Size-selective predation accounts for intra- and inter-specific variation of inducible morphological defense ofDaphnia

Inducible defense is adaptive when prey organisms cope with fluctuations of predation risk, and there exists ample variations of inducible defense both within a species and between species even in a single habitat.Daphnia, a freshwater zooplankton genus, shows remarkable inducible morphological defense when exposed to predators. This study explored the intra- and inter-specific variations of inducible defense ofDaphniaand tested whether the variations can be explained by the size-selective nature of predation. In Lake Fukami-ike, Japan,Daphnia ambiguaandDaphnia pulexcoexist with planktivorous fish andChaoboruslarvae that have different preferences for prey size. We experimentally examined the ontogenetic change of inducible defense for ten clones of each of the twoDaphniaspecies against the two different predators. The degree of defense expression evaluated based on the induction of multiple morphological traits showed remarkable differences betweenDaphniaspecies, instars and clones, and the intra- and inter-specific variations were attributed to size-selective predation from the different predators. The inducible defense ofDaphniashowed the adaptive variations depending on the predation pressure that resulted from the size-selective predation by the coexisting predators.