A common urban legend describes a tiny fish that can swim up a human urine stream and become lodged inside the body. This fear has created a widespread belief about a hidden danger. This article explores the truth behind this claim, examining the fish’s behavior and the biological realities that make such an event highly improbable.
The Candiru: Myth vs. Reality
The fish at the center of this legend is the Candiru (Vandellia cirrhosa). This small, translucent catfish is native to the Amazon and Orinoco basins, found across countries like Brazil, Bolivia, and Peru. Measuring 2.5 to 15 centimeters, its slender, eel-like appearance makes it nearly invisible in murky waters.
In its natural environment, the Candiru is a hematophagous parasite, meaning it feeds on blood. Its primary hosts are larger fish. The Candiru navigates towards a host fish, forcing itself under the gill cover to make an incision in an artery. The host’s blood pressure then fills the Candiru’s gut with blood, a process taking just minutes.
The myth suggests the Candiru is attracted to urine, following its scent upstream to enter the human urethra. However, scientific studies challenge this belief. A 2001 study indicated that Candiru primarily hunt by sight and showed no attraction to chemical cues like human urine. This research suggests any alleged human invasions are likely accidental, not a targeted response to urination.
Why the Myth is Biologically Implausible
The idea of a fish swimming up a urine stream into the human body faces significant hurdles. Human urination involves a forceful and turbulent flow of liquid. A normal urine flow rate averages 11 to 20 milliliters per second, creating a dynamic current. The pressure exerted by a urine stream, while variable, is relatively low, typically ranging from 0.095 to 0.5 pounds per square inch.
The human urethra also presents a formidable barrier. The male urethra, 15 to 20 centimeters long, is not a straight tube and includes distinct sections. Its membranous portion is notably narrow, making it difficult for an object to pass through. The female urethra is much shorter, around 4 centimeters, also presenting a narrow passage.
For a fish, especially one equipped with backward-pointing spines on its gill covers, navigating against such a turbulent, confined, and non-linear flow is physically improbable. The Candiru lacks the adaptations or mechanisms required to propel itself against this counter-current into a small, constricting orifice. Limited space and lack of oxygen within the human urethra would render survival nearly impossible for an aquatic organism.
Actual Risks in Amazonian Waters
While the “swimming up the urine stream” myth is largely unfounded, interacting with Amazonian waters presents genuine, rare risks. There is one highly publicized, though controversial, case from 1997 in Brazil where a Candiru was reportedly removed from a human urethra. Many historical accounts of Candiru entering human orifices are anecdotal, lacking definitive scientific corroboration. When such an event occurs, it is generally considered an accidental intrusion. If a Candiru enters an orifice, its backward-facing spines can cause it to become painfully lodged, necessitating surgical removal.
Beyond the Candiru, the Amazon River basin is home to diverse hazards. Swimmers face risks from strong currents and submerged debris, difficult to spot in murky water. Water quality also poses a concern; polluted areas near populated regions increase the risk of bacterial and parasitic infections.
The region’s wildlife also contributes to dangers. Piranhas, though often sensationalized, can inflict bites if provoked or in conditions like low water levels. Other creatures like Caimans, electric eels, and freshwater bull sharks inhabit these waters and pose threats. Freshwater stingrays are common and deliver painful stings if stepped on. Certain scavenger catfish species, distinct from the Candiru, are known to cause significant injuries.