Wow. For my dissertation I’m studying the behavioral effects of a trematode parasite (Euhaplorchis californiensis or EUHA) infecting the brains of California killifish. I was dissecting a killifish to count the number of parasites in its brain case and discovered that this particular head was home to two very large isopods.

The picture below is a California killifish brain (center) and 2 isopods (left and right). The brain is a bit beat up because I was searching in the crevices for EUHA. The isopod on the right is missing its back end.

These parasitic isopods were found underneath the fish’s operculum, which I suppose is the fish equivalent of our cheeks. Look at the size of those isopods! Can you imagine having parasites the size of your brain living in your cheeks!

I believe that findings such as this support my argument that researchers should be paying more attention to the parasites in their study organisms. These parasites were not visible externally, but were easy to find after a quick dissection. It’s hard for me to imagine that parasites of this size living off of a small fish have no effects on its behavior or fitness. I know that learning parasitology may seem daunting, but just a little parasitological knowledge would likely provide major insights into explaining individual differences in behavior. ::Jumps off soapbox::

27 Responses to “Isopods as big as your brain!”

  1. Hanninen

    Uh…that is NOT real. You made that up. That is horrifying. AWESOME!

  2. weinersmith

    It’s very real! 🙂

  3. Matt Simmons

    To be fair, I have no conclusive evidence that my brain itself isn’t a parasite. I mean, sure, you can tell me that it isn’t, but really, who’s interpreting what you’re saying? My brain. That’s hardly unbiased evidence.

  4. sharkey


    nice find. I sent it to my kids 🙂

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  7. stefan

    nice photo! this is the beauty of any research work..
    I am not sure that the operculum is equivalent to the mamalian cheek – i think most probably matches the mandible or other bone(s) of the viscerocranium (no comparative embriology class in med school :()

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  9. pheret

    er, do we consume these fish? are the parasites just included with the fish, perhaps as a delicacy, fish cheeks? 😛

    • weinersmith

      🙂 We don’t typically consume California killifish, but isopods are common fish parasites!

  10. John Swensen

    In all honesty, isn’t this really just the size of the cross sectional area of the brain? It is still quite remarkable, but clarification is in order.

  11. Eric Ettner

    The Operculum is the bony gill cover, and so is evolutionarily more related to parts of the human neck (as the gills evolved into parts of our voice-box, vocal chords, etc.) Exactly WHAT part i’m not sure, perhaps the hyoid bone? I believe the book Your Inner Fish (Neil Shubin) would have the answer, but I gave my copy away…
    Anyway, good find! I love isopods (Plankton nerd here), you are now RSS’d!

  12. Snod

    My ex-wife was bigger than that!

  13. tsanko

    Wonderful ..thanks a lot for posting a good informitive blog

  14. ibroxmassive

    This has opened my eyes wide open! Wow!

  15. Quick Links….a lot of them…all good. | A Blog Around The Clock

    […] Isopods as big as your brain! […]

  16. NHL Hockey Forum

    This is the perfect post and may be one that should be followed up to see what goes on

    A friend emailed this link the other day and I will be eagerly anticipating your next blog post. Continue on the wonderful work.

  17. Ciliates! | Weinersmith

    […] killifish have it rough. The picture of the isopods that were as big as their host’s brain came from the killies and that is only […]

  18. lalala

    on an unrelated note cool story….but on SMBC it was mentioned its your bday sooooo HAPPY BIRTHDAY NERD!!! (it said to say that)

  19. Jordan

    Parasites are definitely fascinating. The emerging evidence about the relationship between T. gondii and human behavior is unexpected but really interesting.

    I also appreciated the Radiolab episode about parasites. It’ll be interesting to see if we can find a way to mimic the anti-allergy properties of hookworms to provide those benefits without the infectious downsides.

  20. Zach

    I really think it would be interesting if I could examine some of the parasites of my study organisms (nematodes). I’m just not enough of a microbiologist to do this, though. I’m sure there are some, but as far as I’m aware nobody’s really looked for them.

    • Jordan

      There are some interesting PCR-based methods for doing that. One quick and dirty method is to snag a primer for the 30S ribosomal subunit and amplify that extracted DNA. The sequencing results can give you an idea of some of the microbial diversity.

  21. Can I get parasites,or bacterial infections from a fish tank by accidentally ingesting water? | Plastic Fish Tanks

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  22. Lord Zoltar

    Parasites as big as the brain, yet hardly noticeable from the outside? Maybe it’s just a proportionately small brain…

    Also, SMBC says it’s your birthday so HAPPY BIRTHDAY!!!

  23. rachat de credit

    Obtain and pick some good things from you and it aids me to solve a problem, thanks.

    – Henry

  24. Jessica

    Ugh, the mere sight of that made me queasy. Even more disturbing is the thought that the fish might have been in pain and/ or no longer in full possession of its faculties.

  25. Mark White

    I think the important finds you’re missing are the parasites within the isopods… PARASITE PARASITES!

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