A Blog by Carl Zimmer

Tongue-Eating Fish Parasites Never Cease to Amaze

NOVA put together a video, embedded below, about one of those animals that you have to keep persuading yourself is real, a parasitic crustacean that lives inside the mouths of fishes, eating–and then taking the place of–its host’s tongue.

I can vouch for these beasts, having written about them off and on since I first encountered them in my research for Parasite Rexmost recently on the Loom last year. But I was not aware that it’s the female that wins the Oscar for best performance as a fish tongue. The males just hang out around the gills of the fish and then–yep–mate with the pseudo-tongue.

Clownfish with parasitic isopod. Copyright Lea Lee. http://www.flickr.com/photos/critter71/
Clownfish with parasitic isopod. Copyright Lea Lee. http://www.flickr.com/photos/critter71/

This discovery led me to wonder about the latest research about tongue-eating isopods. I came across a 2012 master’s thesis by Colt William Cook of the University of Texas, which confirms what you see in the video–that the parasites are born as males, and then when they enter a fish, one turns female. This switch only occurs if there’s no female already installed in the host–otherwise, the males stay male. As this transformation takes place, Cook adds, the female’s body grows enormously. Its eyes shrink, since it no longer has to hunt for a home. Its legs stretch out, to help it anchor itself in the mouth.

gilligan isopod
Courtesy of Matthew Gilligan

After one of the males mates with the female, she gives birth to a brood of live male parasites. For their first few days, Cook found, they search madly for another host (each species of parasite seems to only live in a single species of fish). They sniff for the scent of their host, and if a shadow passes overhead when the odor is strong, they shoot upwards through the water. They burn through a lot of energy in the process; if they fail to find a host in the first few days, they settle down and hope they can ambush a fish that happens to be swimming by. It’s a hard way to start your life, and it may explain why several males will huddle inside a fish with only a single female in the offing. Looking for another fish with a single female parasite might be a less promising strategy than competing with the males you’re with.

Of course, these rules may only apply to the species that Cook studied, which infects Atlantic croakers off the coast of Florida. The full diversity of these tongue parasites is probably enormous. A 2012 study puts the total species at 280, but that’s just known species. A team of scientists from Annamalai University in India recently did a survey of the parasites in fishes off the coast of India. Before their study, scientists knew of 47 species of parasites in Indian waters. In just nine fishes, the scientists discovered ten new parasite species. I’d wager that some of the species waiting to be discovered will prove to be even more surreal.

[Clownfish photo: Copyright Lea Lee. via Flickr ]


48 thoughts on “Tongue-Eating Fish Parasites Never Cease to Amaze

  1. I didn’t even know that fish had tongues. Does the pseudo-tongue somehow help the fish to survive, as I assume the original tongue must help? I mean, is this a working tongue or just an ornamental fake?

  2. Cynotho exigua extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. The parasite then replaces the fish’s tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish.[2] Once C. exigua replaces the tongue, some feed on the host’s blood and many others feed on fish mucus. This is the only known case of a parasite functionally replacing a host organ.[2] There are many species of Cymothoa,[3] but only C. exigua is known to consume and replace its host’s tongue….copied from wikipedia

  3. Movie “The Bay” (2012, USA) – is based on a true story telling about these creatures attacking people in one peaceful bay in USA on 4th of July. Horrifying.. a disaster which used to be hidden from the public..

  4. Do the parasite and the fish share some kind of mutual symbiosis or does the parasite harm the fish in any way (apart from eating its tongue)? What’s the life span of these parasites, and if less than the fish’s, does the fish remain tongue-less? Do the fish have any kind of defense against these parasites? If the parasite were to somehow leave the fish’s tongue spot, would it die or try to find another fish? If the fish dies does the parasite die too?

  5. It’s amazing how parasites invade…This is horrific, but atleast the parasite doesn’t kill the fish, it just eats and decays the tongue and then it becomes the fish’s new tongue. The fish use the parasite as its own tongue so its still eating and surviving. Interesting and yet Gross!

  6. I recently did a study on several species of sea threadfin breams,when I am about to dissect some of the individuals,I found a strange crustaceans like creature inside the fish’s mouth,I thought it was just a non finished eaten food of the fish,after reading this article I found my answer,but does this parasite affect the fish growth or development?

  7. Ralph Dratman:

    While a lot is still unknown about these parasites, these replacement tongues have indeed been observed to be used just like the normal tongue.


