Zip up your pants in the Amazon River!

Parasitic behavior of the Candirú towards humans

Throughout the Amazon valley for more than a hundred years, tales has been told of a fish that has the uncanny behaviour of penetrating the urethra of men and women bathers. Local fishermen claimed it was very dangerous to urinate into the river because of a type of catfish which launches itself out of the water and penetrates the urethra by ascending the length of urine stream (Gudger, 1930).

Introduction to the Candirú: Horror story: Candirú: the Toothpick Fish – Weird Nature (Video)

Meet the Candirú (Trichomycteridae), a blood sucker which is commonly found feeding in gill cavities of other fishes. The Candirú can be described as a translucent, eel-like fish of about a couple of inches long as illustrated in the figure. The feeding process of the parasitic Candirú can be described briefly in the following steps:

  1. The scent trail of ammonia which streams from the host (larger fishes) gills is subsequently picked up by the Candirú. Ammonia is the principal form of nitrogenous waste produced by fish and excreted primarily at gills (Spotte, 2002)
  2. The Candirú inserts itself inside the gill flap and the spines around its head latches onto the scale of the host. This helps anchor the Candirú in place while feeding from its hosts by biting the proximal and medial parts of the gill filaments, which would then bleed freely into the alimentary tract of the candiru (Machado & Sazima, 1983).
Another noted behaviour of the Candirú is swimming against the water current, as the same action is required get past the water which is squeezed outwards from the gills in the larger fish which they parasitize (Breault, 1991).
Q. So why does the Candirú display parasitic behavior towards a human host?

This would be related to the cues which the Candirú gathers before feeding. While urea is the primary form of nitrogenous waste excreted by humans, ammonia, which serves as the attractant to the  Candirú, is also present. Moreover, Burton (1972) suggested that the Candirú would mistake the flow of urine for the exhalant stream of water from a fish’s gills. A combination of these two factors is the main cause of the numerous parasitic attacks on humans.

The ordeal is described to be so painful and traumatic, that the victim’s sins debts for his current and following life would have been paid off. An unfortunate case of mistaken identity indeed.

Attack on human (Case study): River Monsters: Fish Swims Up Urine Stream (Video)




  • Breault, J.L., 1991. Candiru. Amazonian Parasitic Catfish. Journal of Wilderness Medicine, 2 (4), 304–312.
  • Burton, M., 1972. The World Encyclopedia ofAnimals. New York: World Publishing, 93 pp.
  • Gudger, E.W., 1930. On the alleged penetration of the human urethra by an Amazonian catfish called candiru with a review of the allied habits of other members of the family pygidiidae, Part I. American Journal of Surgery , 8, 170-88.
  • Machado, F. A. & I. Sazima., 1983. Comportamento alimentar do peixe hematófago Branchioica bertonii (Siluriformes, Trichomycteridae). Ciência e Cultura, 35 (3): 344-348
  • Spotte, S., 2002. Candiru: life and legend of the bloodsucking catfishes. Creative Art Book, Berkeley, 322 pp.

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