Candiru also known as toothpick fish, is a parasitic freshwater catfish native to the Amazon River. Although some candiru species have been known to grow to a size of 16 inches (~41 cm) in length, others are considerably smaller. These smaller species are known for an alleged tendency to invade and parasitize the human urethra.

Candirus are small fish. Adults can grow to around 15 cm with a rather small head and a belly that can appear distended, especially after a large blood meal. The body is translucent making it quite difficult to spot in the turbid waters of its home.

Although there are plenty of scary anecdotes of attacks on humans, there is only one documented case of a candiru entering a human orifice. In this instance, the victim had a candiru swim into his urethra as he urinated while thigh-deep in a river. The victim underwent a two-hour urological surgery to remove the candiru. Apparently, the candiru has spikes to assist it clinging to its host.

The biologist who investigated the incident said: "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."

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.