Bone worm slideshow (Flash). Credit: Monterey Bay Aquarium Research Institute.
Just 7 years ago, scientists discovered some of the strangest denizens of the deep sea: eyeless, mouthless worms feeding on whale skeletons in the waters of Monterey Canyon, California. The "boneworms" were living on the remnants of a carcass that had settled 3000 meters down and were so numerous that they looked like "a shaggy carpet," says Robert Vrijenhoek, an evolutionary biologist at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California, who led the discovery team. Now Vrijenhoek and colleagues have discovered several new members of this unusual group of organisms.
Curious to see if the worms, which the scientists described as a new genus, Osedax (meaning bone-eater), lived at other depths, the MBARI researchers dropped five dead whales from Monterey Bay beaches at different locations in the canyon's waters between 2004 and 2008. Over the intervening years, they discovered a jackpot of previously unknown worms on the bones. "We now have 15 species of boneworms from Monterey Canyon," living at depths from 300 to 3000 meters, says Vrijenhoek. Two additional species have also been discovered on whale carcasses that other researchers sank off of Sweden and Japan.
An analysis of the evolutionary relationships among the 17 known Osedax worms, published in the current issue of BMC Biology, suggests that they have been munching on bones for at least 45 million years. Some may have been dining on ancient marine reptiles from the Cretaceous era, the researchers say.
The worms arrive as larvae at the carcasses, perhaps carried by ocean currents or on the mouth-parts of crustaceans, which feed on the whale meat. Soon after the worm larvae settle on the bones, they mature into adult females; larvae that arrive later and fall on the adult females turn into microscopic male worms that spend their lives in a state of arrested development and in harems inside a female's tube, where they release sperm to fertilize her plentiful eggs. The females sprout rootlike structures at one end of their tubes to attach to and penetrate the bones. Lacking a mouth and gut, the worms rely on bacteria in their roots to digest proteins and lipids from the bones. At the other end of their tubes, the worms grow feathery palps that extract oxygen from the seawater. The worms also happily settle on elephant seal and cow bones, which the scientists also sank offshore. "It doesn't seem to matter what kind of bone," says Vrijenhoek. "They're after the collagen and cholesterol."
Different species of worms appear at various stages of the bones' disintegration. "Some have very shallow roots and are good at colonizing the entire carcass; later they're replaced by others with deeper roots that can mine much deeper in the bone. It's almost like the succession of plants in a forest," says Vrijenhoek. There is also one with a pigtail-like coiled trunk that sinks its roots into bones buried beneath anaerobic mud.
After further study, the scientists will describe and name the many new species. They expect that even more new worms will be discovered because other research teams have now dropped whale carcasses into the watery depths off of Antarctica and France.
"We often talk about the deep sea being an unknown environment, and this discovery points out [how true that is]," says Kenneth Halanych, a marine biologist at Auburn University in Alabama. "Here is a huge diversity of organisms that live a short distance off the California coast, and until recently, we didn’t even know they were there." The many species discovered to date "has huge implications," he adds, for the conservation of whale species. "It's not just whales that are affected" when their populations decline, but "all the fauna that are clearly dependent on them."