DISTRIBUTION 1 45 



in Australia and Tasmania; the latter island claims the distinction 

 of having the world's largest crayfish, Astacopsis franklinii, which 

 reaches a weight of eight or nine pounds. Parastacus is found in 

 South America, while Madagascar has the genus Astacoides. 



In North America the Potamobiidae are represented by Potamo- 

 bius ( == Astacus) to the west of the Rocky Mountains, while the 

 sub-family Cambarinae, which includes Troglocambarus, Orcon- 

 ectes and other genera as well as Cambarus, is found only to the 

 east of these mountains. The European species belong to the genus 

 Potamobius, but a species of Cambarus has been introduced into 

 Germany. There are no crayfish in the greater part of Asia, but 

 the genus Cambaroides is found in Korea and the northern islands 

 of Japan. Some authorities believe that Cambaroides is allied to 

 Cambarus', if this is so the distribution of the northern crayfish is 

 extremely puzzling because it means that two parts of the sub- 

 family Cambarinae are isolated by Potamobius, which occupies the 

 west of the North American continent. 



A most striking feature of the distribution of fresh- water 

 decapods is that it is only in the regions where crayfish are absent 

 that other groups have become established in large numbers. 



The two most important families of fresh-water shrimps are the 

 Palaemonidae and the Atyidae. The latter are small, but some 

 palaemonids reach a length of about a foot. Both families occur 

 throughout the tropics, but a remarkable flock of atyids has arisen 

 in Lake Tanganyika; eleven endemic species are found. This is in 

 great contrast with the other African lakes, such as Nyasa and 

 Victoria, each of which have only a single, non-endemic, species of 

 prawn. 



Only one anomuran genus is known from fresh water, this is 

 Aegla, which looks like Galathea, and has about twenty species 

 living in streams in southern Brazil and Chile. 



Two families of crabs have representatives in fresh water: the 

 Grapsidae and the Potamonidae. The latter are the true river crabs, 

 spending all their lives in fresh water. This is also done by some of 

 the grapsids, but many of them, like Eriocheir sinensis, have to 

 return to the sea to breed (see p. 136). Both families are well 

 represented throughout the tropics, and one true river crab, 

 Telphusa fluviatilis, is found in countries on the northern shores of 

 the Mediterranean. 



