Pacu fish found in Sweden

The sharp-toothed pacu is the mostly vegetarian cousin of the piranha. (Henrik Carl )

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Swedish skinny dippers, you may want to pull on your swim trunks.

A sharp-toothed pacu fish has been caught in the Danish-Swedish strait of Oresund, putting swimmers on edge because of its resemblance to a piranha.

But Scandinavian fish experts say this fish is a mostly vegetarian cousin of the piranha. Still, they have advised swimmers that if they do venture in the water they should "keep their pants on."

It's an eyebrow-raising directive from the University of Copenhagen. Are the professors there having a bit of fun, suggesting that the fish pose a particular danger to male genitalia?

"All we suggested was that swimmers keep their pants on until we know if there are more of these fish out in our brackish waters," Peter Rask Møller, a professor at the University of Copenhagen, said in an email to the Los Angeles Times. 

Like its cousin the piranha, the pacu is native to South America, specifically the Amazon basin. It is mostly a vegetarian fish, munching on fruits, nuts and the occasional fish or small invertebrate. But its human-like teeth are sharp enough to chomp through a fishing line or even a finger, according to a release put out by the university.

So far only one pacu has been caught in Scandinavia, and it is only the second to have ever been caught in Europe. But pacu have been found in waters throughout the world, probably because they have been dumped there by overwhelmed aquarium owners.

"Pacu are commonly kept in aquaria because they are fairly docile, but look like piranhas," explains Peter Moyle, a fish biologist at UC Davis. "They grow quickly in captivity and, unlike goldfish, will outgrow aquaria, so owners release them into whatever pond they can find locally."

Although there have been some rumors of fishermen in Papua New Guinea having their genitals attacked by pacu fish, Moyle said he cannot verify that.

Both Moyle and Rask Møller agreed that the tropical pacu is unlikely to become established in Scandinavia's chilly waters, unless it can find pockets of warmer waters in which to thrive.

"Our winters are cold of course, but fishes like the pacu might survive in outlets from power plants," Rask Møller said. "So for now, we keep our pants on."

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