Constraints on the plasticity of Daphnia magna influenced by fish-kairomones

1. Daphnia magna reproduced at a smaller size (body mass) when exposed to fish-kairomone than D. magna growing up without this cue. 2. The total egg mass for first reproduction was the same in both groups, but the proportion of clutch mass to total body mass was about 11% higher in daphniids exposed to fish-kairomone. This difference was about the same for low and high food concentrations (0.5 or 1.5 mgC l(-1)). 3. Although egg mass was the same, triglyceride (TAG) content of the egg material was significantly lower when daphniids were exposed to fish-kairomones for both food concentrations. 4. The TAG content of the clutch was lowered to the extent that the TAG proportion in the clutch in relation to the somatic mass of the animal was the same for animals treated with fish-kairomone and controls (5% of body mass at first reproduction). 5. The lower TAG content per clutch resulted in a lower quality of the individual egg in daphniids exposed to fish-kairomone. The resulting higher susceptibility to starvation under low food conditions is a substantial cost of the plastic response to fish-kairomones. 6. This cost may set a limit on the plastic response in resource allocation of Daphnia magna exposed to fish-kairomones. However, daphniids living in a system with fish predation are more likely to be eaten by fish than to starve to death.