Let us ruin your day with this tongue-eating parasite - Niko Zlotnik

13,477 views ・ 2025-05-08

TED-Ed


Please double-click on the English subtitles below to play the video.

00:07
One night in 2013, a man in Northern Ireland
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was shocked to discover something bizarre protruding from the mouth of the sea bass
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he was preparing for dinner.
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Peering closer, he came face-to-face
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with none other than the infamous “tongue biter,”
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also known as the tongue-eating louse.
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This species, Cymothoa exigua,
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is one of nearly 400 belonging to a family of fish parasites
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called cymothoids.
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They’re crustaceans, and more specifically, isopods,
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like pill bugs.
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Some latch onto their host's head, others suck blood from fish gills,
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and still others burrow into fish flesh
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and form sac-like membranes around themselves.
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01:01
Clearly, there are many delightful ways to be a cymothoid,
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but Cymothoa exigua and others go specifically for fish tongues—
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or at least what pass as tongues among fish.
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Because instead of what we think of as a tongue,
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fish have a bony structure called a basihyal,
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protruding from the floor of their mouths,
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which they use to move oxygen-rich water towards their gills
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and to swallow food.
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Tongue biters start out looking for a fish to call home for life,
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which can be up to around three years.
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They may swim for days, surviving off stored yolk reserves—
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until finally, they sense a change in the light
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and pick up on a nearby fish’s chemical cues.
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They burst into action, hook onto the fish’s gills
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using the sharp, curved tips of their limbs, and crawl on in.
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There, they molt,
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and since they won’t be living independently again,
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their eyes recede and their limbs lose their hair-like swimming structures.
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Meanwhile, their sperm producing and transporting organs also mature.
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If they’ve moved into a fish that doesn’t already have a tongue biter at the helm,
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they’ll speed right through this male phase and develop into a female.
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They grow much larger,
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their testes shrink, their ovaries become active,
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they form a kangaroo-like brood pouch,
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and they enter their host’s mouth and clamp down on its basihyal.
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Here, they’re thought to suck blood
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from the fleshier part of the fish’s tongue-like structure.
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Over time, the isopod’s habits can cause the fish’s basihyal to wither away,
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leaving the fish with a parasitic crustacean
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acting as a tongue-like appendage.
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In this form,
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the tongue biter is suspected to functionally replace the basihyal,
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moving food and water through the fish’s mouth.
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Only mature female tongue biters take hold of the basihyal,
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and only one at a time gets a taste.
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In the event that a few tongue biters share the same lucky fish
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and the tongue is already taken,
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the others will remain male till the opportunity to switch arises.
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If the female dies, a nearby male can develop into a female
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and fill the recently vacated position.
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Otherwise, males mostly remain in the gills,
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though they may climb into the mouth to mate.
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And when that happens, a female can produce hundreds of tiny tongue biters,
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carrying them inside a special pouch until they’re ready to swim off
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and find fish of their own to make home sweet home.
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Fish can survive years parasitized by cymothoids,
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but it may come as no surprise that having a blood-sucking isopod
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for a tongue isn't ideal.
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While some seem largely unaffected aside from the obvious, localized damage,
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others grow slower, become anemic,
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and are more susceptible to environmental stressors.
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Parasitism rates can get surprisingly high among cymothoids’ preferred hosts,
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which includes snappers, trout, and croakers—
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also commonly consumed by people.
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Surveys of Mediterranean striped sea breams
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and South African largespot pompanos
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found that nearly half the fish carried cymothoids in their mouths.
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And the number can climb even higher in fish farms.
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They’re normally removed,
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but they've occasionally made their way into grocery stores and home kitchens.
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And while theoretically eating a tongue biter doesn't pose any health risks,
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it might not be the most pleasant surprise.
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What? Louse got your tongue?
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