Embryonic and larval defensive responses of agile frog ( Rana dalmatina ) to alien crayfish

Red swamp crayfish Procambarus clarkii, a widespread invasive alien crayfish, represents a serious threat for several freshwater species, including amphibians, which are declining at a global scale. As a shared coevolutionary history is the main factor determining the emergence of antipredator responses, Anuran tadpoles may not be able to cope effectively with this introduced predator. We performed two experiments to assess agile frog's (Rana dalmatina) defensive responses to both P.clarkii and native dragonfly larvae (Anax imperator). First, we conditioned embryos (collected from two ponds 30km away from each other) with predators' chemical cues to explore possible variation in hatching time caused by predation risk. In the second experiment, to evaluate how predators' diet affects tadpole behavior, we conditioned tadpoles for a 5-week period with cues from tadpole-fed and gammarid-fed predators and recorded behavioral and morphological responses. Embryos did not alter hatching time in the presence of any predator cue, while tadpoles from both populations strongly reduced activity and visibility when raised in the presence of tadpole-fed dragonfly larvae. Morphological changes were less straightforward and were induced only in one population, for which broader tails and a slight increase in body size of tadpoles exposed to tadpole-fed predators were observed. The lack of defensive responses in crayfish-exposed tadpoles suggests that the spreading of this invasive species in agricultural lowlands of northern Italy may represent a further threat to their conservation