On this page, click on the fish mouth picture, which brings you to the explanation:

Tongue-eating isopod, Cymothoa exigua (middle)
This isopod causes degeneration of the tongue of its host fish the rose snapper, Lutjanus guttatus, and it then attaches to the remaining tongue stub and floor of the fish's mouth by hook-like pereopods. In this position the isopod superficially resembles its host's missing tongue. Brusca & Gilligan (1983) hypothesize that these isopods serve as a mechanical replacement for the fish's tongue and represent the first known case in animals of functional replacement of a host structure by a parasite. This relationship is so-far known only from the Gulf of California.