Not so tough: Pygocentrus nattereri, the red-bellied piranha.

Credit: Wikipedia

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PERTH: Piranhas have a fierce reputation - but it's a myth, say researchers who claim that the species shoals to evade predators not to engage in feeding frenzies.

"Previously it was thought piranhas shoaled as it enabled them to form a cooperative hunting group," said biologist Anne Magurran. "However we have found that it is primarily a defensive behaviour, and quite a complex one."

Magurran, from the University of St Andrews in Scotland, and Helder Queiroz of the Mamiraua Institute in Brazil, have found that piranhas seek safety in numbers. Though the Amazonian fish are depicted in popular culture as voracious man-eaters, the pair's research reveals that this isn't true.

Scavenging off carcasses

The findings are presented this week as part of the Summer Exhibition of the U.K.'s Royal Society, in London, along with a tank of live piranhas.

See a video of piranhas feeding, here on the Royal Society web site.

Magurran has been studying the Red-Bellied piranha for 12 years, while living in floating houses in the Amazon rainforest. The forest annually floods with waters rising and falling up to 12 meters.

During this time, she has found no evidence of piranhas ever devouring a live human – although the pair have seen piranhas scavenge off carcasses of humans and other animals already dead in the water. The fish typically just "graze" on their prey as it swims past, say the researchers, taking small, circular, bite-size chunks out of fish rather than consuming them whole.

"Contrary to popular belief – and their sharp teeth – piranhas are omnivores," Magurran says. "They are scavengers more than predators, eating mainly fish, plant material and insects."

Far from the villainous beasts that disposed of people in the 1967 James Bond film You Only Live Twice, Magurran found that piranhas are much like other fish, travelling in schools to minimize threats from predators such as river dolphins, caiman, aquatic birds and larger fish.

Fearful of humans

In fact, the little fish are so fearful of human contact that while Magurran and Queiroz were studying wild-caught fish in a tank, they had to erect screens to stop the fish hyperventilating (flapping their gill flaps more rapidly, indicating stress) every time the researchers came too close.

Further study of the shoaling behaviour suggested that shoal size depends on the threat level at the time: when waters are high, predation risk is lower and shoals tend to be small, but in drier conditions, the group size increases. These groups can range in number from as few as ten to as many as 100 fish at a time.

The team also found that shoals have a distinct structure, with older, reproductively mature fish protected in the middle of the group, while younger fish swim on the boundaries, where they have greater access to food.

"Their cautious behaviour is crucial to avoid being eaten," Magurran says.