Quick, name a vicious fish from the Amazon River that kills humans.

Pffft! You said, “Piranha,” didn’t you? Don’t lie, you did.

Well, I don’t know if the piranha just has a poor press agent (or a good one, depending on the fishy creds we’re trying to establish), but under most circumstances, piranhas don’t kill.

That’s not to say that they aren’t dangerous. Those teeth certainly are sharp, and people occasionally lose fingers and toes to piranhas. Most attacks occur when there is a lot of other food such as fish entrails floating about, but their reputation as a fearless killer is a myth.

The piranha is a scavenger, mainly eating off of things that have already died. They only rarely attack live prey, and almost never kill.

But wait a minute, you say, didn’t Teddy Roosevelt witness an entire cow being devoured in “under a minute”? He did indeed, but that was a setup, with purposely starved fish. Despite their reputation from the movies, you don’t have much to fear from these sharp-toothed fish unless you are wading through chum.

But there is another fish in the Amazon River that poses a serious risk to humans, especially to guys.

Meet the pacu.

Chris, pictured here, is now known around the village as Christine

Now you might already be getting an uneasy feeling just from the sheer size of that thing. While the pacu are related to the piranha, they are much larger, reaching up to three feet in length and 55 pounds. You could feed a large family with just one of these monsters.

Teddy Roosevelt also wrote about the pacu in his book Through the Brazilian Wilderness, but only to pronounce them “delicious eating.” You would think he would have mentioned the teeth.

[ We're the pacu! We're the pacu! We're the pacu! Chomp, chomp, chomp! ]

And as weird and disturbing as that mouth full of human-like molars looks, what they do with those teeth is even worse. Although the pacu is not native to Papua New Guinea, it was released there in the 90′s as a food source and has since been dubbed the “ball cutter,” which is every bit as bad as it sounds.

While the pacu mainly use their teeth for cracking seeds and nuts, it’s apparently not too selective about which nuts to crack.

At least two fishermen in Papua New Guinea have been castrated by the pacu since it’s introduction and subsequently bled to death.

There is even an unsubstantiated report that a 24-inch pacu actually jumped out of it’s aquarium in Fort Worth, Texas, in order to bite its owner on the testicles. These fish apparently zero in on the crotch like an ornery three-year-old. Losing a finger doesn’t sound so bad now, does it?

The pacu are sold by pet stores as a “vegetarian piranha,” but they can quickly outgrow a home aquarium. Some unprepared owners have been known to release their fish into lakes and streams when they grow too large for their tanks.

When the pacu recently made headlines after having been found in an Illinois lake, biologists were quick to point out that it is a tropical fish and could not possibly survive the winter.

More ominously, for US swimmers, pacus have been found in 19 states, including warmer states like Florida, Texas, and California, where it’s chances of establishing a large population would presumably be much better.

It sounds like shrinkage could actually be desirable if you’re swimming with a fish like that.