FRESHWATER ANIMALS 



that the various species of a river system often exhibit similari- 

 es which, aside from mutual introgressions, are difficult to 

 understand. Further investigations on hybridization and intro- 

 gression apparently are in progress at Drottningholm. 



Summary 



1. The systematics of two "difficult" genera of freshwater ani- 

 mals, the crustacean Daphnia and the whitefish Coregonus, have 

 been examined in an attempt to discover any common sources 

 of the species problem in these groups that might be considered 

 as peculiar, at least in their intensity, to freshwater animals. 



2. The extraordinarily wide range of phenotypic variants that 

 may develop from the same genotype emerges as the chief 

 source of the species problem in both Daphnia and Coregonus. 

 For the systematist this means that the phenotypic character- 

 istics that are taken as indicators of the genotype, i.e., the species, 

 must be chosen with great care. From an evolutionary point of 

 view, this wide range of phenotypic variation is a reflection of 

 the ability of the genotype to prosecute successful development 

 over the wide range of environmental conditions occurring in 

 the fresh waters that these animals inhabit. It is probably not 

 fortuitous that animals with this ability are among the more 

 successful inhabitants of these inland waters (both Daphnia and 

 Coregonus and the respectively related groups, Bosmina and the 

 chars, for example). For Daphnia the extreme environmental 

 conditions succeed each other cyclically, with the seasons, so 

 that the entire range of phenotypic diversity is seen in the an- 

 nual succession of generations of these short-lived animals. In 

 Coregonus the extraordinary phenotypic diversity, while in part 

 expressed in different year classes of the same population, is 

 principally expressed by allopatric populations. The conditions 

 of life are sufficiently different from one lake to another so that 

 nearly all the phenotypic characteristics of a species are different 

 from lake to lake. 



3. A second source of difficulty in the recognition of species in 

 both genera has been that in many bodies of fresh water several 

 highly variable species may coexist. In North America as many 



