Cymothoa exigua
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. This is the only known case of a parasite replacing the function of a host organ. There are many species of Cymothoa, but only C. exigua is known to consume and replace its host's tongue.