Hide, rest or die: a light-mediated diapause response in Daphnia magna to the threat of fish predation

1. In a laboratory batch culture experiment, a diapause response of Daphnia magna to a simulated threat of fish predation was tested at various light intensities, which under natural conditions determine potential vulnerability of Daphnia to visual planktivorous fish. 2. Under moderate light intensity (1.4 lmol m s) that allows effective predation by fish, the proportion of females producing dormant eggs was significantly higher than under dim light conditions (0.001 lmol m s) that are not favourable for visual detection of prey. Production of dormant eggs was not observed in complete darkness or in treatments missing fish kairomones, irrespective of tested light conditions. 3. The observed phenomenon is interpreted as a flexible response of prey to the conditional risk of predation assessed by Daphnia according to the presence of fish-derived cues on the one hand and the presence of dark refugia on the other. Irrespective of the presence of fish kairomones, Daphnia may not produce resting eggs as long as a safe, dark, bottom zone is accessible.