Thursday, 11 December 2014

Researchers Discover Well-Endowed Bone Eating Worm

A Male of the recently discovered Osedax priapus
Male Osedax priapus
The entire body of males has
evolved  as a tool for mating
Osedax is a genus of weird, deep-sea polychaetes worms, commonly known as boneworms, zombie worms, or bone-eating worms.

The story of these creatures began twelve years ago, when researchers from the Monterey Bay Aquarium Research Institute (MBARI) first discovered them, using the submarine ROV Tiburon in Monterey Bay, California, in February 2002.

The worms were found living on the bones of a decaying gray whale, thus the name "bone-eating", which alludes to how the worms bore into the bones of whale carcasses to reach enclosed lipids, on which they rely for sustenance. Since then, researchers have described more than 15 bone-eating species of Osedax.

What is so interesting about these animals is the extreme sexual size dimorphism they exhibit. Male Osedax are microscopic dwarfs that live as "harems" inside the lumen of the gelatinous tube that surrounds each female. An individual female can house hundreds or even thousands of these males in her tube.

Now, a new study that appeared yesterday on the Current Biology, gives an interesting weird twist to the Osedax story. Marine biologist Greg Rouse at Scripps Institution of Oceanography at UC San Diego and his colleagues have discovered an "evolutionary oddity unlike any other in the animal kingdom".

Examining specimens collected at 700 meters (~2,300 ft) with the help of a MBARI remotely operated vehicle (ROV) Rouse, observed a surprising new type of Osedax species, Osedax priapus.

Although females of the species are roughly the same size as their previously studied relatives, males are tens of thousands of times larger than those of other Osedax worms, and have roughly the same size as females. But why is this so strange? The authors explain:

"This discovery was very unexpected. It's the first known example of such a dramatic evolutionary reversal from dwarf males."  said Rouse.
"Evolutionary reversals to ancestral states are very rare in the animal kingdom. This case is exceptional because the genes for producing full-sized adult males should have deteriorated over time due to disuse. But apparently the genes are still there." added co-author of the study,  Robert Vrijenhoek

Another surprising find was the discovery that males of the new species consume bone on their own, something their dwarf relatives never do.

Adding even more peculiarity to the discovery is the mating process of Osedax priapus. Previously studied Osedax male dwarfs are permanently attached to their female hosts, and therefore do not need mobility to mate, so the researchers wondered how the males of the new species seek out for a mate, given their independence.

"The evolutionary solution (the new species) found was to actually make the male's body very extendable so he can reach far out to find females to mate with -- he can extend his body ten-times its contracted state." said Rouse.

In essence, Rouse said, the entire worm's body has evolved as a tool for mating, adding that this is  "why we named it Osedax 'priapus,' the mythological god of fertility,".
Rouse and his team speculate that less competition for space on certain animal bones allowed the evolutionary introduction of Osedax primps.
"This worm was weird enough as it was and now it's even weirder. This shows us that there continue to be mysteries in the sea and there is still so much more to discover, especially since we only found these creatures 12 years ago." said Rouse.

- Funding support was provided by the David and Lucile Packard Foundation via MBARI, Scripps Institution of Oceanography, National Science Foundation, and the Faculty of Science at the University of Copenhagen.

The closely related Osedax frankpressi
By Robert C. Vrijenhoek, Shannon B. Johnson & Greg W. Rouse
via Wikimedia Commons

- Rouse, G., Wilson, N., Worsaae, K., & Vrijenhoek, R. (2014). A Dwarf Male Reversal in Bone-Eating Worms Current Biology DOI: 10.1016/j.cub.2014.11.032
- Rouse GW, Worsaae K, Johnson SB, Jones WJ, & Vrijenhoek RC (2008). Acquisition of dwarf male "harems" by recently settled females of Osedax roseus n. sp. (Siboglinidae; Annelida). The Biological bulletin, 214 (1), 67-82 PMID: 18258777
- Vrijenhoek RC, Johnson SB, & Rouse GW (2008). Bone-eating Osedax females and their 'harems' of dwarf males are recruited from a common larval pool. Molecular ecology, 17 (20), 4535-44 PMID: 18986498

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