The candiru is a real fish: a tiny, slender South American catfish that lives by drinking blood from the gills of larger fish. Its fame comes from a story that it can swim up a stream of urine and into a human body, and the evidence says that story is far more legend than fact.1The Australian Museum. Candiru – careful where you go… The myth also fell apart partly because of how the fish finds its food, which points it toward other fish rather than people.
What a Candiru Looks Like
Candiru are catfish with no scales and thin, nearly see-through bodies.2Animal Diversity Web. Vandellia cirrhosa They look colorless until they feed, when the blood they’ve swallowed shows through.3Animal Diversity Web. Vandellia cirrhosa The largest reach about 17 cm, roughly the length of a pencil stub, but most are much smaller.4Animal Diversity Web. Vandellia cirrhosa
The body is narrow and tube-shaped, with a slightly flattened head and large black eyes on top.5Animal Diversity Web. Vandellia cirrhosa Whisker-like barbels sit near the mouth, which has tiny needle-like teeth.6Animal Diversity Web. Vandellia cirrhosa Those teeth suit piercing or scraping thin gill tissue, not tearing off chunks of flesh the way a piranha does.
Short, backward-pointing spines sit on the gill cover, the flap that protects a fish’s gills.7Animal Diversity Web. Vandellia cirrhosa Once the candiru is in place, the spines anchor it so it’s hard to shake loose.8Animal Diversity Web. Vandellia cirrhosa
The name “candiru” covers a group of small parasitic catfish, not a single species. There are several kinds, and scientists are still naming new ones from Brazilian rivers.9PubMed. A new candiru of the genus Paracanthopoma (Siluriformes: Trichomycteridae) from the Araguaia River basin, central Brazil The best-known, behind most of the stories, is Vandellia cirrhosa.
How Candiru Feed on Other Fish
Candiru live in shallow, slow, acidic rivers with muddy or sandy bottoms.10Animal Diversity Web. Vandellia cirrhosa They spend most of their time buried in the riverbed, coming out only to feed or mate.11Animal Diversity Web. Vandellia cirrhosa When a larger fish swims overhead, the candiru’s eyes pick it up and it leaves the bottom to approach.
When a candiru spots a host fish, it heads straight for the gills.12Animal Diversity Web. Vandellia cirrhosa It either forces its way under the gill cover or waits for the cover to open as the fish breathes.13Animal Diversity Web. Vandellia cirrhosa Inside, it latches onto the large arteries that run through the gills.14Animal Diversity Web. Vandellia cirrhosa The gill-cover spines hold it in place and help open the blood flow.15Animal Diversity Web. Vandellia cirrhosa
Gill tissue is thin and full of blood vessels so oxygen can pass from water into blood, which leaves blood just beneath the surface. The host’s breathing pulls water through the gill chamber, and that steady current gives a small fish a way to slip inside. Blood in the gill arteries is also under pressure from the host’s heart, so the candiru may not need to suck hard.
The candiru is a parasite rather than a predator. It takes blood instead of killing and eating its host, feeding in short visits before it detaches and drops back to the riverbed. Fishermen mostly know it as a pest of the fish they catch.
Is the Urethra Invasion Story True
People long believed candiru follow the chemicals in urine, because the fish they feed on release waste through their gills.16Wikipedia. Candiru (fish) But a 2001 lab study found candiru hunt by sight.17Talk Science To Me. The Candiru: A Six-Inch SciCom Failure – Talk Science To Me Researchers offered the fish different cues and watched which ones drew them. The candiru responded to the sight of a potential host and showed no interest in urine, ammonia, fish slime, or other chemical lures.18Wikipedia. Candiru (fish) With the scent trigger missing, the story loses its main reason for a candiru to approach a swimmer, and the belief that peeing in the Amazon is especially dangerous loses its footing too.
One old claim said the fish leaps out of the water and swims up the urine stream itself. That claim defies the laws of physics, since a falling stream can’t carry a fish upward.19Journal of Urology. FRI-03 CANDIRU: THE ‘URETHRAL INVADER’ FISH – MYTHS AND FACTS
Despite centuries of stories, the first documented case in a human wasn’t until 1997.20The Australian Museum. Candiru – careful where you go… Most famous candiru stories were relayed secondhand and never checked by a doctor. A single case doesn’t show the fish seeks out people or urine, and even if it’s real, an accidental entry while bathing is very different from a fish hunting a human. A travel-medicine review concluded there’s no need to include candiru in health advice for Amazon travelers.21Oxford Academic (Journal of Travel Medicine). Candiru–a little fish with bad habits: need travel health professionals worry? A review.
Where the Candiru Legend Came From
The first published report came from German biologist Carl von Martius in 1829.22Wikipedia. Candiru (fish) He never saw an attack. An interpreter relayed what local people told him, so each step added a chance for error.23Wikipedia. Candiru (fish) Men reportedly tied cords around their penises before entering rivers, which sounded like proof of a real danger.24Journal of Urology. FRI-03 CANDIRU: THE ‘URETHRAL INVADER’ FISH – MYTHS AND FACTS Yet the genital coverings some tribes wore may have been meant to guard against piranha bites instead.25Wikipedia. Candiru (fish) Martius also guessed that the smell of urine lured the fish, an idea later disproved.26Journal of Urology. FRI-03 CANDIRU: THE ‘URETHRAL INVADER’ FISH – MYTHS AND FACTS
In 1855, French naturalist Francis de Castelnau heard from fishermen that the fish leaps up the urine stream.27Journal of Urology. FRI-03 CANDIRU: THE ‘URETHRAL INVADER’ FISH – MYTHS AND FACTS These claims spread through local communities and drew worldwide attention.28Journal of Urology. FRI-03 CANDIRU: THE ‘URETHRAL INVADER’ FISH – MYTHS AND FACTS Each new version came from a European scholar, so readers far from the Amazon had no way to check it.
Should You Worry When Swimming in the Amazon
Among fishermen in the western Brazilian Amazon, most fish injuries were to the hands.29PubMed Central. Injuries caused by fish to fishermen in the Vale do Alto Juruá, Western Brazilian Amazon Candiru caused none of those; they were blamed only for a tiny share, 0.5%, of injuries to the abdomen.30PubMed Central. Injuries caused by fish to fishermen in the Vale do Alto Juruá, Western Brazilian Amazon Stingrays, piranhas, and other fish caused most of the foot injuries.31PubMed Central. Injuries caused by fish to fishermen in the Vale do Alto Juruá, Western Brazilian Amazon
Stingrays rest half-buried in sandy or muddy shallows, where a wading person can step on one without seeing it. A stung foot is painful and can become infected.
A few simple habits cover the real hazards. A snug swimsuit can keep any small fish from reaching the urethra.32Journal of Wilderness Medicine. Candirú: Amazonian parasitic catfish Shuffling your feet in shallow water helps warn stingrays away, so the ray usually swims off before you land on it. Swim where local people and guides swim, which tends to be clear or moving water rather than stagnant pools. Avoid wading barefoot in muddy shallows, and ask a guide before entering any unfamiliar spot, since they know where currents and animals are.
If a fish bites or stings you, clean the wound and watch for swelling, redness, or fever. Stingray stings need prompt medical care for pain and infection. A fish lodged in the body needs a doctor, never self-removal.