104 FRESHWATER ANIMALS 



this phenotypic variation, we should expect to find it whenever 

 short-lived animals have to develop and remain active under a 

 wide range of thermal conditions. The plankters of freshwater 

 lakes of the temperate zone might be expected to exhibit this 

 variation to a pronounced degree, and they do. Short-lived plank- 

 tonic Cladocera (Bosmina, Daphnia), rotifers, and copepods, all 

 show striking morphologic variation, whereas related species and 

 genera which, for example, are benthic or longer-lived show 

 much less pronounced variation. 



Species Associations. The confusion which the wide intra- 

 specific range of body shapes causes for the investigator seeking 

 to delimit the species is confounded by the frequent coexistence 

 of large populations of several species of Daphnia in the same 

 body of water. Although the associations in lakes tend to be 

 larger, even small, temporary ponds commonly have two species 

 living at the same time. In addition to the fact that several 

 species may occur in a lake at the same time, different species 

 may comprise the dominant associations in different years. Vari- 

 ous evidence indicates that such shifts are possible because a 

 species may exist in a lake in a population so small as to be 

 undetected except with very extensive sampling and careful 

 examination of the material collected (Brooks, MS). However, 

 under favorable conditions this population may attain sufficient 

 size to be readily collected. 



Figure 8 indicates the array of species that can be found living 

 in New England lakes. In some parts of North America even 

 larger arrays of species can be found, but this will suffice as an 

 illustration of the problem. Collections in the upper waters of any 

 New England lake may disclose populations of any one or more 

 of the first five species living in the upper waters. In the bottom 

 waters in midsummer usually cither D. ambigua or D. longiremis 

 will be found, with D. ambigua more common in the lakes of 

 southern New England. There is no indication of preferential 

 associations. In a survey of 107 lakes in southern and central 

 Maine, it was found that the seven species occurred with tlio 

 approximate percentages indicated in Fig. 8 (Brooks, 1956, MS). 

 The most frequent associations were between the species most 



