Male skinny dippers have been warned in Sweden about the dangers of the pacu, or “ball-cutter” fish, a relative of the piranha, The Guardian reported.

It’s a fish that’s become infamous for attacking testicles, which has been known to kill its victims by its vicious attacks.

The warning was issued by Denmark’s Natural History Museum after one of the eight-inch (21 cm) fish was caught in an eel trap located in the Oresund Sound off the south coast of Sweden.

A fish expert, Henrick Carl, at the Natural History Museum of Denmark told Swedish news site The Local that the pacu bite ”because they’re hungry, and testicles sit nicely in their mouth.”

Keep your swimwear on if you’re bathing in the Sound these days – maybe there are more out there!,” swimmers were warned.

Normally found in the warmer waters of the Amazon, the pacu are voracious and can grow up to 90cm (35 inches) and 25kg.