by Bill Schutt
Vendellia cirrhossa is probably the best-known species of candiru, although Vendellia wieneri has the coolest name. Candiru (which typically range from one to six inches in length) prey on larger fish and they feed by wiggling under their hosts’ gill covers (opercula), the flaplike structures that shield the gill chambers. These opercula open and close as the fish breathes, and once inside, the tiny vampires secure themselves to the delicate gill lamellae (which are arranged like pages in a book) by a series of tiny, backward-facing hooks. These integumentary teeth or odontodes (sometimes called denticles) are found in patches around the candiru’s head (including their own gill-covering opercular and interopercular bones). Once secured, the candiru utilize two or more rows of needlelike teeth to bite through one of the blood vessels that function in gas exchange between the feathery, high surface area gills and the fish’s body.*145 Apparently, muscles in the candiru’s mouth and pharynx pump the blood into the digestive tract.†146
The candiru feeds for somewhere between thirty seconds and nearly three minutes, the blood clearly visible through its engorged, nearly transparent body. Since they sometimes feed in groups, the blood loss and damage to the host’s gills can be considerable as pieces of shredded gill lamellae are sliced off by the teeth and rasped off by the odontodes.
Do candirus ever swim up the human urethra? Apparently, it does happen, although thankfully, occurrences are extremely rare. There have been numerous anecdotal descriptions, reviewed in depth by Spotte, but the first confirmed attack was reported by Drs. Spotte, Paulo Petry, and Anoar Samad at the 2001 meeting of the American Society of Herpetologists and Ichthyologists. Dr. Samad, a urologist, had treated a young man who showed up several days after a swim in the Amazon during which he had decided to remove his swim trunks before urinating.
On 28 October 1997, one of us (Samad) attended a 23-year-old man who sought medical attention with extreme swelling and bleeding, having been attacked by a candiru. After extraction by endoscopy, the candiru measured 134 mm SL and 11.5 mm width across the head. Perfusion of the urethra with sterile distilled water prior to endoscopy induced immediate and pronounced scrotal edema. The candiru’s penetration had been blocked by the sphincter separating the penile and bulbar urethras. Subsequently, the fish had bitten through the tissue into the corpus spongiosum, and the opening had allowed the perfusate to enter the scrotum. Some coagulated material was removed, revealing a wound on the bulbar urethra 1 cm in diameter and associated with a small amount of local bleeding. Because of its poorly preserved condition, the specimen could not be positively identified. However, it is likely that it is either a species of Vandellia or Plectrochilus.
According to a November 26, 2002, article in the Niagara Falls Reporter, by Pulitzer Prize winner John Hanchette, the man, identified only as FBC*147 was discharged after five days in the hospital, reportedly with no long-term effects from the encounter. The interesting thing, however, was that the patient’s story had changed, igniting a firestorm of controversy that swept through candiru conspiracy buffs. FBC now claimed that he had not been submerged in the river when the attack occurred. Rather, he was standing in thigh-high water when the candiru leaped from the river, darted through the urine stream, and lodged itself in his penis. After losing what can only be envisioned as a furious, but short-lived battle, FBC watched in horror as the five-and-a-half-inch candiru disappeared into this traumatized chouriço.
Could this happen? University of Calgary biomechanics expert Dr. John E. A. Bertram doesn’t think so.
“In order to swim up a pee stream, the fish would have to swim faster than the stream flow. Additionally, to climb the stream, the candiru would have to lift itself out of the water against gravity. And while a small fish might be able to jump a surprising distance through the relatively low resistance air—a stream of urine would be another story.”
“Why’s that?” I queried.
“Because the penis tip acts as a very efficient nozzle,” he said, “creating a stream of specific form, largely due to its high velocity of flow. Presumably, this keeps us from urinating on our feet. 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.”
“How come?” I asked.
