: 19: 20: 22: 24:

1 1 1 1 1 1 1 1 1

Tote1

10.3 0.6 12.4

10-9

0.6

1.3 1.7

Tr

Tr

23.9

27.7

31.1

13.3

2.2

0.6 10.4 11.6 1.6 1-6

Tr 25.8

cod.

TABLEXVIII-contd.

Polyunsaturated 16 : 2 16 : 3 18 : 2 18 : 3 18 : 4 20 : 2 20 : 3 20 : 4 20 : 5 22 : 4 22 : 5 22 : 6 Total

Tr 0-2 0.7 0.4 0.3 1.7 21.5

Tr

3.0 3.4 1.5 2.3 0.9 Tr 17.5

1.8 1.0 2.8

1.5 0.6 2.6 Tr 0.7 Tr 21.1

Tr 24.2

Tr 11.4 37.0

Tr 10.5 40.0

9.1 39.9

Data from: Yemade, 1964; Lewis, 1967; Morris, 1971.

Tr

0.8 0.7 0.9 Tr

3.7 1.6

0.5

Tr

1.0

0.5

Tr 14.7 40.0

1.0 0.8 0.8 0.2 2.6 16.2 Tr 2.1 22.9 46.6

2.0 12.1 1.0 0.2 23.0 40.7

4.8

5-9

6.3 22.8

6.

111

CHEMICAL COMPOSITION

TABLEXIX. FATTY ACID COMPOSITION OF LIPIDFRACTIONS

Fatty acid

Neomysis Praunus integer flexuosus (yomolar concentration)

Gnathophausia 8p. (fatty acid and fatty alcohol) (yocomposition) ~~

PL 14 : 0 15 : 0 16 : 0 16: 1 16 : 2 17 : 0 l? :1 18 : 0 18: 1 18 : 2 18 : 3 18 : 4 20: 1 20 : 4 20 : 5 22 : 1 22 : 6 22 : 6 Unidentified

4.3 0.6 26.1 9.2 0.1 2.3 20.6 1.1 0.8

TG 6.9 0.5 21-3 4.5 4.3 4.2 14.7 1.5 0.5

PL 3.9 25.1 6.9 0.4

3.6 23.4 0.6 1.7

TG 5.3 0.2 23.2 5.1 2.3

4.9 20.7 2.0 1.1

21.3

25.3

19.3

22.1

13-1 9.5

9.8 6.5

14.2 0.9

11.4 1.7

PL 1-7 0.8 15.9 3.9 0.7 3.1 1.1 17.3 1.4 1.2 1.0 Tr 5.1 14.3 Tr Tr 32-5

TG WE" WEb 2.3 0.4 4-6 0.9 16.5 2.0 82.6 6.6 10.7

Tr 4.0

2.3 31.7 1.3 0.6 2.0 2.0 9.9 2.3 Tr 17.5

73.7

10.2 2,5

8.6 0.6 3.7 0.3

PL, phospholipids: TG, triacylglycerols; WE, wax esters. Data from: Morris and Stlrgent, 1973; Moore, 1970. acid. alcohol.

larger triacylglycerol fraction. The larger triacylglycerol fraction resulted in increased levels of mono-unsaturated acids in the total lipids of the females which were further found to have reduced levels of C-22 : 6 polyunsaturated acids, when compared with the compositions of males and juveniles. Sterols have noti been studied to any degree in mysids. Morris (1973) found 4.8% of the lipids of N . integer as sterols. Cholesterol and provitamin D were identified in the sterols extracted from a mixed sample of Mysis microphthalma and Paramysis loxolepis by Vinogradova and Kandiuk (1967). Teshtimrt and Kanazawa (1971) found that the concentration of sterols in unnamed mysids was 0.21y0 of the body wet weight; this sterol mixture consisted of 3% as 22-dehydrocholesterol, 78% as cholesterol, 8% as brassicaterol, 10% as 24-methylenecholesterol, and a trace of B-sitosterol.

112

THE BIOLOGY O F MYSIDS

The amino acid composition of the protein hydrolysates of Neomysis integer and Mesopodopsis stubberi has been investigated and the predominant acids, in descending order of importance, found to be glutamic acid, aspartic acid, lysine and arginine (Table XX). There is a close similarity between the amino acid composition of these species and that of the freshwater Mysis relicta from Cayuga Lake, New York (Table XX). The most important free amino acids are glycine, taurine, and arginine (Table XXI). The general pattern of amino acids present in the two species are closely similar. Raymont et al. (1968, 1973) and Armitage et at. (1977, 1978) discuss the factors producing variation in the concentrations of individual acids. They also examined transamination between 17 amino acids and the keto acids, a-keto-glutarate, pyruvate, and glyoxylate in Neomysis integer. TABLEXX. AMINOACID COMPOSITIONOF PROTEIN HYDROLYSATES OF MYSIDS (yoOF TOTAL) Neomysis integer

0uig erow Amino acids

Cysteic acid Taurine Aspartic acid Threonine Serine Glutamic acid Proline GI ycine Alanine Valine Cystine Methionine Isoleucine Leucine Tyrosine Phenylalanine Lysine Histidine Tryptophan Arginine Ornithine Total (mg/g dry weight) yo dry weight

females

11.2 4.1 3-8 17.2 3.0 3.6 5.2 5,2 1.1 2.3 5.3 7.7 4.0 5.1 10.2 3.5 7.3 0.2 464.9 46.5

Less mature

Mesopodopsis slabberi

10.6 3.9 3.5 19.1 2.4 4.1 5.4 5.4 0.7 1.7 5.3 8.4 3.9 5.0 10.2 2.4

-

7.8 0.2 510.1 51.0

Data from: Raymont et al., 1973);Tracy and Vallentyne, 1969.

11.0 4.4 4.6 14.8 3-2 4.0 5.8 5.6 1.4 2.4 5.3 7.8

4.3 4.6 10.4 3. ” 6.7 0.2 548.4 54.8

Mysis relicta

11.7 5.4 6.1 14.7 3.8 7.4 8.0

5.5 0.8 1.7 4-9 9.1 2.1 3.9 7.5 1.3

-

5.7

6, CHEMICAL COMPOSITION

113

TABLE XXI. FREEAMINO ACIDSOF MYSIDS (yoOF TOTAL). ONLY EIGHTOF THE MAJORACIDSARE LISTEDIN THREESETSOF DATA Neomysis integer Anzino acids

Cysteic acid Taurine Aspartic acid Threonine Serine Glutamic acid Proline Glycine Alanine Valine Cystine Methionine Isoleucine Leucine Tyrosine Phenylalanine Lysine Histidine Tryptophan Arginine Ornithine Total (mg/g dry weight) yo dry weight

Neomysis integer 0-8 23.1 1.9 1.3 1.4 5.5 4.2 30.9 3.8 1.0 0.6 0.2 0.6 0.7

Owigerow females

Lem mature

Mesopodopsis slabberi

26.4 3.1

26.6 2.9

14.7 4.7

6-0 5.9 27.4 4.8

5.6 2.4 29.6 5-2

22.5 2.9

1.3

2.2

9.0

17.6

12.0

25-5

7.9

-

0.6 0.4 1.5 1.5 0.7 19.1 0.1 95.7 9.6

~~

Data from: Srinivasagm et al., 1971; Raymont et at., 1973.

The a-keto-glutarate transaminase reaction was strong but a weak glyoxylate transamination was also present suggesting some direct production pathways for the free amino acid glycine. Vitamin B,, was found in Neomysis intermedia at concentrations similar to those in euphausiids and markedly higher than in the other crustaceans examined (Hirano et al., 1964b). The occurrence of vitamin C was examined in mysids by Santa and Bacescu (1942). They found the following quantities, in rng/100 g wet weight, near Roscoff, France : Leptomysis lingvura, males and females, 0.14; Mesopodopsis slabberi, males and females, 0.10; Neomysis integer, males and non-ovigerous females, 0.06-0.07, ovigerous females, 0.09 ; Paramysis arenosa, juveniles 0.30, males, 0.21 and L,

114

THE BIOLOGY OF MYSIDS

ovigerous females, 0.13 ; Praunus Jlexuosus, males, 0.1 1, females, 0.13 ; Schistomysis parkeri, ovigerous females, 0.20; S. spiritus, males and females, 0.15. Vitamin A occurs in some species of mysids and not in others. L. R. Fisher (see Mauchline and Fisher, 1969) studied its occurrence in euphausiids where it is predominantly located in the eyes. He published the results of analyses of a few species of mysids (Fisher et al., 1953; Fisher and Kon, 1959); other unpublished data are included in Table XXII. No vitamin A was found by him in the following species : Amblyops kempi, Anchialina agilis, Boreomysis arctica, B. microps, Erythrops erythrophthalma, E. serrata, Euchaetomera tenuis, Eucopia grimaldii, Gastrosaccus spinifer, Hemimysis lamornae, Leptomysis mediterranea, Lophogaster spinosus, L. typicus, Meterythrops picta, Paramysis arenosa, Xiriella armata, S. clausii, S. jaltensis and S . norvegica. Vitamin A was not detected in one sample of Gnathophausia zoea, as shown in Table XXII. Fisher concluded from his studies on Crustacea that pelagic species were richer in vitamin A than neritic or benthic species; there ia no clear evidence of this in mysids. His data show that more vitamin A was present in the eyes than in the bodies of only four species of mysids-Gnathophausia zoea, Eucopia sculpticauda, E . unguicubta and Mysidopsis didelphys (Table XXII). The concentration of vitamin A shows seasonal changes in euphausiids and this may also be true in some species of mysidsj the data for Mysidopsis didelphys (Table XXII) suggest the occurrence of high concentrations in the winter to spring period. Carotenoids were first identified in mysids by Lonnberg (1931a, b, 1934) who examined Praunus Jlexuosus. Fisher estimated the concentrations of astaxanthin in a few species (Table XXII); in addition to these, he found that Leptomysis mediterranea had 39 pg, Paramysis arenosa 37 pg and Xiriella armata 16 pg astaxanthinlg wet weight. These concentrations are within the ranges occurring in euphausiids (Mauchline and Fisher, 1969), except for the t w o very high values found in Erythrops erythrophthulma which were sampled in the Barents Sea, and Mysidopsis didelphys sampled in March in Loch Fyne, Scotland. Seasonal fluctuations in concentrations are suggested by the data on M . didelphys (Table XXII). The pigments of mysids have received little attention. Fisher (unpublished) examined the eye pigments of Eucopia sculpticauda and found evidence suggesting that the cone pigment is melanin and that the inner pigment surrounding the rhabdoms is also melanin. Elofsson and Hallberg (1973), however, state that the black pigment in the eyes and integument of Neomysis integer is an ommochrome.

6. TABLEXXII. VITAMIN A

Species

115

CHEMICAL COMPOSITION AND

ASTAXANTHIN (L. R. Fisher, unpublished)

Vitamin A (pg/g wet wt)

Gnathophausia gigas G. gracilis (I.ingens

G. zoea Eucopia sculpticauda E. unguiculata Boreomysis calif ornica Boreomysis sp. Erythrops erythrophthalma Katerythrops sp. Leptomysis gracilis Mysidopsis didelphys (March) (June) (September) (September) (November) (December) Schistomyeis ornata

% in eyes

3.1 1.7 0.9 1.7 0.6 2.0 0.2 0.9 1.4 0 1.3 1-7 0.2 0.3 3.0 0.9 0 0.9 0.6 0.7

0 19 30 47 0 4.4 13 100

6.5 3.2 0.8 0.8 0.6 0.8 0.3

100 100 50

Astaxanthin ( w / qwet wt)

57 60 76 87

Habit" Bathypelagic 1000-2000 m 350-400 m

400-2000 m

100

850-2600 m

54

700-2500 m

10

Bathypelagio 125

40-500 m 5-500 m

100 100 0

112 29 63 64 42 31

60-300 m

50-150 m

0.5

Neomysis americana Acanthomysis longicornis

0.7 1.6

0-240 m

< 100 m

Mauchline end Murano (1977).

The chromatophores of N . integer and Praunus JEexuosus contain many microtubules and several chromatophores of different colours unite to form chromatosomes. Some contain granules characteristic of red carotenoid pigments. Three white pteridines, dominated by isoxanthopterin, probably constitute the reflecting pigments of the eyes. The chemical nature of the yellow pigment that is also present in mysids remains unknown. The haemolymph of Gnathophausia ingens has an absorption spectrum that suggests the presence of a haemocyanin (Freel, 1978).

116

THE BIOLOGY OF MYSIDS

Succinic dehydrogenase activity was found to be inversely proportional to body weight in Neomysis integer by Raymont et al. (1967). Temperature accelerated the activity and there was evidence of seasonal variation. Adenine nucleotides have been examined in Boreomysis arctica and Hemimysis abyssicola by Skjoldal and Bamstedt (1977). A seasonal fluctuation in adenosine triphosphate (ATP) concentration was observed, a maximal concentration of 6-7 pg ATP/mg dry body weight being present in April-May in the Norwegian populations. Comparable concentrations of adenosine diphosphate (ADP) and adenosine monophosphate (AMP) were 3-4 and 1-3 pg/mg dry body weight respectively. Homarine, a betaine that is supposed to have an osmoregulatory function, was identified in Neomysis intermedia by Hirano (1975). It is N-methyl picolinic acid betaine and occurred at a concentration of 0.58 mg/g wet body weight, a concentration equivalent to about one fifth of that present in the euphausiid, Euphuusia pacijica.

CHAPTER I

INTERNAL ANATOMY The internal anatomy of a few species of mysids, such as Praunus jlexuosus and Gnathophausia species, has been examined in some detail. The structure of the male and female reproductive system has already been described, while discussing the marsupium, and the histology and form of the alimentary tract is described in the section on feeding. Information on other aspects of the internal anatomy is reviewed here. Delage (1883) describes the circulatory system of “Mysis” and his diagrams suggest that the species was Praunus Jlexuosus; it is certainly a species of the tribe Mysini. Siewing (1956) describes the system of Eucopia sculpticauda and E . unguiculata. Mayrat (1956b) examines the blood supply to the head while Alexandrowicz (1955) studies the nervous innervation of the heart of P . jlexuosus. More recently, Belman and Childress (1976) have examined the circulatory system of the lophogastrid mysid, Gnathophausia ingens. The heart is a single chamber, tube-like in form and situated in the posterior dorsal region of the thorax. It is suspended within the pericardium (Fig. 30). The heart is relatively large, Belman and Childress (1976) finding that its wet weight represents 0.20-0.32% of the wet body weight of G. ingens. Valves are present at each end and mark the extent of the heart because the “tube” continues anteriorly as the aorta cephalica and posteriorly as the arteria abdominalis (Fig. 30). Paired ostia, a left and a right, are located dorso-laterally in the median region of the heart; blood enters the heart through them from the pericardium. There are three systems of nerve elements in the heart of Praunusflexuosus (Alexandrowicz, 1955). The first is a local system situated on the outside of the dorsal wall of the heart, the second consists of the nerves connecting this local system to the central nervous system, and the third is the nerves of the arterial valves. The heart first, starts to beat in the Stage I1 larva within the marsupium but beating is discontinuous, occurring in rhythmic bouts lasting a few seconds. Berrill (1969) found that the bouts would occur every 3-4 min a t 4°C and every 1-2 min at 15OC in larvae of Mysis relicta.

118

THE BIOLOGY OF MYSIDS

f

C FIG.30. The circulatory system of mysids. A and B, the system in a species of the tribe Mysini. C, the system of Gnathophausia ingens. The arteries are shown in black; the pericardium is stippled in B and C and the afferent and efferent vessels and gills are indicated in C by dotted vessels. A and B are constnicted from figures of Delage (1883) and Mayrat (1956b); C is after Belman and Childress (1976). aa, arteria abdominalis; ab, arteries to the cerebral ganglia; ac, aorta cephalica; ae, afferent and efferent blood vessels of gill; al, left arterk lateralis; an, antenndar and antenna1 arteries; e, efferent branchial channel; fh, frontal heart; g, gill; h, heart; ha, hepatic arteries; hal, left hepatic artery; har, right hepatic artery; oa, optic artery; p, pericardium; a,sternal artery; sal, left sternal artery; sar, right sternal artery; ua, arteries of unknown destination; va, ventral abdominal artery.

The heart commences beating continuously in the late Stage I11 larva, immediately before it emerges from the marsupium. Henderson (1927), working with what he calls Michtheimysis (probably Mysis relicts), found the heart of the adults beating a t a rate of 120-380 beatslmin, over a temperature range of - 2-18°C; there was a linear relationship

7. INTERNAL ANATOMY

119

between increasing rate of heart beat and increasing temperature. Belman and Childress (1976) found that larger Gnathophuusia ingens have a heart rate varying from 128 to 178 (mean 110) beatslmin a t 4OC and a t 1 atmosphere hydrostatic pressure. The heart rate of G . zoea was approximately 60 beatslmin a t 5°C and 200 atmospheres pressiire during compression experiments made by Macdonald and Gilchrist (1978). The arterial system of mysids is shown in Fig. 30 but some of the descriptions appear to be contradictory and further investigation is required. The aorta cephalica travels anteriorly round the dorsal side of the stomach t o the head supplying small branches to the intestine, cardiac stomach and various muscles (Belman and Childress, 1976). It then divides, a branch descending ventrally to the cerebral ganglia. A frontal heart is located antero-dorsally to this branch (Sewing, 1966; Mayrat, 1956b; Belman and Childress, 1976). This is a swelling of the aorta cephalica associated in Praunus jexmsus with the division of the artery into the branch to the cerebral ganglia and those to the eyes. The optic arteries, however, do not arise from the aorta cephalica in Gnathphausia ingens (Belman and Childress, 1976) and this is surprising because the eyes are supplied by this artery in other mysids, and in euphausiids (Mauchline and Fisher, 1969) and in some decapods (R. S. Pillai, 1965). The frontal heart contracts rhythmically, according to Belman and Childress, more or less in phase with the heart. Ventral to the optic arteries, the aorta cephulica supplies the antennules and antennae in Praunus jexuosus (Delage, 1883) but not in Gnathophuusia ingens (Belman and Childress, 1976). Delage (1883) figures paired hepatic arteries a t the site of the anterior end of the heart where Belrnan and Childress figure paired arteriae laterales. The arteriae laterales in G . ingens supply small branches to the digestive glands, anterior regions of the stomach and various muscles and then supply the eyes, antennules and antennae. These arteries supply the antennae, but not the eyes, in euphausiids and other decapods. In the case of Praunusjexuosus (Fig. 30) it is possible that Delage has wrongly named the arteriae laterales as hepatic arteries. The hepatic arteries originate in the region between the arteriae laterales and the anterior end of the heart in Gnathphuusia ingens (Fig. 30), in euphausiids (Mauchline and Fisher, 1969) and in decapods (R. S. Pillai, 1965). Delage (1883) figures two arteries in this region (Fig. 30) going to unknown destinations; Alexandrowicz (1955) mentions these arteries of Delage, but provides no further information. These arteries of Delage could be the true hepatic arteries of Praunus jlexuosus.

120

THE BIOLOGY OF MYSIDS

The aorta descendens or sternal artery originates at the posterior end of the heart. It is single in PraunusJlexuosus (Fig. 30),in Eucopia species (Siewing, 1956), in euphausiids (Mauchline and Fisher, 1969) and in decapods (R. S. Pillai, 1965) but paired in Gnathophausia ingens (Fig. 30). An anterior ventral branch supplies the ventral region of the thorax, thoracic legs and maxillae. The more anterior mouthparts are probably supplied by the arteriae laterales. A posterior branch of the aorta descendens (sternal artery) is shown by Delage as not extending into the abdomen in Praunus Jlexuosus and similarly by Siewing in Eucopia species, but described by Belman and Childress as extending posteriorly as the ventral abdominal artery in Gnathophausia ingens (Fig. 30). Dorsal and lateral regions of the abdomen are supplied by the arteria abdominalis which terminates in branches to the telson and uropods. The lateral branches of these arteries also supply the pleopods. The various arteries sub-divide into arterioles within the tissues that they supply and the blood is voided from their ends into a complex system of sinuses. Belman and Childress briefly describe the form of the sinuses in G. ingens. Blood from the anterior thoracic sinus and from the abdominal sinuses travels to the infrabranchial sinus. It then enters the afferent branchial channels and so into the afferent branchial veins within the gills. There it circulates, returning through the efferent branchial veins out of the gills into the efferent branchial channels which lead into the pericardium (Fig. 30). Once there, it enters the heart through the paired ostia which are closed by valves during the systolic contraction, Praunus Jlexuosus, like all non-lophogastrid mysids, has no gills. Delage concludes that oxygenation of the blood takes place in the sinuses under the carapace. There are various marked differences between the circulatory systems described in Praunus jlexuosus and Gnathophausia ingens noted in the above account. These may be real differences and R. S. Pillai (1965) found such differences existing between descriptions of the circulatory systems of several decapod crustaceans. Elucidation of the circulatory system is difficult, especially the tracing of the destinations .of the smaller arteries. There does appear, however, to be a general pattern of blood vessels common to mysids and decapods. This basic pattern is also evident in the arterial system of the gammarid, Gammarus pulex (Cussans, 1904) and the isopod, Ligia oceanica (Hewitt, 1907). Consequently, further investigation of the circulatory system of mysids may confirm that there are errors in the present descriptions. The blood pressures in various regions of the circulatory system of Cnathophausia ingens have been measured by Belman and Childress

7.

INTERN&

ANATOMY

121

(1976). The maximum intracardiac systolic pressure was 27.0-20.5 cm H,O with a mean vhlue of 24cm H,O. The range of diastolic pressures was 21.0 to 16.0 cm H,O with a mean of 19 cm H,O. High arterial pressures, almost equal to that of the heart at systole, are maintained in the aorta cephalica and arteria abdominalis. The blood velocities had a mean systolic value of 2-71 cmjs. Osmotic regulation of the blood concentration takes place in brackish water species of mysids. Hyperosmoticity has been demonstrated in low environmental salinities and hypo-osmoticity in 'high environmental salinities (Khlebovich et al., 1970; McLusky and Heard, 1971; Dormaar and Corey, 1973). The iso-osmotic point for Praunus Jlexuosus was at 25%,, while that of the more brackish living species Neomysis integer was at 19X0 (McLusky and Heard, 1971). Another coastal species, Mysis stenolepis, had an iso-osmotic point a t 2 l%,,(Dormaar and Corey, 1973). Mysis relicta, collected from Canadian freshwater lakes, hy-perosmoregulates a t salinities below about 25%, and is isoosmotic to its environment at salinities between 25 and 30X0 (Dormaar and Corey, 1978). G. A. Vinogradov (1976) compares the ability of M . relicta and M . oculata to osmoregulate. The concentrations of sodium, potassium and chloride ions in the haemolymph are isoionic to dilute sea water but hypo-ionic to full strength sea water. The abdominal muscular systems of Gnathphausia zoea, Lophogaster typicus and Praunus flexzlosua are described by Daniel (1928, 1931, 1932, 1933) and that of Spelaeomysis longipes by Nath (1974). Manton (1928a) compares the musculature of the fifth and sixth abdominal segments of Gnathophausia zoea, Lophogaster typicus, L. interrnedius and Hemimysis lamornae. The general arrangements of the muscles are shown in Figs 31-33. The system is extremely complex and the original descriptions cited above are detailed, Consequently, no adequate r6sum6 can be given here and reference must be made to the original papers for detailed comparative information. The system of muscles in the cave-dwelling Spelaeomysis longipes is simplified according to Nath (1974). This is apparently a result of its lethargic life style in that swimming muscles are either absent or atrophied. The muscles of the head of Praunus JIexuosus are described by Mayrat (1955).

Descriptions of the gross morphology of the nerve cord of Gnathophausia longispina and Eucopia australis are given by Sars (1885). Chun (1896) describes the thoracic portion of the cord of Arachnomysis leuckartii. The cerebral ganglia lie anteriorly and dorsally to the mouth and the large paired optic nerves and the smaller paired antennulax and antenna1 nerves originate from its anterior end (Fig.

7.

INTERNAL ANATOMY

123

34). Circumoesophageal commissures connect the cerebral ganglia with the nerve cord. Some of the thoracic ganglia are fused together but the six abdominal ganglia are distinct in Gnathophausia longispina (Fig. 34). Dahl (1956) shows a sagittal section through the cerebral ganglia of Praunus Jlexuosus to demonstrate the positions of the tritocerebrum, deutocerebrum and the anterior and more dorsally sited protocerebrum. The histological structure of the cerebral ganglia of Eucopia and Boreomysis species is described by Hanstrom (1948) and Siewing (1956). The eyes of mysids vary considerably in their degree of development and size relative to the body and gross form. They are reduced to a coalesced plate, dioptric elements usually being absent, in species of the genera Scolamblyops, Michthyops, Pseudomma and Parapseudomma. The eyes exist as separate plates, again with the visual elements reduced or absent, in species of the genera Amblyops Ambtyopsoides, Mysimenzies, Paramblyops, Dactylerythrops and Calyptomma. The eyes of one species of Ceratomysis and Hansenomysis are separate but reduced to spinous processes (Fig. 4.10). Petalophthalmus species have very small eyes (Fig. 4.12) while those of Hansenomysis species are even more reduced and plate-like (Fig. 4.13). Other genera that FIQ.31. Musculature of Gnathophausia zoea (after Daniel, 1933). m, muscle. TT, thoracic transverse m; I , thoracico-abdominal m; 11, anterior oblique rn 1; IIa, external arm of anterior oblique m 1; IIb, oblique transverse m, anterior oblique m 1;IV, anterior oblique m 2; IVa, external arm of anterior oblique m 2; IVb,oblique transverse m, anterior oblique m 2; IVc, posterior auxiliary m, anterior oblique m 2 ; V, central m 1; Vs, dorso-lateral m, abdominal segment 1; VI, transverse m, abdominal segment 1; VIII, central m 2; IX, oblique transverse m 1, anterior oblique m 3; X, anterior oblique m 3; Xa, external arm of anterior oblique m 3 ; Xb, auxiliary m, anterior oblique m 3; Xc, posterior auxiliary m, anterior oblique m 3; XII, dorsolateral m, abdominal segment 2; XIII, transverse m, abdominal segment 2; XIV, posterior oblique m 3; XV, central m 3; XVI, anterior oblique m 4; XVIa, external arm of anterior oblique m 4; XVIb, auxiliary m, anterior oblique m 4; XVIc, posterior auxil'iary m, anterior {obliquem 4; XVII, ,oblique transverse m 2, anterior oblique m 4; XVIII, dorso-lateral m, abdominal segment 3; XIX, transverse m, abdominal segment 3; XX, posterior oblique m 4; XXI, central m 4; XXII, anterior oblique m 5; XXIIa, external arm ,of anterior oblique m 6; XXIIb, auxiliary m, anterior oblique m5; XXIIc, posterior auxiliary m, anterior oblique m 5; XXIII, oblique transverse m 3, anterior oblique m 6; XXIV, dorso-lateral m, abdominal segment 4 ; XXV, transverse m, abdominal segment 4; XXVIa, oblique transverse m 4, anterior oblique m 6; XXMI, central m 5; XXVIII, anterior oblique m 6; XXVIIIa, external ,arm of anterior oblique m 6; XXVIIIb, auxiliary m, anterior oblique m 6; XXVIIIc, posterior auxiliary m, anterior oblique m 6; XXIX, central m 6; XXX, anterior oblique m 7; XXXa, external arm of anterior oblique m 7 ; XXXb, auxiliary m, ,anterior oblique m 7; XXXc, posterior auxiliary m, anterior oblique m 7; XXXIII, posterior auxiliary m, abdominal segment 6; XXXIIIa, flexor muscle of telson; XXXIIIb, longitudinal muscle, abdominal segment 6; XXXIV, auxiliary ,muscles, abdominal \segment 6 ; XXXVIII, dorsal flexor muscles of telson; XXXIX, ventral flexor muscle of telson.

Superficial ventrot musEles

Thwack muscles \ 1-6 \

,'

FIG.32. Musculature of Lop?wga&er t y p i c w . Upper figure showing dorsal, lateral and superficial ventral muscles; lower figure showing main ventral muscles. Nomenclature of muscles is given in the legend to Fig. 31 (after Daniel, 1933).

-

Lateral muscles

sor muscle of tail lobe

flexor muscle of tail lobe

J

FIG.33. Musculature of P r a u n u s j l m w . Upper figure showing main and lateral dorsal and lateral muscles; lower figure showing main ventral muscles (after Daniel, 1928). I, anterior thoracic m; Ia, thoracic longitudinal m 1; Ib, thoracic longitudinal m 2; 11, oblique “transverse” m 1 ;111, anterior oblique m 1 ; IV, thoracic central m ; IVa, thoracic transverse m; V, posterior oblique m 1; VI, central m 1 ; VII, oblique “transverse” m 2; VIII, anterior oblique m 2 ; IX, posterior oblique m 2; X, anterior central m 2; XI, oblique “transverse” m 3; XII, anterior oblique m 3; XIII, posterior central m 2; XIV, posterior oblique m 3; XV, subsidiary m, anterior oblique m 3; XVI, anterior central m 3; XVII, oblique transverse m 4; XVIII, anterior oblique m 4; XIX, posterior central m 3; XX, posterior oblique m 4 ; XXI, subsidiary m, anterior oblique m 4 ; XXII, anterior central m 4; XXIII, oblique transverse m 5; XXIV, anterior oblique m 5; XXV, posterior central m 4; XXVI, posterior oblique n, 5; XXVII. subsidiary m, anterior oblique m 5; XXVIII, anterior central m 5; XXIX, oblique transverse m 6; XXX, anterior oblique m 6; XXXI, posterior central m 5; XXXII, XXXIV, XXXV, flexor muscles of abdominal segment 6; XXXIII, transverse m, abdominal segment 5; XXXVI, subsidiary flexor m, abdominal segment 6; XXXVII, transverse m, abdominal segment 6; XXXVIII, dorsal flexor muscles of telson; XXXIX, ventrsl flexor muscle of telson.

