The Tongue That Wasn't: A Fish's Living Tenant

Claire Brooks

Somewhere along the Pacific coast of Latin America, right now, someone is buying a snapper. Fresh catch, on ice, good price.

Oliver Hart

Set the scene.

Claire Brooks

They carry it home, lay it on the cutting board, and because they are thorough, they open the fish's mouth to check the gills. And something looks back at them.

Oliver Hart

Something looks back.

Claire Brooks

About the size of the last joint of your thumb. Pale. Armored. Seven pairs of hooked legs folded neatly underneath, like a living Swiss Army knife. And it is sitting exactly where the fish's tongue should be.

Oliver Hart

Sitting next to the tongue?

Claire Brooks

No. Where the tongue should be. Because it is where the tongue should be. It ate the original, moved in, clamped onto the stump, and took over the job.

Oliver Hart

And the fish?

Claire Brooks

The fish, for the record, was fine. Swimming, eating, living its life. With a crustacean for a tongue.

Oliver Hart

Last episode we did parasites that hijack behavior. Puppet masters pulling strings from inside.

Claire Brooks

Today is a different rule, and it is one you probably did not know you believed. Your organs are yours. A body part is not a studio apartment. Another animal cannot move in, change the locks, and start paying the utilities.

Oliver Hart

And the animal disagreeing with me is called?

Claire Brooks

Cymothoa exigua. The tongue eating louse. And before you picture a louse, it is not an insect.

Oliver Hart

What is it?

Claire Brooks

An isopod. A crustacean. Its cousins include the roly poly in your garden and the giant isopods of the deep sea, which look like roly polies that got into a growth hormone.

Oliver Hart

The family has range.

Claire Brooks

The family has range. And by the end of this you will know how it pulls off the only known organ replacement in the animal kingdom, what the fish gets out of the deal, spoiler, almost nothing, and why scientists described a brand new species of tongue eater just last year.

Oliver Hart

Where do we start?

Claire Brooks

We have to get the isopod into the fish first. That journey is its own story.

Oliver Hart

Okay. Beginning of the story.

Claire Brooks

Not in a mouth. In open water, with a larva the size of a grain of rice. This is the free swimming stage, the only moment in the entire life of this animal when it is not attached to a fish.

Oliver Hart

And it has one job.

Claire Brooks

Find a snapper. Miss, and that is the end of the story. Hit, and things get interesting.

Oliver Hart

Does it go straight for the mouth?

Claire Brooks

No, and that surprises people. It goes for the gills. Slips in with the flow of water the fish is breathing, and clamps on with seven pairs of hooked feet.

Oliver Hart

Are the hooks specialized?

Claire Brooks

The hooks are a whole field of study, actually. Researchers have found that the shape of an isopod's attachment claws matches its lifestyle. Mouth dwellers carry a different toolkit than body clingers.

Oliver Hart

So you can read the address off the feet.

Claire Brooks

These animals are quite literally shaped by where they hold on. And here is where the story takes its first weird turn. That larva will grow up male.

Oliver Hart

Some of them.

Oliver Hart

All of them. Every one.

Claire Brooks

Then where do the females come from?

Oliver Hart

They are what biologists call protandric hermaphrodites. They start life as males and, under the right circumstances, transform into females.

Claire Brooks

And the right circumstances are?

Oliver Hart

Real estate.

Claire Brooks

I am sorry?

Oliver Hart

The first young male to make the move from the gills into the mouth and claim the tongue gets the promotion. It settles onto the tongue, starts feeding, starts growing, and becomes female. Any males that arrive later stay out on the gills, smaller, waiting.

Claire Brooks

So the mouth is a studio apartment with one permanent tenant.

Oliver Hart

And the gills are the lobby where the boyfriends wait.

Claire Brooks

Let me get the architecture straight. The female is inside the mouth, on the tongue, drinking blood.

Oliver Hart

She will grow to about three centimeters. The males on the gills stay much smaller. And the fish keeps swimming around, mouth full of crustaceans, apparently unbothered.

Claire Brooks

Unbothered. If I have a popcorn kernel stuck in my teeth I am unable to function.

Oliver Hart

Take a moment to appreciate your life.

Claire Brooks

Genuine question though. Why the tongue? Why not just stay on the gills like a sensible parasite?

Oliver Hart

Because the tongue is prime property. It is bathed in blood, it is protected by the mouth, and crucially, it is a piece of the fish that the fish cannot scratch, bite, or rub off.

Claire Brooks

It found the one room in the house the owner cannot reach.

Oliver Hart

Which brings us to the heist.

Claire Brooks

Alright. How do you steal a tongue?

Oliver Hart

Slowly. The female clamps onto the base of the tongue and starts feeding, piercing the tissue and drawing blood. Not enough to harm the fish overall.

Claire Brooks

Just enough to.

