Cymothoa exigua is a parasitic crustacean of the family Cymothoidae. It tends to be 3 to 4 cm long. This parasite attaches itself at the base of the spotted rose snapper's (Lutjanus guttatus) tongue, entering the fish's mouth through its gills. It then proceeds to extract blood through the claws on its front three pairs of legs. As the parasite grows, less and less blood reaches the tongue, and eventually the organ atrophies from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish. Once C. exigua replaces the tongue, some feed on the host's blood and many others feed on fish mucus. They do not eat scraps of the fish's food. This is the only known case of a parasite functionally replacing a host organ.
There are many species of Cymothoa, 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. However, it is also possible that the isopod traveled from the Gulf of California in the snapper's mouth, and its appearance in the UK is an isolated incident. The animal in question will be put on display in the Horniman Museum.