Swedish men have been warned by experts to keep their swimming shorts on in the Øresund channel between Denmark and Sweden after a "ball cutter" pacu fish was discovered by a fisherman.
The sharp-toothed pacu fish is known for attacking men's testicles and is a relative of the infamous piranha.
According to the Daily Telegraph, a 21cm pacu, which is mostly found in South America in the Amazon, was found last week.
Denmark's National History Museum warned: "Keep your swimwear on if you're bathing in the Sound these days - maybe there are more out there!"
Henrik Carl, fish expert at the museum, told The Local: "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."
Carl said he was unsure if there were more of the fish in Swedish waters but explained that the pacu looks just like a piranha.
"They are almost identical to the piranha, you couldn't even tell from the outside. It's just that they have different teeth. Flatter and stronger, perfect for crushing."
Win! A five-star British seaside break for two
Over 20 people attacked by piranhas in 'feeding frenzy' in Brazil
Giant fish-eating spiders reintroduced to British countryside