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Abstract

A chemical signal, released by a fish predator under summer-like high water temperature and long-day photoperiod, caused the formation of resting eggs in a clone of Daphnia magna. No ephippial females were recorded and no ephippia were released in the control treatment during 45 days of the experiment. When exposed to fish water, the fraction of ephippial females reached a maximum of 3.7%,a value comparable to that registered in summer in the Grosser Binnensee (Northern Germany), a hypertrophic lake inhabited by fish, which was the source lake for our experimental clone. The number of ephippia released within 45 days was on average 34 +/- 22. Ephippia formation could not result from the between-treatment differences in population density, and related patterns of food depletion, since no substantial difference between control and fish treatment was observed. Instead, specific information on the presence of a predator provided a cue which induced the formation of resting eggs in Daphnia. Under heavy predation and very low survival probability of parthenogenetic females, ephippia formation in summer can be adaptive, i.e. higher fitness can be achieved through survival in the diapausing state than through the immediate reproductive gain via parthenogenesis