Cymothoa exigua

From wikipedia:

"Cymothoa exigua is a parasitic crustacean of the family Cymothoidae. This parasite attaches itself at the base of the tongue of the spotted rose snapper, Lutjanus guttatus, with the claws on its front three pairs of legs, and extracts blood. As the parasite grows, less and less blood is able to reach the tongue, and eventually the organ atrophies from lack of blood. The parasite then replaces the fish's tongue with its own body, by attaching to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue, except that it has to share its food with the parasite. It appears that the parasite does not cause any other damage to the host fish. Once C. exigua replaces the tongue, it supplements its diet with food particles, thereby relieving strain on the host's circulatory system. This is the only known case of a parasite functionally replacing a host organ."