In a nutshell, almost probably not. Sorry to all the gore aficionados out there, but there are several issues with this concept, ranging from “mildly dubious” to “patently absurd.” Let’s begin with the least heinous and work our way up. “Shouldn’t one expect a few proven cases in the medical literature by now, given the small fish’s reputed ravenous behavior, the geographical vastness of its habitat, and the significant number of people living along the river system?” Bauer wondered.
It’s a nice point: practically all of the “proof” for alleged urethral candiru assaults comes from 19th and early 20th century explorers, and the most of it comes from shakily-translated stories from Indigenous people who, let’s face it, had excellent reason to want all these Europeans to go (or at the very least, to stop peeing in their river.) Furthermore, Bauer points out that there are a number of instances when people who you’d expect to at least acknowledge the presence of a mental penis-invading fish have apparently had nothing to say about it.
Victorian explorers such as Alexander von Humboldt and Alfred Russel Wallace, who spent years in the South American rainforests, never mentioned the candiru, and Henry Walter Bates, who accompanied Wallace on his exploration of the region, wrote that the locals were “almost amphibious” – an unusual lifestyle for a people apparently concerned with a fish swimming up their urethra. Then there are the biological ramifications to consider. Consider yourself a little fish acclimated to the vastness of the Amazon or the Rio Negro, and you find yourself abruptly within a human’s urethra: a tube less than 8 millimeters (0.3 inches) in diameter and conspicuously devoid of fresh water.
You’ll be dead in a few minutes if you try to wiggle your way into a bladder and reproduce. Sure, you would argue, but according to the page I saw, it’s drawn to the scent of pee! It’s unable to stop it! That leads us to another reason to be skeptical of the claims: candiru don’t appear to be very fond of the scent of human pee.
According to a 2015 research in the Journal of Urology, “in 1829 German naturalist and explorer Carl Friedrick Philipp von Martius… theorized that the stench of urine attracted candirus, as principal food of the fish exude urea from their gills.” “However, according to current study, fish hunt by sight and are not attracted to human pee.” Then there’s the real kicker: remember the fish swimming up your pee stream? It isn’t possible.
We don’t just mean “unheard of”; we mean “impossible.” According to the Journal of Urology, “during an expedition in… 1855, local fisherman told French scientist Francis de Laporte de Castelnau that candirus “rise out of the water and enter into the urethra by rising the length of the liquid column.” The authors quickly state that “these statements… contradict the rules of physics.”