Piranhas not so fearsome?

Conservationists working in Brazil have uncovered evidence that indicates that the red bellied piranha may not warrant its grisly image. A team led by Anne Magurran at the University of St Andrews studied the fish in their natural habitat and in large tanks at the Mamiraua Sustainable Development Institute in Brazil. Their observations of piranha shoals suggest the fish may have a nervous disposition and huddle into shoals out of constant fear of being eaten by other predators.

In the Amazon, when water levels are shallow, the fish are more easily picked off by large predators including river dolphins, caymans (a relative of the crocodile) and big fish such as the pirarucu. Piranhas formed large shoals, gaining safety in numbers. Although larger shoals are easier to spot, predators find it harder to pick off a single fish because of the 'confusion effect', caused by the erratic movements of the shoal. Using giant water tanks and models of birds of prey, the team simulated airborne attacks on different shoal sizes.

They found piranhas in smaller shoals were more distressed and took longer to calm down. The nervousness was monitored by observing a structure near the gills which quivers rapidly with stress. "Our study has given us a better understanding of the function of shoaling. Previously, it was thought piranhas shoaled as it enabled them to form a cooperative hunting group. However, we have found it is primarily a defensive behaviour," said Professor Magurran.

Hmm, they’ll be drummed out of the predator club for this sort of timidity, I'm sure!