The Tongue Eating Louse is a gross parasite living in a fish's mouth
Cymothoa exigua or the Tongue eating louse is a parasitic crustacean. It tends to be 3 to 4 cm long. This parasite attaches itself at the base of the spotted rose snapper's tongue, entering the fish's mouth through its gills. It then proceeds to extract blood through the claws on its front three pairs of legs. As the parasite grows, less and less blood reaches the tongue, and eventually the organ atrophies from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish. This is the only known case of a parasite functionally replacing a host organ.