The story is that the fish swims up a stream of urine into a man's penis, then eats it from the inside. But is there any truth to it?

Of all the denizens of the Amazon basin, there is none more feared than the tiny fish known as the candiru. Since coming to the attention of science in the early 19th century, this creature has occupied the very darkest recesses of the popular imagination.

The reason for this is the candiru's supposed habit of entering the human penis, lodging itself in place with sharp barbs, and feasting on it from the inside – a horror story that is enough to keep your legs firmly crossed for days.

This tale has been told everywhere: from documentaries on the BBC and Animal Planet to Grey's Anatomy; from William S. Burroughs' Naked Lunch to Chuck Palahniuk's Fight Club; and invariably it serves as shorthand for the worst thing that could possibly happen to a human being. Internet forums abound with references to the fish, as well as grisly embellishments concerning its activities – laying eggs in bladders and suchlike.

So far, so disgusting. But it is not at all clear that any of this is true.

"Throughout the Amazon valley for more than a hundred years, the tale has been told of a fish that has the uncanny habit of penetrating the urethra of men and women bathers, particularly if they should pass urine while in the water."

A typical early account describes the candiru as "very small, but uniquely occupied in doing evil"

So begins a paper published in 1930 in the American Journal of Surgery by Eugene Willis Gudger. While this piece stands as a veritable candiru Bible, collecting and analysing accounts from as early as 1829, it mostly deals with with speculation, hysteria and urban legend.

The first challenge is identifying the fish. The Amazon is home to countless species of catfish, and several tiny, virtually transparent species have been labelled as candirus over the years. The one that is generally thought to be the culprit is the 5cm-long Vandellia cirrhosa. It is known to parasitise the gills of larger fish, feeding on their blood and generally making their lives a misery.

Despite their vampire-like feeding habits, these parasites are insignificant in the vast, muddy depths of the Amazon. Were it not for their fame as man-eaters, they would undoubtedly have sunk into obscurity in some Brazilian taxonomist's bottom drawer.

However, this is exactly the point where the story begins to veer into the realms of speculation.

A typical early account describes the candiru as "very small, but uniquely occupied in doing evil". Such melodrama is common in 19th-century writings on the fish, when a handful of European explorers came across people in the Amazon who regaled them with stories of this real-life river monster.

The fish do not discriminate, and will happily enter a convenient vagina or anus

In their tales, local communities lived in fear of a fish that in a moment could inflict upon them the most excruciating pain conceivable. These creatures, it was claimed, were even more feared than the ferocious schools of piranha that also inhabited these waters.

The German botanist Carl Friedrich Philipp von Martius was the first European to document candirus in the Amazon. He described how local men tied their urethras shut when spending time around the water.

George Albert Boulenger, Curator of Fishes at the British Museum, outlined an intricate device assembled from coconut shells and palm fronds. Even more elaborate is a description of a convoluted system of bath houses that had apparently sprung up in the depths of the Amazon, allowing natives to draw water from the river without ever having to immerse themselves in it.

The authors mostly focus on the vulnerability of penises to attack, perhaps because they themselves are all men. But many emphasise that the fish do not discriminate, and will happily enter a convenient vagina or anus.

It gets worse. Some writers claim that candirus can leap from the water and vigorously ascend streams of urine to reach their target. There are also stories of candirus biting holes in the flesh of passers-by in order to enter and feed on their blood.

Despite all the graphic depictions of genital mutilation, not one of these men ever witnessed a candiru attack

Perhaps most horrifying of all are the remedies offered for a candiru in the privates. While some sources optimistically recommend a hot bath, or herbs capable of dissolving the fish, the verdict is virtually unanimous: the best way to get rid of the parasite is to remove the offending member altogether.

"The only means of preventing it from reaching the bladder, where it causes inflammation and ultimately death, is to instantly amputate the penis," declared Boulenger, describing a trip taken by one of his colleagues. "At Tres Unidos, Dr Bach had actually examined a man and three boys with amputated penes [sic] as a resuIt of this dreadful accident."

The thing is, despite all the graphic depictions of genital mutilation, not one of these men ever witnessed a candiru attack. There are dozens of reports from the 19th and early 20th centuries of candiru behaviour, and every one relies exclusively on hearsay.

As WR Allen, a renowned Amazonian ichthyologist, put it: "I was toId of numerous cases of the candirus entering the urethra, but they were always some distance downstream, and when I arrived downstream I was told of many such cases upstream".

So has the candiru been miscast as a penis-chomping villain?

According to the limited contemporary research that has been undertaken, it would seem so. As more consideration has been given to the candiru, scientists have attempted to explain why these fish would attack humans in such a way.

The candirus seemed totally uninterested in any chemical cues

After all, in reality doing so would mean certain death for the fish. Away from their aquatic environment, and trapped in a small tube no larger than the width of their body, they would not stand a chance.

The most compelling idea is that the waste ammonia secreted through the gills of fish is the means by which candirus locate their prey. If that is true, urea could be similar enough to confuse these parasites into swimming up a stream of urine.

In 2001, Stephen Spotte of the University of Connecticut at Avery Point and his colleagues set out to test this idea, without putting anyone's genitals at risk.