“Because the narrow urine stream has boundaries with the relatively low-density air. If the fish wandered into this boundary, the low resistance flow of the air would destabilize it—essentially pulling the candiru out of the high-drag pee stream.”
I mentioned this explanation to Stephen Spotte, who agreed with Bertram. “I just don’t see how it’s possible,” he said. “The candiru would be trying to swim, and the lateral sweeps of its tail would be wider than the urine stream. So in addition to destabilizing its body, it wouldn’t be able to generate the thrust it needed.”
“So was this Brazilian guy just making it up?”
“I don’t think so,” Spotte said. “I mean he didn’t even know what candirus were, so it’s hard to believe that he invented the story. I still think he was, you know, pissing right at the surface—and then it would be possible.”
“How so?”
“If you’ve ever watched these things feeding in an aquarium, they shove themselves right under a fish gill—I mean it’s a rapid, violent motion. It happens instantaneously—you can’t even see it. So it’s possible that this fish got up there so quickly that the guy didn’t even have time to react. That part I believe.”
“How long do you think a candiru could survive inside a human urethra?” I asked.
“Catfish can live a long time in pretty dire circumstances,” Spotte replied, and I noticed that his voice had taken on an ominous tone.
“How long are we talking about?” I asked, a little fearfully. “A couple of minutes?”
“More like a couple of hours,” he said, “although this one was certainly dead by the time they pulled it out of the victim.”
I was almost afraid to ask the next question. “When was that?”
“They did the surgery three days after the guy came in. Besides the pain in his penis, I can’t even imagine what it was like not being able to urinate for three days. Dr. Samad said that his abdomen was swollen like a soccer ball. He was a tough dude.”
So why would a candiru abandon its normal, gill-feeding lifestyle for a visit to the Telegraph Office? In his book, Stephen Spotte reviewed several current hypotheses.
In the “urine-loving hypothesis,” the candiru seeks to embed itself in a mammalian urethra with its ultimate destination being the urinary bladder.
“This whole idea of candirus being attracted to urine is problematic,” he told me. “For one, urine is not listed among the major food groups.” And in fact, no other vertebrates are known to feed solely or even predominantly on urine.
Then there was the oxygen—or lack of it actually. Oxygen is a requirement for every other vertebrate on the planet, and in a urine-filled bladder there wouldn’t be any. Additionally, urine would be far saltier than the fresh water of the Amazon and the temperature would be significantly higher as well. All in all, it was a trip that sounded more suicidal (or accidental) the more I learned about it.
Interestingly, these apparent drawbacks didn’t stop Carl Eigenmann, one of the top ichthyologists of his day, from proposing that “further study may demonstrate that some species of Candirús have become parasitic in the bladders of large fishes and aquatic mammals.”
Once again, Stephen Spotte was not convinced. “Paulo Petri and I did some experiments, both in the field in sort of a quasi-laboratory situation, and we got no response at all.”*148
Another hypothesis proposes that candirus become accidentally embedded in the human urinary tract owing to a behavior called rheotropism, which is defined as the movement of an organism in response to a current. The idea here is that the candiru mistakes the flow of human urine for that of a natural flow of water (like the current produced as water leaves the gills of a large host fish)
. In what has become known as the “wrong turn hypothesis,” after the candiru mistakenly enters the urethra, its backward-curved odontodes prevent it from turning around. The trapped creature continues forward until it dies from a lack of oxygen.
Yet another hypothesis posits that the candiru tracks its piscine prey by the chemical trail the larger fish leaves in its wake. If chemicals typically found in human urine (possibly ammonia, the protein albumin, and creatinine—a breakdown product of muscle physiology) also serve to stimulate the candiru’s host-hunting behavior—this might explain the creature’s attraction to the human urethra. Sweat has also been suggested as an attractant, although this doesn’t explain why the candiru headed for the tip of FBC’s penis.
“It could certainly be something in the urine,” Spotte told me. “It could also be the urine flow itself.”
