Cymothoa Exigua

Also called the "tongue-eating louse," this parasitic crustacean of the family Cymothoidae enters fish through the gills, then attaches itself at the base of the fish's tongue. Once there, it extracts blood through its front claws, causing the tongue to atrophy from lack of blood, then it replaces the fish's tongue by attaching its own body to the muscles at the tongue's stub. They are supposedly not harmful to humans unless picked up alive, in which case they can bite.