128

THE BIOLOGY O F MYSIDS C

\

9

Fra. 34. The nerve cord of (7nathphazcsia Zongispim (after Sam, 1885). a, f h t abdominal ganglion; c, cerebral ganglia.

have species with markedly reduced eyes that are more or less normal in shape are : Antromysis, Bathymysis, Chalaraspidum, Dactylamblyops, H yperamblyops , Parambl yops , Pseudambl yops , Pseudochalarasp idum, Pseudomysis, Spelaeomysis, Stygiomysis, Teraterythrops and Troglomysis. Zharkova (1970) has studied the histology and reduction of the eyes in Boreomysis californica, B . inermis, Eucopia australis, E . grimaldii, Hyperamblyops nana, Petalophthalmus armiger, Dactylamblyops pellucida and D . tenella. Various stages of reduction are evident ranging from an ommatidium that is complete except that there is no lens, through an ommatidium with no lens and no nerve connection to the rhabdom, t o an ommatidium where both the lens and the rhabdom are absent. Some species have eyes of peculiar shape; those of some species in the genera Erythrops and Hyperythrops are kidney-shaped and red in colour, while the eyes of Heteromysoides species are of the form shown in Fig. 10.26. Divided eyes where the visual elements occur in two quite distinct regions (Figs 8.20, 8.22) occur in species of the genera Anisomysis, Caesaromysis, Carnegiemysis, Echinomysides, Echinomysis, Euchaetomera and Euchaetomeropsis. A few other species also have modified compound eyes that are usually described as having accessory eyes (Fig. 7.12); these are in the genera Dioptromysis, Kainmatomysis and Leptomysis. Casonova (1977) found that the eyes of Eucopia unguiculata (hanseni) are larger, relative t o the body size, in individuals living in the north-eastern Atlantic than those living in the Mediterranean. The number of ommatidia in the eye is related to carapace length of the animal, Atlantic individuals of carapace length 3.5-7-5 mm having 50-220 ommatidia while the Mediterranean individuals of carapace length 3.5-6.5 mm have 50-130 ommatidia. According to Hallberg (1977), there are about 1500 ommatidia in each eye of Erythrops serrata, about 600 in each eye of Mysidopsis gibbosa, some 1600 in the eye of Neomysis integer, about 200 in the eye of Praunusflexuosus and about 5000 in the eye of Siriella norvegica. Thus the numbers of ommatidia present in an eye vary widely between species but the significance of this is not known. The most detailed studies of the general histology of mysid eyes are those of Mayrat (1956a, 1962) on Praunus jlexuosus, Eguchi and

7. INTERNAL

ANATOMY

129

Waterman (1966, 1973) on those of Mysidium gracile and Mysis relicta, Kulakovskii (1969) on 'those of M . oculata and Hallberg (1977) on those of Erythrops serrata, Mysidopsis gibbosa, Neomysis integer, Praunus Jlexuosw and Siriella norvegica. There is little remarkable about the general structure (Fig, 35) because it is comparable to that of the compound eyes of other crustrtceans. Hallberg (19771, in his comparative study of five species, found that the ommatidia of the different species are constructed on the same general plan although they vary considerably in detail. Variation exists in the number of retinular cells, the presence or absence of an epirhabdom and in the arrangement, and distribution of &hepigment. The detailed structure of the optic neuropiles (lamina ganglionaris, medulla externa and medulla interna) of Lophogaster longirostris, Mysis oculata, M . relicta, M . litoralis, Neomysis integer, Praunus Jlexuosus and Erythrops servatus (serrata) was examined by Elofsson and Dahl (1970). These ganglia completely fill the eyestalk (Fig. 35) and conform to the general malacostracan pattern. In reduced eyes, such as those studied by Zharkova (1970), there is a tendency for the medulla terminalis and medulla interna t o be closer to the cerebral ganglia. The reduced eyes of Amblyops abbreviata, Boreomysis scyphops and Pseudomma afine were examined histologically by Elofsson and Hallberg (1977). They show that this reduction is not a negative response to an aphotic environment but that the eyes are adapted to reception of low intensities of non-polarized light. The eye of Boreomysis scyphops is dish-shaped (Fig. 35), with an enlarged retinular layer consisting of some 3000 ommatidia. The optic neuropiles are well developed although small relative to the size of the retinular.layer. The ommatidia lack a dioptric apparatus and are composed primarily of retinular cells forming 8 cylinder with microvilli protruding into the central chamber. The axons from the ornmatidia pass through a layer of red pigment cells, which serve as a basement membrane, to the lamina ganglionaris. The eye of Amblyops abbreviata (Fig. 35) is different, contains some 1000 ommatidia and also has a well-developed retinular layer, pigment cell layer and optic neuropiles. The eye of Pseudomma afine is of similar construction to that of Amblyops abbreviata but has no pigment cell layer adjacent to the retinular layer. The enlargement of the photosensitive areas by the development of microvilli enhances the reception of low levels of light intensity. Consequently, Elofsson and Hallberg argue that these eyes are positively adapted to the low light regimes of the deep environments. The retinal pigments (Fig. 35) allow dark adaptations of the eyes, as demonstrated by Beeton (1959) in Mysis relicta. Nicol (1959) tested

D

FIQ.35. Compound eyes. P r a u n w $ e m o s w : A, schematic section of an ommatidium and B, of the eye. Boreomysis scyphops: C, horizontal section of the eye. Amblyops abbreviata: D, sagittal section of the eye. (A and B after Mayrat, 195th; C and D after Elofsson and Hallberg, 1977). bm, basement membrane; bv, blood vessel; oc, corneagenous cells and cone cells; cor, cornea; cr cn, crystalline cone, dp; distal reflecting pigment cell; dpc, distal screening pigment; lg, luminu ganglionurb; m, muscle; me, medullu externu; mi, medullu i n t e r n ; mia, accessory medulla i n t e r n ; mt, medulla terminalis; oa, optic artery; p, pigment layer; pc, proximal pigment, r, retinular layer; rh, rhabdom; sg, sinus gland; sp, sensory pore; X, sensory pore X-organ (organ of Bellonci).

7.

INTERNAL ANATOMY

131

Praunus neglectzls and found that the minimum intensity of light to which it responded was 29 x 10" pW/cm2 receptor surface a t 10 cm; the colour of the light was blue, the filter used having a spectral range of 420-540 nm with a maximal emissioh at 475 nm. The reaction time of Mysis relicta t o light of various wavelengths was tested by Beeton (1959) who found that they reacted most quickly to wavelengths close to 515 nm and 395 nm. He suggests that this species probably has two visual pigments. Herman (1962) investigated the spectral sensitivity of the eyes of Nwmysis americana over a narrower spectral range that did not include wavelengths as short as 400 nm. He fhund highest sensitivity at a wavelength of 515 nm. Mysids do not have a nauplius eye nor a dorsal frontal organ (Elofsson, 1965). They do, however, have paired ventral frontal organs situated within the bec ocellaire, which is located a t the anterior end of the body between the bases of the eyestalks and antennules (Fig. 36). Elofsson examined this structure in Lophogaster longirostris, Eucopia sp., Boreomysis arctica, Arcitaeomysis sp., Mysis litoralis, M .

FIG.36. Sagittal section through the anterior part of the head of B o r w m y A a r c t h to show bec owllaire (after Elofsson, 1965). a, antennular nerve; b, cerebral ganglia; c, cells of ventral frontal organ; om, compressor muscles; r, rostrum; vf, ventral frontal organ.

132

THE BIOLOGY OF MYSIDS

oculata, Schistomysis ornata, Praunus Jlexuosus and Neomysis integer. The organ has a similar structure in all species with cells along its length except in Boreomysis arctica (Fig. 36) where the cells are restricted to the distal region. The sensory pore X-organ (SPX-organ) and sinus gland occur within the eyestalk (Fig. 35), the SPX-organ being located towards the dorsal side of the eye, the sinus gland towards the ventral side. Mayrat (1956a), who examined these organs in Praunw Jlexuosw, reviews the earlier literature. Kulakovskii (1969, 1971, 1978) describes their occurrence in the eyestalk of Mysis oculata and their hormonal control over the chromatophores. The SPX-organ of Boreomysis arctica consists of a group of cells and a vesicle, surrounded by a connective tissue sheath, located near the base of the sensory papilla of the eyestalk. The organ is connected by afferent nerves with the medulla terminalis. Neurosecretions produced by the cells of the organ are transported to the vesicle where release is probably effected by the afferent nerves. Dahl and Mecklenburg (1969) suggest that this organ in mysids should be considered not only as a neurosecretory organ but also as a neurohaemal organ. The SPX-organ of Mysis relicta, connected with the medulla terminalis, has “onion bodies” present (Hogstad, 1969) although these were not found to be present within the organ of Boreomysis arctica (Dahl and Mecklenburg, 1969). Kulakovskii (1969) shows an X-organ in the medulla terminalis (MTGXorgan) of Mysis oculata and Gabe (1966) found this ganglionic organ in Siriella sp., Praunus Jlexuosus and Paramysis sp. Hogstad (1969) did not observe it in the eye of Mysis relicta; there was, however, a group of cells situated medio-laterally between the medulla externa and medulla interna, that were assumed to be the medulla interna ganglionic X-organ (MIGX-organ). The sinus gland, described by Hogstad as biscuit-shaped, lies in the lateral-ventral region of the eyestalk close to the medulla interna and medutla terminalis (Fig. 35). Bioluminescence has been reported in several mysids. Illig ( 1905) describes a bioluminescent gland in the maxilla of Gnathophausia ingens. Glandular cells secrete material into a reservoir, the walls of which have muscles attached to them. The reservoir opens to the exterior near the base of the exopod of the maxilla. Tett and Kelly (1973), quoting Harvey (1952), state that, in addition to Gnathophausia species, species in the genus Gastrosaccus and possibly Siriella are luminescent. No modern information is available and investigations of the occurrence of luminescent organs in the Mysidacea are required. Another pair of glands, of a very different nature, the excretory or antenna1 glands are present in the basal segments of the antennae.

7.

133

INTERNAL ANATOMY

They are of the form found in other crustaceans, consisting of an end sac connected by a urinary duct to a terminal vesicle that opens at a papilla to the outside of the animal (Fig. 37). The glands of Lophogaster typicus and Hemimysis Eamornae are described by Cannon and Manton ( 1927b), those of Gastrosaccus spinifer, Mysis relicta, Praunus jlexuosus, P. inermis and Mesopodopsis slabberi by Vogt (1932, 1933) and those of Eucopia species by Siewing (1956). The end sac is oval in shape and has an internal network of blood lacunae. It connects with an irregularly shaped urinary duct, short in Mysis relicta but notably longer in Mesopodopsis slabberi.

P

t

es

m

es

m

FIQ.37. The excretory organs (antenna1glands) of left, Guatrosaccus spinifer and right, Myaie relicta (after Vogt, 1932, 1933). es, end sac; m, mandible;p, external opening; t, terminal vesicle; ud, urinary duct.

A gland, termed a Y-organ, is described in the ventral region of the head of Praunus jlexuosus, Paramysis sp. and Siriella sp. by Gabe (1952, 1956). This gland appears to be analagous with the ventral gland of insects and secretes a hormone controlling the moulting cycle. There is some evidence suggesting that mysids are able to locate, by chemosensory mechanisms, sources of food. The presence of excretory material, detected by similar sensors or sensilla, may also convey useful information to the mysid. Clutter (1969) hypothesized the presence of chemo- and mechanoreceptors in mysids when examining the swarming behaviour of Metamysidopsis elongata. Fuzessery and Childress (1975) investigated the amino acid feeding response thresholds of Gnatlwphausia ingens and some other crustaceans. Concentrations of amino acids (L-glutamic acid, taurine and DL alpha aminon-butyric acid) of 10-10-10-11 M initiated feeding behaviour. The dactyl receptors of G, ingens had a response threshold at 6 x 10-8 M, while antenna1 receptors had a threshold a t 5 x lo-' M. Fuzessery and Childress discuss the implications of these observations including the

134

THE BIOLOGY OF MYSIDS

possibility that this species, and other species, can orientate to a chemical gradient in order to locate food. No description of the chemosensors involved in the detection of the amino acids is given by Fuzessery and Childress. The existence of sensilla on the antennae, mouthparts and thoracic legs of crustaceans has been known for some time. Crouau (1978) and Guse (1978) describe the histological structure and nervous innervation of hair sensilla on the antennae of Neomysis integer. The double and triple innervated hairs are probably mechanoreceptors, while the hairs with many sensory cells are probably mechanochemoreceptors or chemoreceptors. There are, however, also many sensilla and openings of sub-integumental glands distributed over the whole of the integument of the body of crustaceans (Mauchline, 1977s; Mauchline and Nemoto, 1977; Mauchline et al., 1977). These often have distinctive patterns of distribution that are peculiar to families, genera and, in some cases, species. The distribution of such organs in the integuments of Eucopia sculpticauda and Praunus jlexuosus is shown in Fig. 38. The integuments have been digested in hot potassium hydroxide which removes all the soft tissues. They are then cut along the ventral midline and the carapace and dorsal and lateral (tergal and pleural) regions of the abdominal segments laid flat, outer surfaces upwards, on microscope slides for examination. The tissues of the glands and sensilla are digested by the hot potassium hydroxide and only a hole (pore) is left at the site in the integuments where the sensilla or gland was present. The hole or pore is only an indication of a connection between the sub-integumental tissue and the outside environment. This connection can be one of the following: a tubular duct of a subintegumental gland, that releases its secretions to the outside environment ; a tube connecting a sensillum with its axons and sensory neurons in the hypodermis; a tube through which nerve fibres from the peripheral nervous system pass outwards to that part of the sensillum that is external to the integument. These pores occur over the whole of the integument, including the exposed tergal areas of the posterior thoracic segments (Fig. 38). There is a varying degree of bilateral symmetry, most evident in the distribution of conspicuously large pores or in that of conspicuous groups of pores. The distributions of conspicuous pores or groups of pores in species in different genera are shown in Fig. 39. Eucopia sculpticauda and Praunus jlexuosw are included. The distributions show considerable variations between genera. Dr M. Murano supplied 11 Siriella spp., 7 Neomysis spp. and 10 Acanthomysis spp., and the distributions of the pores were examined. The distributions shown in

7.

INTERNAL ANATOMY

135

FIG.38. Distribution of pores in the treated integument of (left) Praunwflexuow and (right) Eucopia aculpticauda (Mauohline, 1977).

Fig. 39 contain pores common to all species examined within each of these genera. There was remarkably little variation in the general pattern between species within these genera and this is emphasized by the identical overall patterns shown for the closely related genera Neomysis and Acanthomysis (Fig. 39). Detailed examination of the distribution apd numbers of pores in species within these genera did produce specific differences in some cases, but not in others. The two species of Schistomysis examined (Fig. 39) are rather different and so larger specific differences may occur in some other genera.

... .... -..

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.%

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Eucopia sculpficauda

Schisfomysis spiritus

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Praunus f lexuosus

Kaf ery fhrops oceanae

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Neomysls soecies

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Mysidopsis gibbasa

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1 FIQ.39. Distribution of groups of pores in the carapace (C), posterior thoracic segment (PT) and the six abdominal segments 1-8.

7.

INTERNAL ANATOMY

137

The sub-cuticular and intra-cuticular structures of the sensilla and glands that were present a t the sites of these pores before digestion of the integument by potassium hydroxide are unknown. The surface of the fresh integument has been examined and a variety of sensilla and gland openings recognized. There are feather-like setae that are probably hydrodynamic receptors. They tend t o occur in dorsal anterior regions of the carapace and other dorsal regions of the body, including the abdomen; Debaisieux (1947) draws some on the dorsal surface of the endopod of the uropod and shows nerves travelling to them (Fig. 40ds). The rows of pores occurring across the dorsal region of the carapace and abdominal segments in many species (Fig. 39) often represent the sites of small conical structures with or without a pore a t the apex; these are thought to be chemosensory in function. Much further work is required, especially on the histological structure of these organs. It seems probable, however, that the mysids are monitoring various parameters of their three-dimensional environment, through the use of integumentory receptors. Clutter (1969) suggests that Metamysidopsis elongata, in common with other crustaceans, uses chemical cues for mgting. The newly moulted female appears to be located along a chemical gradient by the male. The specific receptors involved have not been identified. One further pair of large sense organs are present in the Sub-Order Mysida except in the family Pekalophthalmidae, but are absent in the other Sub-order, Lophogastrida. They are the statocysts, or gravity receptors, present in the endopods of the uropods (Fig. 40). The histology of the statocyst of Praunus jlexuosus is described by Debaisieux (1947, 1949) who reviews the earlier literature. The statocyst comprises a vesicle inside which is a lith suspended on sensory hairs. The lith is secreted by the mysid a t the moult that terminates Stage I11 of the larval development ; this moult takes place shortly after emergence from the marsupium. The lith consists of a protein matrix surrounded by a calcareous shell that is rich in calcium fluoride (Enbysk and Linger, 1966) and has an undetermined organic base. The shell is penetrated by the 50-60 sensory hairs by which the lith is suspended in the vesicle. These hairs originate from a ventral sensory cushion (Fig. 40) and are inserted along the outside lateral edge, relative t o the animal's body, of the lith (Fig. 40). The planes of the sensory cushion and lith in the two statocysts are inclined towards each other, ventromedially, so that they are a t an angle of 50-55" t o the horizontal longitudinal plane of the animal. Consequently, the sensory hairs are under tension when the animal is orientated horizontally. Neil (1975~) discusses the shearing forces acting on the sensory hairs during

138

THE BIOLOGY OF MYSIDS

A

\

C

\ \ \

,'550',

FIQ.40. The statocyst of Praunus ~ V e x w a w r .A, uropods showing position of stetocysts in the endopods. B, sagittal section through the lith within its vesicle. C, diegrammetic transverse sections of the endopods to show the orientation of the litha relative to each other and to the horizontal. (A end B after Debaisieux, 1947; C, after Neil, 1975~).c, sensory cushion; ds, dorsal sensory aetae; 1, lith; m, muscle; n, nerve; sh, sensory hairs of lith; v, vesicle.

rotation and tilting of the animal and shows that the neural control system sunimates the afferent signals from both statocysts. The statocysts operate in conjunction with the eyes to control orientation of the mysid in space (Neil, 1975a-c). The shearing stimuli over the sensory cushion in each of the two statocysts result in opposite turning reactions because of the mirror inclinations of the cushions. This bi-directional input to the statocyst control system, however, results in an essentially uni-directional output to the eye stalks. Applying visual stimuli, such as a vertical light beam, modifies the eye movements but leaves their general pattern unaltered, and the orientation of the animal is normal. Orientation was altered in Mysidium gracile by applying a horizontal light beam; the mysids compromised and orientated a t about$45" to the horizontal (Jander, 1962). Rice (1961, 1964) found that the effects of gravity dominated those of light in the orientation process. Increase in hydrostatic pressure caused the mysids to move upwards in the water column, even when illuminated from below. This reaction took place in Leptomysis lingvura, Schistomysis spiritus, Praunus jiexuosus, P . neglectus and to a, considerable

7. INTERN&

ANATOMY

139

but lesser extent in SIiriella urmatu. An additional sensory input is probably used for purposes of orientation by, for example, epibenthic species ; this input is from tactile receptors on the dactyls of the thoracic legs (Foxon, 1940). Consequently, orientational responses probably result from integration of inputs not only from the statocysts and eyes, but also in certain circumstances, from additional inputs from integumental receptors, especially tactile receptors on the thoracic legs.

CHAPTER 8

PHYSIOLOGY A N D RESPONSES TO PHYSICAL-CHEMICAL PARAMETERS OF THE ENVlRONMENT Mysids exhibit a diversity of behavioural patterns. Their behavioural responses are elicited by two general classes of environmental stimuli; the first consists of changes in the physical and chemical parameters, while the second consists of stimuli originating from other organisms, including other mysids. In addition, of course, the internal physiological state of the individual also has an influence on its behaviour a t any one instant of time. Consequently, the physiology of the animals and their responses to the physical-chemical parameters of the environment will be discussed in this chapter. This is followed by an examination of more general aspects of the behaviour of individuals and populations in the next chapter. Swimming is effected by beating the exopodites of the thoracic legs. Many female mysids have rudimentary pleopods while in many males the pleopods, if developed, are modified as secondary sexual characters, and are not used in swimming. The beating of the thoracic endopodites generates the respiratory currents as well as providing propulsion for the body. Laverack et al. (1977) have shown that the beat of an exopodite has a modified rowing action. It consists of a power stroke, a return stroke, and a pause. The beat frequency in Praunus jlexuosus is about 4 Hz. The co-ordination between successive exopodites on the same side of the thorax and between the exopodites of corresponding legs on opposite sides of the body is strong. The speed of swimming of mysids has not been measured in many species. Metamysidopsis elongata of 4-7 mm total body length (a length representitive of many mysids, see Fig. 20, p. 72) can swim at speeds up to 13cm/s but normal cruising speed is probably within the range of 5-6 cm/s (Clutter, 1969). Acanthomysis species of 5-9 mm total length can maintain speeds of up to 20 cm/s in the laboratory, although Clutter considers that they normally swim at speeds of 5-7 cm/s in the sea. Another species of comparable body size, Mysidium columbiae, can swim at speeds of

8.

PHYSIOLOGY AND RESPONSES TO THE ENVIRONMENT

141

up to 15 cmjs (Steven, 1961). Much slower swimming speeds are mentioned by Foulds and Roff (1976) in Mysis relicta of 10 mm total body length; they measured respiration rates a t swimming speeds in the ranges 14-1.6 and 2.1-3.6 cm/s. Robertson et al. (1968) estimated a swimming speed of 5-10cm/s in this species while observing its behaviour from a submarine. Consequently, the maximum swimming speed of this species remains unknown. Paramysis intermedia could not maintain its position in water currents exceeding velocities of 40 cm/s (Boroditch and Havlena, 1973). Species and individuals vary in the amount of time they spend swimming. All species are slightly denser than the surrounding seawater; Morris (1972) found that the density of Eucopia sculpticauda was 1.024. An epibenthic habit influences the amount of time spent swimming and low light intensity has been shown to have an inhibitory effect on swimming. Some species appear to swim almost continuously while others have what are probably endogenous rhythms of activity. For instance, Hobson and Chess (1976) found that Siriella pacijica is a nocturnal carnivore that lives on the surface of the seabed among the kelp, Macrocystis pyrifera, during the day, but forages outside the kelp beds at night. A co-occurring species Acanthomysis sculpta, however, fed by day and night usually entirely within the confines of the kelp beds and showed no periods of increased or decreased activity. Burrowing species of Gastrosaccus rest during the day in the sediment, but emerge at night to swim; this is an endogenous rhythm of nocturnal emergence and Macquart-Moulin ( 1 9 7 7 ~ ) found that this pattern of swimming activity persists under experimental conditions of constant darkness for 10 days in G. mediterraneus and for about 50 days in G. spinifera. Most hyperbenthic species probably rest for periods on the surface of the sediment or local algae. Species such as Praunus$exuosus can be observed doing this in shallow water during the day for one to several hours a t a time. Two hyperbenthic species, Leptomysis gracilis and Siriella jaltensis, were found to have no endogenous rhythm of activity and a further species, S . clausii, only had a weak rhythm (Macquart-Moulin, 1973b). Anchiah a agilis, which according to Macquart-Moulin, is hyperbenthic in the Mediterranean but which is more strictly pelagic in British waters, also exhibited no endogenous rhythm of swimming activity. Consequently, it is probable that endogenous rhythms are confined to hyperbenthic species living among the macro-algae in shallow habitats and other species living in specialized situations, such as burrows. Hyperbenthic species living in deeper habitats without sheltered regions such as those provided by macro-algal beds, probably have

142

THE BIOLOGY OF MYSIDS

irregular periods of rest on the sediment surface. Pelagic species probably swim more or less ‘continuously. The levels of energy expended in swimming have been measured in two mysids. Klyashtorin and Kuz’micheva (1975) found that the coastal Neomysis mirabilis expended energy during swimming a t a level equivalent to 1.3 times that of the rate of standard metabolism. They conclude that planktonic crustaceans weighing 1-100 mg in body weight and swimming a t rates characteristic of periods of prolonged continuous swimming have total energy expenditures that exceed levels of energy consumption at relative rest by factors of 1.5-2.0. Comparable factors were determined for the large bathypelagic species Gnathophausia ingens by Quetin et al. (1978). They found a factor of 2.3 a t a rate of swimming activity that the animal could maintain continuously for a period of a t least 14 days. This factor increased to 2.9 a t the maximum rate of swimming activity observed in this species. Some 3% of its metabolic scope was required by G . ingens to maintain its position in the water column and about 40% for other swimming activity. Quetin et al. concluded, contrary to their previous convictions, that G. ingens swims more or less continuously a t a steady and relatively rapid rate and does not have a lethargic life style. Foulds and Roff (1976) also examined levels of energy expenditure during swimming. They, however, used the rate of routine metabolism as the reference rate. They defined routine metabolism as present when “a behaviour pattern of resting interspersed with spontaneous activity under conditions of minimal external stimulation” existed. A maximum increase of 1.1 times the routine rate waa present in the freshwater Mysis relicta when swimming a t speeds of 1-0-1-6 cm/s and 1.2 times the routine rate a t swimming speeds in the range 1.2-3.6 cm/s. These swimming speeds are probably low relative to the maximum speeds of this species. Rhythms of activity may be interrupted temporarily by moulting although there is no information on this aspect in mysids. Nouvel (1957b) discusses the process of moulting in a range of species and Saudray (1972) examines the effect of moulting on the functioning of the mandibles of Praunus jlexuosus. Holmquist ( 1959b) considered that Mysis relicta maintained in aquaria at 4°C had intermoult periods ranging from 17 to 24 days and sometimes from 28 to 30 days. Blegvad (1922), rearing individual Praunus inermis and P . Jtexwsus in aquaria, observed intermoult periods of irregular duration; his data are not presented in a form that will allow more detailed exatmination. The intermoult periods appear to range from 15 to about 60 days, a modal range being approximately 20-30 days. The duration of the

8. PHYSIOLOGY AND RESPONSES

TO THE ENVIRONMENT

143

intermoult period was found to increase logarithmically when plotted against body length or successive moult number in euphausiids and other decapod crustaceans (Mauchline, 1977b, c). The intermoult periods determined in Praunus JEexuosusby Nouvel and Nouvel (1939) and Metamysidopsis elongata by Clutter and Theilacker (1971) are shown in Fig. 41 in relation to total body length. Dr R. Gaudy kindly

30r 1-003

10

Carapace length ( rnm) 20 30 40

50

Gnathophausis ingens

5 5°C 6 5°C 7 5OC

20 -200

10-100 P L a Q ,

4-

Praunus f/exuosus

-

0 Mefarnysidopsis elongofa

A

Melongofa

2 ' 1 2

I

4

'

I

6

I

I

8

I

"

10

]

12

I

14

1

I

16

I

Total length (rnrn)

FIQ.41. The durations of the intermoult period related t o body size of three species of mysids. The durations of the periods of fhzathophausia ingens are 100-250 days and are related to carapace length and determined a t 3 temperatures. The regression, equations of the lines are given in Table XXIII. Data on Praunw fZexuosus from Nouvel and Nouvel (1939), on Mebmyeidopsis elongab from Clutter and Theilacker (1971) and on G m t h p h a w i a ingem from Childress and Price (1978).

allowed me to examine his data (Gaudy and Guerin, 1979) prior to publication ; the relationship between the duration of the intermoult period and successive moult number a t several temperatures is shown in Fig. 42. The regression equations for these relationships are given in Table XXIII, the regression coefficients all being significant a t the 1.0-0-1% level. Childress and Price (1978) have determined the intermoult periods of the bathypelagic Gnathophauaia ingens. These ranged

144

THE BIOLOGY OF MYSIDS

30 r

Y 20 -

-"z 10-

0

-

-

0

u

0 .-

b a

c

-

7-

-

3

0

E

4-

0)

c

C -

Lepfomysis hnqvuro 10°C A 14°C o 18°C A 22°C

214°C 018°C A 22°C A

A

1 -

1

1 2

1

1 4

1

1 6

1

1 8

1

1

1

1 10 Moult number

3

5

7

9

FIG.42. Duration of the intermoult period related to successive moult number at various experimental temperatures in Hemimysis spetuncola and Leptotmysis lingvura. Data from: Gaudy and Guerin (1979).

from about 105 to 210 days, dependent upon the sizes of the individuals and the experimental temperatures. They expressed the relationships at the three temperatures as:

+ 0.00051L3 6.5"C D = 137.7 + 0*00050L3 7 ~ 5 ° C D = 120.5 + 0*00038L3 5.5"C

D

= 166.3

where D is the duration of the intermoult period in days and L3 is carapace length to the third power. These equations have been transformed in Table XXIII for comparison with those of the other species. The most notable feature of the equations for this relationship in G. ingens is that their slope ( b ) is markedly reduced, not only relative to that of the other mysids in Table XXIII but also relative to other

TABLEXXIII. REGRESSION ANALYSES OF THE LOGARITHM OF THE ~ ~ R M PERIOD O ~ T (IN DAYS)ON BODYLENUTEOR SUCCESS~VE MOULTNUMBER Regression constants logy = bx f a Species

Independent measurement

Gnuthophauaia ingens a t 5.5"C a t 6.5OC at 76°C

CL CL CL

Hemimysis speluncoh at 10°C at 14°C at 18OC a t 22°C

MN

m

MN MN

M~ASWMENT OF

a

b

2.1700 2.0804 2-0289

0.0040

10 10 9 10

0.5575 0.3517 0.1602 0.1 112

0.0831 0.0870 0.0964 0.0907

0.9169* * * 0*9249*** 0*9693*** 0.9315***

Gaudy and Gaudy and Gaudy and Gaudy and

Guerin (1979) Guerin (1979) Guerin (1979) Guerin (1979)

5 7

0.6975 0.4239 0-3036 0.0117

0.1305 0.0780 0.0661 0.0873

0.9874** 0*8090*** 0.8795*** 0.9359***

Gaudy and Gaudy and Gaudy and Gaudy and

Guerin Guerin Guerin Guerin

0.3604 0.3886

0.0699 0.0850 0.0676

0.9795** * 0.5987*** 0.9717* * *

Clutter and Theilacker (1971) Clutter and Theilacker (1971) Nouvel and Nouvel (1939)

n

Leptomysis lingvura a t 10°C a t 14°C at 18°C at 22°C

MN MN MN

10

Metamydopsis elon.gata females males Praunua jlexuo;lu8

TL TL TL

5 6 11

MN

Value and signijkance of

A

7

0.0048

Source of data

Childress and Price (1978) Childress and Price (1978)Childress and Price (1978)

0.0035 0.0035

(1979) (1979) (1979) (1979)

TL, total length; CL, carapace length; MN. moult number; n, number of observations. The value and significance of the regression coefficient t is given.

146

THE BIOLOGY OF MYSIDS

crustaceans (Mauchline, 197713, c). The relationships in adult euphausiids have values of b,ranging from 0.0058 t o 0-147, in decapods other than panulirid from 0.010 to 0.054 and in panulirids from 0.004 to 0.009. Hartnoll (1978) suggests that b decreases in larger decapods and this may also be true in mysids. Conversely, it may be related in some way to the conditions of the bathypelagic environment in the case of G. ingens. Further discussion of this aspect and its effect on growth rates is presented in Chapter 10. The rates of oxygen consumption of mysids during respiration are comparable to those of other crustaceans (Table XXIV). The TABLEXXIV. OXYGEN

Species Gnathophausia ingens Gnathophausia zoea Gnathophawia gigas Gnathophawia gracilis Boreomysis californica Archaeomysis grebnitzkii" Gastrosaccua simulansa Hemimysis lamornae Holmsiella anomla" Hypererythrops sp. Metamysidopeis elongata" Mysis relictaa

Temperature ("C)

pl OJmg wet wt/h

5.5 5.5 5.0 5.5 4.0

0'024-0'48 0.035 0.031 0.027 0.032 0.022 0.033 0.17 0.27 0.49 1.54-2.40 1.8 3.6 0.3 0.79 0.5-0.7

4.0

5.5 5 10 15 27.5 10 20 13 17 13.8 5 4

Neomysis americana Neornysis awatschensisa Neomysis in,teger

a

CONSUMPTION OF

5 5 10 15 10 18 f 0.5

MYSIDS

Reference

Childress (1971a) Childress (1975) Quetin et al. (1978) Childress (1975) Childress (1975) Childress (1975) Childress (1975) Jawed (1973) Jawed (1973) Jawed (1973) Gauld and Raju (1975) Grainger (1956) Grainger (1956) Ikeda (1977) Ikeda (1970) Clutter and Theilacker (1971) Lasenby and Langford 0.4 (1972) Foulds and Roff (1976) 0.21 0.26-0.38 Raymont end Conover (1961) Jawed (1973) 0.21 Jawed (1973) 0.29 Jawed (1973) 0.35 0.15-0.46 Raymont et al. (1966) 0.18- 1.29 Raymont and Krishnaswamy (1968)

Original data on dry weight basis; dry weight equivalent to 20% wet weight.

8.

PHYSIOLOGY AND RESPONSES TO THE ENVIRONMENT

147

bathypelagic species of the genera Gnathophausia and Boreomysis have lower weight specific respiration rates as discussed by Childress (1971b, 1975). Their increased body size relative to that of coastal and epipelagic species does not account for this decrease (Fig. 43) as pointed out by Childress. The haemolymph, which probably contains a haemocyanin, of Gnathophausia ingens has an unusually high affinity for oxygen. Free1 (1978) found that the partial pressure of oxygen a t half saturation (P50)is 1.5 k 0.19 mmHg, and suggests that the P,, may be well below 1 mmHg when this species is living in its normal habitat a t temperatures of 5°C and less. The rates measured in Gastrosaccus simulans by Gauld and Raju (1975) are exceptionally high, but may be partly accounted for by the high environmental temperature, 27.5"C in which this species lives. Chin (1976) obtained rates of respiration in Neomysis awatschensis two or more times higher than those determined by Jawed (1973) in Table XXIV; the conditions of salinity in the experiments differed. Comparable rates of respiration occurred in Paracanthomysis hispida (Lee and Chin, 1976). Klyashtorin and Musayeva (1977) compare estimations of respiration rates of Neomysis mirabilis using an oxygen electrode and the standard Winkler chemical method. Lower rates, lower by 8%, were measured with the electrode but they conclude that its use makes such measurements easier and faster to carry out. Various factors, in addition to swimming activity, affect the respiration rate. The most important is the environmental temperature, increasing temperature being associated with higher respiration rates (Grainger, 1956). Clutter and Theilacker (1971) estimate a Qlo for Neomysis americana of 1.6 and for N . integer and Hemimysis lamornae of 1.9; this latter factor was calculated from the data of Grainger (1956) on H . lamornae. A Qlo of 2.9 for Archaeomysis grebnitzkii and of 1.7 for Neomysis awatschensis are present in Table XXIV. Chin (1971) found the Qlo for Neomysis awatschensis, in experiments where individuals were acclimatized to 5" intervals of temperature in the range 5-25"C, to be between 1-56 and 2.18 over the 5-15"C range and to be 1.17-2.64 over the 15-25°C range. The effect of thermal acclimatization on respiration rates of this species was investigated further by Chin (1974). A Qlo of approximately 2.0 was found by Lasenby and Langford (1972) for the freshwater Mysis relicta. The effects of varying combinations of temperature and salinity or of varying salinity independent of temperature have been investigated by Chin (1972, 1974), Jawed (1973), Foul& and*Roff (1976) and Vlasblom and Elgershuizen (1977). There are optimal combinations of temperature-salinity for each species, as would be expected.

