Muckraking Turns Up Deep-Sea Dwellers

BOSTON--Ecotourists flock to tropical rain forests and coral reefs, hoping to glimpse all sorts of colorful creatures. But for sheer biodiversity, mud from part of the sea floor rivals those terrestrial hot spots. Now, researchers have discovered the source of mollusk species--a 200-kilometer-wide band of ocean that rings the edges of the continents.

For decades, researchers thought the deep-sea floor was a barren expanse of mud that housed just a handful of hardy species. But about 10 years ago, marine ecologist Ron Etter of the University of Massachusetts, Boston, and colleagues discovered a wide variety of differently shaped clams and snails in mud that had been previously collected from the margins of the North Atlantic.

To find out how this panoply of tiny mollusks evolved, Etter's team adopted a technique used by biologists to assess the genetic diversity of animals on land--they compared the sequence of a common gene called 16S RNA. Because the mollusks had been chemically preserved decades ago, the team had to figure out how to extract DNA samples.

The researchers then compared the 16S RNA gene sequence in populations of four species of tiny clams and snails that live in the North Atlantic. To their surprise, the diversity varied across the deep sea. Clam populations on the continental slope--the region of sea floor from 200 to 2000 meters deep--were much more diverse than populations on the continental rise, a deeper region that slopes gently out to the abyss. The team presented its finding here today at the annual meeting of the American Association of the Advancement of Science, which publishes ScienceNOW.

Etter says the results indicate that new species arise on the continental slope and make their way to the vast plains that blanket the ocean bottom. The work represents an important step toward uncovering the forces that promote the evolution of new marine species, says oceanographer Lisa Levin of the Scripps Institution of Oceanography. "What's exciting is knowing where to look," she says. "I think he's opened a whole new door."

Etter's site
Diagram showing the continental slope and rise