SYSTEMATIC REPORT Hi 



Tribe LEPTOMYSINI 

 Genus Mysidetes Holt and Tattersall, 1906 



1885 a ? Mysidopsis G. O. Sars, p. 202. 



1905 Mysideis (pars) Holt and Tattersall, p. 127. 



1906 Mysidetes Holt and Tattersall, a, p. 39; b, p. 10. 

 1906a Metamysidella Illig, p. 210. 



1908 Mysidetes, Tattersall, p. 32. 



Remarks. G. O. Sars (1885a, p. 202) formed a new species for a very damaged female specimen 

 collected by ' Challenger ' off Australia and doubtfully referred it to the genus Mysidopsis as M. incisa. 

 From the form of the telson and the endopods of the uropods it is probable that this specimen should 

 be referred to the genus Mysidetes, but in the absence of males it is impossible to be quite certain on 

 this point. 



Illig (1906a, p. 210) founded a new genus and species, Metamysidella kerguelensis, for specimens 

 collected by the ' Tiefsee-Expedition ', 1898-9. He did not mention the form of the male pleopods nor 

 the male genital organ, but there is no doubt that his species must be referred to the genus Mysidetes, 

 which had been founded earlier in the same year by Holt and Tattersall (1906 a, p. 39) for specimens 

 collected in the Antarctic. 



These authors considered that the rudimentary form of the pleopods in both sexes was sufficiently 

 distinctive a character to warrant the institution of a new subfamily which they named Mysidetinae. 

 This suggestion has not been adopted by subsequent workers and the genus Mysidetes remains in the 

 tribe Leptomysini of the subfamily Mysinae. 



The genus Mysidetes can be distinguished from all other genera of the tribe Leptomysini by 

 the rudimentary form of the male pleopods, and by the larger number of sub-segments into which 

 the carpo-propodus of the third to the eighth thoracic endopods is divided. In this respect the 

 genus resembles the genus Pseudomysis, but in that genus the male pleopods are normal and not 

 reduced. 



One of the most remarkable features of Mysidetes is the very long, forwardly directed, tubular 

 genital organ of the male, which in some species, notably M. kerguelensis (Illig), is enormously elon- 

 gate and may extend beyond the anterior margin of the antennular peduncle. Similar long tubular male 

 genital organs are found in two other genera of mysids, Heteromysis and Mysidella, although these 

 belong to widely separated tribes and subfamilies. It may be difficult to distinguish females of these 

 genera, but males may be recognized by the form of the first and third thoracic endopods. In Mysidetes 

 the endopods of both these appendages are normal and not swollen or modified ; in Heteromysis the 

 carpo-propodus of the third thoracic endopod is unsegmented and very swollen with the dactylus 

 bending over to form a strong sub-chela, while in Mysidella the endopod of the first thoracic appen- 

 dage has the propodus very swollen and armed with strong spines. 



Six species, five of which are from Antarctic waters, have been referred to this genus, but the 

 Discovery material is so rich that, in addition to adding some details to already known species, I have 

 been obliged to found no fewer than six new species. There is considerable individual variation in the 

 members of the species of Mysidetes and much evidence that growth changes take place after maturity 

 has been reached. It may be that later workers will consider that in some cases further divisions should 

 be made within the new species here described. I have tried to find clear and definite specific charac- 

 ters for these new species and have in no case relied upon a single specimen. 



All the five species previously recorded from Antarctic waters, except M. hanseni Zimmer, are 

 represented in this collection. The new species, which I have named M. microps, resembles Zimmer's 



