This ocean parasite kills a fish's tongue and ten takes its place; what makes the tongue-eating louse so different

A parasite that destroys a fish’s tongue and then takes over its place sounds almost impossible, but that is exactly what the tongue-eating louse is known for. Cymothoa exigua enters through a fish’s gills, cuts off the tongue’s blood supply and attaches itself to what remains.

The strange part is that the parasite does not simply kill its host. It stays where the tongue was and functions as a replacement, feeding on blood and mucus while the fish continues to eat, as per a report by Science Daily.

How does the parasite replace the tongue?

Cymothoa exigua is an isopod, making it a saltwater relative of the familiar woodlice found on land. Its common name, the tongue-eating louse, comes from its unusual relationship with fish.

The process begins when a juvenile isopod enters through the gill slit. It moves along the gill arch before attaching itself to the base of the tongue with its hooked legs. But the parasite does not actually chew through the tongue. Stefanie Kaiser, then a postdoctoral fellow at New Zealand’s National Institute of Water and Atmospheric Research, explained to the American Association for the Advancement of Science that the animal draws blood from the tongue.

As circulation is cut off, the tongue gradually withers and eventually falls away. The parasite then anchors itself to the remaining tongue stub.

It stays there for the rest of the fish’s life, feeding on blood and mucus. Meanwhile, the fish continues moving food around its mouth using the parasite in place of its lost tongue. Whether the fish is truly unaffected by the arrangement is much harder to establish.

What did scientists actually observe?

Much of what is known about the famous tongue-replacing parasite traces back to a remarkably small study.

In November 1979, researchers Richard Brusca and Matthew Gilligan examined 37 spotted rose snappers caught near Guaymas in the Gulf of California. Two of those fish contained a female C. exigua in their mouths, and both had lost more than 90 percent of their tongues.

Despite that damage, both fish had body fat and full stomachs. One even had a piece of fish measuring about 20 by 50 millimetres in its stomach.

Brusca and Gilligan published their findings in Copeia in 1983. That study became the basis for the familiar story of a parasite becoming a fish’s replacement tongue.

The observation remains important, but it involved only two fish. That means the popular image of a fish comfortably living with a crustacean tongue rests on a very limited documented sample.

How does the parasite reproduce?

The reproductive biology of these parasites is unusual too.

An infected fish can carry a larger female attached to the tongue stump and a smaller male inside the gill chamber behind her. Fertilisation takes place there.

Cymothoids are protandrous hermaphrodites, meaning they first mature as males and some later become females. According to Kaiser’s account, juveniles enter fish through the gills as males. The first one to reach the tongue typically changes into a female and settles there.

Other individuals remain male in the gill chamber. Pheromones from the female may prevent them from changing sex, although that mechanism has not been established.

After fertilisation, the female carries her eggs in a pouch underneath her body before releasing juveniles that search for their own fish hosts.

Is every tongue-eating louse really Cymothoa exigua?

Not necessarily.

The family Cymothoidae contains 383 species across 40 genera, with roughly 100 species living inside fish mouths. Many can look similar, making identification difficult without detailed examination.

That became particularly clear in 2025, when José Salgado-Barragán and Mayra Ixchel Grano-Maldonado examined 32 mouth-dwelling isopods taken from snappers sold in markets in Mazatlán. They described a species new to science, Cymothoa facimar, in Systematic Parasitology.

The species lives in Pacific red snapper and mullet snapper and had previously been grouped under the more familiar name. The distinction is based mainly on body proportions and the structure of its trunk segments. The species has since been formally recognized in the World Register of Marine Species.

Can fishing pressure make these parasites more harmful?

Another cymothoid species provides a striking example of how the same type of parasite can have different effects depending on the condition of its host.

Maria Sala-Bozano, Cock van Oosterhout and Stefano Mariani studied striped sea bream from two Mediterranean locations. One was beside a marine protected area, while the other was exposed to heavy commercial fishing.

Their research, published in the Biological Journal of the Linnean Society, found that the mouth parasite Ceratothoa italica caused substantially more damage in the heavily fished population and almost none near the protected area.

Parasite load accounted for 34.4 percent of the variation in liver condition among the fished bream, compared with just 0.3 percent inside the protected zone.

Can the parasite turn up in fish sold for food?

It can.

In 2005, a shopper in southeast London bought a red snapper and discovered a crustacean inside its mouth after taking the fish home. The incident was reported to the local council, whose environmental health team passed the specimen to the Horniman Museum.

Jim Brock, the museum’s keeper of natural history, identified the animal and told Practical Fishkeeping that he had never encountered anything like it during his 13 years on the job.

The tongue-eating louse may sound like an animal belonging to some remote corner of the ocean, but its hosts can make their way into ordinary fish markets.

FAQs

What does the tongue-eating louse do?
It destroys a fish’s tongue and attaches itself to the remaining stump.

Does Cymothoa exigua eat the tongue?
No. It drains blood from the tongue until the tissue withers.