  File under: You learn something new every day. The candiru is one creature you definitely don't want to get too close to. Like many of the more than 2,000 species of catfish in the world (there are about 1,200 in South America alone), Vandellia cirrhosa possesses a set of barbels that resemble feline whiskers.
But this diminutive specimen -- it is only about half an inch to three inches long -- has a rather dicey trait not shared with others in its family: It is the world's only vertebrate that parasitizes people. It can anchor itself in a most intimate part of the human anatomy, leading one jokester to dub it Urinophilus diabolicus.
In English it is candidly known as the urethra fish, because it can actually swim up the urethra of an unsuspecting bather and lodge itself firmly in place by flaring sharp spines along its gills. Not only does this make for a most excruciating ordeal, but once embedded, the cold-blooded little terror can't be removed by anything short of surgery. Candirus primarily set up house inside larger fish, where they feast on the host's blood. They are attracted to nitrogen, which usually leads them to a gill chamber, but apparently they can't distinguish between one nitrogen-emitting orifice and another: They have been known to follow a stream of urine right to its source.
Candirus are hard to spot because they are nearly transparent. So if you're swimming in an Amazon jungle, here's a tip: Don't micturate in the water, and be sure to wear tight swimwear. 
