Chemical Cues from Piscivores Induce a Change in Morphology in Crucian Carp

Crucian carp (Carassius carassius) develop a deeper body in the presence of northern pike (Esox lucius) and this has been suggested to be a predator-induced morphological defense. The evolution of inducible defenses 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 depth of crucian carp when exposed to visual and chemical cues from predators and to alarm substances from conspecifics. Results show that chemical 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 aquaria, induced an increase in body depth