A new creature emerges from a forest drowned by the Gulf of Mexico
The creature was small, about the size and color of a grain of rice. Dan Distel, director of the Ocean Genome Legacy Center at Northeastern University in Boston, wasn't sure what it was, other than a type of mussel. He put the small bivalve in a petri dish and asked a colleague to put it aside.
“By the time we got back to the lab, the little bugger had crawled out of the shell,” he recalls with some annoyance. “And we couldn't find it.”
Months later they found another, and Dr. Distel realized that the mussel looked strangely familiar. It looked like the giant mussels found at hydrothermal vents in the deep sea 1,500 feet below the ocean's surface, which have gills that contain bacteria that allow the mussels to obtain nutrients from corrosive hydrogen sulfide bubbling from the Earth's crust. But this mussel was small and pale and, strangest of all, it only lived about 60 feet deep. DNA analysis quickly confirmed that this mussel was a new species, which the scientists named Vadumiolus teredinicola. It is the first mussel of this group ever seen at depths of less than 90 meters. The existence of this shallow cousin, the researchers suggest, could help explain how the giant clams ended up deeper.
Dr. Distel and his colleagues discovered the mussel while investigating an ancient underwater forest off the coast of Alabama. During the last ice age, bald cypress trees grew in what was then a swamp, sixty miles from the ocean. Then, sometime between 45,000 and 70,000 years ago, as sea levels rose, the trees were swallowed up by the advancing sea. Swirling sand wrapped the dead trees in a natural sarcophagus. For thousands of years, everything was still in the forest, until heavy waves, fueled by one of the 2004 hurricanes, swept away the sand. Fishermen were amazed to discover trees on the otherwise featureless bottom of the Gulf of Mexico, 10 miles from dry land, and a journalist, Ben Raines, helped bring the site to the attention of scientists.
Since then, the ancient wood has provided a beautiful buffet for all kinds of organisms, and Dr. Distel and his colleagues collected and characterized them as quickly as possible. The wood won't last forever, and the forest could be re-buried by another big storm. But the scientists believe this unusual environment could harbor organisms with unsuspected talents. Dr. Distel focuses mainly on shipworms, a group of mussels that tunnel through waterlogged wood and which could be a source of new antibiotics.
These newly discovered mussels appear to live in burrows left by dead shipworms. The 124 individuals identified during the investigation were all found in these tunnels. They fit very snugly and must have crawled in when they were smaller, said Dr. Thistle.
“Once they start growing, they can't get out,” he said. “They're stuck there.”
This fits in with how scientists suspect that the mussels earn their livelihood. Like their deep-sea brethren, V. teredinicola harbors nutrients and is nourished by symbiotic bacteria that require an environment with little or no oxygen. In a shipworm burrow, a mussel could plug the hole with its body, creating an oxygen-poor environment for its symbionts, while still having access to the oxygen-rich ocean water it needs outside the burrow.
Another sign that the mussels live permanently in protective burrows is their extreme vulnerability.
“Their shells are paper thin,” said Dr. Thistle. “To pick them up, I used some paintbrushes, like chopsticks. If you try to pick them up with tweezers or your fingers, you'll crush them.”
The existence of these new mussels lends credence to an older hypothesis by Dr. Distel and his colleagues. In a article published in 2000they suggested that deep-sea mussels may have evolved from shallow-water individuals that reached the seafloor by hitching a ride on falling pieces of waterlogged wood. At the time, there were no shallow-water mussels known to digest sulfides. But this discovery suggests that there may have been sulfide-eating mussels closer to the surface, ancestors of these new mussels, and those in the depths.
There may be more: according to the Ocean Census project, between 75 and 90 percent of species suspected of living in the ocean are still undiscovered. In support of this goal, Dr. Distel and his colleagues submitted V. teredinicola to the Ocean Census. It is the first new ocean species to be listed.