Candiru Fish

It was also once thought that the fish was attracted to urine, as the candiru's primary prey emits urea from its gills, but this was later discredited in formal experimentation.
"The candiru usually targets a big fish as its host, drinking blood from its gills, but occasionally they make mistakes. This has resulted in one of the most infamous legends to emerge from the Amazon: a man urinating in the river who has a fish swim up his penis." – Jeremy Wade, biologist.