Piranhas have been unfairly demonised by Hollywood and are far from the voracious killers of popular imagination, research has shown. The South American fish have a reputation for ganging up in killer shoals to tear the living flesh from anything daft enough to swim within reach.

In the James Bond film You Only Live Twice, a tank full of piranhas was the villainous Blofeld’s favourite means of disposing of unwanted guests and inefficient staff. And in the 1978 B-movie Piranha they terrorised a holiday resort.

But according to scientists, the fish, despite mouths bursting with sharp teeth, have been given a raw deal and, if not exactly cuddly, are more victim than villain.

Rather than form aggressive shoals of more than 50 fish to search and destroy larger animals in the waters of the Amazon, they group together as a defensive measure, said Professor Anne Magurran, of the University of St Andrews.

There are so many large predators, notably caiman and river dolphins, trying to eat them that they seek safety in numbers. “Contrary to popular belief — and their sharp teeth — piranhas are omnivores. They are scavengers more than predators, eating mainly fish, plant material and insects,” Professor Magurran said.

“Piranhas are under constant attack from a number of large predators including river dolphins, caiman and some of the world’s largest fish, such as the piracucu — which can grow up to three metres long. Their cautious behaviour is crucial to avoid being eaten.”

She worked with Helder Queiroz of the Mamirauá Institute to investigate piranha behaviour in a flooded forest forming a reserve half the size of Wales in Amazonian Brazil. Their findings will be displayed from today at the Royal Society’s annual Summer Science Exhibition. They were able to dispel “the myth” that the only reason piranhas group together is because they intend to go hunting in a pack. When the fish have formed their defensive shoals, the biggest and strongest piranhas are found at the centre where it is safest.

Smaller, less mature fish are left to take their chances at the edge of the shoal, but there are advantages because this means that they get first bite at any food the shoal comes across.

Professor Magurran added: “Previously it was thought piranhas shoaled as it enabled them to form a co-operative hunting group. However, we have found that it is primarily a defensive behaviour and quite a complex one.

“There is a distinct structure. But there is no ‘lead fish’ or co-operation — the piranhas pretty much look out for themselves. The bigger, older fish tend to swim in the middle as they are reproductively mature and need to keep safe, whereas the outer layer of the shoal is made up of smaller, younger piranhas.

“Being at the edge of the group means they can get access to food sooner — essential if they are to grow more quickly and mature.”

Shoal sizes are determined by the risk of being attacked by predators, which in turn is influenced by water level. The Amazon goes through wet and dry seasons and when the water levels are lowest, the threat to piranha is at its highest.

Low water levels mean all the animals in the river system are much more bunched up, making it easier for predators to find food and giving the prey fewer places to hide. Water levels can rise 12 metres (40ft)each year because of seasonal rain and meltwater from the Andes.

The exhibition opens at the Royal Society in Carlton House Terrace, London, from today until Thursday.

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