Inducible defenses in multipredator environments:: Cyclomorphosis in Daphnia cucullata

Phenotypically plastic defenses are strategies to reduce predation risk in variable environments. Predator-induced formations of protective devices in cladocerans are prominent examples of phenotypically plastic defenses. To understand the adaptive value of a defense, it is better to investigate its effects in a multipredator context, instead of merely concentrating on a two-species interaction. Small cladocerans such as Daphnia cucullata are prey items for many aquatic invertebrate predators and thus comprise a useful model system to investigate defenses and their effects. In this study, we tested the hypothesis that the helmets of Daphnia cucullata are inducible with chemical cues from different kinds of predators and that they act as a generalized defense offering protection against several predators, each using a different hunting strategy. Results from our induction experiment show that chemical cues released from Chaoborus flavicans, Leptodora kindtii, and Cyclops sp. induce significantly longer helmets and tail spines and thus act as proximate factors for cyclomorphosis in Daphnia cucullata. Our predation experiments revealed that the induced morphological changes offered protection against each of the predators tested. Interestingly, the protective mechanisms and the prey size classes which were protected differed between predator systems. Our results suggest that phenotypic plasticity in Daphnia cucullata evolved as a "diffuse" coevolution against different invertebrate predators which selectively feed on small prey items. The additive benefits may increase the adaptive value and thus facilitate the evolution and persistence of this generalized defense.