  8. (português)Eu já vi esse parasita no peixe vermelho mais conhecido como olho-de-cão (Priacanthus arenatus) quando fui pescar na costa de Angra dos Reis.

    I have seen this parasite in fish commonly known as red-eye dog (Priacanthus arenatus) when I went fishing off the coast of Angra dos Reis

  9. Nataliya said – “Movie “The Bay” (2012, USA) – is based on a true story telling about these creatures attacking people in one peaceful bay in USA on 4th of July. Horrifying.. a disaster which used to be hidden from the public..”

    not a true story at all. the trope of filming is “found footage” that implies actual events but it is all fiction.

  10. Avon – parasites are a natural part of life, and that “poor fish” eats other animals whole. Parasites are not evil and mean, they deserve to survive as much as any other creature.

  11. I was handed a frozen dead fish from another fisho one arvo, as the fish defrosted next to me one of these suckers crawled out, sure are tough.

  12. wow! a NEW discovery! it shows we people are still learning, but what a parasite, amazing and … amazing…

  13. I am a nine year old student in Qatar and I would like to know if the jewel wasp sting only works on cockroaches or another animal.

  14. ick. ick. ick . . .ooooohhh ICK! and yet I could NOT stop reading until I got to the end of the article!! IIIIICCKKK!! *shudder*

  15. I agree with *shudder* about this thing! The idea of it is so awful. At least this nightmare only happens to fish! Not to us.

  16. And that explains how humans came about. Aliens hosted with chimps. Only problem is the experiment eventually failed since we’re dumber than chimps based on how we treat the planet.

    The above comment is tongue-in-cheek, take it for what its worth. :)

  17. It’s amazing how this works. I wonder, is there further study showing how long the host would live when the parasite is still inside it?

  18. I cannot help wondering whether this particular parasite might be infecting more fish these days in our damaged oceans. Does anyone know whether this little monster has been playing the tongue game throughout recorded history (I hope so) or whether, rather, this is only a recently-observed pattern (please, deity, if any, let this not be new)… ?

  19. Forget what I said about a possible deity. No dice, because I just located this: http://io9.com/5890093/fishing-linked-to-higher-percentage-of-tongue+eating-blood+drinking-mouth-leeches-from-hell

    Let us hope the post I just linked is totally wrong. Stupid writer, imagining that OUR FISHING IS DESTROYING THE OCEANS. That just can’t be true… please? No, really, please. I don’t want to see the end of oceans as we know them, within my lifetime. I really don’t.

  20. My friend doesnt believe me on this so thanks now I can say I’m right but it makes me iffy on eating cod tongues now but meh there amazing

  21. Just watched “The Bay” I was curious and had to look up the parasite .
    So this has no effect on humans?
    Reading the comments about people finding them in the fish they’re eating is disgusting!

  22. Sweet Baby Jesus…

    I will only ever be able to eat fish again if I am so fortunate as to COMPLETELY forget everything I just read.

    And WHAT kind of person finds THIS appetizing?! Bet there’s bodies in his basement. Honestly, if my husband said such a thing I’d divorce him.

  23. I remember seeing this parasite one time while fishing in the San Francisco Bay. I caught a Croker (kingfish) and while pulling the fishing hook, saw this hard shelled bug in there too.

    I was about 12 years old at that time and thought it was simply a type of water bug the fish ate but had to use pliers to pull it out of the mouth. Last I remember was stabbing parasite. And tossing it overboard.

    Thinking back it grossed me out then and now decades later, I find out what it was. Amazing how nature survives.

  24. Along comes a marine biologist to answer a few questions on this years-old post!

    – No, they do not harm humans even if you ate the isopod by accident or purposefully.
    – There is no known evidence that the isopods actually consume the tongue, just that they cut off the blood supply so that it falls off.
    – Yes, the fish is still generally healthy and can eat okay. Maybe not as great as before, but okay.
    – No, you should not remove the parasite and throw it back. This would be nearly as harmful as cutting the tongue out of a fish and throwing it back, since either way you have a tongue-less fish.
    – This strategy (the “tongue game”) seems to have been around for quite a while in this group of isopods, but evolution happens pretty slowly no matter what.
    – I don’t know whether the prevalence of cymothoid isopods is increasing from pollution/fishing/etc., because the results vary a lot between environments and between organisms.


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