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Cymothoa exigua is a parasitic crustacean of the family Cymothoidae. This parasite attaches itself at the base of the tongue of the spotted rose snapper, Lutjanus guttatus, with the claws on its front three pairs of legs, and extracts blood. As the parasite grows, less and less blood is able to reach the tongue, and eventually the organ atrophies from lack of blood. The parasite then replaces the fish's tongue with its own body, by attaching to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue, except that it has to share its food with the parasite. It appears that the parasite does not cause any other damage to the host fish ^[1]. Once C. exigua replaces the tongue, it supplements its diet with food particles, thereby relieving strain on the host's circulatory system. This is the only known case of a parasite functionally replacing a host organ.

A stage play involving this phenomenon has been written and performed. The play focuses on a dream that playwright Tennessee Williams has, in which his tongue is consumed by a parasite and he loses the ability to speak. The play's author is the American playwright R. J. Tsarov, and the play first premiered at the annual Tennessee Williams Festival in New Orleans in March 2003 ^[2].

There are many species of Cymothoa ^[3], but only C. exigua is known to consume and replace its host's tongue.

In 2005, a fish parasitised by what could be Cymothoa exigua was discovered in the United Kingdom. As the fish is normally found off the coast of California, this led to speculation that the parasite's range may be expanding^[4]. The animal in question will be put on display in the Horniman Museum ^[5].