98 



FRESHWATER ANIMALS 



emperatures is an almost inevitable result of the action of a 

 genotype which has to be able to maintain activities at slightly 

 above 0° C. 



In the past it has usually been assumed that the ability to 

 produce these extreme helmets has been retained in the genetic 

 makeup because of selective advantage conferred by the pos- 

 session of such a bizarre body shape. Although this is not the 

 place to develop the inadequacies of this group of hypotheses, 



largest helmet 

 commonly occurring 



form characteristic 

 of entire range 



DAPHNIA PARVULA 



Fig. 5. Geographical variation in the shape of the helmets <>i maximum 

 size in Daphnia retrocurva Forbes. The three specimens with large 1 helmets 

 typify the helmet shapes developed in the three regions indicated. The 

 specimen at the top is a reminder that the vast majority of specimens 

 throughout the range of the species have helmets smaller than the ones 

 depicted. For comparison the distribution in North America and the charac- 

 teristic invariant shape of D. parvula Fordyce are indicated. D. parvula is 

 the only species closely related to D. retrocurva. 



