The Candiru: A Six-Inch SciCom Failure

This is one story that should never have been.

Science communication is a careful interplay of priorities. Words and approaches must match the knowledge base of the audience, always bringing them to a better-informed place than where they were before, and align with the writer’s goals. Scientific knowledge is our collective best estimate of the grand truth of the cosmos, and it demands respect even when its finer points must be left aside to get messages across. When truth gets in the way of a good story, it is truth that must prevail, and our role as science communicators is to make sure a good story happens anyway.

The people who introduced the Western world to the candiru had other priorities.

Fair warning: This story deals with some difficult medical details involving human genitals and catfish.

“Candiru” is the common name for several species of South American catfish, deriving from a name in the Tupí language (candirú) spoken throughout South America. In English-language writing, candiru usually refers to several catfish in the family Trichomycteridae, and especially to Vandellia cirrhosa, also known as the vampire fish. The name provides some useful information about this fascinating creature: this fish drinks the blood of other fish. The natural habit of V. cirrhosa and a few other trichomycterid catfish is to slip into the gill chambers of other fish, attach to their blood-rich gills using spines, and drink.

This fascinating and rare diet for a fish was, however, not enough for the people who published initial Western accounts of the candiru. Rather than seek truth and present it in the form in which they found it, these writers mingled half-truths, far-reaching conclusions, and failures to ask follow-up questions into a lurid tale that has since proven to be one of the most stubborn urban legends about nature ever conceived. Ask a layperson about the candiru, and if they recognize the name at all, they will ask, “Isn’t that the fish that lodges itself in human urethras?”

Such is the tale of the candiru: the fish that, supposedly, makes a habit of sniffing for human urine and charging headlong into it to lodge itself in human urethras, to the point of being able to ascend out of a body of water through a urine stream to find its human prey.

This is a good time to mention that, although there are several parasitic candiru species of varying sizes, V. cirrhosa approaches 15 centimetres in length as an adult.

Nothing about this story is true. The candiru is totally uninterested in the smell of urine; it rarely attacks humans; it is completely incapable of ascending a urine stream; and on the rare occasions that a human does encounter a candiru orifice-first, urethras are not involved. But the sheer graphic spectacle has driven the idea into the public imagination as surely as the candiru drives its spines into the gill chambers of its intended prey, and the myth persists to this day in books, television shows and other media.

Stories like this do not come from nowhere. Longstanding misapprehensions of nature often evolve from misinterpreted folk wisdom, but this time, total breakdowns of scientific communication going back over two centuries are squarely to blame. A timeline:

1. In 1829, German biologist Carl Friedrich Philipp von Martius published an account of Indigenous customs from the candiru’s range. His account included that they would make a point to cover their genitals underwater to protect themselves against candiru attacks. In addition, von Martius stated that the candiru was drawn to the smell of urine. It turned out that von Martius did not speak the local language and misunderstood precautions against piranha attacks, and that his idea that the candiru was drawn to urine was pure speculation.
2. In 1836, German botanist Eduard Poeppig relayed a secondhand story about a woman who had had a candiru lodged in her vagina. Per the report, locals used plant extracts to help dislodge the errant fish.
3. Around the same time, biologist George A. Boulanger described groups of Indigenous men who had had their penises amputated and attributed these surgeries to the aftermath of candiru attacks. As before, these accounts did not involve speaking to the people being described and almost certainly referred to the aftermath of piranha attacks. It was later verified that candiru did not inhabit the relevant streams.
4. In 1855, another naturalist, Francis de Castelnau, published a hearsay account that the candiru could and would swim up urine streams to lodge itself in human urethras. De Castelnau himself dismissed this story as preposterous in the same text in which he related it.
5. It was not until 1891 that a firsthand account of candiru-human parasitism would appear, thanks to naturalist Paul Le Cointe, whose story was much like Eduard Poeppig’s, including the candiru’s orifice of choice.
6. Similarly, it was not until 1997 that a modern account of a candiru attacking a human would appear, written by Dr. Anoar Samad, and this story ticked every box of the candiru urban legend. In Samad’s account, the candiru swam up a urine stream into a man’s urethra and Samad himself performed the surgical extraction. The publicity surrounding this incident likely did more than any of the previous stories to cement the candiru in the public imagination as a species of urethra-seeking spine missiles.
7. However, inquiry into this incident by Stephen Spotte in 1999 revealed numerous inconsistencies between the written report, video of the surgery and the candiru specimen itself. Spotte also pointed out the reappearance of long-discredited ideas such as the journey up a urine stream, which is physically impossible. Taken together, the inconsistencies Spotte discovered suggest that this account was at least partly fabricated or embellished.
8. In 2001, Stephen Spotte showed in a lab that the smell of urine does not draw candiru and the species seeks its prey by sight.

For two centuries, researchers spread stories of genital-attacking catfish in far-off lands without consulting local experts, verifying details or separating speculation from data. Their half-baked stories, preserved specifically for their lurid appeal, created a persistent error in laypeople’s minds about what a candiru is and what it can do, facilitating modern grifters in the process. The researchers’ failure to prioritize accurate, effective scientific communication made these fascinating creatures into fairy-tale monsters. However, the most recent public-facing work on the candiru is finally setting the record straight, thanks to communicators like Dr. Spotte.

So, here’s the candiru’s reality:

Vandellia cirrhosa and other fish like it are part of an extremely small number of vertebrate species that are obligate parasites. As adults, they are comparable in size to pencils. They seek out larger fish by sight and lodge themselves in their targets’ gills to drink their blood. On vanishingly rare occasions, they slip into other human orifices comparable in size to the gills of large fish. None of the historical accounts of candiru behaviour reflect their actual biology. The candiru’s relevance to humans who live where they live is minimal. Piranhas, on the other hand, are pack-hunting carnivorous scissors attached to a fish’s body—and they demand to be taken seriously.

Among our hopes here at Talk Science to Me Inc. is that future scientific communication about species like the candiru avoids sensationalizing them and presents their reality in accurate terms that convey the natural splendour of our world as it is, not as the next generation of clickbait.