I decided to try out my own hypothesis. “What about the possibility that the candiru are reacting to a disturbance in the water—at least initially. They think they’re reacting to a large catfish, and by the time they get close enough, it’s too late.”
“The problem with that is the fact that Paulo and I caught many of these fish in the middle of the night, in rapids—I mean you couldn’t even stand up in this water. And yet we tethered a catfish out there and within thirty minutes the candirus found it. Now they weren’t detecting movement because this catfish wasn’t swimming.”
“So what do you think the candiru are detecting?” I asked.
“Nobody really knows, but my best guess would be that it’s a combination of things. Catfish can taste and smell—they’ve got all these really well-defined sense organs, specialized for dealing with low-visibility environments. Probably, they sense something in the fishes’ protective slime—which is sloughed off continuously, just like we drop old hairs. Candirus also have an array of pores around their heads, so it’s also likely that they’re detecting the electromagnetic charges generated by things like muscle contraction. In any event, it’s an interesting problem and it just goes to show catfish are amazing creatures.”
“What about attacks on nonhuman mammals,” I asked, adding that I thought it strange not to have come across any in the literature.
“None,” Spotte replied. “We haven’t got any evidence. But you’re right. Why don’t they attack dolphins and manatees and river otters—they’re certainly releasing stuff into the water—and they’ve got some very large orifices.”
The scientist continued. “As far as human encounters with candirus, I just can’t think of it as anything other than an accident. But how it arrives there—that’s still a mystery.”
I asked Dr. Spotte a final question. “What do you think the odds are that someone submerged in a stream where candiru live, and deciding to take a pee, would get attacked by these creatures?”
Dr. Spotte replied without hesitation. “About the same as being struck by lightning while simultaneously being eaten by a shark.”
What was so thought provoking about all sorts of Galápagos finches to young Charles Darwin…was that they were behaving as best they could like a wide variety of much more specialized birds on the continents. He was still prepared to believe, if it turned out to make sense, that God Almighty had created all the creatures just as Darwin found them on his trip around the world. But his big brain had to wonder why the Creator in the case of the Galápagos Islands would have given every conceivable job for a small land bird to an often ill-adapted finch? What would have prevented the Creator, if he thought that the islands should have a woodpecker-type bird, from creating a real woodpecker? If he thought a vampire was a good idea, why didn’t he give the job to a vampire bat instead of a finch, for heaven’s sake? A vampire finch?
—Kurt Vonnegut
10.
A TOUGH WAY TO MAKE A LIVING
In addition to vampire bats, leeches, bed bugs, ticks, mites, and candirus, there are literally thousands of species that feed on blood. They range from intestinal nematodes called hookworms, that produce iron-deficiency anemia in their hosts,*149 to mosquitoes, one of the eleven families of fly-relatives (dipterans) with blood-feeding members. There are assassin bugs (which may have plagued Charles Darwin) and vampire moths (seven species in the Asian genus Calyptra). These insects use their sharpened mouthparts to pierce the skin of animals like water buffalo, elephants, tapirs, and even humans.
Then there’s Geospiza difficilis, the pint-sized vampire wannabe that so impressed author Kurt Vonnegut. Although dramatic, “vampire finch” is not a terribly accurate name for this bird, which is also known as the sharp-beaked ground finch. Basically, the vampire tag is questionable, because unlike the other sanguivores mentioned thus far, Geospiza difficilis is not an obligate blood feeder.*150 The birds merit mention here, however, because they appear to be giving part-time vampirism a serious shot. In that regard, they occasionally supplement their normal diet of small seeds and nectar by eating eggs and by pecking at the wings, body, and tail regions of another Galápagos resident—the blue-footed booby (Sula nebouxi). Once Geospiza difficilis has inflicted a small wound with its beak, it begins sipping the booby’s blood, hopping out of the way if the larger bird gets annoyed. The finches feed in this manner for several minutes, giving way to other individuals who line up, waiting their turn like customers at a deli counter.
