There's a parasite in the ocean that doesn't just feed on its host, it becomes part of it — Cymothoa exigua enters a fish through the gills, kills off its tongue by cutting the blood supply, and then

The Cymothoa exigua is a parasitic isopod that enters a fish through its gills and replaces the host's tongue. It feeds on the fish's blood and mucus while effectively performing the function of the tongue for the remainder of the host's life.