Why No Fish Wants a Tongue-eating Parasitic Louse in its Mouth

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.

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.

"Isopods in the family Cymothoidae are parasites of fishes, and as juveniles all cymothoids must find and attach to their host," says Wetzer. "In the case of Cymothoa exigua — the species that is attached to the tongue — males enter the body via the gills, mature, mate and females move to the tongue."

How, you might ask, do the females enter the fish if only male juveniles enter the fish's gill slits? Cymothoa exigua is a protandric hermaphrodite, meaning it has the ability to change from male to female once it is an adult. In other words, all of them are born male, and as a baby enters a fish's gills, it keeps maturing as a male, but once another male juvenile shows up, the first one gets the signal that it's time to transform into a female. After this business has been sorted out, the female crawls into the fish host's mouth and starts feeding on its tongue. Once she has replaced the tongue, she is free to mate with any of the males hanging out in the fish's gill chamber and raise babies in a nice, safe cave.

Cymothoa exigua is a powerful little crustacean, with seven pairs of legs tipped with spines, which help her anchor into the fish's mouth. However, the first step in the process is to use her five sets of jaws modified with a variety of ice-pick-like tubes to puncture the fish's tongue and suck out the fish's blood. This process, by the way, is not thought to be very pleasant for the fish.

As the isopod drains the fish's tongue of blood, the muscle itself atrophies and withers away. At this point, she grasps what remains of the tongue stub with three or four of her spined leg sets and digs in, functionally replacing the tongue all together.

As unpleasant as this is, these isopods generally don't kill their host. However, Cymothoa exigua does not survive well without a host.

"Without its host adult, fully mature isopods would not survive well, as it's an obligate parasite," says Wetzer. "It has lousy swimming capabilities and gravid females — females with eggs and juveniles in her pouch — are especially non-agile. This is in contrast to some species [of the same family of isopods] which are free-living and can occur in such large numbers that they can deflesh a fish or body entirely."