Chemical alarm signalling in aquatic predator-prey systems: a review and prospectus

The importance of chemical cues in predator-prey interactions has recently received increasing attention from ecologists. The sources of chemicals to which prey species respond often originate as cues released by the predator (reviewed by Kats and Dill, this issue). Alternatively, cues may be released by other prey animals when they detect or are attacked by a predator. Such cues, known as chemical alarm signals, are particularly common in aquatic systems. These signals provide the basis of our current review. Short-term behavioural responses of prey animals to alarm signals have received the most attention. Behavioural responses of prey resemble those exhibited to known predators, and are therefore likely to make receivers less vulnerable to predation. More recently, studies have shown that benefits to alarm signal receivers extend beyond the immediate behavioural response of nearby conspecifics over a few minutes. For example, alarm signals are important in mediating the learning of unknown predators and dangerous habitats. Furthermore, alarm signals have been implicated in induced morphological defences, and defences that involve shifts in life history characteristics. In some systems, predators may have ways to counteract alarm signals used by their prey. We suggest that researchers must consider alarm signalling and associated learning phenomena when formulating hypotheses and designing future experiments.