Candiru

The candiru (Vandellia cirrhosa), a diminutive parasitic catfish from the Amazon Basin, inspires a unique and deeply psychological form of dread. Rarely exceeding 15 centimeters, it is infamous for anecdotal reports of swimming into and lodging in human urethral or other bodily openings, attracted by the scent of urine or other fluids.

While such extreme incidents are poorly documented and debated scientifically, the confirmed danger of the candiru lies in its parasitism of fish. It uses sharp opercular spines to anchor itself inside the gill cavities of host fish to feed on blood. For humans, any attempted removal from a sensitive orifice would be agonizing and could cause tearing, hemorrhage, infection, and potentially require surgical intervention.

The sheer horror of its purported modus operandi has cemented its legendary status. Its danger, therefore, is a blend of verified biological parasitism and potent mythological fear, representing a nightmare scenario of a small, invasive creature exploiting the human body, making it one of the most psychologically terrifying freshwater fish.

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(National Geographic)
Candiru Perhaps the most feared freshwater species of all, candiru are parasitic catfish in the family Trichomycteridae. These small, narrow fish normally feed on the gills of larger fish in the Amazon. However, over the past few centuries there have been reports of these creatures lodging in the urethras of men and women. Some native peoples along the Amazon have described means of preventing such a disturbing infection, which is said to be hard to remedy, even with surgery.

Protection measures are said to include tough clothing, tying off extremities with ligatures, and avoiding urination around rivers. It was long believed that urine attracted the candiru, although a recent study showed that the animals displayed no interest in it. There are very few documented cases of candiru attacks on human beings, and a much discussed incident from 1997 has been met with considerable skepticism. Further, with an average width of a quarter of an inch and average length of three inches, the fish are a little large to enter a urethra. Still, the prospect remains frightening enough to keep many people out of tropical waters, even far away from the candiru's range.