Catfish are a fairly diverse group of fish, familar to aquarium keepers and fans of fried food. They vary in size from the over two meter long Giant Mekong Catfish (Pangasius gigas; a record 646 pound specimen is above) to a finger-length parasitic species, the candiru (Vandellia cirrhosa, see below). Thanks to a reminder by Tara (and ultimately, PZ), it’s the candiru that I want to write about today.
The candiru (as Fishbase notes) “[e]nters the gill cavity of larger fish to suck blood … Forces itself under the gill cover of host fish to enter gill chamber during ventilation of the latter. Bites mostly at the ventral or dorsal aorta arteries, and the blood is pumped into its gut by the host’s blood pressure. It does not need any special sucking or pumping mechanism to quickly engorge itself with blood, but simply uses its needle-like teeth to make an incision in an artery.”
Fair enough – the fish enters the host through the gill chamber and latches on for a meal – nothing terribly horrible or worrying here, particularly as this is a relatively quick process – the “[t]ime required to engorge itself with blood and leave host’s gill chamber ranges from 30 to 145 seconds.” However, as Fishbase also note, the species is known “to enter the urethra of humans urinating under water; presumably it mistakes the urea for water exhausted from gills.”
My problem is not with what the candiru does, after all it’s just doing what natural selection molded it to do, and it certainly didn’t evolve to parasitize humans. No, my problem is the legend that seem to have grown up around the wee beastie. Take, for example, the website that Tara and PZ mention which presents the fish as an active seeker of human victims:
The Candiru, you see, has a nose for urine. When it gets hungry, it sniffs the current of its stream or river for a urine trail, then follows the trail upstream to the source: someone pissing into the water … You may have heard a simpler version, in which the Candiru swims right up the Urethra, right up somebody’s penis. That would actually be less horrible than the real method. Instead of wriggling right up the urethra and lodging there, the Candiru wriggles up the victim’s anus, then gnaws its way into the urinary tract.
Or this version:
However the reason that the Candiru is most feared by humans is because it is the only vertebrate known to parasitize humans! The fish is said to be addicted to the taste and smell of human urine. Candirus parasitize humans, when they are skinny-dipping while urinating in the water. The candiru tastes the urine stream and follows it back to the human. It then swims up the anus and lodges itself somewhere in the urinary tract with its spines. Blood is drawn, and the candiru gorges itself on the blood and body tissue, its body sometimes expanding due to the amount of blood consumed.
Once inside it would eat away the mucous membranes and tissues until hemorrhage would kill it or the host. It was also said that even if one caught the fish by the tail, once in the urethra it could not be pulled out because it would spread itself like an umbrella. The Candiru can attack both men and women. Penectomy is generally preferred to the misery and pain associated with leaving the fish in the urethra.
Horrific stuff … but perhaps a tad hyperbolic when you consider that recent investigations “could find no candiru allegedly removed from a human urogenital or rectal orifice preserved in a reputable museum, hospital, or academy for scientific identification” and that there has been only one confirmed candiru “attack” on humans (in 1997)(source). Indeed, that incident did not involve any entry through the anus:
A candiru had entered the urethral canal through the penis, made its way through, [and being blocked by the sphincter separating the penile urethra from the bulbar urethra] taken a hard lateral turn and bitten into the spongy tissue [corpus spongiosum] of the scrotum.
Nasty stuff indeed. Of note is that, if the fish was blocked by the sphincter in the urethra, it would more than likely not be able to penetrate the anal sphincter, move into the rectum, and burrow in to the urinary tract.
In the candiru we see a species wonderfully adapted for what it does. What we don’t see – by all the available evidence – is a species that regularly feeds on humans. As David Hume said in On Miracles, “[a] wise man, therefore, proportions his belief to the evidence.” A wise man, it appears, has little to fear from this particular catfish.
On to more metaphysical matters. Let us allow the legends to be true. Is one justified in claiming that
[t]he Candiru is so exaggeratedly awful that it would be wasted as evidence against the idea of a benevolent God. It works better as evidence for Schopenhauer‘s version of the natural world against Darwin’s. … This, after all, is not just another beastie doing its best to get by in a tough old world. It’s too gratuitously awful for that. It’s proof of a bad world, made by a giggling psycho who was proud of his work.
No, it’s not. Candiru is not “gratuitously awful” and mapping moral ideas onto an amoral natural world (though tempting) is flawed. The only thing it is evidence for is the power of evolutionary mechanisms to shape organismal diversity.