The Origin and Ecological Function of an Ion Inducing Anti-Predator Behavior in Lithobates Tadpoles

In aquatic environments, chemical cues are believed to be associated with prey response to predation risk, yet few basic cue compositions are known despite the pronounced ecological and evolutionary significance of such cues. Previous work indicated that negatively-charged ions of m/z 501 are possibly a kairomone that induces anti-predator responses in amphibian tadpoles. However, work described here confirms that this specific ion species m/z 501.2886 is produced by injured tadpoles, exhibits increased spectral intensity with higher tadpole biomass, and is not produced by starved predators. These results indicate the anion is an alarm cue released from tadpoles. High resolution mass spectrometry (HR-MS) revealed a unique elemental composition for [M-H]^-, m/z 501.2886, of C₂₆H₄₅O₇S^- which could not be determined in previous studies using low resolution instruments. Collision induced dissociation of m/z 501 ions formed product ions of m/z 97 and m/z 80, HSO₄^- and SO₃^-, respectively, showing the presence of sulfate. Green frog tadpoles, Lithobates clamitans, exposed to the m/z 501 anion or sodium dodecyl sulfate exhibited similar anti-predator responses, suggesting organic sulfate is a tadpole behavior modifier.