Parasites can be nasty little critters, but there is at least one species of isopod that "repays" its host in a bizarre way. An isopod called Cymothoa exigua targets the rose snapper, slipping through its gills and clamping onto its tongue. After eating the organ, the contented parasite remains in the fish's mouth, providing the fish with all the functions of the original tongue, including grinding food against the teeth on the roof of the snapper's mouth.
Marine biologist Rick Brusca of the University of Arizona says that while there are hundreds of such tongue-targeting isopods in nature, only C. exigua can actually assume the duties of the consumed organ.
A parasite with a thing for snappers: