The vast freshwater ecosystem of the Amazon River is home to abundant animal life, and many of its species thrive by virtue of their ferocity. If one were to ask the locals which of the river's indigenous species is the most treacherous, a few might describe the roaming packs of carnivorous piranhas, or the massive anaconda snakes; but based on the general sentiment of the region, the most frequently uttered response would be "candirú."
The candirú is a tiny catfish which dwells in the depths of the Amazon River. These fish do not hunt in packs like the piranha, nor are they exceptionally large like the anaconda. In fact, the candirú is among the tiniest vertebrates on the planet, and it is sometimes referred to as the "toothpick fish" due to its small size and slender shape. Only a handful of people have had the misfortune of crossing paths with the candirú, but their experiences serve as cautionary tales to any who venture into the mighty river.
Though the candirú is a parasite, humans are not among its viable hosts. It lingers in the murky darkness at the river's bottom, quietly stalking its neighboring fish. Light is scarce in the soupy deep, but the candirú does not need to see... it can taste the traces of urea and ammonia that are expelled from breathing gills.
The tiny hunter shadows its prey, almost invisible due to its translucent body and small size. When the target fish exhales, the candirú detects the resulting flow of water and makes a dash for the exposed gill cavity with remarkable speed. Within less than a second it penetrates the gill and wriggles its way into place, erecting an umbrella-like array of spines to secure its position.
Unconcerned with the host's panicked thrashing, the firmly anchored parasite immediately nibbles a hole in a nearby artery with its needle-like teeth, feasting upon the bounty that gushes forth. Within two minutes the candirú's belly is swollen with the blood of its victim, and it retracts its gripping barbs.
Though it may seem that the exploited host fish has escaped, its injuries are so extensive that chances of survival are grim. Meanwhile the victorious attacker slinks back into the river's dark places to digest its meal.
There are many troubling stories regarding human run-ins with the candirú, though until recent years these were not given much credence by the medical community. It is not uncommon for people swimming or bathing in the river to urinate in the water, an action which creates tiny water currents that are rich in urea and ammonia. It seems that the tiny, slender catfish cannot always distinguish a urinating human from an exhaling fish gill, and on occasion it will attempt its trademark high-speed attack on some unfortunate soul.
Silvio Barbossa was one such soul. He was swimming in the Amazon River when he went head to head with the tiny parasite:
"I felt like urinating. I stood up, and it was then it attacked me. The candirú attacked me. [...] When I saw it, I was terrified. I grabbed it quickly so it couldn't go deeper inside. I could only see the end of its tail flapping. I tried to grab it, but it slipped away from me and went in. [...] I was very afraid, because the candirú bites."
When the candirú successfully invades a human, it proceeds exactly as it would with a fish host. After entering the misidentified orifice, it quickly wriggles its way in as far as possible, often accompanied by the victim's frantic attempts to grip the slippery, mucus-coated tail. In the unlikely event that the panicked victim manages to grasp the fish, its backwards-pointing barbs would cause excruciating pain at each pull, and bring a quick end to the dramatic tug-of-war. Once inside, the parasite inches its way up the urethra to the nearest blood-gorged membrane, extends its spines into the surrounding tissue, and starts feasting.
For the candirú, this misguided journey is a one-way trip; its bloody banquet leaves it too swollen to escape. The only known retaliation against the invader is delicate and expensive surgery, or failing that, a folk remedy which combines two herbs to very slowly kill and dissolve the fish. Silvio was fortunate enough to have access to modern medical facilities, though he had to endure three days of profound agony before the fish was extracted by an awestruck urogenital surgeon.
Silvio's incident was the first officially confirmed report of a candirú attacking a human, but such leg-crossingly horrific tales have haunted the region for generations. According to legend, many men chose castration as an alternative to a slow, excruciating death back before surgery was an option.
Though such brushes with the candirú are exceedingly rare in statistical terms, it is wise to heed the advice of the locals, and avoid urinating in the Amazon River at all costs. When the natives of the Amazon speak, one would be foolish not to listen. They are privy to some of the world's most horrible truths.