Tongue-Eating Louse
(Cymothoa exigua)

Found in oceans worldwide, the Tongue-Eating Louse is a parasitic isopod with one of the most unsettling life strategies in nature. It enters a fish through the gills, severs the blood supply to the tongue until it falls off, and then attaches itself in its place — effectively becoming a functional replacement tongue for the fish, while the fish continues to use it to eat. It is the only known parasite to replace a host organ!