Give a marine snail an easy life, and it will take its time drilling into a clam. Put it under competitive stress, and it will look for a faster route. Those changes, scarred into fossils, show that an unknown catastrophe nearly two million years ago changed the competitive balance in the Western Atlantic and the ecosystem has yet to fully recover, according to research published this week in the journal Science.

In the seagrass meadows of the Gulf of Mexico, Chicoreus and Phyllonotus marine snails feed on Chione clams by slowly drilling a hole through the shell wall. That process can take a week, while the snails risk losing their prey to another snail or being attacked themselves by fish, crabs or other predators.

High levels of competition should favor faster feeding, said Geerat Vermeij, professor of geology at UC Davis and an author on the paper. The snails can get a quicker meal by drilling through the thinnest part at the shell's edge -- but risk getting their feeding proboscis nipped off by the closing shell.

The pattern of drill holes in fossil shells can give insight into what life in the ocean was like millions of years ago and how it compares to today.

In the laboratory, Gregory Dietl, a postdoctoral researcher at the University of North Carolina who is now at Yale University, UC Davis graduate student Gregory Herbert (now at the University of South Florida) and Vermeij found that when modern-day snails had to compete for food with other snails, they began edge-drilling their prey. When they were separated, they went back to slow wall drilling.

"They have the same gene pool, but you can elicit different behaviors depending on the competitive environment," Vermeij said.

A severe but regional extinction event at the end of the Pliocene Epoch 1.7 million years ago seems to have tilted the balance from high competition to low competition, according to the researchers. At that time, up to 70 percent of marine species in the Western Atlantic Ocean disappeared, with some parts of the world affected to a lesser extent and others unscathed.

The researchers looked at thousands of fossil clam shells from before and after the extinction and compared them with modern shells. Edge-drilled shells are abundant up to 1.7 million years ago, and then disappear entirely. None of the modern shells they looked at show edge-drilled holes.

The results show that competition intensity has not returned to pre-extinction levels even though a long time has passed since the event, Vermeij said.

The cause of the Pliocene extinction remains unknown. The work is published in the Dec. 24 issue of Science.