Researchers at Scripps Institution of Oceanography in La Jolla have found a new species of “bone-eating” worms living deep underwater and uncovered an evolutionary oddity.
The Osedax “bone-eating” worms were first discovered 12 years ago. These deep-sea creatures feast on the bones of dead animals. Normally female worms are larger, and have “harems” of tiny dwarf males.
But after examining worms collected 2,296 feet down by a remotely operated vehicle, marine biologist Greg Rouse observed a surprising new type of Osedax species. Females of the new species are roughly the same size as their previously studied relatives, but males are tens of thousands of times larger than those of other species, and are roughly the same size as the females.
“This discovery was very unexpected,” said Rouse. “It’s the first known example of such a dramatic evolutionary reversal from dwarf males.”
Rouse’s findings were published last week in the journal Current Biology. Rouse’s collaborators included Nerida Wilson (formerly at Scripps and now at the Western Australian Museum), Katrine Worsaae of the University of Copenhagen, and Robert Vrijenhoek of the Monterey Bay Aquarium Research Institute.
“Evolutionary reversals to ancestral states are very rare in the animal kingdom,” noted Vrijenhoek. “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.”
Also surprising is the mating process for the new species. Previously studied Osedax male dwarfs are permanently attached to their female hosts, and therefore do not need mobility to mate, so the scientists wondered how the newly discovered males are able to seek out 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, “and that’s why we named it Osedax ‘priapus,’ the mythological god of fertility.”
The scientists speculate that less competition for space on certain animal bones allowed the evolutionary introduction of Osedax priapus.
“This worm was weird enough as it was and now it’s even weirder,” said Rouse. “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.”
