A Blog by Carl Zimmer

Tongue Parasites to People of Earth: Thank You For Your Overfishing

Whenever I give a talk about my book Parasite Rex, I try to gather together the creepiest images of parasites that I can. Every time, there’s one kind of parasite that summons an instant reaction: a mix of laughter, sucked-in breaths, and gasps of recognition. I speak, of course, of the parasites that eat your tongue.

I only mean you if you’re a fish. Some species of isopods (crustaceans related to the less creepy crabs and lobsters) will swim into the gill of a fish, make their way to its mouth, and devour its tongue. It will jam its legs into the gills to hold itself in place, facing forward, its eyes gazing out of the fish’s mouth, taking the very place of the tongue it just ate.

I was first introduced to these disturbing creatures by Matthew Gilligan, an invertebrate zoologist at  Savannah State University. I had come across a disturbing picture of one of these parasitic isopods in a paper he published in 1983 and sent him an email, asking questions about it. I wondered what the isopods did once they were done devouring the tongue. As far as Gilligan could tell, they stopped eating the fish once they had made a place for themselves in its mouth. Perhaps afterwards, they fed on the animals that the fish itself caught. After all, the fish that Gilligan and others had caught with these isopods were still alive and seemed healthy. Gilligan’s response made me wonder if perhaps the fish simply used the hard-shelled back of the parasite as its own tongue.

Since then, a new generation of scientists have studied these mysterious parasites, and it looks as if their dealings with their hosts are not as peaceful as once thought. In 2003, for example, scientists studying isopods in a fish farm off the coast of Turkey found that sea bass with the parasites in their mouths had lower blood counts than ones that still had their tongues intact. It seems that the isopods act like blood-drinking mouth leeches.

In a new paper in the Biological Journal of the Linnean Society, British researchers have followed up on these studies with a big survey of the Mediterranean, inspecting striped sea bream for tongue parasites. They compared two populations of the fish that were closely related to each other but lived in very different environment. One population, off the coast of France, lived in a marine protected area. The other, off the coast of Italy, is heavily fished. In the protected waters, the scientists found, 30 percent of fish had the parasites in their mouth. In the fished waters, 47 percent did.

Take a moment to let that sink in. Almost a third to almost half of these fish open their mouth and look as if they came out of a science fiction movie. Having recently just eaten bream for the first time, I’m relieved to find out that the parasite does not cause human disease. Still, next time I eat fish I may have some disturbing images in my head.

The scientists found not only that there are more parasites in the heavily fished waters. They also found that parasites took a heavier toll there. Fish with tongue parasites ended up smaller and lighter in the heavily fished waters than infested fish in the refuge.

These differences all seem to come down to fishing. A lot of studies on many species of fish have shown that heavy fishing can drive the evolution of small fish. Fish that get to be sexually mature faster may be more likely to have offspring than fish that take their time to reach bigger sizes. In that rush, harvested fish may end up unable to defend themselves against enemies, including tongue-eating parasites.

It would be interesting to see what the rates of tongue parasites are in other heavily fished and protected parts of the world. If this pattern holds up, it may turn out that overfishing has made the world a more alien place.

[Image: University of Salford]

21 thoughts on “Tongue Parasites to People of Earth: Thank You For Your Overfishing

  1. Eat your heart out, Stephen King, you just couldn’t make this stuff up.
    Man seems to be making life easier for (other) parasites.

  2. 30 percent Vs 47 percent is based on a single study. There needs to be more follow-up studies to make sure this number is statistically significant and did not arise by chance!

    [CZ: Chris. It’s certainly true that this study ought to be followed up with more like it. But this study was not tiny. From the paper: “Parasite prevalence was significantly higher in the TYR (47%; 52 infected fish of a total of 90) than in the BA (30%; 58 of 194 fish) population sample (c2 = 20.139, d.f. = 1, P < 0.0001)." As statistics go, that's a P you definitely want!]

  3. Alternatively…

    Fish to tongue parasite: Thank you for eating my tongue and thus keeping me small and malnourished enough to slip through the holes of the fishing net.

    Ironically, our overfishing of these stocks may have turned what was once a serious problem (parasite ate your tongue) into a positive survival advantage (at least you now won’t get caught and eaten).

  4. That was an interesting paper, and nice to see something written about those tongue-biter which was about their actual ecology rather than another “Check out this gross thing! Urrgghh!” article. I have a question about the study (the new one on Biological Journal of the Linnean Society) – is there any particular reason why the researcher chose to only looked at the tongue-biter?
    I know that the study you cited in Journal of Fish Disease indicates that they can cause anaemia, but so can a number of other blood-feeding parasites. Like many other fish, Lithognathus mormyrus is infected by a whole community of other parasites may also inflict various pathologies, so why did they chose to look at the tongue-biter? Okay, I must admit they are pretty cool, so the answer might “why *wouldn’t* you want you look at the tongue-biter?” – but why chose that *particular* parasite as the indicator for this study?
    Maybe I should just give them the benefit of the doubt – maybe from their past experience looking at parasites in Lithognathus mormyrus, they found that the tongue-biter was a good overall indicator for the ecology of the host fish. Perhaps they are saving the data they have on the other parasites for their next paper?

