Cymothoa exigua, or the tongue-eating louse, is a parasitic crustacean that enters fish through the gills and then attaches itself at the base of the fish's tongue. It extracts blood from the fisht causing the tongue to atrophy from lack of blood. The parasite then replaces the fish's tongue by attaching itself 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.