THE TOUNGE-EATING LOUSE

Cymothoa exigua, or the tongue-eating louse, is a parasite of the family  Cymothoidae. This parasite enters fish through the gills, and then attaches itself to the fish's tongue. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female. Females are 8–29mm and 4–14mm in maximum width. Males are approximately 7.5–15mm and 3–7mm wide. The parasite severs the blood vessels in the fish's tongue, causing the tongue to fall off. It then attaches itself to the stub of what was once its tongue and becomes the fish's new tongue.

They cause loss of blood by using their claws to extract blood from the host after which they replace the host's tongue with their body. *weird*  

It reproduces sexually,  It is likely that juveniles first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. If there is no female present, within a pair of two males, one male can turn into a female after it grows to 10mm. The female then makes its way to the fish's mouth where it uses its front claws to attach to the fish's tongue. 

It is of no record that this creature is dangerous to man, except if caught alive which may cause it to bite.