Cymothoa exigua (Tongue-Eating Louse)
These parasites are a part of the crustacean family, believe it or not. It enters the fish through the gills and attaches itself to the base of the tongue. It uses its front pair of claws to suck out blood and this causes the tongue to atrophy, basically, it wastes away until its nonexistent. Once the entire tongue has atrophied, the fish can use the parasite as a perfectly functioning tongue. This is the only known example of a parasite replacing an organ of the host.
