The Mystery at the Base of One of Biology’s Strangest Relationships

Little crustaceans called tongue biters drain the blood from the tongues of fish. Then things get weird.

a tongue-biting isopod in a clownfish
Biosphoto / Alamy

For starters, you need to know that a fish tongue is not like a human tongue. Our tongues are flexible, muscular, and magnificently mobile; they help us speak, suck, swallow, whistle, lick, taste, and tease our friends. Fish tongues—properly called basihyals—don’t do a lot of those things. They are, in their most basic form, just flat stubs of bone, perhaps topped with a scant pad of soft tissue, that protrude from the base of the mouth. They help fish shuttle food down and push water through gills, and don’t do a ton else.

But like a human tongue, the fish tongue does offer a highly accessible strip of blood-rich meat, parked in an oft-opened hole in the head—excellent bait for a parasite. Some eons ago, a few pillbug-like crustaceans called isopods happened upon this revelation. They became teeny terrors known as tongue biters, and several have since gained a reputation for nomming away at lingual appendages until little to none of the soft tissue is left. Their host’s mutilated mouth becomes their forever home, to the detriment of fish and to the shock of many of the humans who stumble upon them.

“You look into a fish’s mouth and there’s eyes staring back at you,” Jimmy Bernot, an evolutionary biologist and parasite expert at the Smithsonian National Museum of Natural History, told me. The isopods are built like wood lice, with rounded, segmented backsides and seven pairs of spindly legs. They’re cute, but also very creepy. “Every time is as bad as the first time,” Kory Evans, a fish biologist at Rice University, told me. “It’s like being Rickrolled.”

The tongue-biter story is speckled with plot twists, bloodshed, and a well-timed sex change. It is also a frustrating and hotly contested mystery that pushes the possibilities of what parasitism can be, and what it means for a creature to fully enmesh itself into the body of its host. Some researchers think that a couple of these parasites can, under rare circumstances, take the place of a fish’s tongue and help the host swallow food. If that’s the case, then these critters have accomplished what no other known parasites have: usurping the role of another animal’s organ and, in effect, forcing their host to almost become dependent on them.

Our tale begins, as so many great stories do, with a coming-of-age at sea. A fresh-faced, dewy-eyed tongue biter, just a few millimeters long, emerges into the water and begins to swim. It has hours, maybe days, to locate permanent digs before it’s eaten or dies of starvation, unmated and alone.

This particular parasite is lucky. It finds a fish, penetrates the body (probably through the gills), and eventually invades the mouth. Now a full-sized adult, the creature curls its jointed little legs around its host’s tongue, like a surfer clinging to a board, and locks stubbornly in.

The isopod will spend the rest of its days there, hoping to mate and reproduce. All tongue biters start out their adult lives as gill-lurking males; a subset later transitions into a female form—the only sex that’s thought to reliably set up shop atop the basihyal. Should another male sidle in through the gills, his best shot at reproductive success is wooing the established occupant and impregnating her so she can spill a new generation of baby isopods into the sea.

Cymothoa epimerica from grouper
Cymothoa epimerica in a grouper (SANDRA CATHERINE ZAINATHAN)

This is where things start to get foggy. Several species of tongue biters exist, each with its own fish hosts. Some will tuck into the tongue with gusto while others are daintier grazers, and researchers still aren’t sure just how bad the damage usually gets. In one common strategy, the isopod lances open the tongue and slurps at its blood. The process can be quite slow, because the isopod “wants its host to stay alive,” says Adrienne Mora, a parasite expert at the Scripps Institute of Oceanography. (Should the fish die, the isopod has no choice but to “jump ship,” says Rachel Welicky, a marine parasitologist at the University of Washington. No longer able to swim as they did in adolescence, most of them just sink.) A fish can actually tolerate an isopod’s lingual embrace for quite some time, perhaps for many months, even years. Plenty of the tongue-bitten fish that researchers have tugged out of the ocean don’t look that much worse for wear, apart from the beady-eyed creature inhabiting their mouth—which “can be much less problematic than you would think,” Bernot said.

In some cases, though, the isopod drinks and drinks until the tissue begins to wither and atrophy, transforming the tongue into a stub. The parasite’s super-strong legs, built like grappling hooks, may contribute to the carnage as they grip and tear into the tissue, Welicky told me. “They leave impressions in the tissue,” Bernot said, like “little footprints.”

