Dark Banquet - Bill Schutt [93]
“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