Peter Rask Moller, a professor at the Copenhagen Museum of Natural History, warned male swimmers to closely protect their private parts this month after a fisherman in Denmark caught a South American pacu, a toothed relative of the piranha, CNN reported Friday.
He said the fish sometimes confuse the male sexual organs for tree nuts.
"Anyone choosing to bathe in the Oresund these days had best keep their swimsuits well tied," Moller said.
However, the professor said his warning was not meant to stir a panic.
"We did say that we recommend men to keep their swimsuits tied up until we know if there are more pacus out there in our waters," Moller said. "Of course, this is half a joke since it is very unlikely that you would actually meet one here and that it would bite you. It's up to people themselves how careful they want to be. I'll keep my shorts on, though."
William Fink, a piranha researcher at the University of Michigan, said the pacus are vegetarians and there have been no reports of the fish attacking humans.
"The nuts that they're eating, the fruits that they're eating, are splashing down from above, and humans don't act like that when they're swimming," Fisk said.