Are those Human Teeth?
The first time I saw these photos I thought it was some kind of hoax.
Pacu fish are several species of freshwater fish from South America. The word pacu is a Brazilian Indian term.
Despite their relation to the piranha, their teeth and jaw alignment differ; the piranha has that scary-looking underbite with sharp pointed teeth while the pacu have either a less-pronounced underbite or a mild overbite with straighter teeth that look creepily human.
Pacu fish are also much larger than piranhas, a troublesome characteristic as they are popular in home aquariums due to the fact that they can be somewhat responsive to their owners. Full-grown pacu can grow up to almost a meter long, much to the distress of aquarium owners who may not have the space for such a large species. This leads to the release of pacu into foreign environments where they either die from cold temperatures, or thrive to out-compete native species as they potentially also bring disease and new parasites with them. Recommended appropriate disposal methods for too-large fish include just eating them.
Fun fact: The pacu was once erroneously reported to commonly mistake testicles for fruit or nuts. This was intended as a joke and not actually reflective of fact. The fish likely won’t bite your testicles unprovoked, though they are omnivorous and attracted to movement. Larger fish have reportedly bitten people, drawn by wiggling fingers in the water. The teeth enable them to eat hard foods like seeds and nuts which can lead to a serious injury if they mistake you for food. They aren’t as aggressive or meat-hungry as piranhas are, just a little stupid in that endearing fishy way.
Additional source: NAS Fact Sheet
Image source: International Business Times,