Rehabilitating piranhas

A University of St Andrews biologist has discovered that, despite their reputation as being fierce and ferocious, piranhas are fearful fish.

Following field work in the Brazilian Amazon, Professor Anne Magurran and her colleague Dr Queiroz have revealed that red- bellied Amazonian piranhas gather in groups for protection rather than hunting their prey in packs. And, despite their sharp teeth and man-eating image, they live on a diet of fish, fruit and small invertebrates like shrimp – and some related species are completely vegetarian.

Professor Magurran, who is based in the University’s Gatty Marine Laboratory, studied the fish with Brazilian scientist Dr Helder Queiroz in the Mamirauá Reserve in Brazil. They found that piranhas swim in large shoals – not to enable them to attack large prey but as a defence mechanism against their many predators, including dolphins, larger fish and birds.

Professor Magurran said, “Many myths have grown up around piranhas – they’ve been in James Bond movies and are portrayed as one of nature’s most feared killers. In actual fact, piranhas are much more worried about being eaten than they are concerned with attack. Under most circumstances, a human walking into a stream full of piranhas would emerge unscathed!”

She also observed that piranhas became nervous in captivity and had to put screens around the tank because the fish began to hyperventilate at the very sight of humans.

The researchers experimented by placing the fish in tanks in groups of two to eight. As reported in the Royal Society journal Biology Letters, they found that breathing rate increased with smaller schools. Another experiment simulated an attack by a cormorant and found that, although all the piranhas breathed faster in response, those in larger schools returned to normal sooner. The fish found safety in numbers.

Professor Magurran said that, in their flooded forest habitat, the fish might be protected by natural cover like branches and other vegetation, possibly making the size of the group less important. Either way, though, the researchers learned something about the fish’s true fearful nature. “We thought it would be quite neat to do work on piranhas because so little is known about them,” Professor Magurran said. “But this notion that they were fearsome fish, frightened of nothing – we had to revise that. They’re basically like regular fish – with sharp teeth”.

ENDS

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