"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

January 24, 2010

January 24 - Enteroxenos oestergreni

Let's say you are dissecting some sea cucumbers and you come across this thing (see photo) - what do you think it is? Is it a worm? No - In fact, it is a snail! You have just found Enteroxenos oestergreni - a species of parasitic gastropod that lives inside the body cavity of sea cucumbers. Evolution has done away with all its apparently superfluous organs like its digestive system, gills, heart, nervous system, and reducing it down to just the bare essentials of a parasite - reproductive organs. The only morphological clue that this organism is a gastropod are its larval stages (veligers), which look like tiny, delicate snails - just like that of many other marine snails. However, after it enters the sea cucumber host, it eventually transforms into the worm-like adult form. An adult E. oestergreni is essentially a long stringy sac of eggs, just floating in the coelomic fluid of its host.

See the original paper here.

Contribution by Tommy Leung.


  1. It's been known for 68 years and I've never heard of it?! Amazing! I'm not spending enough time in marine bio labs ;-)

  2. Very nice. This is not something that was mentioned in my Invert. Zoo. class and likely will not be mentioned in Parasitology (no human aspect, you know). I appreciate the link to paper.

  3. Great series!

    I was a big fan of the book Parasite Rex. Do you have any other suggestions for interesting parasite books?

  4. Cool blog! The original description of Enteroxenos oestergreni has been scanned by BHL, I've extracted the article Enteroxenos östergreni, ein neuer, in Holothurien schmarotzender Gastropode as part of my BioStor project. The paper is by Kristine Bonnevie, who -- according to Wikipedia -- was Norway's first woman professor.

  5. Thanks for the link Rod, that's awesome. There's just so little known about this species (and others from its family).

    There's so much more cool things that we can find out about these crazy snails - comparative genetics, evo-devo, or just some basic eco-parasitology.

    If I find out that there's a location where these parasites are commonly found and I can find interested collaborators, I'm putting together a grant proposal!

  6. Oops ... 114 years!! Thanks Rod! :-D

  7. I wonder what creationists would make of a creature consisting only of sex organs? The explanation would likely be convoluted, like this gastropod!

  8. man...parasites are crazy! What a cool story...thanks for sharing. I normally read (and blog) about macro-organisms, but this is weirds me out, in a good way haha