"How candirus can feed successfully in turbulent, muddy water, and often at night, suggests refined sensory adaptations, possibly the capacity to detect characteristic tastes and odours emanating from the prey," they reasoned.

Should one not expect by now a few confirmed cases in the medical literature?

The researchers compared the behaviour of candirus when presented with live fish and with potential chemical attractants, such as ammonia. Their findings were fairly conclusive: the candirus seemed totally uninterested in any chemical cues, instead appearing to respond with gusto to the sight of a delicious goldfish.

So, in the absence of any quantifiable reason why a candiru would confuse a human with food, we at least need to see some well-documented contemporary examples of candiru attacks if the whole thing is to stand up to scrutiny.

"Considering the alleged voracious habit of the little fish, the geographical size of its habitat, and the considerable number of people living along the river system, should one not expect by now a few confirmed cases in the medical literature?" asks Irmgard Bauer of James Cook University in Townsville, Australia in a 2013 paper.

Yet in the past few decades there has only been one solid report of a candiru attack.

The year was 1997. In Manaus, the isolated capital of the Brazilian state of Amazonas, a patient was wheeled in with a candiru in his urethra. After hours of surgery, urologist Dr Anoar Samad managed to extract the fish, dragging its carcass back out through the unfortunate patient's penis.

In order to swim up a pee stream, the fish would have to swim faster than the stream flow

This incident was subsequently published by Samad. It remains the only first-hand example of such a procedure in medical literature.

This could very well be the end of the story, were it not for one man. Stephen Spotte, who performed the chemical attractant experiment, met Samad in 1999 to discuss the incident.

Spotte is the only person to have ever dedicated themselves to seriously investigating the candiru myth. When he approached Samad, he was presented with photos, a video of the procedure and even a preserved specimen. Despite this, he had reservations about the story.

First, the patient insisted that the candiru had ascended his urine stream before violently lodging itself in his urethra. This may accord with travellers' tales, but according to biomechanics expert John Bertram of the University of Calgary in Alberta, Canada, it is also patently ridiculous.

"In order to swim up a pee stream, the fish would have to swim faster than the stream flow [and] lift itself out of the water against gravity," says Bertram. "In any event, even if the candiru could power itself up the stream, it would have to stay completely within the urine and that would be difficult."

Unless we write these men off as liars, it is important to investigate their claims further

Essentially, the idea that a candiru can propel itself into a penis in this way is not just unlikely, it is against the laws of fluid dynamics.

Next, there is the specimen. Not only is Samad's proudly preserved fish far too large, it bears no signs of having inserted itself anywhere. The doctor claimed that he had snipped the spines off the fish, which would have been crucial if the fish were to be removed successfully, but the preserved fish was entirely unblemished.

Still, Spotte is unwilling to entirely write the incident off. "I mean, [the patient] didn't even know what candirus were, so it's hard to believe that he invented the story," he says. Nevertheless, when pressed on how likely he feels a candiru attack actually is for someone urinating in the Amazon, Spotte describes the chances as being "about the same as being struck by lightning while simultaneously eaten by a shark".

But what about all of those accounts by early explorers?

What emerges from their writing is a nightmarish picture of riverside communities scared to go near the water for fear of candiru attack. They describe infrastructure, rituals and medical procedures, all established to deal with these parasites. Unless we write these men off as liars, it is important to investigate their claims further.

There is still a great potential for misinterpretation of language, postures, and gestures

In her 2013 analysis of candiru literature, Bauer asks a seemingly obvious question: do these parasites pose a threat to the millions of people visiting the Amazon region every year? In doing so, she compares the experience of travelling in the Amazon now with how it was in the 19th century.

For those intrepid early travellers, penetrating the dense forest and hearing endless stories of bizarre creatures, it would have been difficult to sort fact from fiction. "In such circumstances, a first report, relayed with caution, can quickly take on a life of its own and, embellished with more and more gruesome details, eventually become a fact," says Bauer.

Language barriers would also have been a problem. A lingua franca is widely used in the Amazon, based on the language spoken by the Tupi, one of the area's most important ethnic groups. The European interlopers may well have spoken it, but we can safely assume they were not fluent. In this scenario, writes Bauer, "there is still a great potential for misinterpretation of language, postures, and gestures."

All this means that genuine practices could have been misinterpreted. Specially-constructed penis guards for protection against, for example, piranhas, could be misidentified as anti-candiru technology.

There are still stories of strange animals that have never been properly investigated

Similarly, tea made from the fruit of the jagua tree was allegedly used by natives to "dissolve" candirus lodged in the urethra. It does nothing of the sort, but it might be useful against a much more common affliction with similar symptoms: kidney stones.

With enough half-truths, a convincing story can be assembled almost by accident.

What ultimately emerges from this fishy tale is a message of science triumphing over superstition. While scientists have long since consigned unicorns and dragons to the story books, there are still stories of strange animals that have never been properly investigated. Some of them, like the candiru, may well turn out not to be true.

It is important never to underestimate the power of the natural world to amaze, but it is equally important to carry on questioning everything, and not to let horror stories scare us away from the water.