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Gutless Wonder: New symbiosis lets worm feed on whale bones

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11:04am, July 28, 2004
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Deep-sea researchers have discovered an oddball worm that uses a previously unknown type of symbiosis to feed on whale skeletons—even though the worms have no mouth or gut.

Some other worms from the deep have no digestive systems but depend on live-in bacteria for nourishment, explains Robert Vrijenhoek of the Monterey Bay Aquarium Research Institute in Moss Landing, California. The whale bone–raiding worms, in the newly named genus Osedax, likewise rely on symbiotic bacteria. The microbes, from the order Oceanospirillales, reside in green, rootlike growths at a female worm's base. However, the symbionts in female Osedax target an unusual food source—lipids from whale bones that have fallen to the ocean floor—Vrijenhoek and his colleagues report in the July 30 Science.

"We know of no other animal symbiont able to extract lipids," says Vrijenhoek.

The males play out an entirely different nutritional tale. More than 100 tiny males can live inside the sheath that surrounds the female's elongated body. The males don't appear to be parasitic and probably feed off yolk left over from early development. Vrijenhoek rates the males as "little more than larvae."

While females can grow to the size of an index finger, a robust male reaches only 0.3 millimeter in length. "It's the most dramatic sexual dimorphism among worms and may be among the most dramatic in the animal kingdom," says Vrijenhoek.

He and his colleagues discovered Osedax during a 2002 research cruise in Monterey Bay. Their remotely operated craft detected a dead gray whale in the mud at 2,800 meters underwater. "The bones were a carpet of red worms," Vrijenhoek remembers.

The red came from hemoglobin-rich plumes topping the females' stalklike bodies. The craft picked up samples, which went to worm expert Greg Rouse of the South Australian Museum in Adelaide, Australia.

Rouse, a coauthor of the new paper, at first hesitated to identify the new creatures as worms. For one thing, the tubeworms known from deep-sea vents live in hard outer casings, yet the new creatures are sheathed in soft mucus. However, DNA evidence convinced Vrijenhoek that these were indeed relatives of worms from hydrothermal vents. Rouse subsequently found additional evidence for this relationship from his own morphological studies.

Even though the animals don't have mouths, their bacteria-bearing projections penetrate the whale bones. Unpublished experiments comparing the nitrogen isotopes in worms and whale bones suggest that the bacteria are indeed metabolizing lipids from the whales, says Vrijenhoek.

Whale bones are some 60 percent lipids, notes Craig Smith of the University of Hawaii in Manoa, a longtime observer of sunken whale carcasses. He has estimated that 600,000 whale skeletons lie on ocean floors, offering feeding bonanzas for a community of creatures that's barely known. "I think there are a lot of discoveries to be made," says Smith.

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