  5. The basihyal of a fish isn’t quite the same as the tongue of a mammal. But I got bit by one of these types of isopods once and it hurt. Can’t imagine its a pleasant experience for the fish.

  6. Yes, 47 vs 30 it’s no huge difference, but the really striking point of the paper is not so much the difference in parasite load, but rather the fact that – after controlling for other interacting factors – the fish from italy suffered a significanly stunted growth and poorer metabolic rate than the protected spanish ones.

  7. How about harvesting the parasite?..with a little butter and touch o f garlic..might be a delicacy like the snail…gives the fishing industry a value added product.once we overfish it, the rest of the fish will be in good shape!
    Once its an endangered species, we might even have parasite farms?

  8. Wasn’t there an article somewhere on the Discover blogs pointing out that the parasite doesn’t actually eat the tongue, but the tongue atrophies as the parasite is taking blood and fluids from it? I thought it was on The Loom, but maybe Ed had it over at NERS??? A quick Google search comes up with a Wiki page also pointing out the tongue atrophies (not that Wiki can be fully trusted but it confirms what I remember reading a while ago).

  9. I think that every medical student should read “parasite Rex” and “Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body” in the first year of graduation. Thinking in evolution perspective is not easy, but once you start it’s become very helpful to understand every subject on biology field

    Keep the wonderful work

    Thanks !

  10. Hello – in reply to Tommy L: you’re right: every fish species has many parasites. In this particular instance, we found that these two areas showed an overall indistinguishable parasite fauna and we reported these results in two previous papers (see Sala-Bozano et al 2009: DOI: 10.1111/j.1365-294X.2009.04404.x and Sala-Bozano & Mariani 2011: http://www.sciencedirect.com/science/article/pii/S0272771411000643 ). Then we focused on the most conspicuous of them, Ceratothoa italica, as it was also the one that we did not find anywhere else across the Mediterranean.

  11. Carl,
    I’ve enjoyed your books on evolution. Recently I read some articles on “funny statistics” and the book “Bad Science”. Which made me wonder about your response to Chris about all the other things that might make that P value look so nice. Why was one area “no fishing”? Was it more pristine to begin with? Was it further from shore? Deeper? More diverse species, etc.? I’m no expert but wonder how many contributing factors might have determined the outcome in addition to fishing.

  12. Hi Stefano, thank you for your reply and directing me to those studies. It appears that my institute does not have access to Sala-Bonzano & Mariani (2011), so I will contact you directly for a reprint of that paper. I have been discussing the idea of fisheries altering the ecology of marine parasites with some collaborators and we find your results to be very interesting – we are interested in doing similar studies in our parts of the world and as Carl mentioned, to see what pattern we may discover in the parasite communities of commercially-targeted species in different regions.
    Perhaps you might have also seen this paper (currently In Press) – Marzoug et al. 2012: http://www.sciencedirect.com/science/article/pii/S1383576912000141

  13. @3 Peter

    At first blush your concept has merit, however once the big fish get caught in the, say 4 inch, mesh and the fishers come up losing, the net mesh size will be reduced as it is imperative t0 maintain a catch. So being smaller confers an immediate advantage, however man will see to it that it doesn’t last.
    We will be competing for krill, then zoo plankton and nothing on earth can stop us except maybe bacti and/or virus’s.

  14. Awesome article. For me it was quite disturbing, especially the photo. Great information but I am suspect of the conclusion fishing has anything to do with it, although this may very well be the case. There certainly isn’t enough data to arrive at that conclusion (at least presented in the article) unless it was looked for in the first place.

    I’d have to agree that a study of 2 populations and less than 300 subjects in terms of statistical data, is a tiny study. Particularly the limited populations studied (two). There are too many variables between the two environments (aside from one being a protected habitat) to make any conclusions related to the one detail of fishing. Are the insect populations identical? If not I would suspect that could be a factor. What about plant life? What about the general population of the parasites and differences in their anatomies? And on and on.

    As a sport fisherman myself I have noticed differences in size of fish more varied than these in a one mile stretch of river simply due to the amount and type of vegetation near the fishing spot and the effect that the vegetation has on the insect and bird (they eat the insects too) populations. I would also say that my ‘study’ involves somewhere close to this amount of fish. no disrespect to the study only want to point out that every single aspect of an ecosystem has an effect on other parts of that ecosystem in a much more intricate and beautiful way than one or two details… And that fishing while being one of these details, is only one of them.

    That said, I would say that over fishing a thousand miles away can have an even bigger effect on the fish here than any of those things…

  15. I got bit by a isopod last month. The isopod climbed right out of the mouth of the fish i just caght. I live in snohomish Wa, Should those parisites be there

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