The candiru became the most famous catfish in the world in 1997, when an icthyologist read a medical report about a man who had to have this toothpick-shaped fish removed from his urethra. The truth of the claim has always been in dispute, but people have feared peeing in the Amazon river ever since.
They needn't have been afraid. Researchers couldn't test whether one candiru had got itself stuck inside one man, but they could test the notion that candiru were urine-seeking missiles. For some time, people who lived by the Amazon believed that urine attracted the fish. There is some scientific evidence to back up the claim. Candiru are vampires; they suck the blood of other fish, usually by attaching themselves to the inside of the fish's gill. The prey fish of the candiru give off urea from their gills. It made sense that candiru would pursue the chemical.
But when scientists actually got a group of candiru in a tank, the bloodsuckers weren't interested in pursuing anything that didn't look like a fish. They were indifferent to all the potential chemical attractants, including "fish slime," ammonia, and, yes, human urine. The only thing that got them excited was a live fish put in the tank. While candiru have been known to try to sink their teeth into humans, urine probably has nothing to do with. Pee away!
Image: Sketch from 1846, Public Domain.