CHEMICAL CUES FROM PISCIVORES INDUCE A CHANGE IN MORPHOLOGY IN CRUCIAN CARP

Crucian carp (Carassius carassius) develop a deeper body in the presen ce of northern pike (Esox lucius) and this has been suggested to be a predator-induced morphological defense. The evolution of inducible def enses requires that prey have reliable cues of detecting the predator. Here we investigate the nature of the cues used by crucian carp in a series of laboratory experiments. We monitored the change in body dept h of crucian carp when exposed to visual and chemical cues from predat ors and to alarm substances from conspecifics. Results show that chemi cal cues are sufficient to induce an increase in body depth of crucian carp. Alarm substances released by injured crucians had no effect on body depth, whereas pike, even when not feeding in the experimental aq uaria, induced an increase in body depth. This suggests that chemical cues released by the predator induce the change in crucian body morpho logy. Chemical cues from perch with a macroinvertebrate diet did not e licit a response, but when perch were fed crucian carp the body depth of crucians increased, i.e. the chemical cue is related to the piscivo rous diet of the predator. Removal of predator cues from the aquaria r esulted in a limited decrease of body depth. Crucian carp responds to chemical cues from predators with a piscivorous diet, rather than to t he predator per se, by increasing body depth. This should be adaptive in habitats with a suite of predators or when predators undergo ontoge netic niche shifts.