Candiru Fish

"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.
Indeed, the fish appears not to have any response to any chemical attractants, and primarily hunts by visual tracking. Bathers in the region are, however, warned not to urinate while swimming, out of concern that the expansion of the urethra during urination will allow the fish to enter the otherwise closed orifice.