Over-wintering Daphnia: uncoupling the effects of temperature and food on offspring size and filtering screen morphology in D. galeata

Abstract

Field investigation of seasonal variation of filtering setae number (FSN) in the filtering screens of D. galeata showed a clear tendency toward low FSN in animals with low temperature and food levels (an over-wintering population and a deep-hypolimnion-inhabiting part of the population). Laboratory experiments were carried out to uncouple the effects of temperature and food level on the size of neonates and FSN in their filtering apparatus. Results of the first experiments indicated that temperature is the main factor inducing the changes in FSN and the induction takes place during embryogenesis. These results were confirmed in the second experiment. Eggs from the same clutch incubated in vitro at temperatures of 19, 10 and 68C developed into neonates with smaller carapace length with decreasing temperature. Smaller neonates had lower FSN in their filtering screens. These results provide experimental evidence of temperature-mediated embryonic induction in Daphnia galeata and contribute to an understanding of the proximate causes of the morphological features observed in over-wintering populations in the field.