The Candiru (Vandellia cirrhosa) is a small, slender catfish native exclusively to the Amazon basin. It has gained global notoriety for the terrifying claim that it can enter the human body, particularly through the urethra, causing excruciating pain and potentially death. The fish’s translucent, eel-like body has fueled this sensationalized myth for centuries. Examining the biology of this fish, however, reveals a significant gap between the folklore and the scientific reality.
The Candiru’s Natural Predation Strategy
The Candiru is a specialized micropredator that is hematophagous, meaning it feeds on blood. In its natural environment, the fish targets larger fish species, often focusing on the gill cavity. The fish is equipped with backward-pointing spines on its operculum (gill covers), which serve as its primary mechanism for attachment.
The Candiru locates its host by sensing the water flow and chemical signals associated with respiration. Historically, it was believed that the fish was strongly attracted to nitrogenous waste products like urea and ammonia expelled by host fish through their gills. Once a suitable host is found, the Candiru forces its way under the gill cover. It then erects its spines, anchoring itself firmly inside the gill chamber against the host’s branchial arteries.
This anchoring allows the fish to feed on the host’s blood, which is pumped directly into the Candiru’s mouth by the host fish’s own arterial pressure. A feeding event is surprisingly quick, typically lasting less than two minutes, after which the engorged parasite detaches and leaves the host. This highly specialized strategy is adapted for the unique vascular environment of a fish gill, where blood flow and chemical signals are concentrated.
Scientific Scrutiny of the Urethral Entry Myth
The widely circulated myth posits that the Candiru mistakes human urine for the chemical output of fish gills and aggressively swims up the urethra. Scientific investigation has cast considerable doubt on the core biological assumptions underlying this claim. Controlled studies have demonstrated that the fish does not exhibit a significant attraction to chemical attractants like human urine or ammonia when introduced into aquarium water.
Furthermore, the physical dynamics of the river environment make the ascent up a stream of urine highly improbable. A human urine stream possesses a strong, directional flow that would require the small, slender fish to overcome significant fluid resistance. It is considered physically impossible for the Candiru to actively swim up this powerful current and penetrate the narrow urethral opening. The fish is far more likely to rely on visual cues and swim into any convenient opening it encounters near the riverbed.
If entry were to occur, the danger largely hinges on subsequent medical complications, not immediate fatality. Once inside the urinary tract, the Candiru’s backward-facing spines would immediately deploy, causing severe trauma to the urethral lining and anchoring the fish in place. This would lead to intense pain, hemorrhage, and an inability to urinate, potentially causing severe infection. The primary danger is the physical obstruction and trauma, which necessitates complex surgical removal.
Analyzing the Single Reported Human Case
The fear surrounding the Candiru largely stems from a single, widely publicized incident reported in 1997 in Manaus, Brazil. The patient, a 23-year-old man, claimed a Candiru “jumped” from the water and entered his urethra while he was urinating in the river. Urologist Dr. Anoar Samad reported performing a two-hour surgical procedure to remove a 133.5-millimeter-long fish from the man’s urinary tract.
The patient’s account, which fully embraced the long-standing folklore of the fish swimming up a urine stream, was immediately met with skepticism by the ichthyological community. Marine biologist Stephen Spotte, who investigated the case, pointed out several biological inconsistencies. The size of the fish reported was significantly larger than what the male human urethra can typically accommodate without massive, immediate tearing, making the patient’s claimed entry mechanism questionable.
Crucially, Dr. Samad claimed he had to snip the fish’s grasping spines to remove it, yet the preserved specimen later examined had all its spines fully intact. This, along with the general implausibility of the fish successfully navigating the physics of the urine stream, led many experts to conclude the case lacked robust scientific confirmation. The prevailing consensus is that while the fish was surgically removed from the man’s body, the circumstances of its entry were likely fabricated or the event was a highly unusual, non-predatory accident.