Bizarre to the Bone

“Stunning.” That’s the descriptor Scripps Institution of Oceanography at UC San Diego marine biologist Greg Rouse used in recounting his first experiences with an eye-opening species of marine worms that live and thrive on the bones of whale carcasses.

Rouse began studying the bizarre creatures in his laboratory in 2002, but a year later he joined colleagues from Monterey Bay Aquarium Research Institute (MBARI) on a research expedition to inspect them up close in their undersea environment.

“To see them alive on a whalefall as well as all the other creatures was a truly stunning experience,” said Rouse.

With each new study since Rouse, Bob Vrijenhoek, and Shana Goffredi first described Osedax, a genus of mouthless and gutless “boneworms,” scientists gain more insight into how the make their living.

In the latest finding, Scripps scientists have uncovered how the wispy worms are able to carry out their bone-drilling activities. As published in the May 1 online edition of the Proceedings of the Royal Society B (Biological Sciences), Martín Tresguerres, Sigrid Katz, and Rouse detail how Osedax excrete a bone-melting acid to gain entry to the nutrients within whale bones.

“The acid presumably allows the worms to release and absorb collagen and lipids that are trapped in bone,” said Tresguerres. “This model is remarkably similar to how mammals repair and remodel bone, however Osedax secrete acid to dissolve foreign bone and access nutrients.”

In their study, supported by Scripps and an Erwin Schrodinger postdoctoral fellowship (Austrian Science Fund), the scientists describe a process in which the worms use a “proton pump” to secrete acid onto the bone. Tresguerres says similar acid-secreting enzymes exist in all other organisms, such as in human kidneys to handle blood and urine functions.

Because they lack mouths, boneworms must use an alternative method of consuming nutrients from whale bones. Bacteria that live symbiotically within the worms are involved in this process, however, the exact mechanism is not yet fully understood. Some evidence suggests that the symbiotic bacteria metabolize bone-derived collagen into other diverse organic compounds, and that the worms subsequently digest the bacteria for their own nutrition.

“The Osedax symbiosis shows that nutrition is even more diverse than we imagined and our results are one step closer in untangling the special relationship between the worm and its bacteria,” said Katz, a Scripps postdoctoral researcher.

A 2011 study led by Rouse found that boneworms have primarily been found attached to whale skeletons, but they are capable of making a living on other bones as well, including fish (http://scrippsnews.ucsd.edu/Releases/?releaseID=1153). That finding supported a hypothesis that Osedax’s bone-eating lifestyle may have evolved millions of years ago, even before the dawn of marine mammals.

“Determining how Osedax gets into bones was the first challenge in understanding the nutrition of these bizarre animals,” said Rouse. “Now we’d like to understand how they transport and utilize the nutrients that they have uncovered.”

To continue learning more about them, the scientists plan to collaborate with MBARI colleagues in the coming months to collect and study additional bone samples with live worm specimens. These plans include a visit to a whalefall established by Rouse and Virgin Oceanic (Chris Welsh and Eddie Kisfaludy). In 2011 the team towed a 23-meter (75-foot), 60-ton deceased fin whale 12 miles off San Diego’s Mission Bay and submerged it to a depth of 850 meters (2,788 feet).

Katz will join Vrijenhoek and his team to visit the site and bring live Osedax samples back to Scripps for further research, including studies of their physiology.

“Osedax continues to provide fascinating new insights into biology and more will come in the future,” said Rouse.