It’s not hard to see how a mouthful of blood-sucking crustacean could ruin an animal’s life, like a never-ending French kiss from hell. Typical fish tongues move like pistons, suctioning food into the mouth and pushing water over the gills. Any blockage to that process can be “catastrophic,” Stacy Farina, a fish morphologist at Howard University, told me. Parasitized fish can become anemic; they may struggle to breathe or obtain adequate nutrition; they may end up stunted or even die young.

But the extent of tongue loss doesn’t necessarily match the suffering of the fish. At least three isopod species, Cymothoa exigua, Cymothoa borbonica, and Ceratothoa oestroides, appear to reduce the basihyal of some hosts to a mangled, nonfunctional nub; nevertheless, many of their fish landlords persist. Some 40 years ago, Rick Brusca, a marine parasitologist at the University of Arizona, discovered some spotted rose snappers whose tongues had been apparently annihilated by C. exiguathere was “nothing whatsoever left,” he told me, of the original structure. And yet, the parasitized fish seemed happy and healthy enough, their digestive tracts full of fresh food. Brusca and his colleague Matthew Gilligan examined the backs of the little crustaceans and found small scrapes and grooves—evidence, Brusca said, that the injured fish was flexing its unwelcome tenant as if it had become the tongue, knocking the parasite up against the roof of its mouth. Brusca calls it “mechanical replacement of the tongue,” and it is, as he assured me, totally weird—exclusive to C. exigua—and hasn’t been scientifically documented since.

This assertion has sparked one of the biggest tongue-biter controversies of all. Most of the fish biologists I spoke with weren’t enthusiastic about the idea of total-tongue “replacement,” a term that implies that even the bony base of the basihyal gets degraded or dislodged. But that bone is the “midline connection” between the lattice-like structures that stratify the gills, says Nicolai Konow, a tongue expert at the University of Massachusetts at Lowell. If that goes, it’s like the last move in Jenga—“the fish is dead.”

Both Farina and Rice University’s Evans have analyzed several isopod-parasitized fish and found the basihyal intact, which means the tongue, however brutalized, is still technically there. An arm, after all, can’t really be replaced if the humerus is still protruding from the shoulder. Maybe what Brusca and Gilligan observed is snapper-specific. But “I’ve seen no evidence of anything being lost skeletally,” Farina told me.

a herring cale from New Zealand, scanned to show a parasite on its tongue
A herring cale from New Zealand, scanned to show a tongue biter in its mouth (KORY EVANS)

Evans thinks there might be a middle ground: After the tongue’s soft tissues erode, the parasite might clamp onto the basihyal, where it can assist in at least some day-to-day lingual duties in certain fish species. “There’s more than 30,000 species of fish out there, and what the tongue does varies quite a bit,” Evans told me. (Some fish tongues, for instance, even sport their own teeth.) “But fish are pretty metal,” Evans added. “I wouldn’t lose an ounce of sleep if a fish was using a parasite as a tool.”

That version of events would still be something of a biological first: a parasite assuming the function of an organ it devastated—the tongue version of the movie Single White Female, an interspecies tissue transplant of nightmares. “That is a pretty dramatic host-parasite interaction,” Mora, from Scripps, said. “I can’t think of any other example of this.”

Perhaps there’s a reason for that. Evolutionarily speaking, what C. exigua does is an arguably terrible idea. In taking over the tongue, the parasite depletes its primary source of food and bankrupts its host of a vital piece of anatomy. Tongues are not known for being a sustainable crop: “It seems like a dead end to eat tissue that isn’t replaced,” Konow told me. Assuming the role of the tongue, Brusca said, might be a Hail Mary for both host and parasite to “live a little longer.” By prolonging the tongueless fish’s life, the isopod might gain enough time to mate and release her eggs. Without more data, it’s hard to say this for sure, Welicky pointed out. Still, the possibility remains, Mora said: “At that point, maybe it benefits the fish to keep the parasite there,” rather than to have no tongue at all.

This is a prime example of the way evolution works—through tinkering, through stumbling, through endless trial and error, often ending in something less than ideal. No relationship is like the one between a parasite and its host; it’s a constant battle as one tries to outmaneuver the other without compromising itself. Some of the most successful parasites barely hurt their hosts at all, choosing instead to strike up a prolonged armistice. Perhaps those poor snappers encountered an isopod that was too gluttonous, forcing both parties to deal with the grisly consequences. The parasite made its tongue-shaped bed, and its best option for survival was to lie right down.