Oliver Hart

Just enough, day after day, to starve the tongue itself. Cut off from its blood supply, the tongue withers. It atrophies. And eventually it falls away. A tiny shriveled thing, gone.

Claire Brooks

And the gap?

Oliver Hart

There is no gap. In its place, already attached to the stump, already positioned, already sized correctly, is the isopod.

Claire Brooks

Wait, say that again, because I think I skipped over it.

Oliver Hart

That is the reason this animal is famous. The parasite does not just destroy the organ. It replaces it. The isopod's armored body now sits where the tongue sat, doing what the tongue did, and the fish goes on using its mouth, trapping and positioning food against this living prosthetic.

Claire Brooks

Is that unique? Genuinely unique, or unique with an asterisk?

Oliver Hart

As far as science knows, it is the only case in the animal kingdom of a parasite functionally replacing a host's organ. Not damaging it. Not disabling it. Becoming it.

Claire Brooks

And the asterisk?

Oliver Hart

I want to be honest about the as far as science knows part, because this show does not deal in exaggeration. It is the only documented case. The ocean is large and full of things we have not opened the mouths of.

Claire Brooks

But for now.

Oliver Hart

For now the tongue eating louse holds the title alone. The only animal that evicts an organ and then covers its shift.

Claire Brooks

Okay, so what does the fish get? Because people love to call this a partnership.

Oliver Hart

Let us not romanticize it. This is not a partnership. The isopod pays no rent. It collects. A steady trickle of blood and mucus for the rest of its life

Claire Brooks

Is there a measurable cost?

Oliver Hart

There is. Studies on wild snapper populations find that carrying these isopods is not free. Fish with tenants show slightly worse condition, and young fish carrying them die a little more often than they should.

Claire Brooks

Anything beyond condition?

Oliver Hart

A related tongue replacing species off South Africa measurably slows the growth of its pompano hosts. Possibly because a mouth full of isopod partially obstructs the water flow the fish breathes with.

Claire Brooks

Slightly worse condition and slower growth. That is the whole bill?

Oliver Hart

That is the astonishing part. That is the whole bill. The fish survives. It feeds. It grows, mostly. It can live for years with a crustacean for a tongue.

Claire Brooks

Compared to last week's ants.

Oliver Hart

Who got one climb and a funeral. The tongue eating louse has pulled off something no puppet master managed. A sustainable takeover.

Claire Brooks

Because a dead fish is a dead end.

Oliver Hart

It keeps the fish alive the way a squatter keeps the lights on. And there is a word for that balance, which is the quiet theme of the whole episode. Not symbiosis, not murder.

Claire Brooks

Tenancy.

Oliver Hart

Permanent, blood sucking, organ replacing tenancy.

Claire Brooks

So what is life actually like in there? For the fish, and for the tongue.

Oliver Hart

For the female, life is stationary. She will never swim again. She clings to the stump, she feeds, she grows. And if there are males on the gills, one will occasionally make the commute into the mouth to mate.

Claire Brooks

The mouth is a venue.

Oliver Hart

The mouth is a venue.

Claire Brooks

And then eggs.

Oliver Hart

She carries them in a brood pouch on her underside. A marsupium. The same basic arrangement as a kangaroo's pouch, which is not a comparison I expected to make today.

Claire Brooks

And when they hatch, they leave how?

Oliver Hart

The way everything leaves. Out through the mouth of the fish, into the ocean, to go find snappers of their own.

Claire Brooks

So a fish can just be swimming along and.

Oliver Hart

And without any say in the matter, give birth to a cloud of isopods. Through its face.

Claire Brooks

Zoom out for me. Is this a one off species or a whole lineage of this behavior?

Oliver Hart

Cymothoa exigua is the celebrity, but it is one of hundreds of cymothoid isopods, and they have opinions about real estate. Some live on the gills. Some clamp onto fins and skin. Some burrow into flesh and live in a pocket with a breathing hole.

Claire Brooks

Like a tick with ambitions.

Oliver Hart

Like a tick with ambitions. And the family tree connects them to the giant isopods of the deep sea. Football sized relatives cruising the abyss, looking like the tongue eater's final form.

Claire Brooks

Where is the tongue eater found?

Oliver Hart

The eastern Pacific, roughly from the Gulf of California down to Peru. Classic hosts are snappers.

Claire Brooks

How common are we talking? Because I assume this is a rare curiosity.

Oliver Hart

It is not. In one recent study of fish sold in markets in Ecuador, more than a quarter of the fish examined carried Cymothoa exigua.

Claire Brooks

One in four?

Oliver Hart

One in four. If you buy snapper on the Pacific coast of the Americas, there is a non trivial chance your dinner has a tenant.

Claire Brooks

And you said there was a new species.

Oliver Hart

Just last year, twenty twenty five, scientists described a brand new tongue eating Cymothoa from the Gulf of California. Hiding in plain sight in a region people have fished for centuries.

