The role of prey and predator identity in eliciting inducible defenses of Daphnia

Predators can modify population and community dynamics not only through direct predation, but also through nonconsumptive effects. Predator-induced changes in the traits of prey species are important components of these nonconsumptive effects. While these are well studied in simplified one-predator one-prey settings, relatively little is known about how kairomones act on prey across heterogeneous aquatic ecosystems with diverse identities of predator and prey. Kairomones are, to some extent, predator specific, and can be classified as diet dependent or diet independent. This classification depends on whether the identity of the prey plays a role in eliciting a defense response. In an effort to elucidate how prey and predator identity determines the defense responses in water fleas, we used inducible morphology and life history traits of Daphnia mitsukuri as a model to systematically explore to what extent predator and prey identity, and species composition of the prey community determined the expression of inducible defenses, and to what extent predator-induced responses differed among genotypes. Our results showed that the defense responses of D. mitsukuri mainly relied on diet-dependent kairomones, which were not influenced by fish species identity but were highly dependent on the phylogenetic distances between D. mitsukuri and the prey ingested by fish. This phylogenetic signal is strong, with D. mitsukuri responding only to the kairomones of fish that feed on cladocerans. We also found that the amplitude of the responses to fish kairomones increased with the amount of cladoceran in the dietary community. We observed significant differences in defensive traits among three D. mitsukuri genotypes, but the differences were minor compared with the effects of kairomones themselves. The results of our systematic analyses point to the role of prey phylogeny in eliciting inducible defenses of D. mitsukuri, thereby enriching our understanding of nonconsumptive effects in aquatic ecosystems.