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The candiru is a legendary monster. This parasitic Amazonian catfish is said to swim up into the urethras of unsuspecting bathers and devour their genitals from the inside out.

It's a terrifying tale—and one that we've actually told here on SciShow a couple of times. But science has shown that this story is just that—a story—and one that is too awful to be true. As far as scholars can tell, the tales began in the early 1800s.

European explorers and naturalists trekking through the Amazon basin heard about the candiru from indigenous communities. As they explained in their reports, they were told the parasitic fish is attracted to human urine, so you should never pee in the Amazon river. You shouldn't even pee into the water from dry land—the fish can supposedly swim up a stream of urine to find its way into a person's urethra.

And once that happens ... well, there were some removal methods suggested. Some herbs can supposedly be used to dissolve the offending parasite. But the most effective method of removal is, of course, prompt amputation.

And all of that makes for a great monster myth—but it doesn't jive with science. The candiru of legend is most likely the South American catfish Vandellia cirrhosa. It's a parasitic catfish that inserts its head into the gills of larger fish to drink their blood, so it might not be that far out to think it could insert itself into people, too.

But it's about five centimeters long and about a centimeter wide. That's small enough to fit in the gill openings of the fish it targets, but not small enough to slip into a human urethra. A urethra could be pried open, in theory, but these fish don't have any appendages to do that with.

Also, if a candiru did somehow enter a human urethra, it would quickly suffocate—so it wouldn't be chilling up there drinking blood. But the biggest problem with this whole story is that the candiru isn't attracted to human urine. Scientists used to think these parasites sought out the nitrogen-containing wastes—basically, pee—that fish excrete from their gills.

That's because these catfish often hunt in turbulent, muddy waters at night, so it would make sense that they'd rely more on chemical attractants rather than visual cues to find their meals. And if they were sniffing for nitrogen-containing compounds like ammonia and urea to find their prey, it follows they'd also zero in on human urine since it contains some of the same stuff. But in 2001, marine scientist Stephen Spotte tested this assumption in the lab.

He and his team exposed candiru to a range of attractants—fish slime, ammonia, amino acids, and, yes, even human urine—and the candiru didn't go for any of them. The catfish did respond to the sight of goldfish, though—which suggests that, despite the murky depths it hunts in, it actually relies on eyesight to find its meals. Which brings us back to all those stories about pee-seeking fish.

As a 2013 review paper notes, medically confirmed attacks by the fish on humans are very hard to find. Most of the stories were perpetuated by a few 19th century European explorers who might have misinterpreted oral accounts—accidentally, or willingly, for the sake of a sensational tale. None of them seems to have witnessed actual cases.

In fact, there's only one candiru-penis incident that's been medically documented. In 1997, a man in the Brazilian city of Manaus was allegedly brought into the hospital with a candiru wedged painfully in his urethra. A urologist was able to remove the fish after hours of surgery, and he wrote the whole thing up and published the case.

But, other scientists question the veracity of this account. For one, the patient insisted that the fish swam up his stream of urine. This is an impossible feat: the small fish would have had to swim faster than the stream of pee to climb it, which is challenging, since the average human urinates at a brisk 8 to 15 milliliters per second.

The fish would have also had to fight the pull of gravity while swimming upward, and to make matters more complicated, it would have had to stay completely submerged in the urine since the air around the urine is not dense enough for the fish's fins to push against. And that's not likely since a person's urine stream is generally pretty thin compared to a centimeter-wide fish. Now, some fish can jump pretty high—salmon, for instance, can leap up to 2 meters—but it's not clear these catfish are able to jump out of the water at all, let alone well.

And even if one did leap at a person's genitalia, as we said before, it has no way to widen the urethral opening to get inside. And there's no reason the fish would even do that in the first place since it isn't attracted to urine. There are also inconsistencies with the urologist's findings.

He apparently kept the surgically removed parasite. And, according to Stephen Spotte—who visited the urologist and saw the infamous intruder first hand—the fish was far too large to fit in a urethra, and didn't show any signs of having been inside a human being. For example, it still had its spines—which the doctor had claimed were removed during the surgery.

With all these inconsistencies, it's almost certain that this case is a hoax—and, again, it's the only documented case. In the end, there are several scientific reasons why these fish wouldn't do this awful behavior. So if you decide to go swimming in the Amazon, your urethra should be safe from parasitic catfish.

You might want to watch out for the piranhas, though. Or the electric eels. Or the bull sharks, or caimans….

I'm just saying, the Amazon is still a dangerous place! Thanks for watching this episode of SciShow! If you liked learning the science behind this below-the-belt myth, you might like the truth-seeking done on our podcast SciShow Tangents.

Every week, four people who work on SciShow get together to talk about science related to a central topic. There are different segments, like one where someone presents one true fact and two fake ones and everyone else has to try and figure out which is true. With stories like the candiru out there, it can be kind of hard to tell what's real and what's made up!

Also, they tend to go on lots of sciency tangents, hence the name “SciShow Tangents”. You can check it out wherever you get your podcasts! [OUTRO ♪].