Field Report: The Vampire Catfish of the Amazon
Dr. Icky investigates the candiru — a tiny, translucent catfish that lodges itself inside the gills of other fish to drink their blood. And yes, we'll talk about the legend.
Specimen classification
Type
Parasitic catfish
Lives
Amazon basin, South America
Size
2 to 3 centimetres long
Diet
Blood
The candiru is a tiny, practically invisible catfish that makes its living by swimming into the gills of larger fish, locking itself in place, and drinking their blood.
But its biological reality is often overshadowed by a famous, terrifying legend about what happens when humans swim in its river.
Candiru Fish
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The Gill Invader
Candiru are so small and translucent that they are nearly invisible in the murky waters of the Amazon. They don't hunt normally. Instead, they track the chemical trails of ammonia that other fish release from their gills while breathing.
When a candiru finds a host, it swims straight into the gill cavity, which is dark, warm, and full of blood-rich tissue.
Science bit
The candiru is the only vertebrate known to parasitize another vertebrate by entering its body cavities to feed on its blood.
The Barbed Grip
Once inside a tight space, the candiru has to make sure it doesn't get washed out. It does this by deploying a set of backwards-facing spines that open like an umbrella.
These spines dig into the host's tissue, locking the candiru firmly in place. Then, it uses its needle-like teeth to bite an artery and feast. The whole meal takes only a few minutes before the candiru drops off and sinks back to the river bottom to digest.
Danger
Because the spines point backwards, pulling on the fish only makes them dig deeper. The only way out is for the candiru to let go.
The Legend
The candiru is most famous for a horrific legend: that it mistakes human urine for fish gills and swims up human urethras. Stories claim that once inside, it locks its spines, requiring painful surgery to remove.
While there is one documented medical case from 1997, many scientists are skeptical that it happens often, or even that candiru are actually attracted to urine. However, the fear alone is enough to keep many people out of the water.
Weird detail
True or not, the candiru has earned a reputation as the one fish locals fear more than piranhas.
Dr. Icky's verdict
“Whether the human stories are completely true or mostly myth, a translucent vampire catfish that lodges itself in gills with backwards spikes is a verified biological nightmare.”
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