"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

April 23, 2010

April 23 - Osedax roseus

The genus Osedax is commonly known as the "bone worms", although some endearingly call them "zombie worms" as well. These marine polychaete worms were discovered only in 2002 in the deepest parts of Monterey Bay, California. What were they doing? If you guessed "parasitizing zombies", you'd be sadly wrong. You should put down your comic book and pick up Greg Rouse's paper here. If you guessed eating bones from dead whales, you'd be correct (you should continue reading also in case there are more gratifying questions to come). The animals were "rooted" (like tiny trees) into the bones and a large "trunk" of the animal above the bone waving in the current.

So at this point you've figured out that they're not a whale parasite because the whale is seriously 'belly-up' at this point. So Osedax is in fact a decomposer. But when Rouse took a close look at these worms, he found that they appeared to have no guts whatsoever. So where were they getting their nutrients? Being an expert on marine polychaetes he knew that some deep sea worms had the ability to garner bacteria that derive nutrients from geochemical vents or seeps. As it turns out Osedax was doing something very similar. Inside the "roots" of the animal, which are inside the whale bone, the bacteria are helping to digest the yummy fats left by the whale.

Oddly, Rouse was only seeing female Osedax in his collections. Where are they males? If you answered, "out for a cold one at the Mos Eisley Cantina" you'd need to turn off the television and pick up a book. If you guessed, "living as a harem inside the body of the female" you'd be correct! Rouse initially thought these microscopic "bags of sperm" were parasitic (yeah!) inside the females, surviving on the nutrients she and her endosymbiont bacteria were producing. However, at closer examination they appear to actually be larvae that never develop feeding structures at all, just living off the yolk for the egg sac. Never the less, they are able to provide sperm to fertilize the female's eggs!

Ok, I know what you're thinking, "its icky and it not even a parasite, why am I still reading?” Because it's fascinating! Survival in a VAST ocean where there are relatively few dead whales on which to live is a tricky thing; reproduction is even trickier. The "dwarf males" can disperse over large distances, because the only part of the life cycle that isn't attached to a whale bone is the larva. Rouse thinks that the sex is environmentally determined: if larvae land on a bone, they become a female, if they land on a female, they become male. If this species only relied on a male settling next to a female to breed, it would seriously diminish the gene pool.

So Osedax is icky and but it's not a whale parasite, though the males are sort of parasites of the females... so it is completely fascinating! Osedax has evolved surprising and strange means to eke out an existence at the bottom of the sea where few organisms (even zombies) fear to tread.

See also: Rouse et al, 2008. Acquisition of Dwarf Male "Harems" by Recently Settled Females of Osedax roseus. Biol Bull. 214, 67-82.

Contributed by Matt Leslie.


  1. As it is an annelid, I am fond of Osedax. But I am disappointed. And I'm calling it: this is the first DEFINIVELY-not-a-parasite-of-the-day in a long unbroken record of parasites of the day. You can't be a parasite of a dead thing, lest we include hagfish, cookie sleeper sharks, and Osteopelta limpets from whale-fall; or even the myriad and sundry critters that feed on other dead animal carcases. As foul as all of this may be, still I cry "foul" on parasite of the day.

  2. One more thing, the "males are sort-of parasites of the females" no more than are gestating offspring.

  3. I'm standing by this. Not only is it a cool invert, but I think that these little males, living solely within and living off their mate's nutrients and protective body constitutes a parasite enough (more sexual parasites are coming). And, besides, it's all about celebrating diversity, especially the fact that we're still discovering really wacky and interesting critters every day.

  4. I don't check this blog for a few days and what do we have here? There's certainly no shortage of whale parasites to warrant throwing in the famous "bone-eater" here.

    While I am somewhat disappointed by this, I suppose if one is to look for legal loopholes, you can always say it's about the symbionts that live in that worm's rootlets...even then they are technically mutualists...

    Perhaps we can make a bonus double-parasite day in the future to make up for this - there is absolutely NO shortage of fascinating *parasites*...

  5. I still like this. It's not as though "parasite" is a monophyletic group, after all. And yes, Tommy - many of the entries pack enough parasite in their punches (how 'bout them P's?) to warrant highlighting a cool worm that can drill into bones and suck out the fat!

  6. i just came across this site.
    great blog. parasites interest me. don't know why but either gross-out or evolution factor.

    another evolution-related interest is the deep sea so i thought i'd point out on some female anglerfish what was thought to be a parasite was actually a male anglerfish.

  7. And if they are present in human bone!
    Did anyone check that?

  8. No they don't - to be honest this was the only time on this blog when we featured a creature that wasn't actually a parasite (see the debate above). These worms only colonise the bones of carcasses - and carcasses in the deep sea for that matter - so no, they won't be present in a human bones.

  9. I am curious!
    Could there be a similar parasite in human bone marrow, that could cause osteoporosis?

  10. No Tiago. please refer to the discussion above your comment - it's not even really a parasite - see the comment by Mark Siddall.