3-00 @

1-00

i $IL

3 c 0 \

3

c

5

- 1

5 010

0 N

F

0.011

n

Arcboeomysis grebnifzkii Gaslrosaccus sirnulans Hemirnysis farnornae Hypererytbrops sp. Metamysidopsis elongafa Mysis relicta 0~ y s i reficta s 0 Neomysis arnericona Neomysis owotscbensis Boreomysis californico

sr

I

I I l l l l 1l0

I

I

I

I I 1 1 1 1

102 Body wet weight ( mg )

t

I

I

G.ingens

I

H

G.qiqas Ggracilis

I

* I

Gnotbopbausia species

I

I

I

I I I I l l

1 o3

I

i

I

I I I I Ill 104

FIG.43. Respiratory rates of mysids related to body wet weight. The two regreasion &es in the left upper part ofthe figure are for Metamy&pia e h g a t a and Mysis d i c t a respectively. The respiratory rat0s of Gnathopha& species are presented in relation to their range of body wet weight. Data from: Grainger, 1956; Raymont and conover, 1961; Ikeda, 1970; Clutter and Theilacker, 1971; LasenbyandLangford, 1972; Jawed, 1973; Childress, 1975; Gauld and Raju, 1975; Foul& and Roff, 1976.

8.

PHYSIOLOOY AND RESPONSES TO THE ENVIRONMENT

149

The different species so far investigated show varying aptitudes for acclimatization to 'extreme values. The degree of oxygen saturation of the water also affects the ra-tes of respiration. Species such as Holmsiella anornula examined by Ikeda (1977) show a high tolerance to oxygen concentrations as low as 10% of saturation. Archaeomysis grebnitzkii and Neomysis awatschensis, two coastal species, were less tolerant, dying at levels of less than 13% saturation (Jawed, 1973). The most tolerant species, with aerobic regulatory abilities, are the bathypelagic GnathophaMia species and Boreomysis calijomica investigated by Childress (1968, 1971a, 1975). Their respiratory rates did not decline markedly until the oxygen concentrations reached the low levels of about 6% saturation. These species inhabit the oxygen minimum layers of the eastern Pacific where oxygen is a t minimal concentrations of about 0.20 ml 0,/1 (equivalent to about 4% saturation). Gradients of pH are associated with these regions of oxygen minimum layers, the pH ranging from 8.3 at the surface to 7.5 or less in the oxygen minimum layers. Mickel and Childress (1978, however, found that changing pH of the water had no effect on the respiratory processes of Gnathphausia ingens. The only activity affected by changing pH was the extraction of oxygen from the respiratory stream; extraction rates were 10-30% higher a t pH 7.1 than at pH of 7.9 and 8-7 but why this was so is unknown. A seasonal change in the respiratory rate of Neomysis integer was demonstrated by Raymont et al. (1966). Minimal levels occurred in September-October followed by successively increasing levels to a pronounced maximum in March. No other information on this aspect is available. The weight specific respiration rates decrease within a species with increasing body size (Chin, 1971, 1974; Clutter and Theilacker, 1971; Jawed, 1973; Lee and Chin, 1976; Vlasblom and Elgershuizen, 1977) and a similar trend is present between species (Fig. 43) as suggested by Ikeda (1970). Shushkina (1973) determined the relationships between respiration and body weight in Neomysis mirabilis in the sea of Japan as:

R

= 0.14

A detailed study has been made of the respiration rate of N . mercedis by Simmons and Knight (1975). They derived a general equation relating its respiration rate (yl O,/animal/h) to changing values of body dry weight (mg), temperature ("C), sex (see below), salinity (% sea water) and month of the year (1-12). Sex was scored 1-5; 1 being for juveniles, 2 for males, 3 for immature females, 4 for non-ovigerous

150

THE BIOLOGY OF MYSIDS

mature females, and 5 for ovigerous females. The equation is:

R

=

2.552+1.779 WT+0-443 T-0-888s-O.Ol8SAL-0.178

M

where R is respiration, W T is dry weight, T is temperature, S is the score for sex, S A L is percentage sea water, and M is the month of the year. They examined the influence of single variables upon the others and upon the respiration rate. The relationship of respiration rate to temperature, salinity and body weight is not constant but varies with season. The simple relationship between respiration and body weight also changes seasonally. They examined all permutations and came to the following general conclusions. The change in the respiration rate in response to changes in either the salinity or temperature is dependent upon the body dry weight of the mysid and its sex. In turn, the influence of body weight and sex depends upon seasonal effects of reproduction and food supply. Changes of temperature a t low salinity and of salinity at low temperature produce larger responses in the respiration rate than the corresponding changes at high salinity and high temperature. An indirect method of measuring respiratory rates is to study the activity of the electron transport system (ETS). Exploratory investigations on other crustaceans have suggested a close correspondence between ETS activity and oxygen consumption (Mauchline, 1980). This is possibly true in Mysis relicta. Borgmann (1978) found that nicotinamide adenine dinucleotide (NADH) oxidation rates were equal to succinate oxidation rates and hence oxidation is also approximately proportional to the rate of oxygen consumption. The rate of production of carbon dioxide by Neomysis integer was measured by Raymont and Krishnaswamy (1968). This was done by determining changes in the pH of the medium. The carbon dioxide output of this species maintained a t 18+0*5"Cranged from 0.10 to 0.62 pl carbon dioxidelmg wet weightlh. General studies of excretion in mysids are few and can be appropriately summarized here. Hargrave and Geen (1968) found that a, mysid (un-named) excreted about, 2.4 pg-at x 10-5 inorganic phosphoruslmg body weightlday. The dominant nitrogenous excretory product is ammonia with lesser quantities of amino-N and apparently no urea (Jawed, 1969). Ammonia amounts to about 75% of the total excreted nitrogen while amino-N represents about 2074. Jawed found that the proportion of nitrogen excreted as ammonia increased at lower temperatures. The average rate of excretion of ammonia a t 10°C by Neomysis rayii wa3 25 pg N/mg body N/day. Chin (1974) found that the rate of ammonia excretion in N . awatchensis changed

8.

151

PHYSIOLOOY AND RESPONSES TO THE ENVIRONMENT

seasonally, average, rates of about 42-58 pg Nfmg body N/day occurring during the summer as compared with rates of 25-42 pg N/mg body N/day during the winter. The amount of total nitrogen excreted at low salinity ( 8 %,) increased with decreasing temperature while the proportion of amino-N increased (Chin, 1976). A similar effect of low salinity and decreasing temperature on the rate of phosphorus excretion occurred. Rates of excretion of ammonia-N were 6-16 pg nitrogen/mg dry weightlday, of amino-N 1-2-58 pg nitrogenfmg dry weightlday, and of total phosphorus 0.2-7.8 pg phosphorus/mg dry weightlday, depending upon conditions of'temperature and salinity. Lee and Chin (1976) found that female Paracanthomysis hispida, over a temperature range of 7-23"C) excreted 2.5-8.0 pg nitrogenlmg dry weightiday as ammonia-N, 1.4-1-9 pg nitrogenfmg dry weightiday as amino-N, whereas comparable rates in males were 4.4-12.6 and 1.9-3.6 respectively. Starvation of the animals significantly decreased the rates of excretion. Raymont et al. (1968) determined the rate of excretion of ammonia-N in terms of body nitrogen in Neomysis integer as approximately 200 pg nitrogenlmg body nitrogenlday. Studies of energy budgets of mysids are few. The partitioning of ingested food between growth, reproduction, respiration, egestion and moulting is shown in Table XXV. The environments of the two species cited are very different. Metamysidopsis elongata is a littoral species living between 35"N and 22"s in the eastern Pacific; the population studied by Clutter and Theilacker (1971) lives in environmental temperatures of 14-20°C. Mysis relicta on the other hand, was studied by Lasenby and Langford in two freshwater lakes, namely Char Lake located in the Arctic at 74" 42") 94" 54'W and the temperate Stony TABLE XXV. PERCENTAGE ENERQY BUDGETS OF MYSIDS Temperature ("C)

Species Metamysidopsis elongata Female Male

14-20 14-20

Mysis relicta, female Stony Lake Char Lake

2-10

0.2-3.8 ~~~~~~~~

G

R

Re

M

36

9 0

49 54

7 10

15.9 11-5

5-7 3.3

63.7 70.2

14.7 14.9

35

~

Ingestion = Growth + Reproduction+ Respiration+ (Moulting+ Egestion), or I = G R + R e +M. Environmental temperature is given. Data from: Clutter and Theilacker, 1971; Lasenby and Langford, 1972.

+

152

THE BIOLOGY OF MYSIDS

Lake at 44" 35'N, 78" 45'W. I n females, the proportional amount of ingested energy ascribed to growth and reproduction decreases with decreasing environmental temperature (Table XXV). This reflects the increased time required for the females to develop at lower temperatures from the egg, attain sexual maturity and produce and lay their own eggs. According to Clutter and Theilacker, the males use a small amount of energy for reproductive products and they assumed it was negligible (Table XXV). The males require less total ingested energy than the females in order t o attain sexual maturity, but expend relatively more of this energy in respirat,ion and moulting. The most important physical parameter of the environment, governing the behaviour and distribution of many species, is the presence or absence of light. Environmental illumination was mentioned previously in the context of various aspects of the animal's physiology. Smith (1970) exposed Hysis relicta to 10 h of normal laboratory levels of illumination during each 24 h period for some 12 weeks; a group of control animals was kept in constant darkness. Survival was high in the control, 93.3%) but only 18.6% of the animals exposed to the periodic illumination survived the experiment. Higher survival rates of many planktonic crustaceans occur under conditions of constant darkness in the laboratory, but it is difficult to relate this result directly to the environment, especially in the context of some of the littoral species examined. Macquart-Moulin (1965, 1972, 1973b, 1977a, b, c) and MacquartMoulin and Patriti (1966) have made the most detailed studies of the behavioural reactions of mysids to light. The phototactic responses of Neomysis americana and Mysis relicta have been examined by Herman (1962) and Beeton (1959) respectively. I n general, mysids are attracted to weak sources of light, but avoid bright light. Bright light often inhibits swimming activity. Normal diurnal light levels of 103-104 lx are inhibitory and produce a negatively phototactic response in most species. This negatively phototactic response was still present at a light intensity of 1 lx in species such as Siriella jaltensis, Anchialina agilis,Gastrosaccus lobatus, G. mediterraneus and G . spinifer. The phototaxis becomes positive a t intensities of 4 x lx or less. Negative phototaxis is present in blue and green monochromatic light of 6 lx, but it is reversed in light of the same intensity but of longer wavelength, as found in Anchialina agilis by Macquart-Moulin (1977b). The 24 h cycle of change in ambient light intensity is the dominant factor controlling the die1 vertical migration of mysids (Beeton, 1960; McNaught and Hasler, 1966; Heubach, 1969; Teraguchi et al., 1975). Mysis relicta migrated towards the surface layers when ambient light

8.

PHYSIOLOOY AND RESPONSES TO THE ENVIRONMENT

153

intensities were decreasing through the range 150-10 lx; the morning descent took place as the light intensities increased through the range 10-3-10-2 lx. Holmquist (1959b) suggests that species such as M . litoralis, which occurs in both freshwater and marine situations, tend to occur deeper in freshwater lakes and a t lower light intensities than comparable populations of the same species in marine habitats. These patterns of responses to changes in environmental illumination result frequently in littoral hyperbenthic species retiring to the shelter of macro-algal beds during the day, where they remain relatively inactive. The nocturnal period of darkness stimulates them to swim and forage outside the beds in the pelagic environment. The earlier work of Foxon (1940) demonstrated this behaviour in various hyperbenthic mysids. The burrowing species of the genus Gastrosaccus usually remain in the sediment during the day but forage in the water column at night (Macquart-Moulin, 1977~).Mysidium columbiae, according to Steven (1961)) appears to be exceptional in that it lives close to the water surface in the tropics, even in the mid-day sun. The positive phototactic response of mysids to weak light sources means that they can be attracted to light lures. Light lures can be used in combination with nets to sample mysids (Brattegard, 1970a; Bacescu, 1975). Brown and Talbot (1972) showed that Gastrosaccus psammodytes were attracted to narrow beams of light in laboratory tanks. This response, however, was absent in the presence of water currents, even weak water currents that did not impede the swimming of the mysids. This die1 pattern of activity affects sampling of mysids, especially hyperbenthic species ; larger numbers often occur in night samples, as instanced by the results of Jansson and Kallander (1968) in sampling Praunus species and Neomysis integer. Polarized light was detected by Mysidium gracile and used for directional orientation (Bainbridge and Waterman, 1957, 1958; Jander and Waterman, 1960). This species tended to orientate its longitudinal axis perpendicular to the plane of polarization. Some terrestrial arthropods have been shown to use linearly polarized light in the sky as a means of navigating during migration but this has not yet been demonstrated in mysids. Environmental temperature affects many of the physiological processes of the animal, as discussed previously. The tolerance of mysids to changes in environmental temperatures varies between species and, to a lesser extent, between populations of the same species in different environments. The freshwater Mysis relicta studied by Smith (1970) in Trout Lake, Minnesota, USA did not tolerate temperature rises, even when these were as small as 1"C/day. Acclimatization

154

THE BIOLOGY OF MYSIDS

of this species to raised or lowered environmental temperatures is probably a slow process and Smith concluded that hypolimnetic temperatures as high as 10°C and epilimnetic temperatures as high as 14°C would adversely affect populations of this species. Holmquist ( 1959b) found that this species in northern Europe and Greenland tolerated a range of O-lS"C, but only if the temperature was kept constant. The vertical and horizontal distribution of temperature in t'he water column was found by Beeton (1960) and Reynolds and De Graeve (1972) to influence the extent of the vertical migration of this species. Thermoclines with gradients of 1.67"C and 2°C acted as barriers to the upward migration of M . relicta; a warm epilimnetic layer was usually avoided although this species is certainly present in such a layer on occasions (Mundie, 1959; Beeton, 1960). Hakala (1978) found that the distribution of M . relicta in Lake Paajarvi, Finland was strictly controlled by the position of the 7°C isotherm, it living a t lower temperatures. The upper lethal temperature for Neom,ysis mercedis (awatschensis)is between 24 and 2505°Cand this species, unlike Mysis relicta, is better able to withstand thermal shocks, even as great as a 14°C rise in temperature (Hair, 1971). Heubach (1969) presents similar results for this species and also points out that Neomysis americana has an upper lethal temperature in this range. The related species, N . intermedia, which occurs through the same range of latitude, is tolerant to a wide range of environmental temperatures, 0-35°C (Murano, 1966d) ; reproduction of this species, however, only takes place between 15°C and 30°C (Murano, 1 9 6 6 ~ ) Neomysis . intermedia may live in freshwater lakes with summer temperatures ranging between about 4°C and 30°C. Very little information on environmental temperatures relating to the distributions of individual species of mysids is available. It is probable, as Kinne (1955) suggests in the case of N . integer, that juveniles have different ranges of tolerance to temperature changes than the adults. This brackish water species was shown by Vlasblom and Elgershuizen (1977) t o be more tolerant of high environmental temperatures than Praunus Jlexuosus ; this is to be expected from comparing the distributions of these two species in coastal regions. The effect of salinity alone, or in combination with temperature, on the physiological processes has been discussed. As with temperature, the tolerances of mysids to changing salinities varies immensely from species to species. McLusky and Heard (1971) point out that Praunus Jlexuosus has been recorded living in varying ranges of salinity, namely 2-18 21.5-27.9 and 35-37-8 They tested the salinity tolerance of this species and found it to range from 2-33 in individuals

x0,

x0

x0.

x0

8.

PHYSIOLOGY AND RESPONSES TO THE ENVIRONMENT

155

from a sea loch in western Scotland. It survived for only 10.75 h at salinities of 0.5 and 50 but tolerated a salinity of 40 for 114 h. Vlasblom and Elgershuizen (1977) found that Neomysis integer was more tolerant of salinities lower than 18 and less tolerant of salinities higher than 18 than Praunus JEexuosus; the latter species tends to inhabit a more bypically marine habitat while the former OCCLUS in brackish and occasionally freshwater habitats. Stepwise did not cause mortality in acclimatization to salinities as low as 21 Leptomysis mediterranea a t 10°C but mortality was present when the salinity was decreased suddenly (Lucu, 1978). Some of the burrowing species of the genus Gastrosaccus may be exceptionally euryhaline. As mentioned earlier, embryo bearing female G. msangii remain buried in the sand, even intertidally after the tide has receded. Consequently, such mysids will be subjected to freshwater seepage, and the effects of rainfall on the beaches. Brown and Talbot (1972) found that G. psammodytes can withstand a 25% dilution of its environmental sea water for at least 42 h with no ill effect. A mortality of 20% took place in 42 h at a dilution of 50%. There was only 30% survival after 42 h in one quarter strength sea water, but survivors recovered completely when returned to normal strength sea water. Fishelson and Loya (1968), on the other hand, found that G . sanctus lives on the Mediterranean shore of Israel in a very high range of salinity of 3638 and could reproduce in even more saline water in the laboratory. Neomysis intermedia, occurring in freshwater and brackish lakes of northern and eastern Japan, tolerates a salinity a0 high as 45 and as low as 0.04-0.06%, (Murano, 1966~).Two species, Mysis litoralis and M . oculata living in the White Sea, were shown by Khlebovich et al. (1970)to be able to tolerate salinities as low as 10 while Holmquist (195913, 1963a) concludes that both these species are euryhaline, M . litoralis certainly occurring in freshwater environments. She further found that the freshwater Mysis relicta could be transferred from fresh water to salinities of 7 and survive there for several months. This predominantly freshwater species occurs in the Baltic Sea in a range of salinity of 3-11.7 and Ackefors (1969) states that it does not occur in regions where the salinity exceeds 12 Very few observations have been made on salinity tolerance of mysids at different environmental temperatures. The tolerance to salinity change is liable to be less at higher temperatures as Vlasblom and Elgershuizen (1977) found in Praunus jiexuosus and Neomysis integer. Likewise, there are few definitive studies of the distributions of mysids in the natural environment in terms of salinity and temperature distributions. A littoral species that appears to be restricted to 8

x0

X0,

x0

x0

x0

x0

x0 x0,

x0.

156

TEE BIOLOGY O F MYSIDS

relatively narrow range of salinities is Metamysidopsis insularis. Quintero and Roa (1973)found that it occurred in littoral mangrove lagoons of Venezuela in a salinity range of 32-7-37.0 Environmental salinity apparently determined its distribution and it did not occur in regions where the salinity was less than 32.7 T u r x r and Heubach (1966)and Heubach (1969)noted that Neomysis mercedis (awatschensis) occurred most commonly in freshwater and brackish regions of salinity 7.5 in the Sacramento-San Joaquin Rivers, USA; population densities decreased in regions where salinities were higher than 18 Neomysis americana however, although occurring in estuaries at salinities in the range 15-20%, (Hulburt, 1957; Cronin et al., 1962) is probably more common outside the estuaries in open coastal situations, including the Georges Bank (Whitely, 1948). Hodge (196313) concluded that Rhopalophthulmus brisbanensis preferred a range of 12.5-33 while Gastrosaccus dakini preferred 5-15 in the Brisbane River, Australia. These salinity preferences appeared to control the seasonal and spatial movements of the populations up and down the river. Ranges of salinity found in the Cananeia region of Brazil did not limit the distributions of Mysidopsis tortonesei and Brasilomysis castroi (Almeida Prado, 1973). Dissolved oxygen concentrations in the water affect the behaviour of mysids to some extent. Childress (1971a, 1975) and Belman and Childress (1976) have shown how Gnathophausia ingens is adapted to live in the oxygen minimum layers of the eastern Pacific. A related species G. zoea, was found to recover even after subjection to anoxic conditions for a period of 4 h (MacDonald and Gilchrist, 1978). NO comparable investigations of littoral or coastal species have been made although some of them are undoubtedly subjected not only to low oxygen concentrations but also to severe changes in concentrations. Ackefors (1969) found that Mysis mixta occurs in regions of the Baltic where the oxygen concentrations are as low as 1-2ml/l water. Holmquist (1959b) found that the freshwater Mysis relicta could probably tolerate levels of dissolved oxygen as low as 29% saturation, or about 2-3 ml O,/l sea water, and Ackefors (1969) suggests that it can live in concentrations down to 1.0m102/1. Other individuals appeared to survive even in the presence of small concentrations of hydrogen sulphide. The behaviour and distributions Of the following species relative to environmental concentrations of dissolved oxygen have been examined : Neomysis mercedis (awatschensis) by Turner and Heubach (1966) and Heubach (1969); Mysis litoralis and M . oculata by Holmquist (1959b); Neomysis intermedia and N . japonica by Murano (1966c, d) ; Metamysidopsis insularis by Quintero

x0. x0.

x0

x0.

x0,

x0

8.

PHYSIOLOGY AND RESPONSES TO THE ENVIRONMENT

157

and Roa ( 1973) ; Bowmaniella brasiliensis, Brasilomysis castroi, Metamysidopsis elongata atlantica, Mysidopsis tortonesei and Promysis atlantica by Almeida Prado (1973). There is no evidence a t present that dissolved oxygen concentrations act as primary controlling factors in the behaviour of the species so far investigated. Variable r a g e s of hydrostatic pressure are experienced by different species of mysids. Bathypelagic mysids are subjected to pressures within the range 100-300 atm and Amblyops magna, caught by Belyaev (1966) at more than 7000m depth, was living at a pressure of some 700 atm. Childress et al. (1978) use a thermally insulated codend on their pelagic trawls to capture Gnathoph-ausia ingens. This cod-end does not protect the animals from the processes of decompression, which they experience during hauling to the surface. This species, however, caught in this manner lives, moults and grows in the laboratory in controlled temperature conditions for many months (Childress and Price, 1978). The effects of the rise in ambient temperature during hauling to the surface are more severe than the effects of decompression, as far as survival of the animals is concerned. Mysids are probably sensitive to small changes of hydrostatic pressure, as Rice (1961) demonstrated in the littoral Schistomysis spiritus. It responded continuously to a cycle of pressure changes comparable to that which it would -experience in its natural habitat on the shore during a tidal cycle. Brown and Talbot (1962), on the other hand, produced no convincing evidence that another littoral species Gastrosaceus psammodytes, responded to changes of the order of 1.5 atm, and Enright (1962) found no response by Archaeomysis maculata to changes to 0.1 atm. Mysids maintained in tanks illuminated from above or below move upwards when the pressure is increased and downwards when it is decreased, the source of the vertical light having little effect. Mysids maintained in horizontally lit tanks respond to increases in pressure by moving towards the bright light or by decreased activity. The species tested by Rice (1967) were Leptomysis lingvura, Praunus jlexuosus, P . neglectus and Schistomysis spiritus. A further species, Siriella armata, behaved differently, the upward movement in increased pressure being more pronounced when lit vertically from above than from below. The tolerance of some littoral species to unrealistic pressures is indicated by the LD,, of Neomysis integer at 160 atm measured by Schlieper (1968). Tidal flows affect the behaviour of littoral and estuarine species. Mysids normally living sub-tidally during daylight are often immigrant in the tidal region a t night (Colman and Segrove, 1955; McLachlan

158

THE BIOLOQY OB MYSIDS

et al., 1979). No endogenous tidal rhythm has been demonstrated in mysids and tidal events appear to modify rather than control aspects of their behaviour. Many littoral species are dispersed between high and low water marks at high tide, but become concentrated a t the water's edge at low tide (Liao, 1951; Matsudaira et al., 1952; Colman and Segrove, 1955 ; Zatkutskiy, 1970; Connell, 1974) ; t,he populations become dispersed again throughout the intertidal area on the following high tide. Flood tides in the Sacramento-San Joaquin River Estuary modify the vertical distribution of Neomysis mercedis (awatschensis); the flood tide has a greater velocity than the ebb tide (Heubach, 1969). This species does not occur in regions of the channel where water velocity exceeds 12cm/s (Turner and Heubach, 1966), a velocity probably equal to or exceeding the maximum cruising speed of the mysid. Mysids orientate in flowing water, facing into the current (rheotaxis). This rheotaxis produces a surprising effect in populations of the burrowing Gastrosacc.tcs psammodytes. Brown and Talbot (1972) demonstrated positive rheotaxis in this species and observed that burrowing takes place as the animals are left dry on the beach by a receding wave. They are thus facing the shore at that time and an examination of the orientation of individuals dug from the foreshore confirms this because they had retained this orientation. Clutter (1969) found that the orientation into water currents by Metarnysidopsis elongata was present under conditions of constant darkness, although less proficient; over 90% of mysids were orientated 15' either side of the direction of the water current when illuminated while only 26% were so orientated in darkness, a further 46% being orientated in the sectors 15' t o 90' either side of the current. Mysis gaspensis was seen by Dadswell (1975) to cling to small pebbles in water currents of 40 cm/s in an estuary in Newfoundland. Observations of the freshwater Mysis relicta from a submarine, by Robertson et al. (1968), showed that it appeared to cling to a hard substrate where possible and were orientated into the current; these observations were made at a depth of approximately 42 m. Wave action in the littoral region affects the distribution of many intertidal species (Boroditch and Havlena, 1973 ; Brown and Talbot, 1972). It tends to depress their vertical range on the shores, the mysids moving offshore to deeper water to avoid the effects of breaking waves. Breaking waves also wash Gastrosaccus species out of their intertidal burrows. Consequently, mysids respond to changes in the great majority of the physical and chemical factors of their environments but the detailed responses can vary between species. This reflects the wide

8. PHYSIOLOGY

AXD RESPONSES TO TEE E ~ O N M E N T

159

variety of environments colonized by mysids. These responses to changing environments govern the distribution of many coastal species present within localized areas. They often make location of the populations and interpretation of the results of sampling a sea area difficult. Mysids respond to one another and their social behaviour also influences their distribution within a sea area. The social behaviour of mysids is discussed in the following chapter.

CHAPTER 9

BEHAVIOUR The changing physical and chemical factors of the environment, as discussed in the last chapter, can therefore enable mysids to occur in alternative parts or areas of the environment at different times. This is most noticeable in coastal and especially the littoral and intertidal environment. Vast numbers of a species can be sampled in a restricted area one day, and be completely absent from that area on the following day. Consequently, quantitative sampling of the populations has to take into account the whole local horizontal and vertical range of the species. This is most easily done by recognizing a coastal area that has a number of relatively discrete environments such that each species of mysid is almost completely restricted to one habitat. A relatively isolated, sheltered bay or inlet in an otherwise rocky coastline usually provides suitably isolated and contained populations of intertidal species. River estuaries or fjords are other areas where populations of a species can usually be easily located and monitored. It is important in studying populations of a species in such an area t o know the locations of other populations of this species in adjacent areas. An estimate can then be made, in the context of the physical, chemical and topographical characteristics of the region as a whole, of the possible amount of communication (immigration and emigration) between the populations under study and those in the adjacent areas of the region. This is necessary to distinguish real from apparent increases in population numbers. Real increases result from active breeding and production of young, while apparent increases can result from one or both of the following : an immigrant population from an adjacent area moving into the area and mixing with the resident population or a disaggregated population aggregating or swarming in a small sea area, so that it is sampled more effectively. The converse of these two processes, emigration and disaggregation, can take place and the result is an apparent decrease in the population numbers. The sizes of relatively isolated populations of coastal species were estimated seasonally in various regions of the west coast of Scotland and 58’N by Mauchline (1971f). The between latitudes 55’30”

J

M

M

J

S

N

J

M

M

J

S

N

700 400 300

200

8000

I 120

8 brasi/iensis

I

4000

40

1600

;$oo

800

60

20

FIG.44. Seasonal occurrence of species of mysids. Anchidim agilis: histogram, Lochs Etive and Creran (Mauchline, 1971f); dotted line and right scale x 2, Gulf of Marseille (Macquart-Moulin, 1965); hatched line and right scale, Plymouth (Tattersall, 1938'. BowmanieEZa brasiliensis, Cananeia Region, Brazil (Almeida Prado, 1973). Brasilomysis castroi, Cananeia Region, Brazil (Almeida Prado, 1973). Erythrops elegans : histogram, Clyde Sea area and Loch Ewe (Mauchline, 1971f); dotted line and right scale, Gulf of Marseille (Macquart-Moulin, 1965). E. serrata, Clyde Sea area (Mauchline, 1971f) ; Qastrosaccus dakini, Brisbane River, Australia (Hodge, 196313). Q. normani: histogram, Clyde Sea area and Loch Ewe (Mauchline, 1971f); hatched line and right scale, Plymouth (Tattersall. 1938). G. spinijer: histogram, Loch Ewe and Gruinard Bay (Mauchline, 1971f); dotted line and right scale, southern North Sea (Boysen, 1977); hatched line and right scale x 10, Kiel Bay, Baltic Sea (Baan and Holthuis, 1971). Hemimysis Zamornae Clyde Sea area (Mauchline, 1971f). Black histograms, ovigerous females and mature males ; cross-hatching, other stages.

J

M

M

J

S

N

FIG.45. Seasonal occurrence of species of mysids. Leptonzysis grucilis : histogram, Clyde Sea area (Mauchline, 1971f); dotted line and right scale, Gulf of Marseille (MacquartMoulin, 1965); hatched line and right scale x 3, Plymouth (Tattersall, 1938). L. lingvura: histogram, Clyde Sea area and Loch Ewe (Mauchline, 1971f); hatched line and right scale, Isle of Man (Liao, 1951). Mesopodopsis slabberi: histogram, southern North Sea (Baan and Holthuis, 1971); dotted line and scale x 10, Gulf of Marseille (Macquart.-Moulin, 1965); hatched line and scale x 0-1, Kiel Bay, Baltic See (Boysen, 1977). Metumysidopsis elongata, California (Fager and Clutter, 1968). M . elongata ntlnntica, Cananeia Region, Brazil (Almeida Prado, 1973). Mysidium columbiae, Jamaica (Goodbody, 1965). Mysidopsis angusta, Clyde Sea area (Mauchline, 1971f). M . clidelphys, Clyde Sen area (Mauchline, 1971f). M. gibbosu, Clyde See area Mauchline, 1971f). M. tortonesei. Cananeia Region, Brazil (Almeida Prado, 1973). Black histograms, ovigerous females and mature males; cross-hatching, other stages.

J

M

M

J

S

N

800

400

4000 2000

3000

I N mercedis

1000

40

20

10000

tc

f?

1SO?

6000

flexuosus

i

u

800 2000 FIQ. 46. Seasonal occurrence of species of mysids. My& mixta, Kiel Bay, Baltic Sea (Boysen, 1977). Neomysis americunu, Indian River Inlet, Delaware (Hopkins, 1965). N . mercedis, Sacramento-San Joaquin River Estuary, California (Heubach, 1969). N . integer: histogram, Loch Etive (Mauchline, 1971f); hatched line, Baltic Sea (Kinne, 1955). Paramysis arenoaa, Clyde Sea &ma and Loch Ewe (Mauchline, 197Lf). P . baceacoi, Roscoff, France (Labat, 1957). P . nouveli, Roscoff, France (Labat, 1957). Pseudomma aflne, Clyde Sea area and Loch Etive (Mauchlinj, 1971f). Praunua Jexuosw,: histogram, Clyde Sea area and Loch Etive (Mauchline, 1971f); hatched line, Denmark (Blegvad, 1922). Black histograms, ovigerous females and mature males ; cross-hatching, other stages.

