14O A BIOLOGY OF CRUSTACEA 



an easy prey for small fish and large water beetles. The lower limits 

 of salinity in which Artemia may be found may be said to cor- 

 respond roughly with the upper limits of salinity tolerance of its 

 predators. Salinity only limits the distribution of Artemia in the 

 sense that a high salinity is necessary to eliminate its predators. 



Inland saline waters often have a chemical composition rather 

 different from that of the sea. In some cases this allows fresh-water 

 Crustacea to live in higher concentrations than they could if the 

 dissolved salts were the same as those in the sea. The importance 

 of the chemical nature of the dissolved salts can be illustrated by 

 reference to Daphnia magna. In natural waters this species has been 

 found living in concentrations up to about a third that of sea water, 

 while pure sodium chloride solutions with one-seventh the con- 

 centration of the sea rapidly kill it. 



It is evident that as well as the amount of salt, its chemical com- 

 position is also an important factor in the distribution of Crustacea. 

 But we cannot make any general statements about the effects of 

 various salts in natural habitats because we do not yet know 

 enough about the subject. 



FRESH-WATER CRUSTACEA 



The geographical units of fresh water are much more clearly 

 defined than the subdivisions of the sea, and there are correspond- 

 ingly clearer faunistic divisions. The ancient lakes provide excellent 

 examples of distinct and characteristic faunas. Lake Baikal has been 

 the scene of almost riotous evolution among the gammarid amphi- 

 pods; about three hundred species are found in the lake, and only 

 one of these, the common Gammarus pulex, is found elsewhere. 

 The rest of the world has just about as many gammarid species as 

 Lake Baikal. An interesting species flock has also appeared in Lake 

 Nyasa, though the numbers involved are not so fantastic. The 

 copepod genus Lernaea is parasitic on fish, and there are about 

 thirty species known in the world; eight of these species are found 

 in Lake Nyasa, and of these seven have not been found elsewhere. 

 Lake Tanganyika has been the site of evolution of a species flock 

 of Argulus, also parasitic on fish, and to some extent replacing 

 parasitic copepods, which appear to be rare in this lake. 



When dealing with the marine Crustacea it was not feasible 

 even to mention all the various groups, because practically all the 

 groups are well represented in the sea. Fewer groups have success- 



