Sulfur-spewing hydrothermal vents at the ocean floor may seem like a strange home for surf-loving animals like mussels, but more than 14 species of mussels thrive several thousand meters deep in the Atlantic and Pacific Oceans. How they got there was mostly speculation--until now. At the Ocean Sciences Meeting in Honolulu on 13 February, researchers reported using a pair of mitochondrial genes to chart the mussels' evolutionary course. Shallow water mussels, they found, hopped from sunken wood to whale bones before settling at the bottom.
Biological oceanographer Craig Smith of the University of Hawaii in Honolulu first proposed that sunken organic remains might serve as a way station for mussels en route to the deep back in the 1980s, when he noticed mussels growing on rotting whale bones on the sea floor off the coast of California. At the meeting, Smith described how colonies that set up shop on whale bones can last for decades, sending out their mobile larvae to colonize nearby organic remains. Such environments are more common than you might think: Smith estimates that nutrient-rich whale skeletons are scattered every 9 kilometers on average along gray whale migration routes, and sunken wood is common off forested coasts.
Earlier work with mussel nuclear DNA unearthed a common ancestor for the mussel species from four distinct niches: sunken wood, whale bones, hydrothermal vents, and cold bottom seeps--regions where hydrocarbons or salts ooze out of the ocean floor. But the DNA hadn't changed enough between the species to allow researchers to figure out how the species were related. Mitochondrial DNA mutates faster than nuclear DNA, however, so University of Hawaii oceanographer Amy Baco sequenced two mitchondrial genes from three to six species from each of the four environments. The evolutionary tree calculated from these genes suggested that mussels moved from rotting wood to sinking whale carcasses to seeps near continental margins and finally to hydrothermal vents further out in the ocean.
Evolutionary biologist Robert Vrijenhoek of the Monterey Bay Research Institute believes clams and other animals may follow the evolutionary path described by Baco. But he suggests it may be an oversimplification to describe the progression as a one-way street. "Other evidence," he says, "suggests mussels may be a bit more clever and opportunistic bunch," moving back and forth between vents and seeps once they've reached the deep-sea neighborhood.