It’s the stuff of science fiction: a parasite that eats, and replaces, the tongue of its host. That’s the very specific, rather icky job of the tongue-eating louse, Cymothoa exigua. These parasites are isopod crustaceans, related to the pill bugs (or roly polies) you can find in your yard. As an isopod, they have a segmented exoskeleton. Coloration varies but typically consists of shades of green or gray. For reasons not yet clear, they appear to be selective of the species of fish the parasitize. To date, they have only been found in eight species of fish, including Atlantic croaker, spotted seatrout, and a few species of snapper.
C. exigua are protandrous hermaphrodites, meaning they begin life as males and mature into females once they reach a certain size. Juveniles have a short, free-swimming stage in the water column. They use their sense of smell to find a new host. Then they wait for the fish to pass above them and launch themselves directly upwards. They enter through the gills. This is pretty normal – it’s a common way for parasites to get in. Once inside, they attach themselves to the gills of their host. If there is already a female in residence in the mouth, they remain in the gills as males, ready to mate with the female. However, if the tongue is still available, then the first male to mature as a female (around 10 mm in length) lays claim, and any others still in the gills remain male. The transition is no small change. Their eyes shrink, their legs get longer, and their body gets three times bigger. If there are males present when the female matures in the gills, mating will take place before the female moves into the mouth. However, mating can also occur in the mouth if a male happens across an established female. The method of reproduction is sexual, and females carry the eggs in a marsupium-like structure until they are ready to hatch. Ready eggs are released into the water column, where juveniles begin their life.
The female attaches herself to the base of the fish’s tongue with the help of her seven pairs of legs, severing the blood vessels. She then feeds on the blood of the doomed part of the tongue, releasing anticoagulants so that the blood flow does not stop, until the majority of the tongue finally atrophies from lack of blood and falls off. The parasite then becomes the functional tongue of the fish, which is able to use the parasite just like a normal tongue. This is the only known case where a parasite functionally replaces a host’s organ. It appears that the louse does not cause any other damage to the host fish. Once she replaces the tongue, she continues to feed on the host's blood and mucus, or that of the fish’s prey. If a fish has multiple lice, they can cause the fish to become malnourished. The relationship between fish and louse can last for years. These lice can live up to three years, so the parasitized fish can sometimes outlive the isopod. When a host fish dies, the louse, after some time, detaches itself from the tongue stub and leaves the fish's mouth. It can then be seen clinging to its head or body externally. What then happens to the louse in the wild is unknown.
Menhaden are so often seen with their tongue replaced by the isopod that they're sometimes called "bug mouths.” And they're so common in snapper that they’ve earned the nickname: snapper-choking isopod. It seems that several parasites in the louse's Cymothoa genus use similar tactics, and different cymothoid genera are adapted to specific areas of attachment on the host. This includes scale-clingers, mouth- or gill-dwellers, and flesh-burrowers. The tongue-eating louse is quite widespread, from the Gulf of California south to north of the Gulf of Guayaquil, Ecuador. Two host records were also recently discovered in Costa Rica. They have been sampled in waters from 6 feet 7 inches to almost 200 feet deep. Though present in most warm oceans, they appear to be most prevalent off the coast of North America. You don’t have to worry about C. exigua
going after your tongue, as they seem to have quite specific tastes in fish – but don’t be surprised if they bite when you pick them up.
Where I learned about tongue-eating lice, and you can too!
Galveston Island State Park FB
Our Breathing Planet
The Fishes of North Carolina