82 FRESHWATER ANIMALS 



about this genus is based upon investigations described more 

 fully in a monograph on the systematics of North American 

 Daphnia (Brooks, 1956). Coregonus was selected as the second 

 group because it represents a very different kind of organism ( one 

 especially different from Daphnia in its reproductive behavior), 

 its systematics has proved particularly thorny, and, possibly 

 most important, it has been the subject of a penetrating study 

 by Gunnar Svardson (1949-1953), of the Institute of Freshwater 

 Research at Drottningholm, Sweden. Whereas the genera chosen 

 represent a reasonable biological diversity, they are both groups 

 which flourish especially in the glaciated areas of North Amer- 

 ica and Eurasia. Thus both groups are alike in having been 

 subjected to the disruptive influences of the Pleistocene glacia- 

 tions. However, as most of the bodies of standing fresh water 

 are now in areas which were glaciated, the rich fauna which has 

 developed in these waters will have, in greater or lesser degree, 

 shared the same history. Any assessment of the role of these 

 Pleistocene events in the evolution of even these two genera is 

 well outside the scope of this study. 



The Species Problem in Daphnia 



Daphnia are small (% to 4 mm. long), planktonic Cladocera 

 common in lakes and small ponds. They feed primarily on small 

 algae which they filter from the water by means of combs on 

 some of their thoracic appendages, which are enclosed within a 

 bivalve carapace. The genus is worldwide in distribution (as 

 many genera of Cladocera tend to be). The ranges of some spe- 

 cies are rather limited, embracing but a portion of a continent, 

 whereas those of others are broad, extending over several conti- 

 nents. 



A recent study (Brooks, 1956) indicates that there arc fifteen 

 species living in North America, out of a total of about thirty 

 well-characterized species. However, it is probable that there 

 are about fifty extant species. "While the distributions even of 

 these thirty known species arc not so completely known as one 

 might wish, a rough estimate, on the assumed basis of forty to 

 filty species, for the other major land areas would be: South 



