The Parasite That Replaces a Fish's Tongue: Cymothoa exigua

A fisherman off the coast of Costa Rica opens a rose snapper's mouth, and where the tongue should sit, there is a small isopod with two dark eyes clamped into the base of the muscle.

The only parasite that replaces an organ

The animal is Cymothoa exigua, a parasitic isopod that Wikipedia summarises as "the only species of parasite known to functionally replace a host organ." The mechanism is unusually literal. A juvenile enters the fish's mouth as a free-swimming male, crawls through the gill arches, and lodges in the buccal cavity near the base of the tongue. Using hooked front claws, it severs the arteries supplying the tongue. Deprived of blood, the tongue tissue necrotizes and sloughs off. The isopod then clamps its own body to the muscular stub with sharp legs, and its dorsal surface takes the physical place of the missing organ.

What makes this remarkable, and worth reporting on its own terms, is the outcome for the host. The snapper does not die. Once the transition is complete, it goes on eating, breathing, and behaving normally, simply using the parasite in place of the organ it lost. The fish's body plan has been rearranged around a small crustacean, and the arrangement holds. An attached female can live fused to her host's mouth for up to about three years.

That combination, catastrophic damage followed by long-term functional integration, is the reason biologists single Cymothoa exigua out from the roughly one hundred cymothoid species that attach inside a fish's mouth. Eight cymothoid genera specialise in the mouth cavity, and none of the others has been documented completing this particular substitution. The tongue is destroyed, and something else keeps the job.

The eastern Pacific and its snappers

The native range of Cymothoa exigua runs along the eastern Pacific Ocean, from the Gulf of California down to Ecuador, with confirmed populations in the Gulf of Guayaquil and along the coasts of Costa Rica. Depth records for the species run from about two metres to sixty metres, which places it firmly on the shallow shelf rather than in the open ocean or the deep.

That depth band matters, because it is also the water in which the parasite's preferred hosts live and feed. The primary hosts are members of the snapper family Lutjanidae, especially the spotted rose snapper, Lutjanus guttatus, in the Gulf of California. The parasite is not exclusively a snapper specialist, however. It has been documented parasitising eight fish species across two orders and four families, a range wide enough to suggest the mechanism is not tuned to a single host anatomy.

The wider family it belongs to, Cymothoidae, is a warm-water group. As Wikipedia notes, "species of the Cymothoidae are generally found in warmer waters and rarely in the cool and cold climates." The eastern Pacific shelf, warm and productive, supplies both the temperature and the host density the animal needs.

There is one indication that this arrangement is not entirely benign. Fish carrying two or more of these parasites tend to be visibly underweight, an early sign that the metabolic drain on the host is more than cosmetic. A single settled female appears to be a tolerable passenger. Two or more begin to show.

Inside the mouth

The physical sequence, once a juvenile finds a host, is orderly. It navigates past the respiratory tissue of the gill arches and lodges in the buccal cavity near the base of the tongue. The hooked front claws sever the arteries feeding the tongue. As the blood supply is cut, the tongue atrophies and dies. The isopod then anchors to the remaining muscular stub with its sharp legs, positioning its dorsal surface where the tongue's upper surface used to be.

Once the substitution is complete, the female no longer eats tongue tissue. There is no tongue tissue left. Instead, she survives on mucus draining from the palate and on small quantities of the fish's blood. It is a modest diet, and it explains how the arrangement can persist for years without killing the host outright.

The size of the animal doing all this is worth stating plainly. Adult females reach 8 to 29 millimetres in length and 4 to 14 millimetres in width. Males remain smaller, at 7.5 to 15 millimetres long and 3 to 7 millimetres wide. This is a crustacean that fits comfortably on the tip of a finger, doing structural work inside the mouth of a snapper.

Sex, eggs, and a lonely ocean

Cymothoa exigua is a protandrous hermaphrodite. Every individual is born male and only switches to female if no female is already present on the host. The first male to arrive on a fish will become the female of the pair. The second, smaller male stays male and lives attached to the gills beneath her.

That rule, first arrival becomes female, second arrival stays male, solves a real problem. Encounters between free-swimming juveniles in the eastern Pacific are rare, and any individual that finds a host cannot afford to wait for a mate of the opposite sex to arrive. Making sex conditional on who else is already there guarantees that any pair that does meet on a host can reproduce.

From that attached position, each female is estimated to produce broods of more than 400 eggs, incubated in a ventral pouch called a marsupium until they hatch as free-swimming larvae and enter the water column. A three-year residency and repeated broods of that size can sustain a species whose entry points, the mouths of specific snapper populations at specific depths, are otherwise vanishingly narrow.

A scientific record and a supermarket

The species entered the scientific record in 1884, described formally by the Danish zoologists Jørgen Christian Schiødte and Frederik Vilhelm August Meinert. Its family had been named earlier still. The Cymothoidae was erected in 1818 by the English naturalist William Elford Leach, and the family today contains around 380 described species across roughly 40 genera. Eight of those genera specialise in the fish mouth, together comprising about 100 species, but Cymothoa exigua is the only one documented to fully replace the tongue.

The parasite occasionally surfaces outside its range through the fish trade. In 2005, a British fishmonger in Belfast discovered a live Cymothoa exigua inside the mouth of an imported red snapper, the first record of the parasite reaching UK shelves. A woman in Puerto Rico once sued a supermarket claiming poisoning by a cooked tongue-eating louse. The case was dismissed. Cymothoa exigua is not poisonous to humans, and it cannot survive on a human host. Handled alive, it can pinch. That is the extent of the danger it poses to people.

Back in the eastern Pacific, the host fish continues its life on the shallow shelf. It swims, it feeds, it breathes. Every meal it takes for the rest of its life passes over the animal that ate its tongue.

Sources

• Wikipedia: Cymothoa exigua
• Wikipedia: Cymothoidae
• Wikipedia: Parasitism