Predator recognition and defence strategies in crucian carp, Carassius carassius

Crucian carp from populations that lack piscivores are extremely vulnerable to predation. However, in the presence of piscivores these fish develop an inducible morphological defence, a deep body. This switch from a vulnerabl e, shallow-bodied morph to a morphologically defended morph makes this spec ies very suitable for investigations of anti-predator strategies, and trade -offs between morphological and behavioural defences. To address these ques tions, we performed eight different experiments. We found that crucian carp exhibited fright responses to chemical cues from unfamiliar predators (nor thern pike, perch) when these were fed prey that contained alarm substance (for northern pike: crucian carp, roach; for perch: crucian carp). Cues fro m small pike that were fed prey that lacked alarm substance (swordtails) ca used no significant fright response whereas cues from larger pike with the same diet did. Perch on a chironomid diet elicited weaker but significant f right responses. Starved predators caused as strong fright reactions as rec ently fed ones did, whereas no response was exhibited towards nonpredatory fish (roach, crucian carp). Crucian carp were able to detect the presence o f pike after cues had been diluted to an equivalent of 21000 l, and larger predators elicited stronger fright responses. Prior experience of predators decreased fright responses. In particular, individuals from populations th at coexisted with northern pike responded less to chemical cues from northe rn pike than individuals without prior experience did. Thus, crucian carp m ay use both alarm-substance related and predator-related cues to identify p redators. Further, they were able to discriminate between large and small p redators. Finally, individuals from populations that coexist with predators exhibit less pronounced fright responses. These fish have an induced morph ological defence, a deep body, which most likely decreases the need for str ong antipredator behaviour.