Creature Feature: Tongue-eating louse

“Hey Mr. Fish, Cat got your tongue?”

Nope! Actually, it’s the parasite Cymothoa exigua, commonly known as the tongue-eating louse. Despite their louse-y nickname, C. exigua are actually isopods (a type of crustacean like crabs or shrimp). And as you probably guessed from their common name, they do some pretty freaky stuff!

Tongue-eating lice (Cymothoa exigua) are parasitic isopods that live in the mouths of marine fish such as this spotted rose snapper (Lutjanus guttatus). Photo by Andy Heyward [Source]

Tongue-eating lice are most often found inhabiting the mouths of their fish hosts, effectively replacing the fishes’ tongues—which is how the parasites got their nickname. Despite the “tongue-eating” part of the name, C. exigua do not technically eat their hosts’ tongues. They actually attach themselves in the buccal (mouth) cavity at the base of the host’s tongue and suck blood from the tongue until it shrivels up and falls out [1]. The now tongue-less fish starts using its new mouth isopod as a replacement tongue, while the parasitic “tongue” continues to feed on its host’s blood and mucus [1]. Believe it or not, the fish can survive and get by just fine with a parasite as a tongue [2].

Cymothoa exigua removed from its fish host [Source].

Now, of course the poor host fish doesn’t just open its mouth and let the louse walk (or swim) in, so you might be wondering how these parasites get to their ultimate oral destination. The answer: they enter through the gills! Often, multiple tongue-eating lice enter the same host, but only one of them will have the privilege of replacing its tongue. So how do they decide? Easy: only the females do it. But for such a freaky animal like C. exigua, that seems too simple. Replacing tongues just isn’t weird enough in the parasite world; C. exigua are also protoandric hermaphrodites, meaning each individual is born male and can become female later in life [1]. All C. exigua individuals that enter through the gills of a host fish are male, and if no female is already present, one individual transforms into a female. She then attaches to her tongue throne and assumes the title of “matriarch of the fish mouth.” The remaining unattached males may stay and live in the fish’s gills chambers [2] and compete to mate with the female louse as she performs her tongue-ly duties.

A mouth-eating louse infecting the mouth of a sand steenbras (Lithognathus mormyrus). The parasite enters through the gills of the fish, attaches at the base of its tongue, and eventually replaces the tongue. Photo by Marco Vinci [Source]

As with most parasites, C. exigua are somewhat particular about their host species, though they are certainly not the pickiest parasites in the world. C. exigua are most frequently found parasitizing snappers like the Colorado snapper, Lutjanus colorado, and Pacific red snapper, Lutjanus peru [2] They can also be found in flatfish, particularly the Pacific sanddab (Citharichthys sordidus), and occasionally in a few other species. C. exigua live along much of the west coast of Mexico, upper South America, and in the Galapagos, and they are especially abundant in the Gulf of California [3]. While this particular species has a pretty contained range, almost 400 other species in the same family (Cymothoidae) exist in various parts of the world. Other cymothoid species infect a variety of marine and freshwater fish and inhabit their gills, mouth, flesh, or external surfaces [3,4].

C. exigua replacing the tongue of a Clark’s anemonefish (Amphiprion clarkii) in Indonesia. Photo by Christian Gloor [Source].

As unsettling (yet awesome) as C. exigua may be, they cannot actually harm humans (unless you count the nightmares they trigger when encountered). However, they can be a nuisance to humans because they parasitize commercial fish around the world. A lawsuit against a supermarket in Puerto Rico emerged after someone claimed they were poisoned from an isopod inside a cooked fish, but since these isopods are not toxic to humans, the lawsuit was unsuccessful [5]. Some people even consider isopods to be quite tasty snacks!

Parasites like C. exigua have inspired horror films and the imaginations of many artists. “Parasitic Takeover” by Emory Allen was inspired by C. exigua [Source].

As with many examples of nightmare-fueling biology, pop culture has taken the concept of C. exigua and run with it: the idea of a “mutant” C. exigua was the premise of the 2012 horror movie The Bay. Fortunately for our peace of mind, this cinematic work of science fiction remains purely fiction. Lots of fish, however, are not so lucky.

Christofer Brothers is a first year graduate student in the Animal Behavior Graduate Group. He is interested in predator-prey interactions in flying insects and environmental factors that influence those dynamics. He also has an unhealthy love of puns.


  1. Brusca, R. C. & Gilligan, M. R. (1983). Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda). Copeia1983(3), 813–816.
  2. Ruiz, A., & Madrid, J. (1992). Studies on the biology of the parasitic isopod Cymothoa exigua Schioedte and Meinert, 1884 and its relationship with the snapper Lutjanus peru (Pisces: Lutjanidae) Nichols and Murphy, 1922, from commercial catch in Michoacan. Ciencias Marinas18(1), 19–34.
  3. Hadfield, K. A. (2012). The biodiversity and systematics of marine fish parasitic isopods of the family Cymothoidae from southern Africa. Doctoral dissertation, University of Johannesburg.
  4. Smit, N. J., Bruce, N. L., & Hadfield, K. A. (2014). Global diversity of fish parasitic isopod crustaceans of the family Cymothoidae. International Journal for Parasitology: Parasites and Wildlife3(2), 188–197.
  5. Williams Jr., E. H. & Bunkley-Williams, L. (2003). New records of fish-parasitic isopods (cymothoidae) in the Eastern Pacific (Galapagos and Costa Rica). Noticias de Galápagos62, 21–23.

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