The Pacu fish, also known as the Ball Cutter, has powerful teeth which look like a set of human molars (projectavalon.net)
Men have been warned to be very careful when swimming in Scandinavia, due to sightings of the pacu fish, which is known to eat male genitals.
The South American pacu has a reputation for devouring the tenderest parts of the male anatomy. The fish have been found in the strait between Sweden and Denmark.
Henrik Carl of Denmark's National History Museum says it normally eats smaller fish but told the Sun that the pacu's teeth were "perfect for crushing".
He explained: "Their mouth is not so big, so it normally eats nuts, fruit, snails and small fish, but human testicles are just a natural target."
Fishermen have bled to death after having their testicles ripped off.
The pacu's normal habitat is the Amazon, and this is the first time the fish has been found in Europe.
Pacu have square, straight teeth with a slight overbite, which look remarkably human. Fully grown pacu are much larger than piranha, reaching up to 0.9m (3 feet) and 25kg (55 pounds) in the wild.
Humans have had tasty revenge on the pacu. Former US president Theodore Roosevelt wrote of catching and eating pacu in his book Through the Brazilian Wilderness. He described them as "good-sized, deep-bodied fish," and noted, "They were delicious eating."
Jeremy Wade, a British angler, specialises in hunting for pacu, also known as 'The Ball Cutter'.
He told the Daily Mail: "I had heard of a couple of fishermen in Papua New Guinea who had been castrated by something in the water.
"The bleeding was so severe that they died. The locals told me that this thing was like a human in the water, biting at the testicles of fishermen. They didn't know what it was.
"It is a hot and dirty area so the people would often go to the water with their children to wash but obviously they were very worried about this thing in the water."
Watch the video about the testicle-eating pacu fish spotted in Illinois, USA