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Cast Your Eye on This

by Brian Dunning, Sep 30 2010
Leucochloridium paradoxum

The long green sporocyst is visible inside this snail, extending all the way up into its larboard eyestalk.

Today we have a look at an especially horrible little beast from the parasite department. Parasites come in all shapes and sizes, but what's most interesting about the best of them is the jaw-dropping life cycles that some of them have developed. Leucochloridium paradoxum (aka the green-banded broodsac) is one of my favorite examples.

This little fellow is a flatworm that lives inside birds, and subsists off of their bodies from the inside out. It has developed a neat trick for getting the next generation of little flatworms into other birds. There's one way out of a bird: through its droppings. The problem is that birds don't typically eat other birds' droppings, so at first glance, this seems to be a poor choice for spreading one's seed.

Fortunately, there are other things out there that do eat bird droppings. Leucochloridium paradoxum puts its larvae into the bird's droppings, where they sit and wait for an unlucky snail to come along. Snails are quite fond of bird droppings, and so they eat them up, along with the broodsac larvae. Once inside the snail, the larvae grow into sporocysts, which are big long tubes containing dozens or even hundreds of little flatworms. The sporocyst grows inside the snail, most obviously up into its eyestalks. Horrifically, the sporocyst pulsates in such a way that the eyestalks look almost like barber poles, flashing bright horizontal bands of color to attract — you guessed it — birds.

A bird comes along, thinks the snail's eyestalk is a caterpillar, and nips it off. The bird is thus full of broodsac flatworms, which then live out their normal lives until it's time to reproduce, and the cycle starts over again.

While infected, the snails can still see, although not very well. They're no longer really able to discern light from dark, and so they end up out where they shouldn't be, out where birds can see them more easily. And, of course, once their eyestalks are bitten off by birds, they can't see at all.

Enjoy this video of a snail so infected:

26 Responses to “Cast Your Eye on This”

  1. Kemi Eke says:

    Wow, so the ‘Alien’ facehugger really does exist! Creepy!

  2. CW says:

    This is probably a dumb question, but don’t other animals, like these birds, have a digestive system that can kill the parasites that they ingest (i.e. stomach acid)?

  3. LovleAnjel says:

    EEEEeeeeeeeeeeewww. As someone who studies snails, I absolutely hate these guys, and the trematodes that neuter snails.

  4. steelsheen11b says:

    I always thought the most horrible little beasts from the parasite department were the John Edwards and James van Praaghs of the world.

  5. Pam says:

    Good gosh, I must remember not to check out Skepticblog at breakfast!

  6. Holy Crap! I knew my fear of zombie snails wasn’t unwarranted, nature is weird.

    So the pulsing is the movement of the collective group of sporocysts?

  7. Beelzebud says:

    LOL there is something utterly horrific about the idea of a parasite taking over a host in such a way. My reptile brain recoils in irrational terror. :D

  8. Max says:

    Do we know how parasites with complex life cycles evolved?

  9. You know when you don’t get enough sleep and your eye twitches? Yeah, well, I have that right now and reading this didn’t help.

  10. Carl says:

    Cool but not very, um, skeptical.

    How about mind-control parasites, like Toxoplasma, which screws with its rodent host’s brain to make it actually move toward cats, because its life cycle requires the parasite to get into the cat’s gut?

  11. Tony says:

    All praise almighty God! Such a complex means of reproduction simply must be the work of an intelligent designer, and one so benevolent (if you are a parasite)!

  12. Zevv says:

    Interesting and all, but how does this relate to skeptisicsm ? Sorry, but completely misplaced on this blog.

  13. citizen wolf says:

    Parasites and parasitoids are clear proof of the kindness and benevolence of a creator. After all, religion is a parasite of the mind.

  14. MaikU says:

    This is disgusting! I was eating for god’s sake!

  15. Shawn S. says:

    @#14, that’s okay, the parasite was too so no manners have been offended.

  16. Mike says:

    How could anyone be critical of this post? This is an awesome example of the seemingly ingenious ways evolution has shaped life on planet earth. As a skeptic, this stuff fills me with amazement when I think about our existence.