252 



R. G. Stress et al. 



TABLE 6-1 Species of Crustacea in the Plankton and Their Life Cycles in a 

 Typical Polygon Pool (IBP Pond C) on the Tundra near Barrow 



All Crustaceans except for the Daphnia are monovoltine (one 

 generation per year); offspring of the overwintering generation enter 

 diapause to survive through the next winter. Usually, the embryonic stages 

 enter diapause, but in the cyclopoid copepods a copepodid (pre-adult) 

 instar overwinters. (The precise overwintering instar was not determined.) 

 In some species, such as Cyclops strenuus, nearly all copepodid instars 

 have the ability to encyst (Elgmork 1959, Szlauer 1963). The advanced 

 stage of development of the overwintering cyclopoids enables them to 

 reach adulthood and reproduce shortly after the ice melts in June. 



The Daphnia are the only taxa at Barrow to produce a generation that 

 does not overwinter. The sequence of events in the Daphnia life cycle 

 follows: first, the overwintering embryos (formerly called ephippial eggs) 

 hatch shortly after the ice thaws; second, the hatchlings, which are all 

 female, reach maturity and in mid-July release a brood of young (Stross 

 1969, Stross and Kangas 1969); third, after this brood of young is 

 produced, the females continue to reproduce but now produce diapausing 

 embryos (Edmondson 1955, Stross 1969). If food is exceptionally 

 abundant, the brood of young (called young-of-the-year) will also produce 

 diapausing embryos. Only females are known for D. middendorffiana but 



