Predator cue detection in Daphnia involves ionotropic receptors IR25a and IR93a

Perceiving environmental information is crucial for organisms to locate resources and avoid predation. In freshwater ecosystems, species of the microcrustacean Daphnia develop inducible defences in response to predator-specific chemical cues, which confer measurable fitness advantages. While some predator-specific chemicals have been identified, the receptors mediating detection of these chemicals remain poorly described. Because these inducible responses depend on the detection of predator-derived chemicals, understanding the underlying chemosensory machinery is critical for elucidating predator-prey dynamics per se and their modification under environmental change. We analysed the contribution of an ancient subfamily of ionotropic receptors co-opted for chemosensation in protostomes. Using evolutionary conservation of IR25a and IR93a across arthropods, we identified and characterized their Daphnia homologues. Multi-species analysis reveals these co-receptors are essential for detecting predator-specific cues. When predators are present, IR25a and IR93a are upregulated, and the corresponding proteins localize to the chemosensory antennules, supporting their involvement in predator cue detection. RNA interference-mediated knockdown of these co-receptors attenuates species-specific defences. These findings identify IR25a and IR93a as required mediators of chemosensory signal transduction in predator detection. Together, our results reveal a conserved ionotropic receptor pathway that mechanistically connects predator cue detection to inducible defensive plasticity, advancing our molecular understanding of predator-prey interactions.