The Amazon is the largest river in South America, as well as a notorious candidate for some of the deadliest fish found in the world. It is home to about 2, 500 species of animals. From the largest freshwater fish, the Arapaima, to the famous red-bellied piranha, with gnashers sharp enough to strip its meal down to the skeleton in just moments, it is not a place you would like to choose for a peaceful swim.
If you have heard of the Amazon River and rainforests, you would have also heard about the Candiru fish. Like the piranha, this tiny little catfish is also the villain of many fearsome stories and a subject of dread among many people.
Also known as “the pencil fish”, “the toothpick fish” and “the vampire fish”, the Candiru is a parasitic freshwater catfish found in the Amazon basin. It is a small species, which can only grow to 40 cm at most. The fish has developed some notoriety, thanks to various stories claiming its jumping into people’s urethra.
The Candiru feeds on the blood of its hosts. It forces the operculum (the gill opening used for breathing and feeding) of its host open, or sometimes waits for it to open, and attaches itself to an internal blood vessel and slashes it. It feeds on the blood coming out from the wound. After about two to three minutes of feeding, it leaves the host and retreats to the river bottom.
The earliest known account of a Candiru attacking a human was made by German biologist Carl Friedrich Philipp von Martius, in 1829. In his report, Martius claimed that the native people of the area would go into the river with a protective cover around their genitals to prevent Candiru attack. Later, many stories of different Candiru attacks started to emerge, but most of them went unverified. In a number of the cases, Candiru entered the vaginal canal, not the supposed urethra.
So far, there has been only a single documented incident of a Candiru entering a human urethra. The incident took place in 1997, at Itacoatiara, Brazil. A 23-year-old man reported a Candiru jumped from the water into his urethra while he was urinating in a river. The man underwent a urological surgery by Dr Anoar Samad to remove the fish from his body immediately. However, there were many discrepancies in the story, as was apparent in the observations made by American marine biologist and Candiru expert Dr Stephen Spotte.
Even though it might seem fascinating, there are a lot of factors that prove the stories surrounding Candiru to be just myths. Due to the presence of urea in urine, some early doctors speculated that the fish was attracted to urine. But experimental studies proved that it was a false assumption. The fish hunted by sight, not smell. Another major myth was that it could swim directly up a stream of human urine. But according to the laws of fluid dynamics, this is impossible. The fish is too fragile and small to be able to withstand both the downwards gravity and the force of falling urine. Also, the act of entering the urethra is suicidal for the fish as well. With no oxygen or space, it won’t survive even a minute.
Besides that, there is a lot of scepticism regarding the fact that the Candiru attacks humans. All the dark rumors surrounding the Candiru might prevent you from knowing the facts, such as that, just like a cheetah, it relies mostly on stealth by camouflaging using its translucent appearance and sneaking up on its prey.
Given the lack of evidence, it is safe to presume that the Candiru is not attracted to human urine, and neither has a tendency to enter into the human urethra. The myth emerged from many false accounts of people over several years. Even the chances of the fish attacking when a person is urinating while submerged in a stream inhabited by Candiru is, as Dr Stephen Spotte stated, “about the same as being struck by lightning while simultaneously being eaten by a shark.”
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