Take, for instance, the parasitic isopod Cymothoa exigua, which parasitizes fish. This isopod, which is a crustacean like a shrimp or a lobster (it looks a bit like a roly poly, or sow bug, which is a terrestrial crustacean), lives in the ocean and makes a living off a few different species in the perch family — mostly snappers and drums. The living they make might seem a little much (read: Boschian horror show) for our refined human tastes, but Cymothoa exigua makes an honest living attaching to a fish's tongue, sucking blood from it until it falls off, and then replacing it by gripping onto the tongue stump and acting as a prosthetic tongue for the rest of its host's life.
"It is now in a position to consume what the fish is eating, or consume its blood and tissue," says Regina Wetzer, curator and director of the Marine Biodiversity Center at the Natural History Museum of Los Angeles County.
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Although there are other mouth-infesting isopods out there feeding on other types of fish like barramundi or mahi-mahi, Cymothoa exigua is the only one known to science that eats and then replaces the tongue.