Predator-induced morphological traits determine escape performance in a larval amphibian species

Although interspecific morphological diversity in aquatic animals often reflects swimming-related functional trade-offs, whether intraspecific morphological variation, often generated by phenotypic plasticity, also reflects similar trade-offs remains unclear. We tested this hypothesis using Rana pirica tadpoles, which exhibit a dragonfly-induced phenotype with a deeper tail and a salamander-induced phenotype with both a deeper tail and a swollen body. Fast-start swimming speed was highest in the dragonfly-induced phenotype, intermediate in the salamander-induced phenotype and lowest in the non-induced phenotype. Because swimming speed is influenced by both morphology and body-motion effort, we quantified motion–speed efficiency (Ems) as an index of morphological effects on swimming performance. Both predator-induced phenotypes exhibited higher Ems than the non-induced phenotype, despite similar tail-motion quantity among phenotypes, with the highest value in the dragonfly-induced phenotype. These results indicate that the shared deeper tail more efficiently converts body-motion effort into swimming speed, whereas in the salamander-induced phenotype, this function is partly offset by increased hydrodynamic drag caused by its swollen body, a morphology that protects against swallowing by gape-limited salamander larvae. These results demonstrate that predator-induced morphologies involve functional trade-offs between swimming escape and physical protection and suggest that such trade-offs contribute to morphological diversification through phenotypic plasticity.