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Swedish men have been told to stay out of the water after a gonad-gobbling fish known as the "ball cutter" was spotted in the wild.

The warning was issued after fisherman caught what appears to be a 21cm-long pacu fish in the Oresund Sound between Denmark and Sweden.

Related to the famous Amazonian piranha, the pacu is famous for its taste for nuts of both the human and vegetable kind.

Unlucky swimmers in the Amazon and Orinoco regions – where the ball-cutter is normally found – have bled to death after the fish chowed down on their crown jewels.

Henrik Carl, a fish expert at the National History Museum in Denmark, issued this stern warning: "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. And its mouth is not so big, so of course it normally eats nuts, fruit, and small fish, but human testicles are just a natural target. It's not normal to get your testicles bitten off, of course, but it can happen, especially now in Sweden."

Swedish men, known for their predilection for naked swimming, have now been advised to cover up when they get into the sea. Although judging by the following description of the pacu fish, Swedes won't need much persuading to keep their trunks on.

"They are almost identical to the piranha; you couldn't even tell from the outside," added Carl. "It's just that they have different teeth. Flatter and stronger, perfect for crushing."

This is the first time a pacu has been spotted in European seas and DNA tests are now being carried out to suss out whether the creature the fisherman caught is a genuine ball-cutter. "This one was the first, but who knows, it's probably not the last," Carl warned. ®

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