Among the crustaceans, a certain family named Cymothoidae are particularly suited to be fish parasites, and different ones are compatible with mullet, grouper, flounder, perch, and other marine or freshwater fishes. Generally they affix to the fish's skin, or perhaps under it, and live off blood sucked from it for the duration of their life. They are often orders of magnitude smaller than the host, and don't cause much strain on its body except for the initial biting damage.

One particular fish, however, the Lutjanus guttatus or spotted rose snapper, acts as the only host to one member of the family, Cymothoa exigua. This particular cymothoid attaches itself at the base of the snapper's tongue with claws, and drinks from the artery which supplies its blood. As the parasite grows, less and less blood is able to reach the tongue, eventually causing it to atrophy and become useless. Fortunately, the parasite is attached to the stub, and is now large enough to operate as a replacement tongue for manipulating the fish's food. What's more, it gets free meals of freed food particles when the fish eats, lessening the drain on its blood manufacture. As far as anybody knows, this is the only case of a parasite fully and functionally replacing one of its host's organs.

Looking in the mouth of a fish which has undergone this process is fairly interesting, as one can see the little white crustacean nestled between red folds of flesh. Pictures are easy to find with a Google search, if you're interested. Also, the critter really does have the shape and look of a tongue, albeit a tongue with two beady little black eyes....

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