A tiny crustacean called Cymothoa exigua enters a fish through its gills, severs the blood vessels of its tongue until the organ withers away, and then latches onto the stub to serve as a functioning replacement tongue for the rest of the fish’s life.

Cymothoa exigua is the only known parasite that anatomically replaces one of its host's organs. It enters a fish through the gills, severs the blood supply to the tongue until it withers, then anchors onto the stub to serve as a living, functional replacement.