144 



A BIOLOGY OF CRUSTACEA 



Fig. 58. Phreatoicus tasmaniae (Isopoda). A fresh-water form which super- 

 ficially resembles an amphipod. Actual length of body about 2 cm. 



only a few non-marine species are known, such as Tanais fluviatilis 

 from the Argentine and T. stanfordi in the Kurile Islands. 



The best known of the fresh-water mysids is undoubtedly Mysis 

 relicta, which is found in various lakes in North America and 

 Europe. Mysis oculata is a close relative which lives in the Arctic 

 Ocean. It is thought that M. relicta has evolved from M. oculata 

 after being isolated in lakes formed during the Ice Ages. The Arctic 

 origin of the species is confirmed by its preference for cold lakes 

 and its winter breeding habits. Mysis relicta starts laying eggs when 

 the temperature falls below 7 C. and stops again when the tempera- 

 ture rises above this level. 



Other mysids are known from fresh waters in caves; Troglomysis 

 vjetrenicensis from Herzegovinia, and Spelaeomysis bottazzii from 

 Italy, though the water in which the latter species was found was 

 slightly brackish. 



There are no fresh-water Euphausiacea, but the Decapoda are 

 quite well represented in fresh water and their distribution is com- 

 paratively well known. Crayfish can be arranged in two families, 

 the Potamobiidae in the Northern Hemisphere and the Parastacidae 

 in the Southern Hemisphere. Both families live mainly in the 

 temperate regions so that there is a vast area of the tropics which 

 lacks crayfish; they are also absent from the whole of Africa, but 

 present in Madagascar. 



Various genera of crayfish characterise different parts of the 

 world. Among the Parastacidae, Astacopsis and Engaeus are found 



