Chemosensory behaviour of juvenile crown-of-thorns sea star (Acanthaster sp.), attraction to algal and coral food and avoidance of adult conspecifics.

Abstract

Intraspecific and habitat-mediated responses to chemical cues play key roles in structuring populations of marine species. We investigated the behaviour of herbivorous-stage juvenile crown-of-thorns sea stars (COTS; Acanthaster sp.) in flow-through choice chambers to determine if chemical cues from their habitat influence movement and their transition to become coral predators. Juveniles at the diet transition stage were exposed to cues from their nursery habitat (coral rubble-crustose coralline algae (CCA)), live coral and adult COTS to determine if waterborne cues influence movement. In response to CCA and coral as sole cues, juveniles moved towards the cue source and when these cues were presented in combination, they exhibited a preference for coral. Juveniles moved away from adult COTS cues. Exposure to food cues (coral, CCA) in the presence of adult cues resulted in variable responses. Our results suggest a feedback mechanism whereby juvenile behaviour is mediated by adult chemical cues. Cues from the adult population may deter juveniles from the switch to corallivory. As outbreaks wane, juveniles released from competition may serve as a proximate source of outbreaks, supporting the juveniles-in-waiting hypothesis. The accumulation of juveniles within the reef infrastructure is an underappreciated potential source of COTS outbreaks that devastate coral reefs.