See the BBC video on YouTube (embedding disabled by request):
"The Candiru fish, or Toothpick fish, is a natural parasite commonly found living inside Amazonian Catfish. Watch this short video from BBC wildlife show 'Weird Nature' to learn how this small fish has become a horror story in its own right when introduced to humans."
Candiru, also known as canero or toothpick fish, are parasitic freshwater catfish found in the Amazon River. They have a reputation among the natives as the most feared fish in its waters, even over piranha. They are eel-shaped and translucent, making them almost impossible to see in the water. Candiru grow to a size of 6 inches (~15 cm) in length and have barbels around the head, together with short, backward pointing spines on the gill covers.
The Candirú lies in wait at the river's murky bottom, searching for its next host by sampling/sniffing the water for expelled chemicals, such as urea and ammonia from the gills of other fish. Once having detected a fish in the vicinity, with a burst of speed the Candirú darts towards the gill cavity and lodges itself in place with its spines. Then, with usually fatal consequences for its victim, the Candirú begins to gnaw a hole towards a major blood vessel and gorges itself for no more than a few minutes. It will then dislodge itself and sink back to the river bed in order to digest its food and wait for its next meal.
This fish is also known to attack humans and animals and swim into an orifice (the vagina, anus, or even the penis—and deep into the urethra). Because of spines protruding from the fish, it is almost impossible to remove except through surgery. It locates its human targets when people urinate near the fish.
Parasites - Candiru fish.
Candiru, from Wikipedia, the free encyclopedia.
Image source: Candiru, Vandellia cirrhosa, Wikipedia, public domain.