cymothoa exigua

Also known as the tongue-eating louse; it is a species of parasitic isopod crustacean whose females attach themselves to a fish host's tongue and feed on blood. Eventually the tongue is reduced to a stub and the parasite effectively becomes the fish's new tongue.
The Cymothoidae family is incorrectly called uonoe (lit. "fish-food") in Japanese. (And its subspecies are called tainoe (bream-food), fugunoe (puffer-fish-food) and tobiuonoe (flying-fish-food))

See also:

• Wikipedia article: Cymothoa exigua species / Cymothoidae family