Claire Brooks

Does it have a name yet?

Oliver Hart

Cymothoa facimar. It honors a marine science faculty.

Claire Brooks

That is touching.

Oliver Hart

It is touching, and it also means a university department is now immortalized as a tongue parasite.

Claire Brooks

Academia is full of tributes like that if you look closely.

Oliver Hart

Segment time. The Internet Lied to You.

Claire Brooks

I had a feeling. Is this the movie?

Oliver Hart

This is the movie. If the tongue eating louse rings a bell for you beyond nature documentaries, it is probably The Bay. Two thousand twelve, found footage horror, mutated isopods supercharged by pollution move into a Chesapeake Bay town and start occupying people.

Claire Brooks

And it works as horror because.

Oliver Hart

Because the monster is real. The isopod footage is real. The tongue eating is real. The human part is not.

Claire Brooks

Three reasons, as usual.

Oliver Hart

Three reasons you are safe. One, these are fish specialists. The entire toolkit, hooked feet, blood feeding mouthparts, is built for fish anatomy, and you are not a snapper.

Claire Brooks

Two.

Oliver Hart

Two, they need to be submerged in seawater, breathing through gill like structures on the abdomen. Your mouth is a dry, warm, oxygen poor cave by their standards.

Claire Brooks

Three.

Oliver Hart

Three, in the entire medical record there is no case of a cymothoid isopod setting up shop in a human. The worst documented outcome of handling one is a defensive pinch.

Claire Brooks

So nothing at all.

Oliver Hart

Well. Here is the truth the horror movie will not tell you. People eat them.

Claire Brooks

Sorry?

Oliver Hart

Not on purpose. But remember, one in four market fish in some places. They are crustaceans. Cooked, they are technically seafood.

Claire Brooks

The world's worst surf and turf.

Oliver Hart

Served inside the surf.

Claire Brooks

Alright, you ended last episode with how much of your behavior is yours. What is the question this week?

Oliver Hart

Creepier one. How much of your body is yours?

Claire Brooks

Go on.

Oliver Hart

Think about what the isopod actually did. It did not just damage the fish. It identified a load bearing organ, removed it, and slotted itself into the gap. Functionally, anatomically, permanently.

Claire Brooks

And the mouth still works.

Oliver Hart

Food still gets positioned, swallowed, processed. If you watched the fish eat you might not notice anything wrong. Somewhere in there, part of the fish quietly changed ownership.

Claire Brooks

Okay, but we do this to ourselves constantly.

Oliver Hart

We do, minus the blood drinking. Artificial hips. Pacemakers. Cochlear implants. Prosthetic heart valves, some of them made from pig tissue.

Claire Brooks

Meaning there are people walking around whose hearts are.

Oliver Hart

In a small but literal sense, part pig. And we do not find that horrifying. We find it miraculous.

Claire Brooks

So what is the actual difference?

Oliver Hart

The difference between medical miracle and body horror turns out to be mostly about who chose the arrangement.

Claire Brooks

Which is a lower bar than I would like.

Oliver Hart

And that is the real lesson here. The boundary between organ and organism is not a law of nature. It is a job description.

Claire Brooks

Unpack that.

Oliver Hart

A tongue is a thing that sits in a mouth and helps you eat. The isopod sits in the mouth and helps the fish eat. Evolution, the ultimate pragmatist, does not care about your categories. It cares whether the mouth still works.

Claire Brooks

The fish was never consulted.

Oliver Hart

Never. And it pays a tax. A little blood, a little growth, a little worse odds when it is young. But it lives. It eats with the thing that ate its tongue.

Claire Brooks

There is something almost philosophical about that level of acceptance.

Oliver Hart

Although I suspect the fish is not doing philosophy.

Claire Brooks

So the takeaway for anyone told to know themselves.

Oliver Hart

Remember the snapper. Swimming through the Pacific, getting on with its life, unaware that one of its organs has a heartbeat of its own. The self is a looser confederation than we like to think.

Claire Brooks

Episode one, your behavior might have co authors.

Oliver Hart

Episode two, your body might have tenants. This show is going to be a long exercise in realizing you were never quite who you thought you were.

Claire Brooks

I promise it is more fun than it sounds. Check your next snapper before you cook it, everybody.

Oliver Hart

Not because it is dangerous. Just because it is polite to know who is coming to dinner.

Claire Brooks

What have we got next time?

Oliver Hart

The eleven thousand year old dog cancer. Canine transmissible venereal tumor is a contagious cancer, an immortal cell line from one ancient dog that has been passing between dogs for millennia. It is arguably the oldest organism alive.

Claire Brooks

Arguably.

Oliver Hart

We will pair it with Tasmanian devil facial tumors and contagious clam leukemia. Stay curious, Nadia.

Claire Brooks

Contagious clam leukemia. Sure. Stay odd.