J

M 1

1

M 1

1

J '

S 1

1

N 1

J 1

~

M

M

J

S

N

1 1 1 " 1 1 1 1 1 1

~

s spirirs 2000

1200

400

120

100 20

40 750

250

R brisbanensis

30

100 10

400

FIG.47. Seasonal occurrence of species of mysids. Pruunus inernzis: histogram, Clyde Sea area (Mauchline, 1971f); hatched line, Denmark (Blegvad, 1922). P . neglectus: histogram, Clyde Sea area (Mauchline, 1971f); hatched line and right scale, Denmark (Blegvad, 1922). RhopuZopthaZnaus brisbunensis, Brisbane River, Australia (Hodge, 1963b). Schistomysis kervillei : histogram, Loch Ewe (Mauchline, 1971f); hatched line, southern North Sea (Baan and Holthuis, 1971). S. ornuta: histogram, Clyde Sea area and Loch Etive (Mauchline, 1971f); hatched line and right scale, Plymouth (Tattersall, 1938). S. spiritus: histogram, Clyde Sea, area and Loch Ewe (Mauchline, 1971f); hatched line, southern North Sea (Baan and Holthuis, 1971). Siriella cZTmUtU: histogram, Loch Ewe (Mauchline, 1971f); hatched line, Gulf of Marseille (MacquartMoulin, 1965).S. cluuaii: histogram, several areas of west coast of Scotland (Mauchline, 1971f); hatched line and right scale, Gulf of Marseille (Macquart-Moulin, 1965). S. @ensis: histogram, several areas of west coast of Scotland (Mauchline, 1971f); dotted line and left scale, Gulf of Marseille (Macquart-Moulin, 1965); hatched line and right scale, Isle of Man (Liao, 1951). S. norwegicu, Clyde Sea area and Loch Etive (Mauchline, 1971f). Black histograms, ovigerous females and mature males; crosshatching, other stages.

9.

BEHAVIOUR

165

occurrences of these species are shown in Figs 44-47 along with the occurrences of other species in other latitudes investigated by Labat (1957), Hodge (1963b), Goodbody (1965), Hopkins, (1965), Fager and Clutter (1968), Heubach (1969), Baan and Holthuis (1971), Almeida Prado (1973) and Boysen (1976). I n addition, Moore et al. (1979) record seasonal changes in the numbers of Neomysis integer present in the Severn Estuary while Hakala (1978) studied the changing number of Mysis relicta in Lake Paajarvi in southern Finland. All species, with the possible exception of the tropical species Mysidium columbiae (Fig. 45), show seasonal periods of maximal and minimal occurrence. The populations of M . columbiae studied occurred at 19"N in Jamaica. Southern sub-tropical species occurring in latitudes 25-27'5, namely Bowmaniella brasiliensis, Brasilomysis castroi, Gastrosaccus dakini, Metamysidopsis elongata,atlantica, Mysidopsis tortonesei and Rhopalophthalmus brisbanensis, all show seasonal maxima and minima of population numbers. Mauchline (1971f) divided the species of the west coast of Scotland into groups according to their seasons of maximal occurrences. These groups have been re-examined in the context of whether the individual species are near their northern or southern limits of geographical distribution, or occur in coastal, littoral or estuarine habitats. The seasonal maxima of occurrence of the Scottish species (Mauchline, 1971f) are listed in Table XXVI, along with the babhymetric habitat in which they were studied; the latitudinal range of the study area is 55"30'N to 58"N. Other species with the latitudes in which they were studied are also listed. No obvious patterns are present in the data and only one aspect is worth mentioning in some detail. The maxima of occurrence of littoral and estuarine species are in the spring, summer and autumn with minima in the winter except in Erythrops elegans (Fig. 44) which occurred maximally in the winter and spring. Gastrosaccus spinifer, a littoral species in the west of Scotland, was found to occur maximally during the winter (Fig. 44) by Boysen (1976) in the southern North Sea and by Baan and Holthuis (1971) in Kiel Bay in the Baltic Sea; the populations studied by these authors were not littoral but neritic. Leptomysis lingvura in a littoral habitat in the Isle of Man (Liao, 1951) has a winter maximum of occurrence (Fig. 45). Mesopodopsis slabberi in a littoral habitat (Macquart-Moulin, 1965) has summer and autumn maxima but in neritic habitats (Baan and Holthuis, 1971; Boysen, 1976), has autumn and winter maxima of occurrence (Fig. 45). Schistomysis kervillei in an offshore habitat (Baan and Holthuis, 1971) has a winter maximum as does 8.spiritus (Fig. 47). Estuarine and littoral species, therefore, tend to occur maximally during the warmer months of the year, but if they occur in offshore

TABLEXXVI. SEASONAL OCCURRENCE AND AGGREGATION OF MYSIDS Most interne breeding periods species

Study area

Species range . Habitat 58N-25N

C

Bowmankllu b r a s i l h s k 25s B r a a i h y s b CaetrOi 25s Erythrops ebgans 58N

25N-20s 23s-25s 58N-25N

L L L

No (Yes 42N) Yes Yes Yes?

E . serrclta Gaatrosmua abkini a. n m n i a. spinijeer

56N 27s 58N 58N

?ON-50N 275-35s 58N-8N 59N-8N

C E C L

Yes Yes ?(Yes 42N) ?(Yes 54N)

Hemimysis l a n a o w

56N

57N-41N

C

Yes?

LeptornysiS grccCiliS L . liTlgVura Mesopodopsb a j k m M . skbberi

56N 56N 32s 54N

58N-25N 58N-25N

-

C L E L

Yes Yes Probably No (Yes 42N)

Metmn.yysidop& elowata M . elongata atlanticr;C My+urn wlumbiae MysidopsG anguata M . didelphys

32N 25s 19N 56N 56N

35N-22s 255 24N-llN 68N-31N 68N-40N

37N-335

of

occurrence

B r d throughout ( W , winter ;Sp, spring; the year Su, summer;A , autumn)

56N

Anchialina agilie

Seasonal mima

L L

ges Yes Yes

C

No

of

Type oj aggregation

W

Breeding aggregation inW A Shoals su Shoals .W, Sp Shoals, breeding aggregation in Sp Sp, Su, A Shoals Sp, Su Shoals W? unknown Sp, Su Breeding aggregation in sp , Sp, SulA Breeding aggregation in Sp and Su W Shoals Sp, Su, A Shoals Su, A Swarms Su, A Shoals and swarms

L L

Source

SP P Sp, Su, A? Sp, Su, A W W ? ?

Swarms and schools Swarms Shoals and schools Unknown Breeding aggregation

inw

10 11 11 10

10 4 10 10 10

10 10 12 1, 13 8 11 3, 6 10 10

M . gibboaa M . tortonesei My& rnixta N m y & amm'cana N.W V d i S N . integer Paramy& arenoaa P . baceswi

P . rwuveli Praunua $exuoaua

P. immk P.neglectus Pae-mu

aflm

s.o m t a s.82yiritu-9 Siriella armata s.clauaii S. j d t e d 8. norvegica

56N 255 38N 38N 56N 58N-56N

58N-29N 23s-25s 78N-40N 5ON-35N 59N-35N 66N40N 57N-32N

46N 46N 56N 56N 56N 56N

50N-42N 50N-28N 58N-40N 58N-48N 68N-48N "ON-40N

21s

54N

L L C C

Yes Yes

Sp, Su, A

A

?

I

Yes Yes Yes Yes

Sp, Su Sp, Su Sp, SU,A Sp, SU

Sp, Su, A Shoals Sp,A Swarms

10 11

unknown

Su, A Su, A Su, A Sp, Su, A

13 5, 7

C

Yes Yes No Yes No No

275

E

Yes

Shoals Shoals Shoals and s w a m Shoals, breeding aggregation in Su Su, A su Shoals Su, A Su, A Shoals Sp, Su, (A) Sp, Su, A Shoals Sp, SU Sp,Su,A Shoals Sp,Su(A) Sp, Su,A Shoals W W Breeding aggregation inW Sp, Su, A SulA Shoals

58N

58N-49N

L

Yes

Sp, Su

Sp, Su

Shoals and swarms in late Su

10

56N

59N-25N

C.

Yes

W

W, Sp

Breeding aggregation

10

Yes Yes ?(Yes42N)

Sp, Su(A) Sp, SU ?

inW Sp, Su, A Shoals and s w a m Sp, Su,A Unknown W unknown

10 10

3 ?

? ?

W W

10 10

58N 58N 58N 58N 56N

59N-28N 58N-30N 58N-32N 58N-27N 58N-27N

L E L L L

L L

L

L L C C C

1

Unknown unknown

9 10

10 2 2 10

10 10

10 4

10

"he latitude of the study areas 21given along with the latitudinal range of the species. "he habitat 2I shown ~8 estuarine (E), Littoral (L) and neritic (c).Breeding Strategy noted for the study area and for 81'088 south of it in parentheses. Sources of data: 1. Blegvad, 1922; 2, Labat, 1957; 3, Steven, 1961; 4, Hodge, 1963b;5, Rioharde and Riley, 1963; 6, Goodbody, 1965; 7,Hopkins, 1965;8,Clutter, 1969;9. Heubach, 1969;10,Mauchline, 1971f; 11, Almeida Prado, 1973;12,Connell, 1974; 13,Boysen, 1977; 14, Wooldridge, 1977.

168

THE BIOLOGY OF MYSIDS

pelagic or neritic environments the seasonal maxima of occurrence tend to occur in the autumn and winter. The neritic species are listed in Table XXVI. Nine of the thirteen species have winter maxima. Three of the remaining four species, Hemimysis lamornae, Mysis mixta and Neomysis americana, range from the littoral to the neritic environment. The fourth species, Erythrops serrata, is strictly neritic. The reason for such a difference between the majority of species or populations of species occurring in the estuarine and littoral environments on the one hand and those occurring in the neritic or coastal environment on the other, is not known. Baan and Holthuis (1971), collecting mysids from the surface plankton near the “Texel” lightship in the southern North Sea, noted this seasonal pattern of occurrence of winter maxima, but could not explain it. They discuss the possibilities of seasonal and reproductive migrations. Such a migration has only been demonstrated within the restricted habitat of a Scottish sea loch; age groups of #chistomysis ornata are distributed within the loch as shown in Fig. 21, p. 78 and an ontogenetic horizontal migration is performed seasonally. One possible explanation of this pattern is that several neritic species have been found to form breeding aggregations. Such aggregation of a previously disaggregated population will result in large samples from a study area that is also a breeding area of the species. Most neritic species breed in the late winter and spring and so aggregation of the populations for purposes of breeding would be present at these seasons. The process of aggregation or disaggregation of a population of mysids can be detected in two types of sampling programmes. The first programme entails a large series of samples taken throughout a region. The period of sampling should extend over one year to describe the cycle within the populations, but sampling can be restricted to shorter periods to describe the existent states of the populations. Samples should be small in that each sample is taken from relatively small volumes of water or the sampler should be towed over 25 or 50 m serial lengths of transects in littoral regions. Mauchline (1971f) used a D-net incorporating a 75 cm diameter tow-net, a conventional 75 cm diameter tow-net, and a 1 m beamtrawl (for neritic species). Other suitable samplers are shown in Fig. 18, p. 67. The environments sampled extended through and beyond the local distributional ranges of each of the species studied. Individuals of a disaggregated species can be expected to occur in samples in small numbers, even as single individuals, while individuals of aggregated species can be expected to occur in larger numbers or not at all in samples. The aggregations and swarms of a species in a

9.

BEHAVIOUR

169

localized area will not contain all individuals of that species occurring in that area. Some inditriduals do not conform to the general behavioural patterns of the majority of individuals in the population. Steven (1961) suggests that about 5 % of the population of Mysidium columbiae remain independent of the shoals of this species. This proportion of non-conformist animals probably varies at different times within and between populations of species. The overall result, however, is that sampling in areas where dense shoals and swarms of a species occur usually produces three types of samples: the first has large numbers of animals, the second none and the third has single or few individuals. The result of such sampling programmes on the west of Scotland are shown in Table XXVII. Taking Siriella species as examples, S. armata and S. clausii occur more commonly in samples in numbers ranging from 2 to 50 while S. jaltensis occurs more commonly as single individuals. Consequently, it is probable that the former two species tend to be sampled from aggregations while S. jaltensis, although sometimes caught from aggregations, is more commonly sampled from disaggregated populations. The numbers of other species in samples, for example Leptomysis gracilis, Neomysis integer, Praunus jiexuosus and Schistomysis spiritus, show bimodal distributions, the species tending to occur in samples in either small or very large numbers. A similar analysis of samples of mysids collected on the Caribbean coast of Colombia was made by Brattegard (1973); his dat’a have been supplemented with data from his other papers and re-analysed (Table XXVIII) to conform with the information on the Scottish species (Table XXVII). A comparable analysis of the samples of mysids, many of them oceanic species, described by Tattersall (1955) from the “Discovery” collections is given in Table XXIX. The interpretation of the data on a few of these species is more difficult. The samplers used range from 50 cm through 4.5 m tow-nets to large rectangular nets. Consequently, the volume of water filtered when taking individual samples probably ranges over several orders of magnitude. The types of nets in which each species was predominantly caught are indicated in Table XXIX. The four species Eucopia australis, E . grimaldii, E. sculpticauda and Gnathophausia gigas were caught by large nets, The rest of the species were taken predominantly in nets of comparable size to those used to catch species listed in Tables XXVII and XXVIII. The da%ain Tables XXVII-XXIX suggest %hatthe great majority of species aggregate, a t least during certain periods of their life cycles. The seasonal maxima and minima of population numbers in Figs 44-47 reflect real increases and decreases in the sizes of the populations as well

170

THE BIOLOGY OF MYSIDS

as aggregation and disaggregation within the populations. Increases in the sizes of samples taken .of populations of most of the littoral species investigated are a result of simultaneous addition of juveniles, through reproduction, and aggregation of both the juveniles and the adults. Aggregation within different populations of mysids can be very different in origin, form and function, and the following terms were defined by Mauchline (1971f):

Aggregation. A population is aggregated when individuals are not randomly distributed throughout the area or volume in which they are found ; this area or volume may be a neritic bathymetric zone parallel to the shore. No inference is made in using this term to the factors, for example physical or chemical parameters of the environment or behavioural reactions between individuals, that might be responsible for this uneven distribution. Breeding aggregation. A breeding aggregation is a grouping together, in a non-random distribution, of sexually mature individuals for the purpose of mating. Restricted distribution. A population has a restricted distribution if it occurs in a defined part or several defined parts of a sea area or volume. The inference here is that environmental parameters are probably responsible for the uneven distribution of the populations and that behavioural reactions between individuals are less important in maintaining the aggregation. Shoal. Clutter (1969) defines shoals as large groups of mysids, ranging from a few metres to tens of metres across, that occur within their habitat zones (areas). He uses this term in the context of species that live in shallow water, probably in open bays. Shoals however, can also occw in more strictly pelagic habitats. Swarm. According t o Clutter, swarms are smaller, more integrated groups of mysids than those comprising shoals. Swarms are often constituent parts of shoals. The term swarm implies greater cohesiveness but not parallel orientation of individuals. School. Like swarms, schools can be constituents of shoals (Clutter, 1969), but schools differ from swarms in consisting of polarized groupings of individuals swimming in the same direction. Wittmann (1977) does not use the above terms as defined by Mauchline (1971f) but re-defines some of them primarily on a geometrical and secondarily on a social basis. The terms are used throughout this discussion in the form defined above. A detailed knowledge of the biology, life history and local distributional patterns of a species is required before the types of aggregation

TABLEXXVII. THENUMBERS OF SAMPLES WHICH CONTAINED THE STATED NUMBERS OF INDMDUALS OF EACH SPECIESOM THE WESTCOASTOF SCOTLAND (after Mauchline, 1971f)

Total nos of Species

1

Anchialina agilis 5 Erythrops elegana 23 E . serrata 14 Bastrosaccus n o r m n i 5 0. spinifer 3 Hemimysis lamornue 13 Leptomysis yracilis 14 L . lingvura 19 Mysidopsis anguata 19 M . didelphys 33 M . gibbosa 20 Neomysis integer 7 Paramysia arenosa 8 Praunus fiexu~aus 18 P . inermis 19 P . neglectua 18 Pseudommu a&e 7 Schistomysis kervillei 9 S . ornuta 46 S. apiritus 11 Siriella armatu 1 S . clauaii 2 S . jaltensis 7 S . norvegiea 3

2-5

6-20

21-50

51-100

101-200

6 15 32 1 5 14 23 15 25 35 31 8 18 29 27 21 6 6 39 11 6 3 3 1

2 4 29 1 4 0 20 9 4 23 34 4 26 23 23 25 5 2 33 8 5

1 4 21

1 1 7

1 10

2 2

2 2 6 2 6 11 10 12 19 19 12 1 4 15 2 1 2

1 2 1

3 1

201-500

501-1500

>1500 Samples

5

23

25

1

3 9 5 9 8 11

2 2 8 8 5 9

2 29 2 19 8

5 12

3

7

1

5 4 3

3 5 7

2 1 13

1

7

6

15 48 118 7 14 38 117 47 48 102 114 86 83 121 124 76 19 32 143 68 13 7 12 6

Indiu-idu.uk 148 407 4222 13 170 276 32890 420 117 1107 7540 32133 3122 7399 9435 775 114 2307 2388 31081 107 64 33 28

Mean Number number of indiof viduals indiin VidWllsJ largest sample sample 10 8 36 2 12 7 281 9 2 11

66 373 38 61 76 10 6 72 17 457 8 9 3 5

65 108 305 6 49 72 1664 122 12 151 930 2871 247 44 1 1623 48 33 599 248 3635 34 25 9 14

TABLEXXVIII. THE NUMBERS OF SAMPLES WHICH CONTAINED THE STATEDNUMBERS OF INDMDUALS OF EACH SPECIES FROM THE CARIBBEANREGION

Total nos of Species Amuthimysis cherados A. gibba A. polita Anchialina typica Antromysis bahamensis Boutanidla d i s s k i l i s B. parageia B. sewelli Brmilom?ysiseastroi Cubanmysis jimenezi Dwptromysis paucispinosa D. spinosa Erythrops parva

1

2-5

6-20

6 9 7 13 21 13 2 8 6 11 14 10 2 4 1 3 2 5 4 3 1 2 8 9 5 5 1 3 7 1 5 7 7 1 2 3

2 5

3 4

21-50 2 5 7 4 5 1

51-100

101-200

201-500

501-1500 ~ 1 5 0 0Samples

2 3

4

3 5

3 3 8 1

4 4

5 1

1 2

1 2

2 1

2 2

2 1

-

1 2

2

1

1

1

-

Individwzls

24 54 26 45 39 12 6 25 41 20 16

175 532 520 2723 8607 91 33 163 4073 968 69

11 18

494 1029

Nean Number number of indiof widwzls indiin viduals/ largest sample sample 7 10 20

6 7 99 48 4

39 82 57 1376 3200 44 19 40 1624 227 30

45 57

184 684

61

221 8

Heteromysis elegans

1

2

H . mayana

5 2 4 5 5 4 2 1

5

Metamysidopsis swifti Mysidella minutu Mysidiurn colurnbiae M . gracile M . integrum Mysidopais almyra M . ankeli M . bispinulata 1 M . brattstroemi 4 M. cultrata 3 M . furcu 2 M . mathewsoni 8 M . mortenseni 3 M . robusta 4 M , taironana 2 &. tortonesei 7 M . velifera 9 M . virgulata Promysis atlantica 3 Siriella chierchiae '73

2 4 8 2 2 3 1 1 3 1 1

7 3 6 3 2 15 3 2 18

5 3 8 8 2 10 1 2 1 5

2 -

1

3 6 1 2

5 3 4 11 4 4 18

5 1 2

10

-

1

8

6

2

5

1

1

1 2 4

2

1

-

2

1

-

-

1

1 4 2 1 4

6 2 6 2 5

1 -

10

1

8

2 1 5

1 6

0

I

2

3

>

Y

~~

Data from: Brattegard, 1969, 1970a, c , 1973, 1974a.

9 19 48 11 27 6 5 7 16 5 6 20 10 10 11 28 49 17 13 77

120 23 455 220 6455 113 2211 20 68 178 201 14 326 90 71 20 344 14190 1020 1135 433 4188

12

2 51 12 134 10 82 3 14 25 13 3 54 5 7 2 31 507 21 67 33 54

37 5

357 48 1472 51 804 11

35 59 48 7 170 18 28 5 262 3899 125 174 122 1321

TABLEXXIX. THE NUMBERSOF SAMPLES WHICH CONTAINEDTHE STATED NUMBERSOF INDIVIDUALS OF EACH SPECIES OF OCEANIC MYSIDS

Total nos of species

Netsa

1

2-5

6-20

21-50

1 18 4 6 9

2 34 3 4 3 13 10 17 7 8 11 6 7 12 16

2 32 1

3 12 4

4 1 7 1 1

1

51-100 101-200

201-500

+ + +

Y Y Y Y Y+ Y Y Y Y+ LY LY LY

19 8 27 9 7 16 20 21 11 12

2 4 1 8

2

2

1 5 3c

4

Individuals

~

~

Anchialina truncata Antarcknnysk maxim A . oh.linii Arachnmysis megalops Boreomyais brucei B. rostrata Caesaromysk hkpidu Dactykmblyop8 hodgsoni Euchaetmera intemedia E . tenuis E . typica E . zurstrmseni Eucopia australis E . grimldii 1. seulpticauda

>500 Samples

Mean Number number of indiof viduals indiin viduals/ largest sample sample

15*

1

9 120 14 11 12 38 19 53 17 16 27 28 34 24 36

204

23

70

26 18 358 44 201 34 38 41 59 133 59 141

2 2 9 2 4 2 2 2 2 4 2 4

11 3 240 6 26

6 6 4 11 44 13 20

E . unguiculata Gnathophausia gigas G. ingens Hansenomysis falklandica Katerythrops oceunae Longithorax capensis Lophogaster challengeri Meterythrops picta Mysidetas d i m r p h a M . macrops M . microps M . patagonica M . posthon Mysidopsis acuta Pseudamma armatum P . calmani P . sarsii SiriZllu thompsonii

Y L Y + Y Y + Y +

++

+ + +

1

5

6

8 22 5 9

9 10 4 3 2 3 7 2 3 3 2

7 3 5 3

-2

6 7 4

+

-

+ + Y

4 6 17

1 5

2 6

-

12

1 1

1

3 1

27 17

114 33

4 2

34 18 1' 11 7

63 192

2 11 1 4 3 3

1'

1 5 0 1

3 4 4

5

3 1 3 2 7

9

1

2

2 4 5 1

1

1 3 2

1

'.

1

1 1

1

9

1

9 13 9 10 27 26 9 10 10 37

17 41

24 30 154 414 540 76 >400 379 270 36 16 175

17 3.' 60 8 15 15 30 4 4 5

21

5 6 47

3 23 11 5 70 92 300 39 >lo0 9s 107 13 3 36

Data from: Tattersall, 1955. a ,Samples in small nets, less than 1 m diameter; Y, young fish trawl (TYF and TYF 70B); L, 2-4.5 m tow-nets, large rectangular nets. b Quantity described as hundreds. Described as many.

+

176

THE BIOLOGY OF MYSIDS

formed can be defined. Consequently, little can be said about the nature of the aggregations of the'oceanic species listed in Table XXIX. The samples of Tattersall (1955) and others, suggest that juveniles, as well as the adults, of many of these species aggregate and it is probable that social behavioural mechanisms in concert with tile physical-chemical parameters of the environment act to maintain these aggregations. The aggregations of the Scottish species (Table XXVII) were studied in the context of the biology of each of the species (Mauchline, 1971f). Some species such as those of the genus Siriella, Gastrosaccus normani and Mysidopsis angusta were rare and the factors involved in their aggregation remain unknown. Breeding aggregations, consisting predominantly of sexually mature adults occurred at certain times of the year in populations of Anchialina agilis, Erythrops elegans, Gastrosaccus spinifer, Hemimysis lamornae; Mysidopsis didelphys, Paramysis arenosa, Pseudomma ajfine and Schistomysis ornata (Table XXVI). The four species, Erythrops elegans, Gastrosaccus spinifer, Hemimysis lamornae and Paramysis arenosa are littoral and usually permanently located near or just below low water mark, even a t high tide. The other species are strictly neritic living to depths as great as 200 m. The populations of these eight species never attain the size and densities reached by populations of strictly littoral species, the mean numbers of individuals per sample ranging from 6 to 38 (Table XXVII). Two of these species, Erythrops elegans and Paramysis arenosa, in addition form shoals. Shoals are also formed by Erythrops serrata, Leptomysis gracilis, L. lingvura, Mysidopsis gibbosa, Praunus jiexuosus, P. inermis and P. neglectus. The mean numbers of individuals of these species per sample ranged from 9 to 281 (Table XXVII). Two of these species, Erythrops serrata and Leptomysis gracilis, are neritic at depths of 30-150m. Very large samples, greater than 500 individuals, of L. gracilis have been caught and it is possible that this species forms localized swarms rather than shoals. Of the Scottish species, only Neomysis integer, Schistomysis kervillei and S . spiritus are known t o form swarms and these can be observed visually in shallow coastal regions. The mean sample size of these species ranged from 72 to 457 (Table XXVII). The most detailed studies of aggregation of mysids have been made on littoral and intertidal species for the obvious reason that they are usually accessible for visual observation. Social behaviour in conjunction with the physical-chemical discontinuities of the environment appear to act together in the formation and maintenance of shoals of several species. Dadswell (1975) found that the entire population of Mysis gaspensis in a Newfoundland estuary formed a shoal 0.1-0.5 m

9.

BEHAVIOUR

177

wide and some 200 m,in length along the eastern side of the estuary at each succeeding low tide. The mysids were dispersed at other stages of the tidal cycles so that they occurred more or less evenly over the entire estuary at each high tide. Dadswell suggests that the shoal is formed a t low water because the mysids select a localized region of the estuary whexe medium water velocities in combination with low salinities occur and these are the factors initiating and maintaining the shoal. This aggregation is then in the category of a restricted distribution rather than a shoal. Whiteley (1948) studied the “large swarms’’of Neomysis americana occurring on the Georges Bank. The greater portion of the Bank has depths in the range 40-100m although it shallows to 5 m depth in its north-western regions. The “swarms” are not swarms as defined above but conform to the definition of shoals. These shoals on the Bank are elongated in shape and occur on the long axis of the Bank within the 75 m depth contour. The species does not occur in numbers outside the 100 m depth contour and this situation indicates a restricted distribution over, howevex, a relatively large area. Separate shoals occur within this area, approximately 20 000 km2, as indicated by the variation in numbers per sample. The samples, however, were taken over a distance of some 1850 m so that little can be deduced about the sizes of individual shoals, nor is there any information on the influences of the physical-chemical parameters of the environment upon the distribution of the shoals. Whitely did find, however, that the average numbers of this species, when it occurred in his samples, were 43.3 animals/m3 water during the day compared with 0-2 animals/m3of water during the night. This situation suggests that dispersion of the shoals takes place at night, probably in the vertical plane through vertical migration. It is common for intertidal species to be “collected” by the receding tide, so that the individuals become concentrated near low water mark (Liao, 1951; Matsudaira et aE., 1952; Colman and Segrove, 1955; Zatkutskyi, 1970; Connell, 1974). Clutter (1967, 1969) observed another aspect of the intertidal, littoral and immediately sub-littoral environment. This is the tendency for many species of mysids to occur in well-defined bathymetric zones roughly parallel to the shore. This coastal zoning extends seawards to depths of 20-30 m in Californian and Scottish waters. Species living in such zones have a relatively, and often rigidly, restricted distribution such that the populations occur in a series of shoals. This is the case with Neomysis integer, Praunus species and Schistomysis spiritus, for example, in Scotland, Metamysidopsis elongata, according to Clutter ( 1967), in California, Gastrosuccus sanctus in Israel (Moran, 1972) and Leptomysis species in the northern

178

THE BIOLOGY OF MYSIDS

Adriabic Sea (Wittmann, 1977). Shoals of Metamysidopsis elongata moved within their zone of occurrence, but there was no correlation between these movements and changing temperature, salinity, light intensity or composition of the substrate. Clutter, however, considered that their movements might be in response to changing concentrations of detrital organic matter. Certainly Mysidopsis gibbosa in the Clyde Sea area is frequently found associated with the area of fallout of terrigenous organic matter on the sea bottom from freshwater streams. Hobson and Chess (1976) found that Siriella pacijica was numerous in shallow depressions in the sea bottom where flocculent organic material had collected. Swarms of Leptomysis species in the Adriatic Sea remained in constant topographical situations, returning to them after disturbance. Wittmann ( 1977) concludes that swarms of different species that occur in overlapping bathymetric zones usually associate with different substrates, while species preferring the same substrate usually occur in different batihymetric zones. Some species are associated with beds of seaweeds or with coral reefs and this association of a species with a substrate, leading to a restriction of its distribution, is discussed further near the end of this chapter. Light has been shown to influence swarm formation (Clutter, 1969; Macquart-Moulin, 1973a; Zelickman, 1974). Swarms of Neomgsis mirabilis persisted from dawn to dusk but dispersed at night fall while those of Metamysidopsis elongata did disperse to some degree during darkness. Dispersion of hyperbenthic populations takes place at night in species that perform a die1 vertical migration. Wittmann (1977) found that at sunset swarms of Leptomysis species expanded, the distance between individuals increasing. Aggregations of different species mixed in the hyperbenthic layer during the night but separated once again at dawn. Wittmann marked individuals in swarms and found that, although the swarm returns to the same topographical region of the bottom at daylight, the individual need not necessarily do so but may have joined an adjacent swarm. Consequently, there appears to be considerable exchange of individuals between swarms of these Leptomysis species. Clutter (1 969) describes the shoals of Metamysidopsis elongata as consisting of schools and swarms. Juveniles appeared to occur in separate swarms from older individuals as Emery (1968) found in populations of Mysidium gracile and Wittmann (1977) found in populations of Leptomysis lingvura. Green (1970), however, describes mixed swarms of Acanthomysis sculpta, and some littoral species form mixed swarms in western Scotland. The shapes of swarms can vary. Clutter found small spherical swarms of about 1000 individuals of an

9.

