Genotype, Fish‐Mediated Chemical, and Phototactic Behavior in Daphnia Magna

Clones were isolated from natural populations with contrasting fish predation pressure and were tested for their phototactic behavior in the presence and absence of fish and/or fish—mediated chemicals. The presence of fish—mediated chemicals induced a more negatively phototactic behavior in all clones tested. This response was observed within 24 h after exposure to these chemicals. Release from the presence of the fish—mediated chemicals resulted for most clones in a gradual change back to control behavior, which took several days. There was no indication of a maternal effect. The response was not enhanced by a more direct exposure to the presence of fish, suggesting that the stimulus is a chemical. The results indicated genetic differences in responsiveness to fish—mediated chemicals in Daphnia. Clones isolated from habitats in which fish were present were on average more responsive than clones isolated from a fishless habitat. Significant genetic differences in phototactic behavior between the clones tested could be observed in the presence as well as in the absence of fish—mediated chemicals.