Long time readers will be all too aware of my absolute fascination with the bone-devouring zombie worms from the ocean’s depths. Since their discovery merely 6 years ago, researchers have uncovered many discoveries about these strange creatures. To review: Fun Facts on the genus Osedax (meaning “bone-devourer”)! Gather round the whole family!
- Not only do they dine on whale bones, they apparently can dine on cow bones too. Make sure you lock up the cattle in the barn at night!
- There are more than one species and they appear in multiple parts of the world! I wouldn’t sit there looking so smug and contect landlocked countries. It is only a matter of time before crawl out the ocean for a taste of human flesh! Yes I’m talking to you Bolivia and Switzerland.
- They lead disgusting lives that would have made Jerry Falwell weep in despair. It is a female dominated society with males relegated to pathetic little sacks of sperm. The females have evolutionarily beaten down the men in the Osedax world to worthless little semi-parasitic dwarves. Alas this appears to be the condition in all Osedax species discovered thus far.
- Not only are the men made into mice, but females exploit them by keeping them confined to harems! Talk about keeping the man down! Up to 114 males have been found in a single female oviduct.
Really, can you take much more of them? Maybe the Japanese and Norwegians are on to something. Kill the whales, destroy the zombie worms, restore decency to the world. Cut them off from the top? Another approach is to cut them off from the bottom, by depleting (freeing?) the females harem of enslaved males. In order to understand how we might fight these most unholy of sea creatures, Vrijenhoek and colleagues sought to understand where the supply of males came from.
Details below the fold!
As Fritz Vollrath so eloquently stated in a fantastic review article on dwarf males in 1998,
“The human analogy for such a strategy would be a gigolo, who, according to Webster’s dictionary, is ‘a man living on the earnings of, or supported by, a woman’. By definition, gigolos are unlike true parasites because their females receive an important benefit: sperm (or sexual satisfaction). If males are small enough (e.g. dwarfs) then a female can support several because each exerts only a comparably small drain on her resources. Gigolism might be favoured in certain circumstances. For example a female might not actively banish a male after fertilization because she chooses a potential gain in reproductive diversity (and the option of future sperm selection) over a present loss in resources. Multiple males would accumulate if the males were fairly immobile or tolerant of each other foregoing vigorous and exclusive competition over their giant mate. Obviously, sexual selection would counteract any unspecified selection that favoured small size if (1) males did compete (for instance, fought to exclude others) or (2) females were choosy (for instance, act decisively on their evaluation of male fitness).”-
Dwarfing males also produces a biased sex ratio in the population. With so many dudes and not enough chicas several questions remain. Where do all the males come from? Are males in a female host closely related? Are they related to the female? Do the males compete within a female for the privilege of fertilization? Does a female allow any Joe inside her or is there a “test” that males have to pass to gain entry? These are all questions that can be answered by TEH AWESUM POWR OF TEH DEE ENN EH!!
Vrijenhoek and colleagues sampled hundreds of whale bone-devouring zombie worms, Osedax rubiplumus, from scenic central California’s Monterey Canyon. Nearly 400 males were dissected out of 20 females and measured. Males have no gut or mouth and appear to live only on stored yolk sacs. They are essentially nothing but sperm vesicles. Among 54 mature females, there was an average of 26.2 males per female! Interestingly, small immature females still contained some males, meaning that male recruitment occurred over the lifespan of the female. The genetic diversity of the mitochondrial gene COI was high. Regardless of sex, depth, and collection date (samples were taken between 2002 and 2007) all worms appeared to be derived from the same mitochondrial pool.
The authors tested 3 hypotheses of larval recruitment. Larvae can be recruited from a common larval pool (males and females). Male larvae could be supplied by neighbouring females. Or, male larvae are the devil spawn of the females they live in (called arrhenotoky) and exist in an incestuous pagan orgy meant to summon Cthulhu from the deepest abyss of the ocean to swallow whole all of mankind in fiery tentacles of Armageddon.
Unfortunately, this study found that the probability of males coming from their female host was low. Any shared alleles were due to chance alone. So, it appears these worms are not summoning our lord and savior Cthulhu. The hypothesis that males are descended from their female hosts is rejected. Additionally, there was no evidence that males within a female host were closely related to each other. They were not produced by neighboring females and crawled away to nestle inside the girl next door, rejecting the neighborhood hypothesis. There is support for the bone-eating zombie worms from hell being drawn from a local common larval pool though.
What are the implications of this study? Vrijenhoek and colleagues calculated how many breeding females would be needed to sustain the genetic diversity seen in their study, called the effective population size. Between 200,000 to 2 million breeding females (depending on the genetic mutation rate) must be present on whale carcasses at any one time in the Monterey Canyon. The deep sea must be covered in millions of Osedax larvae floating around waiting for the next bone to rain down from surface like mana from heaven. Earlier studies by these authors showed that Osedax will settle on cow bones. It is an argument in favor of habitat generalism. Whale carcasses are larger and longer lived making it more likely that such a site will be found. But the bones from smaller marine mammals, sea turtles or large fish may fall down in enough quantities in various parts of the world, like Monterey Canyon or other productive seas, to sustain large genetically diverse populations of Osedax.
R. C. VRIJENHOEK, S. B. JOHNSON, G. W. ROUSE (2008). Bone-eating females and their ‘harems’ of dwarf males are recruited from a common larval pool
Molecular Ecology, 17 (20), 4535-4544 DOI: 10.1111/j.1365-294X.2008.03937.x