One of the animal kingdom’s strangest relationships - Niko Zlotnik

One night in 2013, a man in Northern Ireland

was shocked to discover something bizarre protruding from the mouth of the sea bass

he was preparing for dinner.

Peering closer, he came face-to-face

with none other than the infamous “tongue biter,”

also known as the tongue-eating louse.

This species, Cymothoa exigua,

is one of nearly 400 belonging to a family of fish parasites

called cymothoids.

They’re crustaceans, and more specifically, isopods,

like pill bugs.

Some latch onto their host's head, others suck blood from fish gills,

and still others burrow into fish flesh

and form sac-like membranes around themselves.

Clearly, there are many delightful ways to be a cymothoid,

but Cymothoa exigua and others go specifically for fish tongues—

or at least what pass as tongues among fish.

Because instead of what we think of as a tongue,

fish have a bony structure called a basihyal,

protruding from the floor of their mouths,

which they use to move oxygen-rich water towards their gills

and to swallow food.

Tongue biters start out looking for a fish to call home for life,

which can be up to around three years.

They may swim for days, surviving off stored yolk reserves—

until finally, they sense a change in the light

and pick up on a nearby fish’s chemical cues.

They burst into action, hook onto the fish’s gills

using the sharp, curved tips of their limbs, and crawl on in.

There, they molt,

and since they won’t be living independently again,

their eyes recede and their limbs lose their hair-like swimming structures.

Meanwhile, their sperm producing and transporting organs also mature.

If they’ve moved into a fish that doesn’t already have a tongue biter at the helm,

they’ll speed right through this male phase and develop into a female.

They grow much larger,

their testes shrink, their ovaries become active,

they form a kangaroo-like brood pouch,

and they enter their host’s mouth and clamp down on its basihyal.

Here, they’re thought to suck blood

from the fleshier part of the fish’s tongue-like structure.

Over time, the isopod’s habits can cause the fish’s basihyal to wither away,

leaving the fish with a parasitic crustacean

acting as a tongue-like appendage.

In this form,

the tongue biter is suspected to functionally replace the basihyal,

moving food and water through the fish’s mouth.

Only mature female tongue biters take hold of the basihyal,

and only one at a time gets a taste.

In the event that a few tongue biters share the same lucky fish

and the tongue is already taken,

the others will remain male till the opportunity to switch arises.

If the female dies, a nearby male can develop into a female

and fill the recently vacated position.

Otherwise, males mostly remain in the gills,

though they may climb into the mouth to mate.

And when that happens, a female can produce hundreds of tiny tongue biters,

carrying them inside a special pouch until they’re ready to swim off

and find fish of their own to make home sweet home.

Fish can survive years parasitized by cymothoids,

but it may come as no surprise that having a blood-sucking isopod

for a tongue isn't ideal.

While some seem largely unaffected aside from the obvious, localized damage,

others grow slower, become anemic,

and are more susceptible to environmental stressors.

Parasitism rates can get surprisingly high among cymothoids’ preferred hosts,

which includes snappers, trout, and croakers—

also commonly consumed by people.

Surveys of Mediterranean striped sea breams

and South African largespot pompanos

found that nearly half the fish carried cymothoids in their mouths.

And the number can climb even higher in fish farms.

They’re normally removed,

but they've occasionally made their way into grocery stores and home kitchens.

And while theoretically eating a tongue biter doesn't pose any health risks,

it might not be the most pleasant surprise.

What? Louse got your tongue?