The tongue-eating louse—Cymothoa exigua—is one of the most disgusting parasites known to man. The louse is found in the Gulf of California and enters its host (the spotted rose snapper) through the fins. Several male lice enter the fish at once and then one of these will develop into a female. The female will then travel to the host’s mouth where it attaches to the base of the tongue. The organ’s blood supply is cut off, causing it to die and detach.

Now the parasite acts as the fish’s tongue by binding to the remaining muscle. It feeds off the host’s blood and also algae and mucus that it collects in its mouth. And what about the other lice? Well, they remain in the fish’s gill chamber and may occasionally leave their new home to mate with the female. The host fish is able to live with the parasite for years and apart from the obvious damage to the tongue, it remains relatively unharmed. The tongue-eating louse is the only known parasite that replaces an organ of its host.