Get your gag reflex ready, you’re going to need it.
The Spotted Rose Snapper Fish, which lives off the coast of California, is plagued by what must be one of the most disturbing parasites in all of nature. The crustacean parasite, called Cymothoa exigua, enters the fish’s mouth and leeches blood from the fish’s tongue until the muscle atrophies and dies. The parasite then attaches itself to the withered tongue-stump, and acts as a working replacement for the organ, spending the rest of its life living off bits of food that enter the fish’s mouth.
The Cymothoa exigua is the only parasite known to effectively replace a body organ.