Hold on to your testicles

The pacu, which has a set of gnashers like a piranha, lives in the Amazon and Orinoco rivers in South America.

Their awesome jaws are able to crack nuts and seeds which drop into the water from overhanging trees.

But their favourite meal is….a human testicle.

Fish expert Henrik Carl said: "The pacu is not normally dangerous to people but it has quite a serious bite.

"There have been incidents in other countries, such as Papua New Guinea, where some men have had their testicles bitten off.

"They bite because they're hungry, and testicles sit nicely in their mouth."

In recent years they have become popular tropical fish for people with exotic aquariums.

Then experts began noticing them cropping up in the wild, first in the US and later in Europe.

A pacu was found three years ago in Denmark, resulting in a warning to men to make sure their swimming trunks were securely fastened before venturing into the water.

In 2013 a pacu was also found in the River Seine in Paris.

The Paris Prefecture of Police newsletter reported "although officers were sceptical of the tropical catch, its human-like teeth marked the species as the feared 'testicle-eating' pacu."

Last year Ron Rossi and his son Frank caught a pacu in a lake.

Ron said: "We did pull the bottom lip down to see what they looked like and they have almost human teeth."

It is thought that in all three cases the pacu was released into the wild because it became too big for the aquarium where it was being kept.

Lars Skou Olsen, curator of Copenhagen's Blue Planet Aquarium, said: "They make excellent pets. The only problem is they grow too big for people's home aquariums. Many people mistake the fish for piranhas, and many pet stores sell them as piranhas.

"Often we have people calling us saying they have some piranhas that have grown too big for their tanks. And when you ask them how big the fish are, you know they can't be piranhas."

Peter Rask Moller, a fish expert at the University of Copenhagen, said: "Male swimmers should keep their pants on in case there are more pacus out there in our cold waters."

(dailystar.co.uk)