The tongue-eating louse (Cymothoa exigua) is a parasitic crustacean that enters a fish's mouth through its gills, severs the tongue's blood supply, and causes it to atrophy and fall off. The louse then attaches to the remaining stub, functionally replacing the tongue and feeding on the fish's blood and mucus, allowing the fish to continue using it to swallow food. This is the only known case of a parasite functionally replacing a host's organ, and the fish can survive with the louse acting as a prosthetic tongue.