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The Debunker: Do South American Fish Want to Swim Up Your Urethra and Bite You?

by Ken Jennings

It's August, and that means one thing in warmer climates: people spend more time in nature, and nature, in turn, tries to bite them. August is mosquito season, it's snakebite season—hell, even Shark Week is in summer. But lots of the thing we know about summer's flesh-nibbling threats are dead wrong, and that's why we have Ken Jennings from Jeopardy! lurking here in the underbrush to tell us, at long last, the real truth about summer's bitey pests.

The Debunker: Do South American Fish Want to Swim Up Your Urethra and Bite You?

If there's one thing the Internet loves, it's a weird animal story—the grosser the better. This is just an escalation of a fascination that spans human history, from Greek philosophers credulously noting hard-to-believe stories of exotic monsters in Asia and Africa to Victorian amateur naturalists heading to the Amazon with butterfly nets. Certainly nature is full of marvels and mysteries, so why shouldn't there be bigger and weirder ones the harder you look?

The Debunker

But technology has given us the power to spread these dubious stories much faster than ever before. The story of the candiru fish—a little Amazon catfish notorious for its habit of swimming up the urethra of male swimmers, planting itself with pointy little barbs, and parasitically feasting on their genitals from within!—was catnip to the Internet, endlessly circulated on listicles of scary animal encounters and hard-to-believe trivia. All that repetition gives the impression of well-established scientific truth. In fact, there's every reason to believe that the tales of the creepy candiru are a myth.

The legend of a fish that would swim up a urine stream and attack an innocent peeing Amazonian is centuries old. No one is 100 percent sure which species the natives were talking about here, but one thing is certain: the laws of fluid dynamics make that kind of upstream swim impossible. Also, all the accounts in the literature rely on hearsay; none has ever been observed firsthand. (There are a few accounts of Amazon Basin fish having to be removed from the human vagina, which are more plausible than the urethra stories from a size perspective, at least.)

The lone documented case of a candiru being removed from a human urethra happened in 1997 in Manaus, Brazil. But marine biologists who have studied the case are skeptical about the surgeon's claims. Besides the physical impossibility of the uphill urine swim, the fish supposedly removed was much too big to have been lodged in the way the doctor claimed, still had all its spikes somehow intact following the removal, and didn't have the teeth to do the kind of tissue damage described. If you ever go swimming in the Amazon, there are plenty of rational things to worry about. But don't worry: being attacked by the mythical penis fish isn't one of them.

Quick Quiz: The candiru is among the smallest of the catfish. The largest catfish in the world, which can grow almost ten feet long, is only found in what Southeast Asian river?

Ken Jennings is the author of eleven books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.