To sink or swim:: Variable diapause strategies among Daphnia species

Although dormancy is assumed to be fundamental to population persistence in temporary ponds, it has been suggested that diapause may be less frequent in populations that inhabit permanent lakes. We compared the seasonal timing and magnitude of investment in diapausing eggs and their subsequent storage in the sediment among Daphnia species from both temporary ponds (Daphnia pulex and Daphnia ephemeralis) and permanent lakes (Daphnia pulicaria and Daphnia dentifera). Species exhibited strong temporal segregation in the timing of diapausing eggs production in both temporary and permanent systems. All populations of D. ephemeralis produced ephippia in early April whereas D. pulex and D. pulicaria produced the majority of ephippia in May-June and D. dentifera did not produce males or ephippia until autumn. Maximum investment in dormant clutches (as opposed to immediately hatching eggs) was always 100% in the temporary pond species but ranged from 3% to 100% among populations living in the lakes. The number of eggs stored in the egg bank varied within and among species but was usually lower in the temporary pond species. Our results indicate that the use of dormancy in Daphnia varies considerably among populations and species and that whether the system is temporary or permanent is not necessarily a good predictor of a population's investment in dormancy.