Chemical alarm cues inform prey of predation threat: the importance of ontogeny and concentration in a coral reef fish

Prey that respond to inappropriate cues in their assessment of predation risk spend more time performing defensive behaviours and less time undertaking behaviours that promote fitness. Hence, prey should respond to cues that are the best predictors of predation risk relevant to the prey individual. Many fish undergo ontogenetic shifts in habitat and resource use during their lifetime; consequently, prey fish are exposed to a variety of predators at different stages of their development. Also, as relative concentration of the alarm cue represents both spatial and temporal information about a predation event, prey should adjust the intensity of their antipredator response in a threat-sensitive manner. We found that the ambon damselfish, Pomacentrus amboinensis, exhibited a threat-sensitive response to damage-released cues of conspecifics, with the magnitude of the response dependent on the cue concentration. Higher cue concentrations (suggesting a greater threat) elicited a stronger avoidance response, while low cue concentrations (weak threats) elicited a weak antipredator response. Moreover, the nature of the response depended on the ontogenetic stage of the cue donor. Reef-naïve recruit individuals exhibited consistent antipredator behaviours when exposed to alarm cue concentrations from individuals of the same ontogenetic stage, modest antipredator responses to juvenile skin extracts, and no antipredator behaviours when exposed to cues from adult fish. Individuals that alter their avoidance response in a manner that reflects the magnitude of risk successfully balance the trade-off between defensive and risky or fitness-promoting behaviours, and natural selection would presumably favour these individuals.