Worm food: these weird creatures dine on bones on the sea floor. Credit: MBARI

When researchers first discovered a strange new genus of marine worms feasting on whale skeletons on the sea floor1, some thought that the creatures had a very specialized habitat and could survive only on the lipid-rich bones of whales. But now it seems that several species of the Osedax worm can dine on cow carcasses as well.

Cow bones might not seem like a natural marine food source, but the researchers who made the find say that carcasses from land animals living near the flood plains of coastal rivers sometimes get washed into submarine canyons. And they suspect that the worms might dine on the remains of other marine mammals, too. ?My guess is that we?ll find that these species are quite general in their ability to exploit any bone that they happen to run into,? says Robert Vrijenhoek, an evolutionary biologist at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California.

With no eyes, mouth or stomach, the tiny Osedax worms burrow into decaying bones and rely on symbiotic bacteria to help them break down the fats in the bone marrow. Until now, they have been found growing only on the bones of ?whale falls? ? whale carcasses that have fallen to the sea floor.

The worms float about in the water and then settle on suitable substrates such as whale falls to feed and reproduce. How they locate these bony meals ? which can be separated by great distances on the sea floor ? is something of a mystery.

Burial ground

Researchers from MBARI and the Scripps Institute of Oceanography in La Jolla, California, planted ?bone trees? made of polyvinyl-chloride (PVC) pipe and cow femurs, and then anchored them in buckets of concrete at varying depths in Monterey Bay, California. After periods ranging from two months to a year, they discovered that six of the eight species present on nearby whale falls were also able to colonize cow bones. In addition, two species were reproducing. The work is published this week in Proceedings of the Royal Society B2.

Vrijenhoek, lead author on the paper, says that this ability to feast on a wider range of bones is in line with the high degree of genetic diversity and the immense population sizes of Osedax. ?It just didn?t make sense to me that they could be restricted only to whales,? says Vrijenhoek.

The cow bones clearly aren?t as tasty as whale. Whale bones can support worm densities of thousands per 5-square-centimetre patch; only a handful of worms were found on patches of cow bones, and two species were not found on the cow bones at all. ?I think the findings are really exciting, but we need to be careful in interpreting them,? says Craig Smith, a biological oceanographer at the University of Hawaii at Manoa. Osedax species are likely to have a range of adaptive feeding strategies, Smith says. ?Some of them are likely to be whale-fall specialists, whereas others are likely to be more generalized.? Smith and other researchers have trawled up other marine bones such as dolphins and porpoises with no signs of Osedax.

Both experts agree that what is needed now is to plant bones from other marine mammals such as elephant seals and sea lions, to see how tasty these are to the worms.