Geospiza difficilis is widely distributed throughout the Galápagos archipelago, but only two populations feed on blood. Researchers have noted differences in feeding behavior, size, and vocalizations in these birds—differences that could indicate that a new finch species is forming before their eyes. For my own part, it’s just as interesting to imagine what might happen if Geospiza difficilis became a bit more adept at obtaining a blood meal, and a bit more dependent on finding one on a daily basis. What if conditions on the tiny islands of Darwin or Wolf changed, and the seeds and flowers these birds needed to survive were no longer available? Would the blood-sipping finches simply relocate to another island? Perhaps, but if they did, they’d certainly run into competition from the well-established finch species already living there. Maybe the vampire wannabes would die out or interbreed with non-blood-feeding members of their own species. Or maybe Geospiza difficilis would accumulate a few more beneficial mutations (to its digestive system perhaps), until it had evolved into a real vampire finch—a feathered version of Desmodus rotundus.
Beyond the hypothetical, though, blood feeding makes absolute sense—and because it does, it should come as no shock that there are thousands of obligate sanguivores out there, as well as others that supplement their diets with blood.
But why does a blood-feeding lifestyle make sense? To address that issue, I’ll start off by repeating the two questions I hear most often about vampires: (1) Why do creatures like ticks, vampire bats, and bed bugs even exist? and (2) What would it matter to the earth if all the blood-feeding creatures suddenly disappeared?
These are questions that I’ve been hearing pretty much since I began working on vampire bats in 1990, and while the first question is certainly valid, the second question illustrates a basic problem that most people have with science. That is, they don’t think like scientists. Don’t get me wrong, though. It’s hard not to empathize with anyone who’s been swarmed by black flies, suffered through malaria or Lyme disease, or experienced the twitching paranoia of a bed bug infestation. For most people, then, it’s only natural to envision a world where these nasty critters didn’t pester, sicken, and kill us with such incredible efficiency.
But vampires, whether they’re bats, leeches, or bed bugs, don’t exist to sicken or kill us. They exist because their ancestors evolved certain characteristics that allowed them access to a highly specific but worldwide resource, a resource that they could utilize as food.*151 That resource, blood, has been vital to the existence of every vertebrate that has ever swum, crawled, walked, run, or flown.†152
With the aid of a muscular pump, blood travels through a wildly complex system of interconnected tub
es. Sanguivores have evolved ways to exploit the accessibility of these tubes and, just as important, the fact that they can be opened and tapped. Given the potential for running into creatures hauling around these tubular filling stations, it would be remarkable if sanguivory hadn’t evolved in a diverse array of taxonomic groups. But here’s the catch. No matter how different blood feeders are from each other (exemplified by leeches and bats), there appear to be a finite number of ways that a sanguivore can successfully gain access to a meal. For this reason, vampires as different as bats and leeches share separately evolved but similar adaptations for their highly specialized lifestyles—that is, they exhibit convergence.
All vampires, for example, are relatively small in size. The largest appears to be the common vampire bat, Desmodus rotundus, tipping the scales at just under an ounce and a half (around forty grams). The reasons for this size constraint are apparently related to my Trinidadian mentor Farouk Muradali’s mantra, “Feeding on blood is a tough way to make a living.” In that regard, the more of the red stuff a vampire requires each day (or night), the less likely it is to obtain it. On a related note, larger vampires would need to drain greater volumes of blood from their hosts, which would increase the likelihood of weakening them to the point of death—a maladaptive trait for a parasite. Additionally, large vampires would be easier to detect by their prey as well as their predators, therefore tending to neutralize another characteristic that all vampires share: stealth. This ability to avoid detection is employed in a variety of ways by blood feeders, both during their approach to a potential meal and during the actual feeding.
The list of convergent characteristics goes on.
All blood feeders also possess finely tuned sensory systems. These allow creatures as different as bed bugs and vampire bats to efficiently locate their potential meals, often in the absence of light.