Inducible defenses: The relevance of chemical alarm cues in Daphnia

Many aquatic organisms use chemical cues to recognize predators and to activate inducible defenses. In zooplankton, most of these cues are thought to be kairomones released by specific predators. However, in several other organisms, e.g., fish and amphibians, alarm signals from conspecifics also provide reliable cues. We tested whether alarm substances also act as chemical cues in Daphnia and assessed their relevance compared with predator kairomones. Water-borne cues from macerated conspecifics altered morphology in all Daphnia species tested (Daphnia cucullata, Daphnia longicephala, and Daphnia lumholtzi). However, kairomones released by feeding predators had distinctly stronger effects. In D. cucullata, we tested for the relevance of predator diet and found that the increase in relative helmet length was most pronounced when the predator was fed with conspecifics instead of heterospecifics. Cross-species experiments with alarm substances also revealed that D. cucullata is more sensitive to info-chemicals from macerated conspecifics than to chemical cues from D. magna. Unspecific alarm cues together with cues released by predators consuming prey may form a blend of chemicals that indicates predation risk and activates inducible defenses in nature.