MYSIDACEA 



By Olive S. Tattersall, d.Sc. 

 (Text-figs. 1-46) 



INTRODUCTION 



Before his death in 1943 my husband, the late Professor W. M. Tattersall, had done a considerable 

 amount of preliminary identification of the very large number of mysids sorted from the 

 collections of the 'Discovery' Investigations, now incorporated in the National Institute of Oceano- 

 graphy. Unfortunately not a single note concerning this work has been found among his papers so 

 that all the counting, measuring and ascertaining the sex and age of the specimens has had to be done 

 again. However, his provisional sorting into genera and, in some cases, into species has been of the 

 utmost help in working on the material. 



This very rich collection, amassed over a number of years, does not comprise all the mysids in the 

 'Discovery' collections, for much of the plankton has yet to be sorted, but there are over 5000 

 specimens, generally in very good condition, from 391 stations which are nearly all situated in the 

 South Atlantic and Southern Ocean. In addition I have included a record of specimens of Boreomysis 

 rostrata Illig, collected in surface tow-nets off the shores of Heard Island, which were sent to me for 

 identification by the courtesy of Dr P. G. Law, Director of the Antarctic Division of the Department 

 of External Affairs, Australian National Antarctic Expedition. This is particularly interesting because 

 I can find no other record of this species from surface waters. 



Identification of the Mysidacea is made difficult by the fact that, in many of the species, the 

 animals continue to grow long after sexual maturity has been attained. This growth is accompanied 

 by considerable changes in the proportions and armature of the body and appendages, so that smaller 

 individuals differ profoundly from larger ones. This disconcerting phenomenon has led to much 

 confusion in the past, because workers with only a few specimens at their command have frequently 

 founded new species, which have subsequently proved to be different growth stages of species already 

 described. The actual size of the animals is, unfortunately, not a reliable guide because specimens 

 living in warmer waters mature more rapidly and reach the various growth stages at a much smaller 

 size than those inhabiting colder regions. 



Only when large numbers of specimens of all sizes are available can the gradual growth changes be 

 traced and the true identity of younger individuals of a species be established. Such growth changes 

 are particularly conspicuous in species of the genera Gnathophausia and Eucopia. The Discovery 

 collection contains over sixty specimens (of all sizes) of Gnathophausia ingens and more than thirty of 

 G. gigas. I have made detailed measurements of these species and the results, which I give in tabular 

 form, fully endorse the valuable work in this field done by Ortmann and others. I have also drawn up 

 a list of the species represented in the Discovery collection together with their synonymies, so that 

 when isolated specimens are found they can the more readily be referred to their true species even 

 though they may represent quite young growth stages (see p. 24-27). 



Another difficulty in working out large collections lies in deciding how much individual variation 

 should be tolerated among members of any one species. In many genera such a mass of slightly 

 differing characters occurs that it seems impossible to find any consistent features whereby the 

 animals can be separated into definite groups. As a result two alternatives arise, (i) making a very 



