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Skinny-dippers in Sweden are quickly putting their trunks back on after a South American fish with a fondness for eating testicles was found in the sea.

A pacu, nicknamed the 'ball cutter', has been found in the sea between Denmark and Sweden - the first time one has been spotted in European waters.

A fisherman landed the eight-inch relative of the piranha in the Oresund Sound, prompting male swimmers to stay on the beach with legs firmly crossed.

A spokesman for the National History Museum in Denmark warned: "Keep your swimwear on if you’re bathing in the Sound these days – maybe there are more out there!"

(Image: Rex Features)

The pacu can grow up to 35 in and can weigh up to 55 lb. and is usually found in the Amazon basin and Papua New Guinea.

It is almost identical to a piranha but has flatter, stronger teeth which are perfect for crushing and fishermen have reportedly bled to death after losing their testicles to the fish’s jaws.

Danish 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.

"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."

Mr Carl said that the discovery of one pacu – which is currently undergoing DNA tests to confirm its identity – should not keep Swedish men out of the water.

But if more were found in the area, it could become a serious issue, he suggested.

‘This one was the first, but who knows, it’s probably not the last,’ he added.