Seasonal changes in the shape of aquatic organisms (cyclomorphosis) have been reported from protozoa to vertebrates. In many cases chemical cues from predators act as inducers and a protective effect against predators has been proved to be the ultimate cause. Phenotypic plasticity in defensive traits evolved as an adaptation against heterogeneity in predation risk. Inducibility of traits is favored if the predation risk is variable, if reliable cues indicate the danger, if effective defenses can be formed within relatively short time spans and if costs are associated with the defenses, which can be saved during times when the defenses are not needed. However, even within a single species not all genotypes respond in the same direction. Local adaptations and even the existence of multiple optima have been shown. Inducible defenses have been reported to bear the potential to dampen predator–prey oscillations, leading to coexistence in bi- and tritrophic experimental systems.