The Candiru, or Toothpick fish. is a parasite that normally lives in the gills of larger fishes where it feeds on the host's blood. Its spiny gill covers anchor it under the operculum of the larger fish. The Candiru was believed to find its host by following the ammonia-scented water ejected from the gills of the larger fish. It is now believed that the fish finds a host by visual cues.
The Candiru was thought to follow the smell of ammonia in human urine. In fact, despite many "stories" of this fish entering the bodies of humans urinating in rivers, the first documented case was not until 1997. Investigation has since revealed that the fish is not 'attracted' to human urine.
So what actually is a Candiru? Opinions seems to vary. Some people regard only Vandellia cirrhosa to be the true Candiru. Others broaden the definition to any species in the genus Vandellia or any fish in the subfamily Vandelliinae. By any definition however, the Candiru is a very slender, translucent fish, growing to about 15 cm in length that is only found in the Amazon River. It is most common in the brown, low pH waters around the junction with the Rio Negro near the city of Manaus.
The exploits of the Candiru often appear, (see the movie above), to be blown out of proportion. Nevertheless, if you plan to swim in the Amazon River, maybe it would be best to go to the bathroom beforehand.
Candiru. Wikipedia. http://en.wikipedia.org/wiki/Candiru.(viewed online 12 September 2010)