Unusual mussels may use whale bones en route to hot sea-floor vents

February 16, 2000

University of Hawaii oceanographers and scientists from two other institutions have announced results of a study that may alter views on the evolution of the unusual animals found at hot vents on the deep-sea floor. The study, co-authored by graduate assistant Amy Baco and Professor Craig R. Smith from UH and researchers at the University of Maine and Harvard University, appear in the Feb. 17 issue of the British journal Nature. "Mussels take wood steps to deep-sea vents?" describes findings that support the controversial theory Smith and co-workers first published in Nature in 1989 and further developed with Baco--that rotting organic material such as whale bones and wood may have served as evolutionary stepping stones for animals to colonize hydrothermal vents. Since the discovery of hydrothermal vents‹where hot, sulfide-rich seawater emanates from the deep-sea floor‹scientists have known that the giant mussels found there were highly unusual. The vent mussels obtain food from sulfide-eating bacteria that grow in their gills, and they appear to be only distantly related to mussels from salt marshes and the rocky intertidal zone. How and when mussels first colonized vents has remained a mystery.

By examining the DNA signatures of many different types of mussels, the authors found that the tiny mussels found on whale falls and wood fragments are closely related to the giant mussels at hydrothermal vents, and that both differ significantly from their shallow-water cousins. One species of mussel found on whale bones and wood fragments appears to be an evolutionary intermediate between the giant vent mussels and their distant shallow-water cousins.

"Rotting whales may provide the missing link between the huge mussels seen at vents and those you find on your plate in a seafood restaurant," Smith said. Whale carcasses that fall to the ocean floor are a huge source of food for numerous scavengers, including huge sleeper sharks and slimy hagfish, that reduce the body to bones within months. After the scavengers, bacteria and diverse invertebrates move in, including worms, mollusks, crustaceans and other animals. These communities can thrive for years, Smith said, and can consist of tens of thousands of animals from hundreds of species located on a single whale skeleton.

Smith's team studied five whale skeletons on the seafloor off Southern California, including three carcasses sunk intentionally after the whales died from natural causes. The carcasses were revisited by manned submersible over a number of years to study the communities of animals on the bones and bacteria that decompose the whale fat and proteins. Researchers have collected at least 11 new species of animals, including a number that appear to specialize on dead whales. They have also collected whale-bone bacteria with enzymes that break down fats rapidly in cold water, which are potentially useful as laundry detergent additives. The work may help explain the evolution of unusual life forms in the deep sea and has yielded novel bacteria that may be useful to humans.

University of Hawaii at Manoa

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