The Spotted Rose Snapper, which lives off the coast of California, is one of a handful of species of fish that are plagued by one of the strangest parasites in all of nature: Cymothoa exigua, a crustacean about the size of a fish’s tongue when fully grown. The young parasites invade their intended host fish by entering the gills. Within the gills, the C. exigua all mature into males. But these parasites are protoandritic hermaphrodites, meaning that the adult males can change to females⁠—and in a fish infected with C. exigua, one of the invading crustaceans does exactly that.

The new female migrates through the innards of the fish into its mouth, and attaches herself to the tongue. Next, she leeches blood from the fish’s tongue until the muscle atrophies and falls off⁠—hence the parasite’s more commonly known name: the tongue-eating louse. The parasite then uses its rear legs to clamp itself onto the withered tongue-stump, and it effectively takes over as a replacement for the organ. The parasite will spend the rest of its life there, living off the blood and/or mucus of the host fish, and occasionally mating with a male that visits from the gills.

Cymothoa exigua is the only parasite known to effectively replace an organ in their host’s body.