179

BEHAVIOUR

Acanthomysis species while schools of this species travelled through the water as elongated bodies. Large swarms are often wafer-shaped and about 5 cm in thickness. The distance between individuals in a swarm varies between species and also between size or age groups within a species. The mean distance to the nearest six neighbours in swarms or schools of Acanthomysis species A, was 1.1-2-7 cm (equivalent to about 3 4 times body length), of Metamysidopsis elongata 3.7 cm and of Mysidium columbiae about 4 cm (Table XXX). Clutter estimated the TABLEXXX. NUMBERS AND DENSITIES OF MYSIDS IN SWARMS AND THE MEAN DISTANCE FROM ONE INDIVIDUAL TO ITS NEAREST SIX NEIGHBOURS (Steven, 1961; Fishelson and Loya, 1968; Clutter, 1969)

Species

Numberlswarm

Number/cms

Acanthomysis sp. A

13 16 22 127 137 e3200

0.1 0.3-0.7 0.3 0.1 0.05 -0.1 % 1'

Qarrtrosaccua sanctus Metamysidopsis elongata Juveniles Mysidium columbiae a

-

14000

-

0.02

M e a n distance (cm) 2-1 1.1-1.5 1.5 2.1 2.7 -2.1 0.22 2-4 3.7 4

In sediment at a density of 18/cm2.

mean distance between individuals as the cube root of the mean volume of water occupied by one individual in a pelagic population. Fishelson and Loya (1968),however, estimated the density of Gastrosaccus sanctus in the sand of a beach on an areal basis. Assuming a more or less even distribution of individuals within the swarm, the mean distance between them is the square root of the area occupied by one animal. There are probably threshold densities within an aggregation that trigger swarming and schooling. This is extremely difficult to study in the laboratory situation, but Clutter ( 1969) estimated such densities to be in the range 0.01-0.70 animals/cms. The minimum density required before Metamysidopsis elongata orientates into schools is probably of the order of 1 mysid/l6 cm3, equivalent to L distance between individuals of about 2.5 cm. Schools can consist of as few as 16 M . elongata, or conversely, of many thousands. Clutter found that

180

THE BIOLOGY OF MYSIDS

small swarms or schools did not disintegrate even when disturbed and could avoid towed nets in a co-ordinated and organized manner. According to Zelickman (1974, rain storms cause swarms of Neomysis mirabilis to disperse, but they reform within a period of 60-90 min after the rain storm has ended. Clutter (1969) and Zelickman (1974) experimentally examined the factors such as population density, illumination and water flow that might affect formation and maintenance of swarms. The main conclusion is that vision is important in the organization of swarms, as Steven (1961) concluded fiom his studies of Mysidium columbiae. Chemical cues and mechanoreception, however, are probably also important, especially in maintaining the relatively high degree of association of individuals that is present in shoals and swarms throughout the hours of darkness. This must be accentuated in species such as Gastrosaccus sanctus which, according to Moran (1972)) probably forms shoals at night, individuals of this species being buried in the sand during the day. Pseudomma ufline has vestigial eyes yet populations of this species form breeding aggregations (Table XXVI). Fage (1925) describes the cavernicolous Spelaeomysis servatus, whose eyes are also extremely reduced, as swimming in large shoals near the surface. Hansenomysis falklandica is yet another species with reduced eyes that probably aggregates (Table XXIX). Consequently, aggregation is probably achieved by several or all of the following processes rather than by any one in isolation. (a) The innate drive towards aggregation, triggered in some species by attainment of a physiological condition, e.g. approaching sexual maturation. (b) The physical-chemical discontinuities and gradientis of the environment in conjunction with the physiology of the species resulting in the population having a restricted distribution : this situation produces populations in which the threshold densities that trigger the formation of swarms and schools occur. (c) Visual cues in the presence of threshhold densities. (d) Chemical cues in the presence of threshhold densities bhat may be an order or several orders of magnitude less than those densities active for visual cues. (e) Mechanoreception by the individuals, probably assuming greatest importance in the processes of maintaining and structuring schools and swarms once formed. The relative importance of any one of the above processes in producing shoals, swarms and schools will depend to a considerable extent upon the species concerned and the habitiat in which it lives.

9.

BEHAVIOUR

181

Wittman (1977) found tihat some species such as Anchialina agilis, Siriella jaltensis and S. armata are not active swimmers and associate strongly with a substrate. These species did not form swarms although his descriptions of their occurrence conform with aggregations within restricted distributions. Other species, such as Leptomysis apiops, are active swimmers but aggregate a t biogenic structures ; they also thus have restricted distributions. A third group of species, examples of which are L. lin,gvura, and L. mediterranea, are active swimmers but still show preferences for certain substrates so restricting their distributions ; these species, however, are active swarmers. A final group, instanced by Mesopodopsis slabberi, show no preference for a substrate, are active swimmers and form schools and swarins. The functional aspects of aggregations of mysids are discussed by Clutter (1969) and Zelickman (1974). One probable function is that shoaling aids in maintaining the population more effectively within a zone of occurrence. This applies particularly t o littoral and sublittoral species that may be subject t o longshore or offshore drift. This would also apply t o estuarine species, especially those that occur in channels with marked tidal flows. Neomysis integer shoals close t o mud banks, in an area studied by Kinne (1955), and probably experiences lower water current velocities as Dadswell (1975) found in the case of a shoal of Mysis gaspensis in a Newfoundland estuary. The social interactions of shoaling would reinforce the environmental parameters in maintaining the integrity of the aggregation. The simultaneous release of all young from the marsupium by a female within a shoal results in the young being potential members of the shoal. The swimming speeds, however, of juveniles and adults of a species differ considerably with the result that a mobile shoal becomes a self-sorting mechanism separating age groups and so producing swarms of individuals of closely similar body size (Clutter, 1969). Some species are more active swimmers than others and the occurrence of mixed age groups within shoals of a species may be indicative of a more lethargic life style. The function of breeding aggregations are self-evident in a three-dimensional fluid environment. This type of aggregation appears t o assume greater importance in the lives of the neritic, and probably offshore pelagic, species than in littoral, intertidal and estuarine species. The only littoral species thought to form a conventional breeding aggregation is Acanthomysis sculpta studied by Green (1970). Shoaling is a more or less permanent feature of the populations of shallow water species and so mating is easily achieved; the primary function of shoaling in these species is probably not for breeding. Shoaling in neritic species, on the other hand, does not seem, in many cases, t o be a permanent state

182

THE BIOLOGY OF MYSIDS

within the populations, although many of them have restricted distributions that residt in densities of animals approaching those that occur within shoals. Densities in populations with restricted distributions are probably within the range 0.01-1.0 individuals11 compared with densities of 50-700/1 in swarms (Table XXX). The mean distances between individuals in these restricted distributions would be within the range 10-50cm compared with distances of 1-4cm in swarms. Wittmann (1977) observed Anchialina agilis, a species that does not form swarms in the Adriatic Sea, clinging to Zostera leaves; the individuals were spaced a t distances of about 3 cm. Clutter (1969) suggests that males detect females receptive to mating at distances of about 5 cm. Consequently, breeding aggregations of a species developing within populations with restricted distributions would produce aggregation with distances of this order between individuals. Shoaling undoubtedly affords protection to individuals and populations against predators (Emery, 1968;Clutter, 1969).Swarming appears to be primarily a behavioural trait of shallow water or surface living crustaceans, these being more or less continuously subjected to visual detection by potential predators. Contacts with an isolated mysid or with a swarm of mysids are probably equivalent events and scored as a single detection by the predator. I n considering a population of mysids as a whole, this results in the frequency of detection being an inverse function of the number of mysids in the swarm. Attacks on swarms by fish often cause the swarms to fragment into smaller swarms that immediately move away from the point of attack, only a few individuals being successfully captured by the fish. The tendency for dispersal of swarms a t night, possibly associated with nocturnal feeding of the mysids, suggests that the avoidance of diurnal predation may be an important, if not primary, benefit conferred on the population by this behaviour. The possible significance of swarming in mechanisms of population control and regulation is briefly discussed by Clutter (1969) but much further information is required before any useful conclusions can be drawn. Mysids exhibit the conventional escape reaction of shrimp-like crustaceans ; that is, they flex the abdomen ventrally and anteriorly and are propelled backwards away from the source of danger. Brown and Talbot (1972) found that Gastrosaccus psammodytes produced this reaction not only when swimming pelagically, but also when buried in its burrow in the sand. A disturbance of the water-saturated sand in the vicinity of its intertidal burrow when the tide has receded caused the mysid to leap “tail first into the air”. It landed a few centimetres from

9. BEHAVIOUR

183

its burrow and immediately buried itself once again in the sand. Gastropod snails elicit this reaction from the mysids when they crawl over the sand close to a burrow. Many mysids can change colour quite remarkably by the expansion or contraction of large chromatophores distributed over the body and appendages. The chromatophores are in three main groups. The neural group is associated with the cerebral ganglia and nerve cord, the visceral group with the gut, hepatopancreas and gonads and the caudal group with the telson and uropods. Tattersall and Tattersall (1951), quoting the detailed investigations of Keeble and Gamble (1903, 1904), have described the system in some detail. The chromatophores contain a large amount of dark brown (red in some genera) pigment and a yellow or whitie (sometimes blue or blue-green) reflecting substance. Keeble and Gamble (1904) describe the histology, development and reactions of the chromatophores to light and changing environmental background colours in detail. The early work of Koller and Meyer (1930) and Kropp and Perkins (1933) demonstrated that eye stalk extracts of Mysis stenolepis, Praunus JEexuosus and P. inermis contain hormones that activate the chromabophores. Praunus JEexuosus is translucent when captured over sand, but dark in colour when captured among seaweed. The related species P . neglectus is dark green or brown, dependent upon the colour of the seaweeds in its habitat. Mysidopsis gibbosa is dark in colour when it lives sub-littorally in the zone of fallout of dark terrigenous material from a freshwater stream. Bacescu (1975) briefly discusses the colour variation occurring in littoral Tanzanian mysids. The range of cryptic coloration varies between species but is limited in many. Zelickman (1974) states that the colour of Neomysis mirabilis changes rapidly as a function of locomotory behaviour. Undisturbed animals in the sea are usually transparent! with a yellowish-grey colour. Their colour darkens a few seconds after capture and, under subsequent quiescent conditions, their normal colour is resumed some 30 min later. Mauchline and Webster (unpublished) examined the time required for several species to change colour when placed against a new background colour. Newly caught Neomysis integer, Praunus jlexuosus, P. inermis and P. neglectus require up to 1 h to adopt a new coloration, a situation noted by Chicewicz (1952) in P. jlexuosus. The time required, however, for the mysids to effect a second change of colour, immediately following the first, is much longer, many individuals not achieving it even after 12 h. This suggests that species adopting cryptic coloration may be relatively static. Cryptic coloration would then have two effects: protection against predators and relative restriction of the animals to a

184

THE BIOLOGY OF MYSIDS

given substrate. A species such as P. neglectus, that lives on several adjacent but different Substrates, would tend to remain spread among the substrates and so inhabit a larger area and volume of the environment. There is evidence that some hyperbenthic species of mysids prefer certain types of substrates, as instanced above by the preference of Mysidopsis gibbosa for areas of fallout or settlement of terrigenous material. Boysen (1976) examined the substrates with which Gastrosaccus spinifer, Mesopodopsis slabberi and Mysis mixta are associated in the Baltic and concludes that only the last species shows a preference, occurring on a mixture of sand and mud. The substrate preferences of a range of western Atlantic species are described by Wigley and Burns (197 1). Bowmaniella portoricensis, Mysidopsis bigelowi and Promysis atlantica prefer sand. Neornysis americana typically associates with sand that is free of silt, while Erythrops erythrophthalma lives over sand that contains small to moderate quantities of silt, but not enough to classify the sediment as silty sand. Two species, Boreomysis tridens and Pseudomma afine, associate with silty sand. Amblyops abbreviata and Bathymysis renoculata live on fine sediments, consisting of fine sand, silt and clay. Only one species was associated with a coarse substrate ; Meterythrops robusta occurs on gravels and other coarse materials. Some of these substrate preferences will result in restricted distributions. The spectrum of particle size of sand may be a limiting factor in the colonization of beaches by burrowing species of the genus Bastrosaccus. Brown and Talbot (1972) found that juvenile G. psammodytes burrowed more frequently into sand with a particle size range of 0.55-0-70 mm in diameter than into sands consisting of particles of larger or smaller sizes. Brattegard in his papers on Caribbean mysids, notes the substrates on which the majority of species were captured; many species appear to show preferences. Many sub-littoral species associate with algae, even with specific kinds of algae, and with corals. Mukai (1971) found mysids that were not identified sheltering among the thalli of Sargassum serratifolium in the Inland Sea of Japan. Xiriella longipes and an Acanthomysis species are commonly associated with the Zostera marina belt in Kyushu, Japan (Kikuchi, 1967). Siriella pacifica and Acanthomysis sculpta live within the California kelp beds (Hobson and Chess, 1976). Praunus neglectus is commonly associated with Fucus and Laminaria beds on the coasts of western Scotland. Coral reefs also provide sheltered areas colonized by mysids. Bacescu (1975) states that Mysidopsis coralicola occurs in areas of sand among corals and algae along with Anisomysis maris rubri, A . sirielloides, Gastrosaccus msangii, Siriella brevicaudata

9.

BEHAVIOUR

185

and Tenagomysis tanzaniana., Mysidium species occur among the reefs of the Florida coast of the United States (Emery, 1968) and Alldredge and King (1977) found species that they did not name to be important constituents of the fauna of the Great Barrier Reef, Australia. Coral reefs and algal beds shelter mysids in every case where they have been searched for ; the individual species are probably distributed between separate microhabitats and substrates rather than occurring in mixed populations. The burrowing behaviour of Gastrosaccus species has been studied in some detail. Macquart-Moulin (1977a, c) has examined the die1 rhythms of activity of several species, that is their habit of remaining quiescent in the burrows during daylight hours but emerging a t night to live pelagically and forage for food. Gastrosaccus psammodytes was found by Brown and Talbot and G. sanctus by Fishelson and Loya (1968)to be able to burrow into sand submerged in water and also into water saturated sand in contact with the air. Burrowing is effected by the thoracic legs, both the endopodites and exopodites, sweeping the sand away from the ventral regions of the thorax to form a depression. The abdomen and telson are moved from side to side with a downward pressure so that they become buried in the sand. There is some forward motion, but of limited extent, during burrowing and all movement ceases once the animal is completely covered with sand. The antennae and antennules seem to have no function in burrowing. Burrowing appears to be initiated immediately the thoracic legs come in contact with sediment, Brown and Talbot describing the reaction as a thigmokinesis. Emergence from the burrows normally takes place as the following high tide floods the beach with water. Some individuals, through the action of the thoracic legs, emerge from the sand spontaneously while others are washed out by wave action. Moran (1972) found that G. sanctus is not evenly distributed in burrows over a beach in Israel; the population is concentrated in a zone of occurrence parallel to the water’s edge. Gastrosaccus species are probably not the only mysids that bury themselves wholly or partially in the sediments. The neritic Lophogaster species may beha.ve in this way. Robertson et al. (1968) observed the freshwater Mysis relicta from a submarine and found that some individuals partially buried themselves in mud at depths of 20-100 m. Some species of mysids live in commensalassociation with other organisms. This commensalism is assumed rather than clearly demonstrated, except in the case of Heterornysis actiniae and the sea anemone Bartholomea annulata. Randall et al. ( 1964) observed Mysidium gracile move to shelter in the spaces between the spines of the echinoderm

186

THE BIOLOGY OF MYSIDS

Diadema antillarum Philippi when danger threatened. Emery (1968) found that schools of Mysidiurn gracile, when threatened by a predatory fish, crowded into the nesting caves of several species of pomacentrid fish that inhabit reefs. The nesting fish made no attempt to eat the mysids, but drove off the potential predator. Other conimensal associations are shown in Table XXXI. The best investigated association is TABLEXXXI. COMMENSAL ASSOCIATIONS OF MYSIDS Mysid

Associated organism

Antromysis anophelinae Land crab Cardisoma guanhumi Anemone Bartholomea annulata Heteromysis actiniae (Leseur) H . gerlachei Gastropod shells of hermit crabs H . gomezi Cavity of a sponge Ophiuroid Astroboa nigra Doderlein H . gymnura H . harpax Gastropod shells of dardanid crabs H . mariarci Cavity of a sponge Gastropod shells of dardanid crabs H . odoiatops H . zeylan ica Cavity of sponge Siphonochalina siphonella Levi Idiomysis tsurriarnali Anemone Megalactis hemprichi Medusa Cassiopea andromeda Spelaeomysis Land crab Cardisoma guan.humi card isomae

Reference

Bowman (1973) Clarke (1955) Tattersall (1967) Bacescu (1970b) Tattersall (1962) Tattersall (1962) Bacescu (1970b) Clarke (1955) Tattersall (1967) Bacescu (1973a) Bacescu (1973a) Bowman (1973)

that between Heteromysis actiniae and the sea anemone Bartholomea nnnulata. Clarke (1955) states that this mysid swims in small groups among the expanded tentacles of the anemone but does not appear to be attacked by the nematocysts. It rests for periods on the basal portions of the tentacles of the anemone where there are fewer arrays of nematocysts. Some B. annulata have no cream coloured crescentic array of nematocysts present on these basal regions of the tentacles and, according to Clarke, the mysids recognize and prefer these individuals. The mysids feed on the undigested food ejected by the anemone. Another mysid, Heteromysis bermudensis, may also occur in association with the anemone (Clarke, 1955). Further investigations of other species, especially in the genus Heteromysis, may provide further examples of commensalism.

CHAPTER 10

POPULATION DYNAMICS The numbers of individuals in populations of most species of mysids fluctuate seasonally in a regular manner. A contributory cause of such fluctuations can be changing degrees of aggregation or disaggregation within the populations as discussed in the last chapter. Such changes in the spatial distributions of individuals can result in apparent increases and decreases in population numbers. The major cause, however, of these seasonal fluctuations is the seasonal pattern of reproduction of the population. Determination of these patberns is often difficult, especially in the cases of neritic and oceanic species. Most investigations so far made have been confined to populations of species living in restricted environments that can be sampled comprehensively. A more or less standard method has evolved for studying the population dynamics of a population of mysids. It involves sampling the population successively ab intervals of one, two or four weeks, the sampling interval depending upon the rates of change in the cornposition of the population being studied. The individuals witihin bhe population can be ascribed to one of the following categories: (a) juveniles, secondary sexual characteristics are absent ; (b) immature males, the secondary sexual characteristics are in process of development ; (c) mature males, the secondary sexual characteristics are fully developed and the gonads are normally functional ; (d) immabure females, the marsupium is developing and is smaller in size than that of the mature female, no young are carried ; (e) mature female, with the marsupium fully developed, but not yet filled with young ; (f) ovigerous females, mature females with young present in the marsupium ; (g) empty females, females with a marsupium from which the young have emerged, these females have not moulted after releasing the young.

188

TEE BIOLOGY OF MYSIDS

All of these categories, are useful in investigating many species. There are species, however, where one of these categories, especially the last, is either missing or of little value in the investigation. The sex of a juvenile cannot normally be determined by an examination of its external morphology. The secondary sexual characteristics that develop are different in different genera. All females have a marsupium that develops in stages a t successive moults and the pleopods may be normally developed or vestigial. The males may or may not have penes, one or more pairs of pleopods fully developed or specially modified, or an enlarged antennular peduncle armed with setae. The form of the secondary sexual characteristics of any one species can be determined from the appropriate descriptive literature listed in Mauchline and Murano (1977). The presence of these characteristics in the process of development is used to ascribe the individual to the category of either immature male or female. The mature males are recognized by the full development of the secondary sexual characteristics and its state of sexual maturity can be confirmed quite easily by examining the gonads. Consequently, the first four categories listed above are relatively easy to recognize. The fifth category, mature females with a marsupium present, but not filled with embryos is more difficult. This category is unimportant in most species studied, because the full development of the marsupium is associated with egg laying. It occurs, however, in a species such as Praunzls jlexuosua in Scotland. The female can release an autumn brood of young and immediately moult to a form with a large marsupium that is not filled with embryos. She is a mature adult that has bred a t lea& once but does not breed again until bhe spring of the following year. This category was not used by Mauchline (1971~) in a study of the biology of this species because it is difficult to distinguish from immature females. They can sometimes be distinguished by their longer body length from immature females with a welldeveloped marsupium. There are species, usually offshore oceanic or neritic species, that may release their embryos prematurely on capture. This results in a larger proportion of “empty females” in the sample than might be expected. This is the last category listed above which, in practice, is usually recognizable. The lamellae of the marsupium are usually distended, thin-walled and irregularly disposed. It$appearance is markedly different from that of the immature or mature female discussed above, that have compact, regularly arranged lamellae. The division of the individuals in a sample among several or all of these categories allows an assessment of the state of the population t o be made. It is important, however, to ensure that the youngest juveniles are being effectively sampled. The size of these juveniles can be

10. POPULATION

189

DYNAMICS

determined by an examination of the oldest larvae, those with stalked eyes, that occur within the marsupia. The youngest juveniles swimming freely in the sea will have a body length that does not exceed that of these larvae by more than 25%. The next step is to measure the sizes of the individuals in the different categories. Body size is usually determined as a measurement of total length: this measurement can be made from either the base of the eyestalk or the apex of the rostrum t o the posterior end of the telson and uropods (excluding the setae); a further measurement, standard length, (Grossnickle and Beeton, 1979) is used and is the distance between the apex of the rostrum and the posterior edge of the sixth abdominal segment. Carapace length has been used by Ishikawa and Oshima (1961), Matsudaira et al. (1952), Murano (1964a), Amaratunga and Corey (1975) and Pezzack and Corey (1979) as an index of total length. The length of the uropod was used by Clutter and Theilacker (197l), total length in Metamysidopsis elongata being equal to 4.5 times the length of the uropod. Total length is the accepted measure of body size in mysids and most, but, not all, authors who have used other indices have defined the relationship of the index used to total length. Such relationships are given in Table XXXII along with TBLE XXXII. REGRESSION CONSTANTSOF BIOMETRICAL RELATIONSHIPS Regression constants y = bx+a or log y = bloga: Species

Parameters

&

a ~ t ~ o s a cvulgaris cus

TL on CL

- 0.538

Myeidopsis didelphys

CL on TL

0.434

juveniles

0-203

females

0.647

- 0.048

males Myaia stenolepis juveniles

Reference

Matsudaira et al. (1952) 0.242 Mauchline (unpub1ished) 0,264 Mauchline (unpublished) 0-230 Mauchline (unpublished) 0-269 Mauchline (unpublished) 3.935

TL on CL 0.037

adult females Praunw inermis

b

+a

2.191

CL on TL

0.051

Amaratunga and Corey (1976) 7-963 Amaratunga and Corey (1975) 0.212 Mauchline (unpublished)

3.034

contd.

TABLEXXXII-contd. Regression constants y = bx+a or log y = b log x Species

M etamysidopsis elongata

Parameters log, WD on log, TL

BD on TL Boreomysis arctica maturing females

a

- 5.436 0.031

b 2.77 0.117 3.42

spent females

- 3.74

3.98

adult males

- 1.68

2.31

log WD on log TL

- 4'5654 2.00

Stony Lake

- 4.4622 2.84

Lake Michigan

-4.1549

2.92

- 1.27 - 1.58 0.66

2-84 3.42 1-10

Finland, < 3 mm CL log WAF,, on log CL on log CL >3mmCL log W, Neomysis americana TL on SL April-September

(log WD + 1)on log SL

- 0.353

0.741

October-March

(log W,

+ 1)on log SL

- 0.598

0.975

log W, on log TL

-4.569

2.691

log WD on log TL

-3.914

2.723

Neomysis m.irabilis

0.018

2.3

Paramysis intermedia

0.002

3.4

Paramysis kowalewski

0.002

3.0

Mesopodopsis slabberi

0.012

2.5

Acanthomysis strauchi

0.020

2.5

Paramysis baeri

Reference Clutter and Theilacker (1972) Clutter and Theilacker (1972)

log WD on log TL

- 2.92

Mysis relicta Char Lake

+a

log W, on log TL

Brattelid and Matthews (1978) Brattelid and Matthews (1978) Brattelid and Matthews (1978) Lasenby and Langford (1972) Lasenby and Langford (1972) Reynolds and De Graeve (1972) Hakala (1978) Hakala (1978) Richards and Riley (1967) Richards and Riley (1967) Richards and Riley (1967) Shushkins et al. (1972) Shushkina et al. (1972) Nekrasova and Rakitina (1968) Nekrasova and Rakitina (1968) Nekrasova and Rakitine (1968) Nekrasova and Rakitina (1968) Nekrasova and Rakitina (1968)

BD, body diameter; CL, carapace length; SL, length measured from tip of rostrum to base of telson ; TL, total length; W,, body dry weight; WW,body wet weight; WAFDW, ash-free dry weight.

10.

191

POPULATION DYNAMICS

those between wet and dry weight and total length. The relationship between antenna1 scale length on the one hand and total and standard lengths on the other are shown graphically for Mysis relicta by Grossnickle and Beeton (1979)but no regression equations are given. A graph relating body dry weight to carapace length in Neomysis intermedia is given by Murano (1964a).Boroditch and Havlena (1973)have published two graphs relating body wet weight to t o h l length in Paramysis intermedia and P . ullskyi. Clarke (1962) measured various parts of the body in four Gnathophausia species (Table XXXIII) ; the lengths of all parts are regressed TABLEXXXIII. REGRESSIONCONSTANTS O F BIOMETRICAL RELATIONSHIPS OF THE BODY AND TOTAL BODYLENGTHIN GNATHOBETWEENVARIOUSPARTS PHAUSIA SPECIES ~~~~~~~~~~~

~

~

Regression constants y = bx+a Species

G . ingena

$+$?

Independent variable CL

cw AL TeL

UL

0. gigas

G . zoea

6+$?

6 ?

6 ? 6+?

RL ASL 70L CL

cw

AL TeL UL RL ASL CL CL AL

AL TeL

UL RL ASL 70L

n 233 229 234 234 234 147 233 28 55

54 58 58 58 37 58 26 22 26 22 48 48 47 48

20

a

-2.4378 0.3276 2.3201 0.5877 -0.2194 12.4393 0.2073 -42.3813 -1-6915 0.3050 0-5158 0.0780 -0.1977 11.4368 1.1871 -1.5758 -0.6897 0.1764 0-4802 0.7512 0.8152 11.3070 3.0234 -22.7793

b

0.4795 0.1677 0-3530 0.2225 0.1750 0.0736 0.1243 0.4628 0-4283 0.1435 0.4063 0.2375 0.1948 0.1345 0.1432 0.4693 0.4995 0,3822 0.3744 0.2026 0.1783 -0.0141 0-0932 0.4629

Regression coescient r

0.9980** * 0*9975*** 0.9981* * * 0*9977*** 0.997 7** * 0.7579*** 0.9951 * * * 0*9551*** 0.9987 * * * 0.9958*** 0*9986*** 0.9973*** 0*9959*** 0-7357*** 0.9945*** 0.9975** * 0*9977*** 0.9910*** 0.9963*** 0.9947*** 0*9928*** - 0.2579 0.9638*** 0.9802* ** contd.

192

THE BIOLOGY OF MYSIDY

TABLEX X X I I I - c o n t d . Regression constants y = bz+a Species

G . gracilis

3

0 d+P 6 ? d+P

G. gracilis (dwarf)

d+? d ? S+P

Independent variable CL CL

cw AL AL TeL UL EL ASL 70L CL CL

cw

AL AL TeL UL RL ASL 70L

Regression coeficient

n

a

b

r

77 130 183 76 131 208 207 110 206 89 19 33 45 19 33 51 52 36 52 33

- 1.1413 -2.3284 -0.2013 0.5441 1.9813 10.2364 -0.2317 6.2667 0.9194 - 25.8196 -0.0461 -3.3350 - 1.6371 1.5013 2.8512 0.3876 9.9140 11.8026 1.1555 - 18.5246

0.3912 0.4130 0.1530 0.4310 0.4019 0.0705 0.2053 0.1623 0.1558 0,4123 0.3704 0.4464 0.1843 0.4127 0.3716 0-1975 - 0'0039 0.0487 0.1450 0.4940

0.9969*** 0*9976*** 0.9965*** 0*9980*** 0.9976*** 0.5362*** 0.9967*** 0-8606*** 0*9949*** 0.9176*** 0*9833*** 0.9896*** 0.9839*** 0*9941*** 0.9879*** 0-9816*** - 0.1597 0*2294*** 0-9608*** 0.9634***

Data from: Clarke, 1962. Independent variables : AL, abdomen length, ASL, antennal scale length; CL, carapace length; CW, carapace width; RL, length of rostrum; Tel. telson length; UL, uropod length; 70L, length of 7th brood lamella (oostegite).

on body length, measured from the base of the eyestalk to the posterior end of the telson excluding the setae. Carapace length is measured from the base of the eyestalk to the posterior lateral edge excluding any spinous process; abdomen length is measured dorsally from the anterior edge of the fist t o the posterior edge of the sixth segment; telson and uropod lengths are measured from the posterior edge of the sixth abdominal segment to the posterior edge of the appendage excluding the setae; antennal scale length is that of the scale alone; the length of the rostrum is from the base of the eyestalk to the anterior tip of the rostrum. No sexual differences were present in the measurements except in those of the carapace and abdomen relative to total length in G . zoea and G. gracilis; separate equations have been calculated for these relationships (Table XXXIII). The scatter within the raw

10.

193

POPULATION DYNAMICS

data is very limited as can be seen in Fig. 48 and deduced from the high values of the correlation coefficients. All the correlation coefficients are significant a t 0.1 yo level except those indicated. The rostrum is subject to damage.

8

20-//:. -30

..:'ES .:
,,..il'-'

10-

0

./"

TG;.z-:e

.,,*rp-'

a

. ::!*H,:~c_.,::;..*ddo-U

4

a v

d,&72*--

-10 a

-3:"-

I

I

1

g c

-20

i---

ropod length

1

-..+ ...." :::... ,,.:

< 2

.

'*o;:' :.

I

I

I

I

I

I

1

I

I

Fro. 48. Regressions of the dimensions of various parts of the body on total body lengt h in Qnathophausia ingens. The respective regression equations are given in Table XXXIII. Data from: Clarke, 1963.

The size of the lith of the statocyst is used by Morgan and Beeton (1978) in analysing the occurrence of individual instars within a

population of Mysis relicta. The frequency distribution of the diameters of the liths was polymodal and the modes appear to correspond with successive instars, although interpretation of them involves the usual difficulties of polymodal analyses. They found that the diameter of the lith ranged from 0.04 to 0-26 mm; their measurements were accurate to 0.01 mm. A decrease in the size interval with a consequent increase in the number of size classes might aid in interpreting the modes, especially among size classes of the smaller individuals. Studies of the populations of a range of species from different latitudes have been made (Table XXXIV). The proportions of different

TABLEXXSIV. POPULATION STUDIESOF MYSIDS IN DIFFERENT GEOGRAPHICAL REGIONS 0bservations Species

Latitude

Anchialina agilis

Region

56'30" 43ON Boreomysis nobilis 70"N Bowmuniella brasiliensis 25"N Brasilomysis castroi 25's Erythrops elegans 56% Erythrops serrata 56"N Gastrosaccus brevifissura 32% Gastrosaccus dakini 27'30's 43'N G . lobatusla. normani Gastrosaccuspsammodytes 35"s Gastrosaccus sanctus 46"N 38"N Gastrosaccus vulgaris Gnathophausia ingens 38"N-32"N

Scotland Marseille W. Greenland Brazil Brazil Scotland Scotland S. Africa Australia France S. Africa Black, Azov Seas Japan E. Pacific

Leptomysis gracilis Leptomysis lingvura

Scotland Scotland, Irish Sea

Mesopodopsis africana Mesopodopsis slabberi

56"N 58"N-5 6"N

32"s 55"N 43"N 46"N Metamysidopsis elongata 33"N M . elongata atlantica 25"s Mysidium columbiae 18"N Mysidopsis bigelowi 38"N Mysidopsis didelphys 56'N Mysidopsis gibbosa 56"N Mysidopsk tortonesei 25"s

S. Africa Denmark France Black, Azov Seas E. Pacific Brazil Jamaica W. Atlantic Scotland Scotland Brazil

Breeding periods

few seasonal

X

A W(?) S P SU A a Su Sa sp su a w sp su a w S P SU a w(?) SPSUaw SPSUAa SPSUAa SPSUAw sp su a w SP SU A sp su a

X

?

2(?) 3 0.12

X

S P SU A w SPSUAw

2 2

X X

X X X X

X X X

X X X

x X X X

X X X X X X X X

Generations1 Year

sp su a a SU A SP SU A a SP SU A SP S U A w sp su a w sp su a w sp SU a AW S P SU A w sp su a w

2 ?

2-3 2 ?

?

2 2 >3

2 1 3

References Mauchline (1971e) Macquart-Moulin (1965) Dunbar (1940) Almeida-Prado (1973) Almeida-Prado (1973) Mauchline (1968) Mauchline (1968) Connell (1974) Hodge (1963b) Macquart-Moulin (1965) Brown and Talbot (1972) Zatkutskiy (1970) Matsudaira et aZ. (1952) Childress and Price (1978) Mauchline (1968a) Liao (1951); Mauchline (1969a) Connell (1974) Blegvad (1922) Macquart-Moulin (1965) Zatkutskiy (1970) Fager and Clutter (1968) Almeida-Prado (1973) Goodbody (1965) Hopkins (1965) Mauchline (1970b) Mauchline (1970b) Almeida-Prado (1973)

75"N-72'N 42"N-4OoN 74'N 74"N

Arctic W. Atlantic Laptev Sea Char Lake

63"N-61°N 61"N 49ON-47'N 46'-43'N 45ON

Great Slave Lake L. PiiiijiLrvi, Finland L. Superior L. Huron Ontario Stony Lake

42'N Mysis stenolepis Neomysis americana

My& Mysis Mysis Mysis

litoralis mixta oculata relicta

SP w ( ? )

-

0.5? 0.5 0.5?

