This afternoon, while I was sweating out my frustrations via one of the gym's cross-trainers, I listened to the May 20th podcast from The Skeptics Guide to the Universe. Among other things, they discussed the candiru, a small freshwater Amazon fish that allegedly swims up the urine stream of someone urinating (stories aren't clear as to whether you have to be waist-deep in the water, or above it). I've always been a bit suspicious of this one - I mean, why do the stories always talk of this odd little fish attacking men, & not women? - so I was interested to hear what the team had to say.
While there are a lot of anecdotes concerning the 'toothpick fish' (the name tells you something about their size & shape), it turns out that there's only one documented case of a candiru swimming up a man's urethra (& having to be surgically extracted). The patient visited a doctor, suffering considerable pain, & reported that he'd been thigh-deep in the river, urinating, when a candiru leapt out of the water & into his urethral opening. Now, while I can't imagine how else a fish got up there (one of the Skeptics made a suggestion), it does sound as if the thing had some sort of internal GPS - how else could you explain its accuracy? And - while the doctor then described removing the (deceased) pesky piscean, there's no other evidence supporting the patient's story of how it arrived there. A more likely explanation was that he was at least waist-deep at the time.
But none of this explains why a candiru would do this. The Skeptics team talked of the fish being parasitic on mammals, swimming into various unprotected orifices & lodging there to feed & breed. And this is where I have to part company with them. This is a fish we're talking about. Fish use their gills to obtain oxygen from the water they swim in. Inside a mammal (& let's face it, we're talking anus/rectum, urethra/bladder, vagina, & possibly mouth/pharynx & ears or nostrils - yeuch, I get squirmy thinking about it!) there is no flow of fresh, oxygenated water &, with the exception of the bladder, no fluid to support the gills. So the fish would suffocate in fairly short order - hardly a strategy for a successful parasite.
When I looked into it a bit further, I found that candiru are parasites - on other fish. They find their hosts by detecting the 'respiratory stream' - the water passing out of the host's gills, which contains traces of urea. They swim into their target's gills, presumably via either the mouth or the gill opening itself, and are anchored there by spines on their heads. The candiru then chews through to one of the gill blood vessels, gorges on blood, & then drops out of the host until hungry again. (If the host is lucky it won't bleed to death from the opened vessel...) Vampire fish!
So candiru are not out there cruising specifically for people, but nonetheless they would probably find the urea present in a human's urine stream, rather attractive. And the ensuing mistake would be fatal for the fish that made it, & extremely painful (& potentially lethal) for the human involved. I guess the moral is: don't go skinny-dipping in the Amazon!