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The saga of the Osedax “bone-eating” worms began 12 years ago, with the first discovery of these deep-sea creatures that feast on the bones of dead animals. The Osedax story grew even stranger when researchers found that the large female worms contained harems of tiny dwarf males.

Examining bone worms collected at 700 meters (2,296 feet) depth by an MBARI remotely operated vehicle, 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 Osedax worms, 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”.

“Evolutionary reversals to ancestral states are very rare in the animal kingdom”, noted coauthor 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”. (1)

Evolution.
Reverse evolution.

Why take one for granted and not the other?

The only rule that life seems to follow, is that there are no rules.
Except for the ones we have in our head…