Predator-avoidance behavior in anuran tadpoles: a new bioassay for characterization of water-soluble cues

In freshwater systems, little is known about the characteristics of chemical cues derived from predators which induce defensive responses in prey species. To elucidate traits of predator chemical cues, we examined chemical cues originating from water incubated by the nymph of the dragonfly Anax parthenope julius, which induces low activity as predator-avoidance behavior in tadpoles of two anuran species, the Japanese tree frog Hyla japonica and the winkled frog Rana (Rugosa) rugosa. H. japonica exhibited a reduction in tail movement time as low activity in response to both untreated incubation water and incubation water that had volatile substances removed by freeze-drying. The response threshold of R.rugosa to chemical cues was determined to be one dragonfly nymph in a water volume between 500 and 5,000 ml. We found that chemical cues inducing predator-avoidance behavior in anuran tadpoles have water-soluble non-volatile characteristics. In this study, we devised both the bioassay to assess the effects of chemical cues and the method to enrich the cues by freeze-drying, which can serve as a tool in the process of identification of unknown chemical cues in freshwater predator-prey interaction.