X

SP

0.5

X

X

SP W SP SU or M7 SP

L. Michigan

X

sp su a w

1

45ON-42"N

W. Atlantic

X

SP w

1

38% 41°N

Delaware, USA Long Island Sound

X X

sp SU a sp SU a w

Neomysis mercedis

45'N 42"N-40°N 38"N

Passamaquoddy Bay Georges Bank E. Pacific

Neomysis integer

56ON-54"N

Nwmysis intermedia Paramysis arenosa Paramysis bacescoi

X X

X

X X X

0.5 0.5- 1.0 0.5 0.7? 1

2-3 2(?)

-

X

sp su a S P SU A (w?) sp su a w

Scotland, Baltic

X

SPSUAw

2-3

54"N 54'N

West Ireland Netherlands, Baltic

X

X

S P SU A usp su a

2-3 3

40°N 58ON-54"N

Japan Scotland, Irish Sea

X

SP su A w SPSUAw

2-3 2-3

X

S P SU A w

2

Roscoff

X X

X

3

3 2(?)

Geiger (1969) Wigley and Burns (1971) Geiger (1969) Lasenby and Langford (1972) Larkin (1948) Hakala (1978) Carpenter et al. (1974) Carpenter et al. (1974) Lasenby and Langford (1972) Reynolds and De Graeve (1972); Morgan and Beeton (1978) Wigley and Burns (1971); Amaratunga and Corey (1975) Hopkins (1965) Richards and Riley (1967) Pezzack and Corey (1979) Wigley and Burns (1971) Turner and Heubach (1966); Heubach (1969) Kinne (1955); Mauchline (1971b) Parker and West (1979) Vorstman (1951); Wiktor (1961) Murano (1964a) Liao (1951); Mauchline (1971a) Labat (1957)

cont.

TABLEXXXIV-contd. Observations speciw

LatitwEe

Paramysis intermedia

Region

few seasonal

Russia

Breeding PETiods

Generations/ Year

SP SU A

29

S P SU A S P su 9 w S P su A w S P SU A

2s

Paramysis kroyeri Paramy& nauveli Paramysia ponticu Paramy& ulkkyi

46'N

Praunus Jee.2cosis

58'N

W. Europe

X

SPSUAa

2

55'N 57ON-54'"

Denmark Scotland, Irish Sea

X

Praunus inermis

sp su a S P SU A w

2 2

55"N 56"30" 55"N 56'N-40°N

Denmark Scotland Denmark Atlantic

X

Praunus neglectus Pseudomma aflne

46"N

Black, Azov Seas Roscoff Black, Azov Seas Russia

Rhopalophthalmus brisbanenaia Schistomysis kervilbi Schietomysia ornakc Schistomysis spiritus

27"30'S

Australia

58"N 56'30" 5BoN-54'N

Scotland Scotland Scotland, Irish Sea

Siriellu annata

58ON-54'N

Scotland, Irish Sea

Siriellu clausii SpelaeQmyiri8 longipw

43'N 10"N

France India

X X

X

X

X X

X X X

X X

sp su a sp su a sp su a w (9)

09

a.

29 2s

2-3 2 2

-

S P SU A w

39

SP SU A w SP su a W S P SU A w

2 1 3-4

all year?

-

SPSUAa S P su

2 2(?)

Referencea Boroditch and Havlena (1973) Zatkutskiy (1970) Labat (1957 Zatkutskiy (1970) Boroditch and Havlena (1973) Liao (1951); McLusky and Heard (1971); Maucbline (1971~) Blegved (1922) Liao (1951); Mauchline (1965) Blegved (1922) Mauchline (1971~) Blegvad (1922) Mauchline (1971e); Wigley and Burns (1971) Hodge (1963b) Mauchline (1971d) Mauchline (1970a) Liao (1951); Mauchline (1967) Liao (1951); Mauchline (19710) Macquart-Moulin (1965) Nath (1973)

The frequency of observations is shown as few or seasonal (many). Periods of intense breeding are indicated by capitals: SP(sp), spring; SU(su), summer; A(a), autumn; W(w), winter.

10.

POPULATION DYNAMICS

197

categories of sexual maturity were determined within the populations of many of the spec?es at frequent intervals throughout the year but only occasionally in the case of other species (few observations). Most of the species investigated have extensive breeding seasons while a few do not. The patterns of reproduction and succession of generations vary between different species and between populations of the same species at different latitudes. The life history of any one species, however, is usually recognizable as one of the following types : (a) species requiring more than two years to achieve sexual maturity such species produce less than 0.5 generations per year ; (b) a species that attains sexual maturity and breeds at an age of' two years, such a species produces 0.5 generations per year; (c) a species that breeds once per year, that is, produces one generation per year ; (d) a species producing two generations per year ; (e) a species producing three generations per year ; (f) a species producing more than three generations per year.

Species producing less than 0.5 generations per year. The bathy pelagic Gnathophausia ingens is the only species so far investigated that has a long life history. Childress and Price (1978) show that it attains sexual maturity at an age of over seven years and that the generation time i s eight years, the young being carried within the marsupium by the female for more than a year. The female produces a, single brood in her lifetime, that is she is semelparous, whereas the majority of other mysids are iberoparous, producing more than one brood per female. Population length-frequency histograms have a peculiar form in this life cycle. Childress and Price found them to be polymodal, the individual modes clearly distinct and corresponding to successive instars. The total number of instars in the life history is 13, the first two living within the marsupium of the female. Species producing less than one generation per year. One species, the freshwater Mysis relicta, has been shown to have a two year life cycle in certain regions (Fig. 49). Lasenby and Langford (1972) found that this species required two years t o attain sexual maturity and breed in Char Lake at 74"N latitude, thus providing confirmation of a two year life cycle first proposed for this species in the Great Slave a t 62'N by Larkin (1948). Carpenter et al. (1974) suggest that this species also has a two year life cycle in Lake Superior. Two other species, M . litoralis and M . oculata, may have two year life cycles in the East Siberian and

L 98

THE BIOLOGY OF MYSIDS

Breeding

ds and 8 s Immature

-

PIG.49. Schematic diagram showing the seasonal composition of a population with a two-year life cycle. The arrow indicates production of the next generation by the breeding adults (closest hatching). The juveniles (open hatching) develop into immature adults (closer hatching) and in some populations may mature at an age of 1 year (broken hatching) t o produce a first brood of young.

Laptev Seas (Geiger, 1969); the data, especially on the latter species, are inconclusive and require confirmation. A species that normally matures to breed at an age of two years may mature and breed a t an age of one year in warmer parts of its distribution; individuals of such populations may survive t o an age of two years and breed a second time. This has not so far been demonstrated in a mysid, Mysis relicta having its life span apparently shortened to approximately one year in warmer regions. A peculiar life cycle, first suggested by Fiirst (quoted by Carpenter et al., 1974) for populations of M . relicta in some Swedish lakes, may also exist in this species in Lakes Huron and Ontario. An alternation of generations on an 18 month time scale may take place (Fig. 50) ; breeding females in the summer may produce a generation that matures at an age of 18 months to breed in the winter and produce a generation that matures in 18 months to breed in the summer. This form of life cycle, producing 0.7 generations per year, requires confirmation. A further variation, described by Hakala (1978),may also be another example of alternation of generations in this species. It breeds in Lake Paajarvi, Finland, in

10.

I I Spring I I

Winter

199

POPULATION DYNAMICS

1

Summer

I

I

Autumn

'

Winter

FIG.60. Schematic diagram of the lifecycle of My& relictu in which there is an alternation of generations. Winter breeders produce a winter generation of young that matures 18 months later to breed in the summer; this summer generation of young matures 18 months later to produce a winter generation and so on. Hatching as in Fig. 49.

winter and two cohorts of young are released in the spring. Young released in April and early May mature during the summer to breed the following winter; they die a t an approximate age of 13-14 months. Young released in late May and early June, forming a second cohort, remain immature throughout the first winter but grow rapidly the following summer to breed in their second winter; females of this cohort die a t an approximate age of 22 months and males at 1&18 months. Dr R. M. Laws of the British Antarctic Survey kindly supplied the two samples of Antarctomysis maxima analysed in Fig. 51. The larvae of tihis species probably emerge from the marsupium at a length of 4-5 mm although they have not been observed, the smallest juvenile caught in these samples having a length of 10 mm. Both samples are trimodal and it seems probable that two years are required t o attain sexual maturity and breed. This species attains a body length of over 70mm and so it is conceivable that some two year old animals may survive to an age of three years and breed for a second time. Several of these longer lived mysids may be semelparous like Gnathophusia ingens. Mysis relicta in northern lakes carries the young in the

200

THE BIOLOGY OF MYSIDS

6

10

14

18

Body length ( mrn) 22 26 30

34

38

42

30 January, 1969

FIG.51. Population histograms o f samples of Antarctomysis m a x i m collected b y the British Antarctic Survey. Juveniles and females are shown above the lines, males below the lines. The transverse hatched lines indicate possible correspondence between modes.

marsupium for a t least 150 days at environmental temperatures below 10°C (Table VI, p. 64). Consequently production of a second brood by females is unlikely. The same is probably true in northern populations of other Mysis species and in Boreomysis arctica and B. nobilis. Antarctomysis maxima, on the other hand, may be interoparous, a proportion of females surviving to be three years of age and produce a second brood. Species having these relatively long life cycles are probably of a larger body size than species producing one or more generations per year. These types of life cycles should be looked for in species living in the deep neritic and the mesopelagic and bathypelagic environments. The life cycle shown in Fig. 49 is probably recognizable in the majority of cases by a clear separation of age groups or modes in population

10.

201

POPULATION DYNAMICS

histograms. Histograms of the cycle in Pig. 50 will contain three or four modes more closely spaced than the modes of the cycle in Fig. 49.

Species producing one generation per year. Mysis stenolepis in Passamaquoddy Bay and the Gulf of Maine has a conventional life cycle spanning one year and illustrated schematically in Fig. 52. The

1

I

I

Winter

I

Spring

Summer

I

I

I Autumn

I

Winter

1

FIQ. 52. Schematic diagram showing the seasonal composition of a population of a species producing one generation per year. The solid arrow indicates production of the next generation by the breeding adults. A second brood (broken arrow), lor cvcn a third brood, may he produced by the females during the breeding 8eason. Significance of hatching as in Fig. 49.

young are released from the marsupium during a restricted period in the spring and attain sexual maturity the following spring when th2y breed (Smith, 1879; Amaratunga and Corey, 1975). Females carrying embryos only occur in the population during the period April to June and no breeding takes place a t other times of the year. Populations of M . mixta in the general area of the Gulf of Maine probably have this type of life history (Smith, 1879; Wigley and Burns, 1971) but confirmation is required. Populations of M . relicta can also have a life cycle spanning one year. Lasenby and Langford (1972), investigating populations in Stony Lake (Table XXXIV), found that eggs passed into the marsupium in December and young developed there to be released

202

THE BIOLOQY OF MYSIDS

in May. The breeding adults survive for some time into the summer but no later. The young attain sexual maturity and breed the following winter. This species is also an annual in soutsh-eastern Lake Michigan where Reynolds and De Graeve (1972) found that it breeds continuously throughout the year, although most intensively in the summer. Morgan and Beeton (1978) note that young are recruited into the population near Milwaukee on Lake Michigan a t distinct intervals throughout the year. The population consequentdy consists of cohorts, the females in any one cohort maturing to breed a t an age of one year and a significant proportion of them surviving, moulting and breeding for a second time four months later. Two coasta.1 neritic species in Scotland, Mysidopsis didelphys and Schistomysis ornata, are annuals. The former breeds in the autumn and winter, while populations of the latter species breed in the winter and spring, although some breeding individuals occur during all seasons of the year. These are probably semelparous species. It appears that some populations of an annual species may be semelparous while others may have interoparity. The factors controlling this are not clear. These life cycles extending over a full year probably apply to certain species in the latitudinal bands 40-60" and to neritic rather than littoral species. The breeding females may produce one or more (hatched arrow in Fig. 52) broods during the breeding season.

Species with two generations per year. The most simple structure of this life history is shown in Fig. 53 by the solid arrows. An overwintering generation matures and breeds in the spring to produce a spring generation. The members of the overwintering generation die after breeding. The spring generation grows and matures during the spring and summer to breed in the late summer to early autumn period and produce a new overwintering generation. The members of the spring generation also die immediately after mating. Consequently, except for short periods of time the population consists of only one age group. The simplicity of structure of this life history depends upon all members of each generation maturing and breeding within a short period of about two months. The life history of none of the species so far investigated conforms to this simple structure, although that of Praunus flexuosus in Scotland and Denmark approximates to it (Mauchline, 1971~).In P. jlexuosus, however, the breeding period is long, extending from February to July and involving the production of successive broods by the females, indicated in Fig. 53 by the hatched arrow. Females reared in the laboratory can produce a brood of young every three or four weeks. There is no method a t present for identifying successive

10.

203

POPULATION DYNAMICS

-

--

I

I

I

Winter

I 1

1

1

Spring

I

+-

Summer

I

I

Autumn

Winter

I

FIG. 53. Schematic diagram showing the seasonal composition of a population of a species producing two generations per year. The breeding females may produce more than one brood of young (additional broken arrows). Body size a t sexual maturity is reduced in summer breeders. Significance of hatching as in Fig. 49.

broods in wild populations. Females in a population of Neomysis americana, studied in Passamaquoddy Bay by Pezzack and Corey (1979), produce broods that are identifiable as modes in the lengthfrequency histograms; this is a result of a high degree of synchronous breeding within the population. This pattern of life history is shown in Fig. 54. The overwintering generation matures in the spring and the first brood is released from the marsupium by all females within a short period of time, giving rise to the first cohort of the spring generation.

Winter

I

I

Spring I

Summer

I I

I

Autumn

I

Winter

FIU.54. Schematic diagram showing an alternative seasonal composition of a population producing two generations per year. See text for further explanation. Significance of hatching 8s in Fig. 49.

204

THE BIOLOGY OF MYSIDS

A second brood is then produced by the females and results in a second cohort of the 'spring generation. The breeding adults die soon after producing the second brood. The first cohort matures and breeds before the second cohort so that the new overwintering generation initially consists of two cohorts. These, however, merge in the winter to form a more or less uniform population. The summer generation does not survive far into the winter. The majority of populations of species listed in Table XXXIV as producing two generations per year have extensive breeding seasons so that size modes within the populations are often partially obscured. This situation is further complicated by a decrease in size of the mysids during the summer period. I n the spring generation, the juveniles develop secondary sexual characteristics and the males and females mature to breed a t smaller body sizes relative to individuals of the overwintering generation. This is indicated in Figs 53 and 54. Lack of synchrony in brood production by the females, combined with the production of successive broods by any one female, causes difficulty in deciding whether there are two generations in the year as opposed to three generations. This is further accentuated in latitudes close to 40" where a population might be expected to have an increased number of generations and individuals a shorter life span. Species having two generations per year appear to occur in latitudes higher than 40".

Species with three generations per year. This form of life history is difficult to determine in natural populations and is illustrated in Fig. 55. Species producing three generations per year usually exhibit more or less continuous breeding with periods of seasonal maxima throughout the year. I n addition, any one female will produce one, two or more broods. Consequently, the populations consist of very mixed age groups and individual generations are obscured. This happens in species such as Mysidopsis gibbosa, Neomysis integer and Schistomysis spiritus in Scotland and in other species listed in Table XXXIV. The overwintering generation produces a few young throughout the winter ; this winter breeding may be absent in populations of the species in an area of more severe climatic conditions. Breeding, however, becomes intensive in the spring and the production of the spring generation can normally be recognized. Most females in species with this life cycle probably produce two broods of young because the resulting spring generation often appears to be bimodal. The spring generation matures and breeds in the summer t o produce a summer generation. Breeding females from the overwintering population may survive t o this time, although in small numbers, and still be producing

10.

Winter

I I

205

POPULATION DYNAMICS

I

I

Spring I

Summer ~

I

I Autumn I Winter

~~

FIG. 55. Schematic diagram showing the seasonal composition of a population of a species producing three generations per year. Format of diagram as in Figs 49, 52 and 53.

young. The breeding females of the spring generation may produce two or more broods, but usually die by early autumn. The summer generation grows during the late summer and early autumn, at least its oldest members (Fig. 5 5 ) maturing to breed in the autumn and produce an overwintering population. Members of the summer generation, especially the younger ones, very frequently also form part of the overwintering population, as indicated in Fig. 55. The proportion of the summer generation that overwinters is probably dependent upon the autumnal climatic conditions. The early onset of cold weather in the autumn may increase the proportion that does not mature to breed in the autumn but that survives the winter to breed either during the winter or in the following spring. The schematic representation of this form of life history is simplified in Fig. 55. Continuous breeding within populations means that there is a continuous, although seasonally variable, production of young. Neverthelees, this basic pattern can often be discerned relatively easily in the populations. Detection of the pattern is simplest in populations where winter breeding is minimal and the spring generation is easily recognizable. This situation usually leads to a summer generation that can also be seen as a mode, although less well defined, in the length-frequency histograms. Development of the secondary sexual characteristics as well as the attainment of sexual maturity and production of young takes place at smaller body sizes in members of the spring and summer generations than in members of the overwintering population (Fig. 55).

206

THE BIOLOGY OF MYSIDS

This type of life history is probably most common among shallow living neritic and littoral species occurring in latitudes between about 25" and 50".

Species with more than three generations per year. The presence of more than three generations per year results in extremely complex population histograms (Fig. 56). Populations of Metamysidopsis elongata, probably of Mysidiurn columbiae and in some years Schistomysis spiritus (Table XXXIV) are in this category. Breeding is more or less continuous throughout the year and each female produces several broods. Generations overlap and cannot be distinguished. The simplified

--------1

I

Winter

I

Spring

I

I

Summer

I

1

Autumn' Winter

pattern shown in Fig. 56 is rarely discernible in the population. Its presence is deduced from laboratory observations on the rates of growth, attainment of sexual maturity and brood production. Most species with this life cycle are littoral or shallow neritic and occur in shoals in latitudes lower than 40".

As explained above, the life histories illustrated in Figs 49-56 are idealized and simplified. Most natural populations, but not all, are less regular owing principally to the presence of extended, if not continuous, breeding seasons and the production of more than one brood of young by most females. Summer breeding individuals, however, are usually smaller than individuals of the same species bseeding at cooler times of the year. Consequently, examination of length-frequency histograms of ovigerous females often aids in the interpretation of the histograms of

10.

POPULATION DYNAMICS

207

the whole population. Most of the species listed in Table XXXIV are shallow coastal and littoral species with the addition of a few species occurring in freshwater habitats. The biology of none of the more strictly offshore species, whether epipelagic or deeper living (100-500 m depth) hyperbenthic, has been examined. Their life cycles are probably simplified, span one or two years, and are comparable with those of species such as Mysis relicta, M . stenolepis, Mysihpsis didelphys, Schistomysis ornata and Leptomysis gracilis. Mesopelagic and bathypelagic species, such as the large sized Cnathophausia species, may all have an increased longevity as shown for C. ingens by Childress and Price (1978). Rates of growth of individuals of various species have been computed by the following authors : Gnathophausia ingens by Childress and Price (1978); Hemimysis speluncola and Leptomysis lingvura by Gaudy and Guerin (1979); Metamysidopsis elongata by Clutter and Theilacker (1971) ; Mysis gaspensis juveniles by Dadswell (1975) ; M . relicta by Larkin (1948), Lasenby and Langford (1972), Reynolds and De Graeve (1972) and Hakala (1978) ; M . stenolepis by Amaratunga and Corey (1975) ; Neomysis intermedia by Murano (1964b) ; Praunus species by Blegvad (1922). Growth rates of many of the species can be computed from the data given in the papers quoted in Table XXXIV. Growth of individuals is faster in generations produced in warmer months of the year than in colder months. Consequently, growth rates of a species can vary between generations. The longevity of individuals of a species can also vary markedly between generations. Individuals in a summer generation may have a life span as short as 6-8 weeks, while overwintering individuals of the same population may live for 5-7 months. Growth in body size of a mysid takes place, as in all crustaceans, principally at moulting and so a truly descriptive growth curve of an individual has a stepwise form. A certain amount of growth in body size can, however, take place between moults. Adult female mysids with embryos in the marsupium do not moult although they may, in the case of Cnathophausia ingens, Boreomysis arctica or Mysis relicta (Table VI, p. 64, carry their embryos for several months. Successive stages in the development of the embryo and larvae can be recognized (Chapter 3) and these are equivalent to successive times in the course of the intermoult period. This situation allows an examination of the potential increases in body size that can be accommodated without the necessity of moulting. Measurements were made of the body lengths of females with embryos or larvae at known stages of development within the marsupium (Table XXXV). Females with eyeless larvae

TABLE XXXV. MEAN BODYLENGTHS f (I AND NUMBERSOF FEMALES WITH DIFFERENT LARVAL STAGES IN THE M A R S ~ I U M . THERATIOS OF THESE LENGTHS ARE GIVEN (Mauchline, 1973a) Lengths (mm)of females with Numbers of femalea with “Eggs”

Eyehs (2)

EYd (3)

Species

(1)

Erythrops serrata Leptomysis gracilis Leptmysis linyvura Mysidopsis didelphys Mysidopsis gibbosa Neomysis integer Paramysis arenosa Praunua flexuosus Praunus inermia Praunus neglectus Schistomysia kervillei Schistomysis o d a Schistmysis apim’tua Clyde Loch Etive

6.3 rf: 0.7 12.0 f 1.4 9.4 f 0.7 11.9 f 1.2 4.7 f 0.6 13.5 f 1.5 7.5 f 0.8 19.2 f 2.2 12.6 f 1.1 19.1 f 1.8 10.8 f 1.1 12.2 rf: 0.7

6.6 f 1.0 7.4 f 1.0 12.9 f 1.5 12.4 f 1.4 10.0 0.4 9.8 f 0.9 11.7 f 1.3 12-1f 1.1 4.9 f 0.6 5.1 f 0.6 13.2 f 1.6 13.6 f 1.7 7.9 f 0.8 7.7 f 0-8 19.1 f 2.0 19.7 f 2.0 12.9 & 1.2 2043 1.7 20.1 f 2.2 11.2 f 1.3 11.3 f 1.2 13.9 f 1.2 12.7 f 0.7

11.4 f 0.9 11-7f 0.9

11.8 f 1.0 12.2 f 1.0

12.1 rf: 1.0 12.2 f 1.0

Ratioa of bngths

(1)

(2)

(3)

2/1

3/1

16 327 12 13 283 353 174 250 318

36 462 12 26 316 338 324 342

13 113 5 12 95 158 99 90

1.05 1.03 1.04 0.98 1-04 0.98 1.03 0.99

1-17 1.08 1-06 1-02 1.09 1.01 1.05 1-03

6

203 6

14 415 21

9 156 7

1.05 1.04 1.04

1.09 1.05 1-14

367 250

347 42 1

133 169

1.04 1.04 Mean 1.03

1.06 1.04 1-07

638

1.02

10.

209

POPULATION DYNAMICS

were, on average, 3 yolonger in body length than females carrying early embryos (“eggs”), while their body length had increased by about 7% ab t,he time when the larvae had developed stalked eyes. The abdomen was found to increase in length relative to the carapace during this time as instanced in Fig. 57 for Praunus jlexuosus. The intermoulb period of females carrying young is undoubtedly extended in duration

-

0

I

I

1

I

I

FIG.57. Abdomen length related to carapace length in female Praunusjexuosw carrying “eggs” (dots) and eyed larvae (triangles)in the marsupium (afterMauchline, 1973a).

relative to that of a female of the same body size but not carrying young. Consequently, a 7 % increase in body size between moults probably overestimates the comparable intermoult increases in body sizes of juveniles, males and immature non-breeding females. Most of the increase in body size is associated with moulting, and a model of growth has been developed that applies to decapod and several other types of crustaceans (Mauchline, 1976, 1977b, c). The duration of the intermoult period was found to increase logarithmically while the

210

THE BIOLOGY OF MYSIDS

growth factors, that is the increment in length expressed as a percentage of the premoult length, decreased logarithmically when related to body length or successive moult number. Very few observations of the intermoult periods of mysids have been made, but the relationship appears to exist in the data in Figs 41 and 42 (p. 143). Growth factors, related to body length, can be calculated from the data of Ishikawa and Oshima (1951) and Matsudaira et aE. (1952) and these are plotted logarithmically against carapace length in Fig. 58. The equations of the two regression lines are:

Neomysis japonica

log G F = 1.423 - 0.194 CL

Gastrosaccus vulgaris

log GF = 1.406 - 0.231 CL

where G F is the percentage growth factor and CL is the carapace length. The correlation coefficients of both regressions are significant at the 0.1% level. Gaudy and Guerin (1979) reared Hemimysis speluncola and Leptomysis lingvura and measured increase in length of the exopod of the uropod at successive moults. The percentage growth factors did not decay logarithmically when related to successive moult number (Fig. 59). The animals were reared at the four temperatures Total length (mrn)

40

20

'

" O r b '

100

60

80

I

'

I

I

1

1

1

1

140

120 I

I

I

1

1

1

'

201.

1

t

1

1.0

1

1

1.5

1

1

2.0

1

1

1

1

1

1

1

J

2.5

Carapace length (mm) FIG. 58. Growth factors expressed as the percentage increme in carapace length of Neomysis japonica (dots and Gmtrosaccus vulgaris (triangles) and as percentage increase in total length of Gnathophawia ingens (stars) related to the increasing size of the animals. The equations of the two lines are given in the text. Data from: Ishikawrt and Oshima, 1951; Matsudairs et al, 1952; Childress and Price, 1978.

10.

POPULATION DYNAMICS

21 1

40r 30 -

-8

20 -

s

c

u

0

5 10-3

2

7-

0 0

4-

10°C 14°C 18°C 22°C

u 3 5 1

7

9 Moult number FIG.59. Growth factors, expressed as percentage increase in the length of the exopod of the uropod, at successive moults in Leptomysis lifl,gvura and Hemimysis speluncola at different temperatures. Data from: Gaudy and Guerin, 1979.

shown in Fig. 42 where the information on the corresponding intermoult periods is given. There was much variation in the growth factors and it does not appear to be related to the environmental temperatures. The data for Hemimysis speluncola are slightly more regular than those for Leptomysis lingvura. The only other measurements of growth factors a t successive moults are for the bathypelagic Gnathophausia ingens (Fig. 5 8 ) by Childress and Price (1978). They were obtained from population length frequency histograms which are polymodal, the modes corresponding to successive instars. The larva within the marsupium moults on emergence and has a growth factor of 24.3%. The subsequent seven moults have factors ranging from 1 9 4 % to 20.4%; in other words, the factor remains constant. The factors then decay from 20.4% to 14.9% over the following three moults, the last three moults of the normal life cycle. In both cases, the growth factors for the final series of moults of Hemimysis speluncola and Gnathophuusia ingens decay in an approximately logarithmic manner.

212

THE BIOLOUY OF MYSIDS

Consequently, the available data on the pattern of increase of the durations of the intermoult periods fit the model of growth. The logarithmic decay of growth factors is present in the data for Neomysis japonica and Gastrosaccus vulgaris. The results for Leptomysis lingvura and Hemimysis speluncola are peculiar in the context of crusbacean growth in general; the growth factors increase to a maximum or modal value and then decay (Fig. 59). This pattern of growth factors occurred in Forster’s data (quoted by Mauchline, 1977b)on the growth of Palaemon serratus in culture. A metamorphosis is involved in this animal and it seems possible that the immediately succeeding larval stages were adversely affected by the conditions of culture; the growth factors were depressed but recovered, by increasing at the successive three moults, after which they decayed in a logarithmic manner. Similar effects of the culture procedure may have affected the growth factors of the early moults in the cultures of Gaudy and Guerin and only further observations on these and other species will clarify the situation. The bathypelagic Gnathophausia ingens may be specially adapted to its environment, as discussed by Childress and Price (1978); the growth factors remain constant until just before the animal starts to mature sexually. The model, therefore, described by Mauchline is probably applicable to all mysids with the exception of bathypelagic species, if G. ingena is typical of them. Assuming that the model is appropriate, laboratory observations of growth factors and intermoult periods can be used in an empirical fashion to construct models of growth rates of individuals and populations. These models can aid in the interpretation of the environmental data, especially where the generations are obscured through continuous breeding of the population. The detailed methods of applying the model are fully explained in the companion work on euphausiids in the second part of this volume, where it is used to examine the growth of Euphausia superba Dana. The number of instars in the life of a mysid appears to be markedly fewer than in many other crustaceans. The first instar lives within the marsupium where it moults and the second instar also dwells within the marsupium. This instar moults as it is released, or shortly after release, from the marsupium. There are a further 10 or 11 instars in Gnathophausia ingens before sexual maturity is achieved (Childress and Price, 1978). Neomysis japonica requires 11 post-marsupial instars to attain sexual maturity (Ishikawa and Oshima, 1951). As mentioned previously, summer generations breed at a smaller body size than overwintering generations. Matsudaira et al. (1952) found that the small sized breeding individuals have 11-13 post-marsupial instars, while the

10.

POPULATION DYNAMICS

213

larger sized animals of the winter generation have 10-21 instars. Similar numbers of instars occur in the corresponding generations of Leptomysis lingvura according to the data given by Gaudy and Guerin (1979). Hemimysis speluncola, however, a cavernicolous species, has 10-11 post-marsupial moults. The difference in body size between breeding adults of different generations is less in this species than in Leptomysis Eingvura and consequentdy only one or two extra moults will be required by overwintering individuals. Moulting, and therefore growth in body size, may be inhibited by low winter temperatures in middle and high latitudes. Such inhibition has to be taken into account in analysing growth of individuals in such regions. Variations in size at sexual maturity not only exist within populations of a species a t different seasons but also between populations in different areas at the same season. Wenner et al. ( 1 974) examine size at sexual maturity and the growth rate of crustaceans in general but include mysids as examples. Their model assumes that the rate of growth and the time required to attain sexual maturity are related. Female mysids generally grow larger than males. Clutter and Theilacker (1971)found that immature male and female Metamysidopsis elongata increase in body size a t the same rates up to an age of about 30 days. Thereafter, although males moult more frequently, as shown by the data in Fig. 41, they grow at a slower rate, This means that the rate of decay of the growth factors at successive moults is faster in males than in females. The growth factors of newly liberated juveniles of Gastrosaccus vulgaris and Neomysis japonica are close to 20% in Fig. 58. The latter species attains a maximum carapace length of about 3.2 mm in females and, by extrapolation in Fig. 58, growth factors will have decayed to values of 6.3%. Gastrosaccus vulgaris attains a maximum carapace length of about 4.0 mm in the females and the growth factors at this size will be about 3%. This is also true of the growth factors of Leptomysis Eingvura and Hemimysis speluncola in Fig. 59. It is not true of the bathpelagic Gnathophausia ingens (Fig. 58). Excepting G . ingens, these ranges and rates of decay of the growth factors are comparable with those found in other crustaceans ; immediate post-larval moults usually have growth factors in the range 20-26% and these decay to factors of 3-6% near maximum size, normally attained after breeding. Sex ratios within populations of mysids are variable and females frequently outnumber males. Brattegard (1973, 1974a) found that the ratios in 15 species ranged from 1.1 to 5.5 in favour of females; one species Heteromysis elegans had a ratio of 0.9 females t o 1 male. A ratio of 1 : 1 appears to exist in populations of Mysis relicta when combined samples for the year are considered (Larkin, 1948; Reynolds and De

214

THE BIOLOGY OF MYSIDS

Graeve, 1972). The ratio, however, normally varies between samples and between seasons (Kinne, 1951 ; Zatkutskiy, 1970; Brown and Talbot, 1972; Reynolds and De Graeve, 1972). This is illustrated well in Figs 60 and 61 where the changing sex ratios found in samples of species from the west coast of Scotland are shown. No consistent reasons for the observed fluctuations have so far been found. Mortality of females after breeding appears to be related to a subsequent dominance of males in

3L

Erythrops serrafa

2

Mysidopsis gibbosa

Neomysis integer

r

..

