ALARM SIGNALS IN DAPHNIA

Daphnia magna can respond to chemical cues from freshly crushed conspe cifics with various behavioural reactions. A shift in vertical distrib ution towards the bottom, the formation of aggregations and direct esc ape responses can all be induced by water-borne signals released from crushed Daphnia. The pattern and strength of the first two behavioural responses (i.e. the persistent tendency to occupy deeper strata in th e experimental columns and to stay within patches) indicate that Daphn ia perceive the signal from crushed conspecifics as nonspecific inform ation, not necessarily associated with any particular kind of danger f rom either vertebrate or invertebrate predators. The adaptive value an d possible costs associated with performing these two behavioural reac tions are discussed. The adaptive value of the induced escape response was directly tested: Daphnia which had experienced the presence of a cue from crushed conspecifics avoided attacks by common bream more eff iciently than naive Daphnia. The recognition of the signal originating from crushed conspecifics can be especially adaptive in encounters wi th unfamiliar predators and with predators that undergo ontogenetic sh ifts in their diet. Under natural conditions, the combination of such a signal with a predator cue can, very reliably, advertise the local s cale of the predatory impact.