Predator-Mediated Plasticity of Prey Life History and Morphology: Chaoborus americanus Predation on Daphnia pulex

Presence of the invertebrate predator Chaoborus americanus induces Daphnia pulex to assume an alternate, less vulnerable morphology, characterized by small protuberances (neck teeth) on the dorsal anterior margin of the head. Whereas previous studies on the demographic characteristics of this induced morphology in single genotypes indicated an unavoidable "cost" (delayed maturity), the results of this study indicate substantial genetic variance for induced life-history response. In addition, the degree of neck-tooth expression, while genetically variable, is a poor predictor of clone-specific life-history changes and fitness gain in the presence of Chaoborus predation. These results reveal several important characteristics of this system of inducible defense. First, the most notable trait of the morph (neck teeth) varies among genotypes. Therefore, the induced morph does not represent an on-off state. Second, the relative expression of neck teeth among clones is a poor predictor of induced life-history changes or of relative gain in fitness in the presence of Chaoborus predation. This fact suggests that induced characteristics other than neck teeth may cause the reduced vulnerability of the induced morph. Third, the life-history consequences of induction are highly variable among genotypes, which indicates substantial genetic variance for life-history plasticity. Although some genotypes express delayed maturity in the induced morph, this is not a general result.