I J l F l M l A l M l J I J l A l S 1 0 " I FIG.60. Sex ratios, expressed as the dominance of females above or of males below the hatched line, of species from the west coast of Scotland in different months of the year. Mysidopsis gibbosa :dots, 1964; triangles, 1965. Neomysis integer: dots, Scotland; triangles the Baltic Sea (Kinne, 1951).

some cases. Hakala (1978) found that males died immediately after copulation in Lake Paajarvi, Finland. Sexual differences in the minimum body lengths at which the secondary sexual characteristics become recognizable may be responsible for other fluctuations. Sexual differences in the longevity of males and females and the larger body size attained by females affect sex ratios in populations dominated by larger size groups. This operates in breeding populations in the spring when, in many cases, only mature adults and juveniles of indeterminable sex are present. The greatest imbalance found in the sex ratio of

10. POPULATION

215

DYNAMICS

a population is 19 : 1 in favour of females in Praunus inermis in November (Fig. 61). Wigley and Burns (1971) observed a ratio of 59 : 1 in favour of females in their collections of Mysis mixta, and suggest that the males may live in a different habitat from the females ; no clear segregation of males and females of a species has yet been demonstrated. A geographical difference in the sex ratio occurred in populations of Eucopia unguiculata (hanseni); Casanova (1977) found that the ratio in favour of females was 1-4 : 1 in the eastern Atlantic off North Africa, 2 : 1 in the Gulf of Gascogne and that a ratio of 1 : 1

Praunus tntvmis

,.A,

J F M A M J J A S 0 N D FIO.61. Sex ratios, expressed as in Fig. 60, of Praunw species from the west coast of

Scotland.

was present in the Mediterranean. Ratios in the oceanic species h t h p h a u s i a gigas and G. zoea were 1 : 1, in G. gracilis 1-5 : 1 but in G. ingens they have been found to vary from 1 : 1 to as high as 7 : 1 (Clarke, 1962). Changes in the sex ratio within populations can result from differential mortality of males and females as mentioned above. Such mortality has been assumed to exist during several investigations. It is always possible that, in some species, the males may have moved to a different sector of the environment during the day and most observations of populations are made during daylight hours. Evidence of

216

TEE BIOLOGY OF MYSIDS

differential mortality is shown in Fig. 61 in populations of Praunus species. The number of males, especially in P . Jlesuosus, decrease markedly during the spring breeding period and again in the late summer and autumn breeding period. These two species are restricted to zones of occurrence among seaweeds and the males were not found in other sectors of the environment. They had apparently mated and died shortly afterwards. This was also found t o happen in populations of Mysis relicta in Lake P&&j&rvi by Hakala (1978). Conclusive estimates of the rates of mortality are difficult to obtain at most stages of the life cycle. Mortality of larvae within the marsupium of eight species of Scottish mysids was estimated by counting the larvae a t successive stages of development at all seasons of the year and determining mean values. This is necessary because of the seasonal changes in brood sizes that occur in these species. The mean number of young per brood can change by as much as 50% in a period as short as four weeks (Fig. 13, p. 47). The development of the larvae within the marsupium takes two to three weeks, depending upon the temperature (Table VI, p. 64). Consequently, the numbers of young in a brood of early embryos and in a brood of eyed larvae, found in females present in the same sample, are not necessarily comparable because the brood size at egg laying may have changed since the brood with eyed larvae was produced. The results of the analysis (Table XXXVI) suggest that mortality of larvae within the marsupium is about 7% up to the eyeless stage and about 10% over the entire period of marsupial development. Amaratunga and Corey ( 1 975) present a graph showing declining numbers of successive larval stages within the marsupium of Nysis stenolepis. This species releases its young from the marsupium any time after the fist marsupial moult, when the appendages become free, and not all larvae are released from the marsupium a t the same time. The rate of mortality during the developmental period up to and including the f i s t marsupial moult is about 13%. This marsupial mortality was ascribed to premature escape or loss of larvae from the marsupium by Mauchline (1973b) but this opinion has had to be revised. Wittmann (1978) demonstrated retrieval of such larvae by the female and adoption of those that had escaped from the marsupium of another female. Mauchline and Webster confirmed these results and consequently marsupial mortality may be a result of death during development rather than of escape. Fager and CluMer (1968), studying the population dynamics of Metamysidqsis elongata, made several assumptions regarding rates of mortality a t different stages of the post marsupial life cycle; they then generated life tables for a population of this species in California. Age

TABLEXXXVI. NUMBERS OF DIFFERENT STAGES OF LARVAE PER BROOD (Mauchline, 1973b) Ratios of

Mean numbers f Q Species

"Eggs"

28.3 f 16.5 Leptomysis gracilis My&ahpsisgibbosa 16.6f 7.6 Neomysis integer 31.6 f 14.2 Paramysis arenosa 14.3f 6.3 Praunwr jlexuoswr 28.2 f 9.6 Praunua inermis 23*9*11.8 Schistomysis kerwilbi 12.6f 6.8 Schistomysis spiritw Clyde 11.3+ 4.9 Loch Ewe 12.4f 4.5

Number of broods examined

EyelRss

EYd

28.2 f 14.5 13-9f 6.3 27.8 f 13.2 12.2f 5-4 25.8 f 9.1 22.3f 11-9 12.7 f 6-5

26.5 k 14.9 13.7+ 6.7 27.6 f 13.1 10-6f 5.4 26.5 f 10.2 20.8f 11.0 124k 7.0

161 135 301 133 205 278 177

340 164 298 267 279 427 355

10.7f 5.3 12.4k 5.4

12*3& 6.4 10.0k 5.0

347 212

313 343

eyehsl"eggs"

eyedl"eggs"

94 71 151 86 81 148 140

0.997 0.837 0.880 0.853 0.915 0-933 1.008

0-936 0.825 0.873 0.741 0.940 0.870 1.016

123 147

0-947 1.000

1.089 0.807

Mean 0.930

0.900

218

THE BIOLOGY OF MYSIDS

specific survival curves were drawn (Clutter and Theilacker, 1971) from estimates of the greatest median and least rates of mortality estimated in the population. Comparable mathematical exercises can be accomplished using various blocks of data referred t o in Table XXXIV. Childress and Price (1978) found that mortality was low in the early post-marsupial instars of Gnathophausia ingens but high in the later instars. They compare this situation with the constanti rate of mortality assumed by Fager and Clutter to be present over the life of Metamysidopsis elongata. No detailed examination of the pattern of mortality over the life history of a coastal species has been made. Mortality of larvae during marsupial development is 10-15% (Table XXXVI). The rate of mortality in later instars probably changes but varies irregularly in time and space. Predators must account for a considerable proportion of mortality in some populations and species and not in others. The.impact of predators may be greater on some instars than on others, through size selection of their prey. Predators are discussed in Chapter 12. The numbers of young per brood are frequently as low as 2-8 (Table 111, p. 48), and so indicative of very low rates of mortality within populations, even if several broods are produced. The number of young per brood increases as the cube of the body length (Fig. 15, p. 56) and an increase in adult body size is usually associated with a longer life history. Consequently, it can be argued that these relationships show mortality is proportional to length of life. The sizes of broods, however, of species of the same body length can vary by one order of magnitude (Fig. 15). Further, as Childress and Price (1978) instance, Metamysidopsis elongata of 5.3 mm body length at maturity produces more than 14 broods containing 5130 eggs so having a fecundity of about 340, while Gnathophausia ingens of 151 mm body length produces 350 larvae in a single brood. The former species has a longevity of just over 200 days, while the latter lives for 2950 days. Consequently, a considerable range of strategies will have been adopted within the Mysidaceae to maintain population numbers. The f i s t estimates of rates of production within mysid populations were made by Blegvad (1922) in Praunus species.' He suggested that single breeding pairs of P. inermis, P. flexuosus and P. neglectus in Denmark could produce 1306, 764 and 588 young respectively. A comparable estimate of production by a breeding pair of P. inermis in Scottish waters was 2450 young (Mauchline, 1965). Estimates of the production of populations of Xchistomysis spiritus in a relatively sheltered bay in the Firth of Clyde and a relatively exposed bay in Loch Ewe in Scotland suggested that production was greater in the

10.POPULATION DYNAMICS

219

exposed situation (Mauchline, 1967). A model of production within a population of Paramysis intermedia has been developed by Miroshnichenko and Vovk (1973). They found a monthly P/B ratio fluctuating between 0.3 and 3.0, being highest in autumn. Hakala (1978) found P/B ratios of 3 . 6 3 - 8 in populations of Mysis relicta in Lake Paajarvi, Finland. Higher ratios could probably be calculated over periods of four to six weeks during production of summer generations of littoral species such as Schistomysis spiritus. Shushkina (1973) estimated daily P/B ratios indirectly in a population of Neomysis mirabilis in the Sea of Japan as 0.13-0.17, while Richards and Riley (1963) noted a ratio of 3.66 in populations of N . americuna in Long Island Sound.

CHAPTER

11

GEOGRAPHICAL DISTRIBUTION The zoogeographical distribution of the majority of species of mysids is either unknown or has not been analysed. Some 65% of the known species live in shallow coastal regions while the remaining 35% live offshore in the deep neritic or oceanic pelagic environments. The oceanic species, about 250 in number, consist of species with restricted distributions as well as the deep living cosmopolitan species that occur in all three oceans. The distributions of about 30 of these species have been described and attempts made t o relate them t o the discontinuities of the oceans. Records of the following species are shown on world maps by the following authors : Archaeomysis species by Murano (1977a); Amblyops abbreviata and several Boreomysis species by Bjmtein and Tchindonova (1958); Caesaromysis hispida by Birstein and Tchindonova (1958) and Murano (1977a);Ceratolepis hamata, Chlaraspidum alatum and various Eucopia species by Fage (1942) and Birstein and Tchindonova (1958); Gnathophausia gigas and G. gracilis by Fage (1941) and Birstein and Tchindonova (1958); G. ingens by Fage (1941)and revised by Pequegnat (1965); Longithorax species by Murano (1976); Lophogaster species by Fage (1942); Katerythrops oceanae by Murano (1976); Meterythrops robusta and Pseudomma truncatum by Birstein and Tchindonova (1958); Mysis relicta by Holmquist (1963b, 1966); Neomysis awatschensis, N . intermedia and N . mercedis by Holmquist (1973); Teraterythrops robusta by Murano (1975). It is possible to collate records from the literature and produce distributional maps for many species. The records, however, are incomplete in most cases as can be seen by examining many of the maps cited above. There are regions of the oceans that have not been sampled comprehensively for mysids. The mysid fauna of a sea area is difficult to sample and a variety of techniques and sampling equipment has to be used (Fig. 18, p. 67). Sampling methods are discussed in Chapter 3. The coastal and littoral nature of the habitats of many species have resulted in unrepresentative sampling of this order during the early historical periods of faunal exploration. Mauchline and

11.

GEOGRAPHIC&

DISTRIBUTION

221

Murano (1977) have shown that over a hundred new species are described in each ten-year period since 1950, owing t o the increased interest in coastal marine science, especially in tropical and subtropical regions. Many coastal regions of South America, Africa, Australasia and Oceania have not been sampled comprehensively and will no doubt yield many new species and extend the known distributions of some of the present species. Consequently, the mapping of xecords of species at the present time is of limited value. It is important, however, to attempt to identify areas for further investigation. There is also value in examining each geographical region, that is areas that approximate to faunal regions. A considerable quantity of information on the mysid fauna of several regions is available. The literature describing the occurrence of mysids is scattered and large. Gordan (1957) produced a classified bibliography of the literature available up to 1955 and this is an accessible paper. A considerable saving of space is effected in the present work by not quoting papers listed by Gordan unless they axe critical to the subject being discussed. She classified the literature according to species and also according to subject. The distributional literature was divided into that describing freshwater and brackish water species and that describing oceanic species and coastal species. A post-1955 bibliography is given in Appendices I1 and 111.The papers are classified according t o genus while the geographical classification is under regions but not divided into oceanic on the one hand and coastal on the other. Extraction of the relevant literature on a species, or a sea ar'ea, is achieved by reference to Gordan (1957) and Appendices I1 and 111. These lists contain the sources of the distributional information given for each species by Mauchline and Murano (1977)but not cited by them. No attempt is made here to produce an authoritative account of the distribution of mysids. Much more information is required before this can be accomplished. It is even difficult to list species occurring within broad geographical regions with any degree of confidence. Such provisional lists can be made by reference to Mauchline and Murano (1977) where each species has been ascribed to one or more of the regions shown in Fig. 62. Further information has become available since the World List was produced, including the discovery of new species (Table 11, p. 14). Some mysids occur in freshwater environments and are of interest in mariculture systems as food for fish. The majority of species, however, are divided into two broad categories. The first consists of species occurring in offshore deep neritic, epipelagic, mesopelagic and bathypelagic environments. The second comprises shallow neritic, coastal, littoral and estuarine species. This latter group

1 20°

160°

160°

1 20°

80°

40°

O0

40°

80°

--60°

5

-1-

I

1200

160°

- --- --

160°

Fro. 62. Map of geographical regions uaed in defining the distribution of the species in Table 11, Appendix I11 end Mauchline and Murano (1977).

11.

223

OEOOBAPHICAL DISTRIBUTION

conhins the majority of species, many of which have, on present information, extremely iocalized distributions. The following discussion attempts to amplify the brief information given by Mauchline and Murano. Comments are made on species occurring in freshwater environments and on the distributions of oceanic species. These are followed by brief discussions of the occurrence of species within several geographical regions.

Species occurring in freshwater There are about 25 species of mysids that occur frequently in freshwater lakes and rivers (Table XXXVII) although most of them TAFILEXXXVII. SPECIESTHAT OCCURFREQUENTLY IN FRESHWATER L m s AND RIVERS Specie8

cfeographical region

Antromysis almyra Antromysis anopblinae Antromysis cenotemis Antromysis peckorum Gangemysis aasimilis Dkrnyds bahirensia Diamysis pengoi Hemimysis anomala Katamysis warpmhowskyi Limnomysie benedini

Surinam Costa Rica Mexico Jamaica India South Europe East Europe East Europe East Europe Europe

My& litoralis Mysia relicta Nanomyeia 8 i W E ? W i S Neomysie awatschensb Neomysie intermedia Neomysie ntercedia Paramy& baeri Paramysis intermedia

Circumpolar, Arctic Circumpolar, Arctic Thailand Japan, China N. America, Kurile Is.,Japan W. N. America East Europe East Europe

Paramysb kessleri Paramyaia lacuatria

East Europe East Europe

Paramysis ullakyi

East Europe

Spelaeomysie c a r d i s o m Taphrmysia b o w n i

Caribbean, Peru Florida, Alabama

Taphrmysis l o u i & w

Texas, Louisiana

Referencee Brattegard (1977) Bowman (19778) Bowman (1977a) Bowman (197710) W. M. Tattersall (1908) Holmquist (1955) Dediu (1966) Dediu (1966) Dediu (1966) Dediu (1966); Weish and Tiirkay (1975); Boroditch (1978) Holmquist (1959b, 1963a) Holmquist (1963a, b, 1966) W. M. Tattersall (1921) Holmquist (1973) Holmquist (1973) Holmquist (1973) Dediu (1966) Dediu (1966); Boroditch (1978) Dediu (1966) Dediu (1966); Boroditch (1978) Dediu (1966); Boroditch (1978) Bowman (1977a) Beck (1977); Bowman (19778) Bowman (1977a)

224

THE BIOLOGY O F MYSIDS

occur in brackish regions as well.,There are also hypogean species, some of which live in freshwater, some in brackish and a few in truly marine situations (Table XXXVIII). Gordan (1957) lists some 300 papers describing the occurrence of fresh and brackish water species. The more recent literature is listed in Appendix 111. The possible function of marginal caves connecting the marine and freshwater habitats is discussed fully by Riedl and Ozretic (1969). TDLE

XxxvIII.

Species

HYPOOEAN MYSIDS LIVINGI N Qeographical region

Aniaomysia vaeaeuri Antromysis cenotensis A . cubanica

Madagascar Yucatan Cuba

A . juberthiei

Cuba

Hemimysia apeluncola Heteromyaoidea w t t i Leptmyaia burgii Spchomyaia bottazzii S. longipes

France Canary Is. France Italy India

S. nuniezi

Cuba

S. olivae S. quinterenaia S. servatua Stygiomysia holthuiai

Mexico Mexico Zanzibar Lesser Antilles, Puerto Rico Italy Jamaica Italy Jugoslavia

S. hydruntina S. major Stygiomysia sp. Troglomysis vjetrenicmsia

CAVES OR WELLS

References Ledoyer (1974) Bowman (1977a) Bacescu and Orghidan ( 1971) Bacescu and Orghidan (1977) Ledoyer (1963, 1965) Calman (1932) Bacescu (1966) Pesce (1976a) Pillai and Mariamma (1964) Bacescu and Orghidan (1971) Bowman (1973) Bowman (1973) Ingle (1972) Bowman (1976) Caroli (1937) Bowman (1976) Pesce (1976b) Stammer (1933)

The four species Anisomysis vasseuri, Hemimysis speluncola and Leptomysis burgii (Table XXXVIII) live in truly marine caves (Ledoyer 1963, 1974; Bacescu, 1966). Heteromysoides cotti is recorded from a, cave some 6 km from the coast but in which the water level fluctuates tidally (Calman, 1932). Two species, the Stygiomysis species described by Pesce (1975b) and Spelaeomysis longipes, occur in wells. The other species in the genera Spelaeomysis and Stygiomysis are confined t o brackish and freshwater caves and wells, have reduced eyes and a body

11.

GEOQRAPHICAL DISTRIBUTION

225

of the general form shown in Fig. 3, p. 16. The remaining species named in Table XXXVIII occur in freshwater caves. The most widely distributed freshwater species is Mysis relicta which occurs in the Great Lakes and other lakes of glaciated North America and in the lakes of northern Europe and Greenland. It also occurs in the brackish environment of the Baltic Sea. Holmquist (1959b, 1962, 1963a, b, 1966)has made a detailed study of the distribution of this species in the context of marine glacial relict faunas whose occurrence is discussed in detail by Segerstrkle (1962).A related species, M . Zitoralis (Table XXXVII), is also circumpolar and occurs in freshwater lakes although it more commonly occurs in brackish or marine environments. Four other species of this genus, all closely related to Mysis relicta, occur in the Caspian Sea, as discussed later. The rivers of eastern Europe, for instance the Dniestr, Pruth and Volga, that flow into the Black and Caspian Seas contain several species living in truly freshwater environments. The species, listed in Table XXXVII, are Diamysis bahirensis, D. pengoi, Limnomysis benedini, Hemimysis a n m a l a , Katarnysis warpachowskyi, Paramysis baeri, P. intermedia, P. kessleri, P. lacustris and P . ullskyi. Several of these species have been introduced as food organisms for fish in various eastern European reservoirs and their original detailed distributions are becoming difficult to determine (Mordukhai-Boltovskoi, 1979). A recent study of the routes of colonization of the freshwater habitats of this region by Limnomysis benedini has been made by Weish and Tiirkay (1975). Populations of all these species are also present w i t h the Black and Caspian Seas as described below. Most of the remaining species occurring in fresh water (Table XXXVII) have marine coastal distributions. They are listed because they have been recorded on a t least some occasions from truly freshwater situations. Other species such as Mesopodopsis orientalis and Tenagomysis chiltoni in India, M . slabberi and Neomysis integer in Europe have also been recorded from freshwater situations but usually there are obvious routes of carriage from adjacent marine habitats. Neomysis integer is often present in freshwater streams a t low tide but its environment becomes brackish at high tide; other estuarine species also undergo these fluctuations. It has been recorded, however, from freshwater lochs immediately adjacent t o the sea in western and northern Scotland (Scott, 1891a, 1894) but there are no recent records of such occurrences. Further investigation of the distribution of estuarine species and of freshwater habitats adjacent to the coast may increase the list of species with resident populations in truly freshwater environments.

226

THE BIOLOGY OF MYSIDS

Oceanic species These species live in the offshore deep neritic, epipelagic, mesopelagic and bathypelagic environments. Some of those listed by Mauchline and Murano as bathypelagic and others, for example many species in the genus Hansenomysis, ascribed a depth of occurrence may be hyperbenthic, living in close association with the sediments. There are relatively few cosmopolitan species occurring in the Atlantic, Pacific and Indian Oceans. Examples are : Anchialina typica Eucopia sculpticauda Arachnomysis leukartii Eucopia unguiculata Caesaromysis hispida Gnathophausiagigm Echinomysis chuni Cnathophausia gracilis Euchaetomera glyphidophthalmica Gnathophausia ingens Euchaetomera tenuis Gnathophausia zoea Euchaetomera typica Katerythrops oceanae Euchaetomeropsis merolepis Meterythrops picta Eucopia australis Petalophthalmus armiger Eucopia grimaldii Siriella thompsonii These are all meso- and bathypelagic species with the exception of Anchialina typicu and Siriella thompsonii, which are both epipelagic in habit. There are about 12 species that live in both the Atlantic and Pacific Oceans but have not so far been recorded from the Indian Ocean. These include five species of the genus Boreomysis. It is surprising. that at least some species in this genus have not been recorded as cosmopolitan because only one of the 35 species in it, B. insolita, is neither meso- nor bathypelagic in distribution. There are some 30 species living in the Pacific and Indian Oceans but about half of these are shallow living, occurring in at least the western Pacific and eastern Indian Ocean. There are, in addition to these offshore species, a further group of coastal and littoral species occurring across the Australasian region and living in both oceans. Only four species have been recorded exclusively from the Atlantic and Indian Oceans; they are Boreomysis illigi , Euchaetomera intermedia, Longithorax capemis and Lophogaster typicw. The remaining offshore species are confined, on present information, t o one ocean. There are about 70 species restricted t o the Atlantic, about 40 of which occur only in the North Atlantic, north of about 40"N. Some 110 species occur only in the Pacific, 60 of them recorded exclusively from the western Pacific, principally through the work of Ii and Murano. The Indian Ocean has 11 species that have not been recorded elsewhere and there are a few species endemic to i;he Red Sea.

11.

OEOORAPHICAL DISTRIBUTION

227

A detailed examination of the regions of occurrence of several species shown by Mauchline and Murano (1977), raises a number of questions. Parapseudomma calloplura occurs in the north-eastern Atlantic and Mediterranean but has also been recorded by Murano off Japan. Further sampling, and in some cases taxonomic revision of species, may alter the known distributions of many offshore species quite markedly. For example, the genus Hansenomysis consists at present of 15 species, all oceanic, and mostly living in the bathypelagic or hadal environments. The Peru Trench contains seven species, the KurileKamchatka Trench three species, the Antarctic three species and one from the equaeorial Pacific (Mauchline and Murano, 1977). They rarely occur in samples, probably because they are hyperbenthic a%depths of 2000m or more; the Antarctic species of this genus live at much shallower depths, 100-500 m. More frequent sampling at depths greater than 2000 m with a variety of samplers may show that these species are much more widely distributed than is indicated a t present. Biogeographic studies of the distributions of mysids have been neglected and little of value can be gained by extending the present discussion. Much further information is required, as will become clear during the following examination of the mysid faunas of the various oceanic and coastal regions shown in Fig. 62, p. 222. The boundaries of the regions are only approximate and many species cross them in coastal regions. These boundaries, however, are used in Appendix I11 to classify the literature on the distribution of the species. Arctic Ocean Few species occur in the Arctic Ocean, Geiger (1969) and Shih and Laubitz (1978) recording the following:

Boreomysis nobilis Michthyops theeli Mysis litoralis Mysis oculata

Mysis polaris Pseudomma frigidurn Parerythrops spectabilis Meterythrops robusta

There are, however, a much larger numbers of species living south of latitude 60"N, some of which intrude into the Arctic Ocean regularly or sporadically. Some of these have colonized coastal or offshore regions of northern Greenland, northern Europe and northern Alaska and Canada. Such species are:

Acanthomysis pseudomacropsis Amblyops abbreviata Amblyopsoides ohlinii

Boreomysis arctica Boreomysis inermis Erythrops abyssorum

228

THE BIOLOGY OF MYSIDS

Erythrops erythroph#halmu Erythrops glacialis Meterythrops robusta Mysis mixta Mysis relicta

Neomysis intermedia Neomysis rayii Pseudomma truncatum Stilomysis grandis

Sampling for mysids north of 60"Nin both the Atlantic and Pacific Oceans has been sporadic. Little is known about the biology of species living in these high latitudes. Several of the southern guests to this region may be expatriate and their breeding inhibited by the prevailing temperature regimes.

North Atlantic, between about 60"N and 40"N The most comprehensive accounts of the species occurring in this region are those of W. Tattersall (1951) and Tattersall and Tattersall (1951). Other papers are listed in Appendix 111. The species are too numerous to list, being more than 100, some 40 of which are restricted to this region. Faunal lists can be made from the publications of the Tattersalls and from Mauchline and Murano (1977). There are faunal differences between the eastern and western coastal areas of this region. The following species are confined to the western Atlantic :

Boreomysis nobilis Meterythrops robusta ,Mysidetes farrani Mysis gaspensis

Mysis stenolepis Neomysis americana Pseudomma truncatum Stilomysis grandis

Several species such as Mysidopsis bigelowi can occur as southern guests in the coastal regions of the northeastern United States and of Canada ; there are many species endemic to the middle latitudes of the western Atlantic. The 40"N latitude appears to be an approximate faunal boundary for mysids in the western Atlantic but the corresponding boundary in the eastern Atlantic appears t o be south of the entrance to the Mediterranean, a t about 30"N. I n the eastern Atlantic, coastal areas are apparently richer in species than comparable areas in the western Atlantic but this may be an artifact arising from the more comprehensive sampling of workers such as the Tattersalls. There are about 50 species that are restricted to the eastern Atlantic, some 30 of these having distributions that extend southwards into the Mediterranean. Information on the occurrence of these species is given by Tattersall and Tattersall (1951) and Mauchline and Murano (1977). Most prominent among these endemic species are those of Praunus, Paramysis and Schistomysis.

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All species in these genera are restricted to this area, the Baltic, Mediterranean, Black and Caspian Seas. Some species occur on both Eides of the northern Atlantic but most of these are offshore species, such as Amblyops abbreviata, A. kempi, Boreomysis arctica, B. microps, B. tridens, Erythrops abyssorum, E . erythrophthalma, E. microps, Eucopia grimaldii, Cnathophausia gigas, C.zoea, Parerythrops obesa. There are, however, also the following that occur in coastal regions:

Heteromysis formosa Mysis litoralis Mysis mixta

Mysis oculata Pseudomma afine Pseudomma roseum

A further European species, Praunus jlexuosus, occurs in the Bay of Fundy and the coasts of Nova Scotia. This species was introduced to this coastline from Europe and is now spreading rapidly (Wigley, 1963; Wigley and Burns, 1971). The Baltic Sea The Baltic is a large, shallow, predominantly brackish water appendage of the north-eastern Atlantic. Its western areas are penetrated by several euryhaline species, but in relatively smell numbers compared with their incidence in the adjacent North Sea. These immiqant species are :

Erythrops elegans Hemimysis lamornae Leptomysis gracilis Mysidopsis gibbosa Praunus inermis

Praunus neglectus Praunus jlexuosus Schistomysis ornata Schistomysis spiritus

The dominant resident species within the western Baltic, according to Hesthagen (1973) and Boysen (1976), are:

Qmtrosaccus spinif er Mesopodopsis slabberi

Mysis mixta Neomysis integer

Of these, only Mysis rnixta and Neomysis integer penetrate the northern and inner regions of the Baltic and there they occur along with Mysis relicta. Ackefors (1969) states that M . mixta is common throughout the Baltic proper and especially in the deeper areas. Mysis relicta is a dominant organism among the hyperfauna of the northern regions of Bothnian Bay, while M . mixta occurs further south in the Quark (Brtgge and Ilus, 1973). Ackefors, quoting earlier workers, states that

230

THE BIOLOUY OF MYSIDS

M . relicta is caught i~ Bothnian Bay at depths between 70 and 150 m and may be pelagic by both day and night. It does not occur in areas where salinities are greater than 12%,. One other species, Praunus inermis, has been recorded from the Gulf of Riga and the entrance to Bothnian Bay (Zmudzinski, 1962). The Caspian Sea species, Paramysis lacustris, used as a food organism in the farming of fish, is reported by Zhuravel(l972) to have entered the Baltic via the Newman River in Lithuania into which it had been transplanted from the Caspian. This species may now be in the process of becoming established within the Baltic where the conditions will favour it. AtZantic Ocean, 40"N to 40"s This vast region can be divided into several areas. Starting in the western Atlantic, the first approximat-e faunal area extends from about 40"N off t,he American coastline southwards into the Gulf of Mexico. Tattersall (1951) and Wigley and Burns (1971) describe the species occurring in it. A second area encompasses the Caribbean and possibly northern regions of the South American continent. It is very rich in species as detailed in papers by Bacescu, Bowman and Brattegard. Conversely, sampling, especially in the Caribbean coastal regions, has been extensive and Brattegard has described some 30 new species endemic to this area and the adjacent coasts of Florida, Mexico, Panama and Colombia. A further faunal area possibly exists on the eastern coastline of South America, from about the equator southwards to 40"s. Almeida Prado details the results of sampling a small area of the Brazilian coast, as does Silva. The species recorded are few; one, Metamysidopsis elongata attantica is a form of the eastern Pacific species M . elongata. Much further sampling off this coastline is required before the affinities of its mysid fauna can be discussed. I n the eastern Atlantic, as stated earlier, the northern fauna intrudes southwards to the entrance to the Mediterranean and probably to about 30"N on the African coastline. The mysids of the western African coastline have probably been incompletely sampled. Information for certain areas is available in papers by Bacescu, 0. S. Tattersall and in Grindley and Penrith ( 1 965) and Wooldridge ( 1977). As in the case of the South American coastline, much further sampling is required. The western and eastern parts of this region each contain over 100 species. About 70 of the species recorded in the western area do not occur in the eastern area while some 40 of the eastern species have not been recorded in the west. This probably reflects the variety of

11.

QEOQRAPHIOAL DISTRLBUTION

231

habitats available in the western area but it may also have resulted from more comprehensive sampling of parts of the western area, e.g. the Caribbean. Lists of species can be drawn up for local areas by reference to the literature in Appendix I11 and by recourse to the papers cited above.

The Mediterranean Sea The mysid fauna of the Mediterranean consists of endemic species and species that also occur in the north-eastern Atlantic. A list of more than 30 Atlantic species can be made, by reference to Mauchline and Murano (1977), that also occur in the western Mediterranean; these species, however, do not in most cases penetrate the eastern Mediterranean. The following species, on the other hand, occur in the northeastern AtIantic and throughout the length of the Mediterranean :

Qastrosaccus normani Gastrosaccus sanctus Gastrosaccw spinifer Hemimysis lamornae

Mesopohpsis slabberi Siriella clausii Siriella jaltensis

There are more than 20 species endemic to the Mediterranean.

Anchialina sanzoi, Kainomatomysis foxi and Schistomysis assimilis are only known from the eastern Mediterranean and the other species are :

Anchialina oculala Calyptomma puritani Erythrops peterdohrni Euchaetomera richardi Gmtrosaccus magnilobatus Gastrosaccus mediterraneus Haplostylus bacescui Heteromysis eideri Heteromysis lybiana Leptomysis burgii

Leptomysis peresi Mysideis parva Neoheteromysis mf.klleri Paramysis festae Paramysis helleri Parerythrops lobiancoi Pseudomma chattoni Rhopalophthalmus mediterraneus Siriella adriatica

The Black Sea Paramysis proconnesia appears to be endemic to the Bosphorus. Salinity in the Black Sea ranges, on average, from about 18 to 23%,. Nine Mediterranean species penetrate the Black Sea. Diamysis bahirensis Qastrosaccus normani

Gastrosaccus sanctus Qmtrosaccusspinifer

232

THE BIOLOGY OF MYSIDS

Hemimysis lamornae Leptomysis lingvura Mesopodopsis slabberi

Siriella clausii Siriella jaltensis

All these species, with the exception of Diamysis bahirensis, are widely distributed in the coastal regions of Europe, occurring throughout the Mediterranean, the adjacent region of the Atlantic northwards t o British waters, including the North Sea and, with the possible exception of Leptomysis lingvura, the Baltic Sea. The following six species are endemic t o the Black Sea although Diamysis pengoi also occurs in the rivers flowing into the Black Sea, as mentioned previously :

Diamysis pengoi Hemimysis serrata Paramysis agigensis

Paramysis kroyeri Paramysis kosswigi Paramysis pontica

The Black Sea also shares the following species with the Caspian Sea : Acanthomysis strauchi Paramysis intermedia Hemimysis anomala Paramysis lacustris Katamysis warpachowskyi Paramysis ullskyii Limnomysis benedini

The Caspian Sea The seven species listed above occur in both the Caspian and Black Seas. There are, however, an additional 15 endemic species living in this brackish environment with an average salinity of 13%,. Acanthomysis grimmi Acanthomysis inflata Caspiomysis knipowitschi Diamysis pusilla Paramysis baeri Paramysis bakuensis Paramysis incerta Paramysis kessleri

Paramysis loxolepis Paramysis eurylepis Schistomysis elegans Mysis amblyops Mysis caspia Mysis mawolepis Mysis microphthalma

The four Mysis species are closely related t o the freshwater Mysis relicta. Maximum depths in the Caspian Sea are about 950m. Mysis amblyops, M . microphthalmu and Paramysis loxolepie have bathymetric ranges of about 200-900 m. Mysis cuspia and M . mawolepis occupy the 100-400 m bathymetric zone while the other species inhabit the shallower coastal and littoral areas.

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233

The Pacific Ocean The earlier distributional literature on Pacific species is listed by Gordan (1957). The most valuable modern works are those of Bacescu, Birstein and Tchindonova, Hodge, Ii, Murano and Wing. The other relevant literature is listed in Appendix 111. Large areas of the Pacific have not been effectively sampled for mysids. This is especially true of the littoral and other shallow environments of the eastern Pacific stlin lines, of Ocean& and the Aushrahsian region. This is reaected i n the following brief description.

North Pacijic, north of about 4O"N The following species were recorded in the Chukchi Sea by Wing and Barr (1977): Acanthomysis pseudomacropsis Acanthomysis sp. Mysis litoralis Mysis oculata

Mysis polaris Neomysis rayii Pseudomma truncatum

Species occurring in the Bering Sea, the Aleutians and southwards to about 40"N number about 60. Knowledge of the distribution of many of these species is incomplete because the littoral habitats of the coasts of Aleutians and western Bering Sea have not been investigated in detail. Ii (1964a) lists nine species occurring in the Sea of Okhotsk, ten species in the Sea of Japan. A list of some 20 species recorded from the Kurile-Kamchatka Trench and the region to the south and west of it, but not found in the north-eastern Pacific, can be obtained from Mauchline and Murano (1977).

The Pacific Ocean between about 40"N and 40"s The north-western part of this region has been investigated in considerable detail by Ii and Murano. There are about 140 species recorded from the region of Japan, the Philippines and the East and South China Seas. These are fully documented by Ii (1964a) and in the papers of Murano. I n the eastern part of this region, the following species, several of them littoral in habit, have only been found on or close to the American coastline : Acanthomysis costata Acanthomysis macropsis

Acanthomysis sculpta Arachnomysis megalops

234

~

THE BIOLOGY OF MYSIDS

E'uchaetornera plebeja Gnathophausia scapularis Heteroerythrops purpura Heteromysis panamaensis Metamysidopsis elongata Metamysidopsis paci$ca

Mysidopsis californica Neomysis mercedis Scolamblyops oculospinum Siriella pacijica Siriella panamensis Siriella roosevelti

Several species have been found by Bacescu (1967, 19718, b) in the hadal environment of the Peru Trench but have not been recorded from elsewhere as yet. They are:

Amblyops ewingi Hansenomysis birsteini Hansenomysis chini Hansenomysis lucifugus Hansenomysis menziesi Hansenomysis peruvianus

Hansenomysis spenceri Hansenomysis tattersallae Iiansenomysis tropicalis Mysidetes peruana Mysimenxies hadalis

Mauchline and Murano (1977) list some 16 species recorded from the central areas of the Pacific-Hawaii, the Gilbert Islands and other regions of Oceania. Further sampling of the shallow environments of these regions will probably produce many more, and probably new, species. Similarly, many coastal regions of Australia have not been investigated although some 40 species are at present recorded from its coastlines. The mysid fauna of New Zealand is dominated by species of the genus Tenagomysis, although others are also present. The study of occurrence and distribution of mysids throughout the Pacific Ocean is a t a very early stage and systematic investigations over bhe greater part of the Ocean are required.

The Indian Ocean north of about 40"N Regions of the coasts of East Africa and Madagascar have been investigated by Nouvel and Bacescu while Pillai provides much information on the Indian species. The distribution of species within the Indian Ocean is not well known and many littoral areas, off mainlands and islands, have been poorly, if at all, investigated. Faunal lists can be made for several areas but how representative they are of the actual list of species present in these areas is questionable. Much more systematic sampling is required over the greater part of the region with particular attention to inshore environments.

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QEOGRAPIIICAL DISTRIBUTION

235

The Red Sea There axe some 20 species that intrude from the Indian Ocean into the Red Sea. These can be obtained from Mauchline and Murano (1977). In addition, the following species appear to be endemic to the Red Sea : Anisomysis levi Paralophogaster i n t e r m d i m Gastrosaccus erythraeus Paralophogaster macrops Gymnerythrops microps Paralophogaster microps Heteromysis digitata Paralophogaster sanzoi Heteromysis gerlachei Pseudan'chialina erythraea Heteromysis kossmunni Siriella dolljksi Idiomysis tsurnamali Siriella serrata Kainomatomysis schieckei Antarctic Ocean Tattersall (1955) describes the species of the Antarctic in considerable detail. She lists eleven species as occurring in the Straits of Magellan; a further 22 occur between 40"s and 60"s (Mauchline and Murano, 1977). The following species penetrate or live permanently in latitudes higher than SO'S : Amblyops antarctica Mysidetes brachytepis Amblyops tuttersalli Mysidetes dimorpha Amblyopsoides crozetii Mpidetes hanseni Antarctomysis maxima Nysidetes microps Antarctomysis ohlinii Mysidetes p o s t h n Boreomysis brucei Paramblyops brevirostris Boreomysis inermis Pseudomma antarcticum Dactylamblyops hodgsoni Pseudomma armatum Eucopia australis Pseudomma belgicae Hansenomysis angusticauda Pseudomma sarsii Hansenomysis antarctica Pseudomma schollaertensis Mysidetes antarctica The above brief account is sufficient to emphasize the lack of information on the geographical faunas of mysids. All faunas are very poorly known with the exception of those of the north-east and north-west Atlantic, Caribbean, Mediterranean and Japanese regions. Many species occur in shallow areas of one to a few metres dept,h. These areas are too shallow to be included within an offshore sampling programme and too deep to be sampled from the shore. This habitat is normally sampled by towing small 0.5 rn diameter nets from boats with outboard engines.

CHAPTER 12

PREDATORS A N D PARASITES Mysids occur widely throughout the marine environment but the most marked concentrations undoubtedly occur in coastal regions. They are, therefore, a potential food resource for coastal fish. This must be especially true of the species of fish that live among the rocks and algal beds in shallow regions where many species of mysids form dense concentrations. Many of these species of fish, however, are of little economic interest and consequently, in many cases, their feeding habits are unknown. Many commercially exploited species of fish migrate into these shallower regions to breed, or for other purposes, and while present there, may feed on the mysids. Several mysids, such as some of those in the genera Leptomysis, Mysidopsis, Neomysis and Schistomysis are known t o form shoals in areas of the continental shelves and are' consequently a potential food resource for a wider range of predatory fish. An epipelagic species such its Pseudanchialina pusilla, which is common in the tropical and subtropical regions of the Indian Ocean, may be preyed upon by the planktivorous fish. Unfortunately, mysids bear a certain resemblance to euphausiids which normally outnumber them in offshore pelagic samples. The mysids tend to be over-looked in such samples and this is probably even more true when examining the stomach contents of pelagic fish. This, in part, may account for the present lack of information on the predation of mysids by planktivorous fish. Miller (1970) found that all age groups of the flathead sole preyed to a considerable extent upon Neomysis kadiakensis but this situation seems to be exceptional among the species of fish so far investigated. The size of prey organism eaten by a fish is often related t o the body size of the fish. Tyler (1972) found that Mysis stenolepis was consumed more frequently by white hake of body length less than 26 cm than by larger fish. Young herring fed on a variety of mysids whereas the adults only ate them occasionally (Hardy, 1924). The olive rockfish studied by Hobson and Chess (1976) preyed most heavily on mysids during the late juvenile period of its life history. Further, many fish perform an ontogenetic migration, the young developing in coastal

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PREDATORS AND PARASITES

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environments while the adults live in more offshore habitats. The European plaice is an example. It has nursery grounds located in sandy coastal bays that are generally inhabited by an assemblage of mysids, often dominated by species in the genera Schistomysis and Paramysis. Edwards and Steele (1968), found that 0-group plaice in one such nursery area of western Scotland consumed Schistomysis spiritus but their diet changed as they developed. This situation could result in predation pressure on the mysids changing seasonally in middle and high latitudes. Moore and Moore (1976) examined the basis of food selection by the flounder and concluded that it depended upon several factors. These included the spatial distribution of the prey, its degree of concealment and its ability to escape and also a conditioning of the fish for certain foods. These factors introduce a further set of variables influencing predation pressure on an individual species of mysid. Moore and Moore concluded that Neomysis integer and Praunus jlexuosus would be quite easily caught by the flounders. The latter species of mysid remains stationary on the surface of the sediment for considerable periods of time and its colouration, or lack of it, may protect it from detection by the fish. The!importance of cryptic coloTation 'in mysids has already been discussed. Neomysis integer occurs frequently in swarms and as Clutter (1969) has pointed out, swarming probably confers protection from predation. This appears to be confirmed by the observations of Hobson and Chess (1978) on swarms of mysids associated with coral reefs. They were surprised that only token numbers of mysids, that occurred in high densities in the water column over the reefs during the day, occurred in the stomachs of planktivorous fish. The information on predation of mysids is widely scattered, and difficult to locate because, of course, the papers are not predominantly about mysids but about one or more of their predators. Consequently, the following account can only be taken as an indication of the widespread importance of mysids in the marine food chains, especially those of coastal regions. Freshwater species of mysids are often introduced into lakes as food for the resident populations of fish, a topic that will be discussed later. Mysis reZictu forms a major component of the diet of several freshwater fish, as detailed in the works of Langford (1938b), Hacker (1956), Rawson (1961), Wells and Beeton (1963), Sparrow et aZ. (1964), Dryer et al. (1965), Grimas and Nilsson (1965), Morswell and Norden (1968), Just (1970), Westin (1970), Northcote (1972, 1973), Reynolds and De Graeve (1972), Nikolaev et al. (1973), Johnson (1975) and Savolainen (1975). Another freshwater mysid, Tenagomysis nowaezealundiae, is

238

THE BIOLOGY O F MYSIDS

eaten by the European perch, Percafluviatilis L., in the Selwyn River, New Zealand (Griffiths, 1976). Epipelagic marine mysids have not been identified among the stomach contents of whales although, as mentioned above, they may have been overlooked among the much more dominant euphausiids that are similar in body form and size. It is, however, more probable that the swarms formed by the mysids are much too small and widely scattered to merit attention by the whales. Nemoto (1959) recorded a young Gnathophausia gigas among the stomach contents of a fin whale caught in the north-eastern region of the Aleutians. Seals, however, apparently feed on mysids to some extent and they have a wider range of swarming species accessible t o them. McLaren (1958) and Dunbar (1941, 1942) found that Mysis oculata is an important element of the diet of the Arctic ringed seal, Phoca hispida Schreber; M . mixta was eaten occasionally. Sergeant (1973) and Dunbar (1942) noted that the harp seal, Pagophilus groenlandicus Erxleben, feeds to a small extent on Mysis oculata and M . mixta. In the Antarctic, the Weddell seal consumes Antarctornysis maxima and A . ohlinii but to what extent remains unknown (0.S. Tattersall, 1965b). Mysids have been recorded in the diets of many species of marine fish. These, however, fall into two broad categories, namely offshore and coastal species. Information on the predation of fish upon offshore species of mysids is lacking. Oceanic species, including bathy-pelagic species, appear to form a minor component of the diet of some planktivorous and deeper living fish. Merrett and Roe (1974), examining the diets of a few mesopelagic species of fish, recorded one unidentified mysid in the stomach contents of the myctophid Lampanyctw cuprarius Tbning as did Clarke (1978) in the diet of L. nobizis off Hawaii. Dragovich (1969) found that the albacore, Thunnus ( G e r m ) alalunga Bonnaterre and the blackfin tuna, Thunnus atlanticw (Lesson), occasionally ate Gnathophausia ingens. I n a later paper (1971) he recorded mysids in the stomachs of yellowfin tuna, ThunPzus (Neothunnus) albacares (Bonnaterre), and the skipjack, Euthynnus (Katsuwonus) pelamis (L.). Macrourid fish are also known to prey upon mysids. Haedrich and Henderson (1 974) found Boreomysis species present among the stomach contents of the benthic rattail Nematonurus armatus (Hector) in the western Atlantic while Pearcy and Ambler (1974) recorded small mysids and Gnathophausia gigas in the food of this species and 'of Chalinura leptolepis (Gunther)iand Coryphaenoides $lifer (Gilbert) in the eastern Pacific. A more recent study by McLellan (1977) lists eleven macrourid fish that include mysids in their diets. Sedberry and Musick (1978) examined the stomach contents of several species of

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fish of the continental slope and rise off the Atlantic coast of the United States. Michthyops parva was eaten by the notacanthid Hatosauropsis macrochir (Giinther), Boreomysis species by the halosaur species of the genus Aldrovandia Goode and Bean, and Boreomysis arctica, B . tridens and Pseudommu roseum by the gadoid longfin hake, Phycis chesteri Goode and Bean. So far, the fish appear to be relatively minor predators of the oceanic mysids but further information is required. Much more information is available about predation of coastal mysids by fish. The most convenient way of examining such records is on a geographical basis because this emphasizes areas and regions where further information is required. The regions used are those shown in Fig. 62. The nomenclature followed for species of fish is that of Bailey et al. (1970) for American species and Hureau and Monod (1973) for north-eastern Atlantic and Mediterranean species.

Arctic Ocean Little information is available on the predation of Arctic mysids by fish. Ponomarenko (1976, 1978) did not identify the species but found that up to 36% by weight of the stomach contents of 0-group cod in the Barents Sea consisted ,of mysids. Herring around Iceland occasionally eat Mysis mixta according to Jespersen (1932).

North Atlantic, between about 60"N and BOON. The following fish of the north-eastern Atlantic include the named mysids in their diet. The list was compiled from the following : T. Scott (19OOa, 1902b), Ray (1914), Todd (1914), Paulsen (1918), Lebour (1919, 1921), Anonymous (1920), A. Scott (1922), Hardy (1924), Savage (1931), Hartley (1940), Redecke (1941), Liao (1951), Tattersall and Tattersall (1951), Jones (1954), Nagabhushanam (1965), Edwards and Steele (1968), Deniel (1974), Kislalioglu and Gibson (1976a), Lande (1976) and Moore and Moore (1976).

Squalus acanthias L., spur dog: Gastrosaccus spinifer. Raja (Raja) brachyura Lafont, blonde ray : Gastrosaccus spinifer. Raja (Raja) clavata L., thornback ray : Gastrosaccus spinifer, other species. Raja (Leucoraja)fullonica I+, shagreen ray: GastrosacczLs spinifer. Raja (Raja) montagui Fowler, spotted ray: Gastrosaccus spinifer. Raja (Amblyraja)radiata Donovan, stamy ray : Boreomysis tridens. Raja (Leucoraja) naevus Miiller and Henle, cuckoo ray : mysids. Raja (Leucoraja) circularis Couch, sandy ray : Erythrops sp.

240

THE BIOLOGY OF MYSIDS

Clupea harengus harengus L., herring : Gastrosaccus sanctus, G. spinifer, Leptomysis gracilis, Mesopodopsis slabberi, Mysis mixta, Neomysis integer, Praunus Jle.xuosus, P. inermis, P. neglectus, Schistomysis ornata, 8. spiritus, Siriella clausii. Sprattus sprattus sprattus (L.), sprat : Mesopodopsis slabberi, M'ysis species, Neomysis integer. Salmo salar L., salmon: Mysis oculata. Salmo trutta trutta L., trout : Neomysis integer. Osmerus eperlanus (L.), smelt : Mysis relicta, Neomysis integer. Syagnathus acus L., great pipefish: young mysids, Neomysis integer. Entelurus aequoreus (L.), snake pipefish : young mysids. Gasterosteus aculeatus L., three spined stickleback : mysids. Spinachia spinachia (L.), fifteen spined stickleback : Neomysis integer, Praunus jlexuosus . Gadus morhua morhm L., cod : Boreomysis arctica, Gastrosaccus species, Mysis rnixta, Jf. oculata, Neornysis integer, Praunus jlexuosus, P. inermis, Schistomysis ornata, S . spiritus, Siriella armata, S . jaltensis, other species. Melanogrammus aeglejnus (L.), haddock : Boreomysis arctica, Erythrops serrata, Gastrosaccus spinifer. Merlangus merlangus merlangus L., whiting : Erythrops elegans, E . erythrophthalma, Gastrosaccus spinifer, Heteromysis formosa, Leptom ysis gracilis, Neomysis integer, Praunus Jlexuosus, P. inermis, Schistomysis ornata, S. spiritus. Pollachius pollachius (L.), pollack: Neomysis integer, Praunus JEexuosus, Schistomysis spiritus, S. ornata. Pollachius virens (L.), saithe : Praunus inermis, Schistomysis ornata, S . spiritus, other species. Trisopterus minutus minutus (L.), poor cod : Erythrops elegans, Praunus Jlexuosus, Schistomysis ornata. Trisopterw esmarki (Nilsson), Norway pout : Erythrops sp., Gastrosaccus spinifer. Trisopterus luscus (L.),bib : Gastrosaccus spinifer, Neomysis integer, Praunus jlexuosus, P. inermis, Schistomysis ornata. Molva molva (L.), ling : Erythrops erythrophthalma. Rhinonemus cimbrius (L.), four bearded rockling : Erythrops erythrophthalma. Gaidropsams mediterraneus (L.), shore ,rockling: Erythrops erythrophthalma. Dicentrarchus labrax (L.), sea, bass : Neomysis integer, Praunus Jlexuosus, Schistornysis ornata.

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Trachurus trachurus (L.), horse mackerel : Mesopodopsis slabberi, Neomysis integer. Boops boops (L.), bogue : Mesopodopsis slabberi. Trachinus vipera Cuvier, lesser weaver : Praunus inermis, Schistomysis spiritus, other species. Swmber (Scomber) scombrus L., mackerel ; Mesopodopsis slabberi, Mysis sp. Gobius niger L., black goby: Praunus jlexuosus. Pomatoschistus microps (Kreryer), common goby : mysids. Pomatoschistus minutus (Pallas), common goby : Neomysis integer, Praunus jlexuosus, Schietomysis ornata. Callionymus lyra L., dragonet : mysids, Neomysis integer. Pholius gunnel1,s (L.),butterfish : mysids. Trigla lucerna L., yellow gurnard 1 Gastrosaccus spinifer, Schistomysis ornata, 8. spiritus. Eutrigla gurnardus (L.), grey gurnard : Erythrops elegans, Neomysis integer, Schistomysis ornata, S. spiritus. Myoxocephalus scorpius scorpius (L.), fatherlasher : Mysis mixta, Praunus jlexuosus. Agonus cataphractus (L.), hook-nose ; Praunus jlexuosus, Xchistomysis ornata. Scophthalmus rhombus (L.), brill ; Neomysis integer. Lepidorhombus whifiagonis (Walbaum), megrim : Erythrops serrata. Psetta maxima (L.), turbot : Gastrosaccus sanctus, Mysis mixta, Paramysis arenosa, Schistomysis spiritus. Arnoglossus laterna (Walbaum), scaldfish : mysids. Pleuronectes platessa L., plaice : Gastrosaccus spinifer, Mesopodopsis slabberi, Neomysis integer, Praunus jlexuosus, P. inermis, Schistomysis ornata, S. spiritus, other species. Glyptocephalus cynogbssus (L.), witch : Mysis mixta, Mysis species. Hippoglossoides platessoides (Fabr.), long rough dab : Erythrops erythrophthalma, Leptomysis lingvura, L. gracilis, Schistomysis ornata, other species. Platichthys jlesus jlesus (L.),flounder : Mesopodopsis slabberi, Mysis mixta, Neomysis integer, Praunus jlexuosus, Schistomysis ornata. Limanda limanda (L.), dab ; Neomysis integer, Praunus JIexuosus, Schistomysis ornata. Solea Eascaris (Risso), sand sole : mysids. The mysids listed above are the commonest species in the northeastern Atlantic with the possible exceptions of Erythrops elegans, which is sometimes locally abundant and Heteromysis formosa, which

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has never been caught in large numbers. Gastrosaccus spinifer is widely distributed, but may be ineffectively sampled because conventional pelagic or hyperbenthic sledges sre usually employed to sample this species which is in close association with the sediments. It is preyed upon by the epibenthic rays and also by other fish, presumably when it has moved out of and off the bottom and is temporarily pelagic. ,The extent of the predation of any one of these fish species on any one of the mysid species is unknown. I n the Baltic Sea, adult cod feed to a small extent on Gastrosaccus spinifer and Mysis mixta while the flounder consumes some Gastrosaccus spinifer according to Arntz (1978a, b). The early work of Jespersen (1928, 1936)showed that herring around Bornholm fed on Mesopodopsis slabberi, Mysis mixta and Praunus inermis. Mysids were more important however, in the diet of populations of herring near the mouth of the Baltic, around the Danish coasts ; the species eaten were : Gastrosaccus spinifer, Leptomysis gracilis, Mesopodopsis stabberi, Mysis mixta, Neomysis integer, Praunus Jlexuosus, P. inermis, P . neglectus, Schistomysis ornata and S . spiritus. Information on the predatory fish that eat mysids in the northwestern Atlantic is scarce. Brunel (1963b) and Jolicoeur and Brunel ( 1 966) found that cod in the Gulf of St Lawrence feed to some extent on Mysis mixta, while F'itt (1973) states that American plaice, Hippoglossoides platessoides (Fabr.), feed occasionally on mysids on the Grand Banks of Newfoundland. Tattersall (1951)records Mysis stenolepis from the stomach of smelt, Osmerus mordax (Mitchill) off New Brunswick. The white hake, Urophycis tenuis (Mitchill), the longhorn sculpin, Myoxocephalus octodecemspinosus (Mitchill), and the American plaice eat mysids in Passamaquoddy Bay (probably H y s i s stenolepis) (Tyler, 1972). Much more information is available on the predation of mysids whose centres of distribution are south of Cape Cod and is detailed below.

Atlantic Ocean between 40"N and 40"5 The fish that prey upon mysids of the western part of this region will be discussed first. One of the most widespread and common mysids of the eastern coastline of the United States is Neomysis americana and much is known about its predators. It is consumed by shad, mackerel, herring and flounder in Long Island Sound (Whiteley, 1948) and by the common anchovy and sea trout in Delaware (Hopkins, 1965). The following list of predators of N . americana in different coastal regions was obtained from Grant (1962), Richards, (1963), McHugh (1967),

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Markel and Grant (1970), Schaefer (1970), June and Carlson (1971), Sikora et al. (1972), Williams et al. (1974), Stickney et al. (1975), McEachran et al. (1976), Grebe (1978).

Raja erinacea Mitchell, little skate: Cape Cod to Cape Hatteras. Raja ocellata Mitchill, winter skate: Cape Cod t o Cape Hatteras. Raja radiata Donovan, thorny skate: Cape Cod to Cape Hatteras. Raja senta Garman, smooth skate : Cape Cod to Cape Hatteras. Alosa pseudoharengus (Wilson), alewife : New York region. Brevoortia tyrannus (Latrove), Atlantic menhaden : New York region. Merluccius bilinearis (Mitchill), silver hake : New York region. Microgadus tomcod (Walbaum), Atlantic tomcod : New York region. Urophycis Jloridunus (Bean and Dresel), southern hake : Georgia estuaries. Urophycis regius (Walbaum), spotted hake : Georgia estuaries. Menidia menidia (L.), Atlantic silverside : New York region. Morone saxatalis (Walbaum), striped bass : Virginia estuaries. Centropristes striata (L.), black sea bass : Long Island Sound. Pomatomus saltatrix (L.), bluefish : Delaware estuaries. Stenotomus chrysops (L.), scup : New York region. Bairdiella chrysura (Lacepede), silver perch : south-eastern States. Leiostomus xanthurus Lacepede, spot : south-eastern States. Micropogon undulatus (L.), Atlantic croaker : south-eastern States. Stellifer lanceolatus (Holbrook), star drum : south-eastern States. Ammodytes americanus DeKay, American sand lance : New York region. Prionotus carolinus (L.),northern searobin ; New York region. Paralichthys dentatus (L.), summer flounder : Georgia. Paralichthys lethostigma Jordan and Gilbert, southern flounder : Georgia. Scophthalmus aquosus (Mitchill), windowpane : New York region. Pseudopleuronectes americanus (Walbaum), winter flounder : New York region. Neomysis americana is a very important part of the diet of the weakfish according to Stickney et al. (1975) and is also a major portion of the diets of the silver perch, Atlantic croaker and star drum in the estuaries of the south-eastern United States. The striped bass and the bluefish are also major estuarine predators of this mysid (Grant, 1962; Markle and Grant, 1970; Schaefer, 1970). The relative importance of the other fish as predators of this mysid is not known.

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Much less information is available concerning the predators of other species of mysids occurring along these coastlines. Erythrops erythrophthalma is eaten by the thorny and smooth skates (McEachran et al., 1976). Heteromysis formosa is consumed by the pollack, Pollachius wirens and the northern sewobin and, along with J f y s E ~stenolepis, is also eaten by the scup and black sea bass in Long'Island Sound (Richards, 1963). Tattersall (1951) notes the occurrence of Metamysidopsis munda in the stomach of a gulf kingfish, Menticirrhus litoralis (Holbrook) off Beaufort, North Carolina. The yellow perch Perca LfZawescens (Mitchill), feeds on mysids in the estuaries of Maryland (Muncy, 1962), and the searobins of the family Triglidae also eat mysids but the species consumed were not identified (Ross, 1977, 1978). The predators of Mysidopsis almyra in the brackish lake Pontchartrain, Louisiana are described by Darnel1 (1958). The following list states whether M . almyra was an important or minor constituent of the diet of the fish concerned:

Anchoa mitchilli diaphana Hildebrand, southern bay anchovy : important. Ictalurus furcatus (Lesueur), blue catfish : important. Arius felis (L.), sea catfish: minor. Menidia beryllina (Cope), tidewater silverside : minor. Morone interrupta Gill, yellow bass; minor. Lagodon rhomboides (L.),pin fish: minor. Bairdiella chrysura (Lacepede), silver perch : important. Cynoscion arenarius Ginsburg, sand seatrout : important. Cynoscion nebulosus (Cuvier), spotted seatrout : important. Leiostomus xanthurus Lacepede, spot : minor. Micropogon undulatus (L.), Atlantic croaker : minor. Paralichthys lethostigma Jordan and Gilbert, southern flounder : minor. The Atlantic croaker, Micropogon undulatus, feeds to a notable extent on Mysidopsis almyra in Mississippi Sound and adjacent regions of the Gulf of Mexico (Overstreet and Heard, 1978). No further information appears to be available on the predators of the mysid fauna of the Gulf of Mexico, Caribbean and the coasts of South America. Similarly, little information appears Go be available about the fish preying upon the mysids of the eastern Atlantic between 40"N and 40"s. Longhurst (1957) and Bainbridge (1960) state that a, major predator of Rhopalophthalmus africana and R. longicauda in the Sierra Leone River Estuary is the sciaenid, the gwangura Pseudotolithus elongatus (Bowditch).Further south, off South Africa, Christensen (1978)

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found that the sparids Diplodus cervinus (Lowe) and D. sargus (L.) feed to a small extent on species of mysids, including Mysidopsis similis. Day 1967) records Johnius hololepidotus, the kob, feeding on mysids in the Knysna Estuary in South Africa.

The Mediterranean The following list of predatory fish and the mysids consumed by them was obtained from Reys (1960), Hoenigman (1963) and Haidar (1963).

Raja (Raja)clavata L., thornback ray : Anchialina agilis, Erythrops erythrophthalma, Erythrops sp., Gastrosaccus normani, Lophogaster typicus, Mysideis parva, Mysidopsis didelphys, Pseudomma chattoni, Siriella norvegica, other species. Raja (Dipturus) oxyrinchus L., long-nosed skate : Lophogaster typicus, other species. Raja (Leucoraja)circularis, sandy ray : Lophogaster typicus. Herluccius merluccius (L.), hake : Lophogaster tyiicus. Mullus barbatus L., striped mullet : Anchialina agilis. Trigla lyra L., piper Lophogaster typicus, other species. Trigla lucerna L., yellow gurnard : Gastrosaccus normani, Lophogaster typicus. Aspitrigla cuculw (L.), red gurnard : Anchialina agilis, Erythrops erythrophthalma, Gastrosaccus normani, Leptomysis gracilis, Lophogaster typicus, Mysideis parva, Mysidopsis didelphys, Parapseudomma calloplura, Pseudomma sp., other species. Aspitrigla obscura (L.), shining gurnard : Anchialina agilis,Erythrops erythrophthalma, Erythrops sp., Gastrosaccus normani. Eutrigla gurnardus (L.), grey gurnard : A n c h i a h a agilis, Erythrops sp., Gastrosaccus normani, Leptomysis gracilis, Lophogaster typicus, Mysidopsis didelphys, other species. Trigloporus lastoviza (Brunnich), rock gurnard : Anchialina agilis, Gastrosaccus normani, Lophogaster typicus, other species. Lepidorhombus boscii (Risso), four-spotted scaldfish : Anchialina agilis, Erythrops erythrophthalma, Erythrops sp., Leptomysis gracilis, Lophogaster typicus, Siriella norvegica, other species. Lophogaster typicus was common in the stomachs of several of the fish, but especially those of the thornback ray. It also occurred, along with Boreomysis megalops and other species, in the stomachs of Micromesistius poutassou (Risso), Gadiculus argenteus argenteus Guichenot,

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TEE BIOLOGY OF MYSIDS

Phycis blennoides (Briinnich), Antonogadus megalokynodon (Kolombatovic) and Symphurus nigrescens Rafinesque in the western Mediterranean (Macpherson, 1978a-c). It is not common in environmental samples because it may live closely associated with the sediment and so i t is not caught by conventional samplers in any great numbers. It must, however, be a relatively numerous species at least in some localized areas. The Black and Caspian Seas Sturgeon feed to some degree on Paramysis species in the Sea of Azov (Savchuk, 1975) while young beluga, Huso huso (L.), feed on Paramysis baeri and P. intermedia in the northern Caspian Sea (Belyayeva et al., 1972). Mikhman and Romanovich (1977) state that the Azov anchovy, Engraulis encrasicolus (L.), feeds on mysids but they did not identify them. Young Percarina demidofli Nordmann consume mysids in the Gulf of Taganrog (Mikhman, 1978). Mysids assume relatively greater importance in the diets of fish in the Black and Caspian Seas than in other regions because there are marked decreases in the variety and numbers of decapod shrimps present in these areas of decreased salinity. Bacescu (1938) describes the extent of this predation on the various endemic mysids in considerable detail.

North Pacific, north of about 40"N

No information has been found regarding fish that prey upon mysids of this region, except in the coastal areas of the north-western United States and in the western Pacific. Neomysis species are eaten by the Pacific sanddab, Citharichthys sordidus (Girard), and the petrale sole, Eopsetta jordani (Lockington), off the coast of Oregon (Kravitz et at., 1976). Neomysis kadiakensis off the coasts of Washi