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MADELEINE BRAND, host:

This is DAY TO DAY from NPR News. I'm Madeleine Brand.

ALEX CHADWICK, host:

And I'm Alex Chadwick. Studying evolution, scientists look for fossils from millions of years ago. That's the sort of bio timescale I associate with Darwin. But along the coast of New England and Atlantic states researchers have found an evolutionary speedster that has changed its ways in less than 20 years, the humble bivalve, the mussel.

With an insight to just what life and death pressures can mean, here is New Hampshire Public Radio's Kerry Grens.

(Soundbite of waves)

KERRY GRENS reporting:

On a clear August day, small waves lap the coastline at Odiorne State Park in Rye, New Hampshire. This tranquil scene belies the ecological and evolutionary bustle going on below the surface. University of New Hampshire Professor Jeb Byers dips his hand into the water and pulls up an example.

Professor JEB BYERS (University of New Hampshire): Oh, one of our culprits just washed up, the Asian shore crab.

GRENS: It's a small, red and tan striped crab shell from a molting juvenile. The shell is only the size of a thumbnail.

Mr. BYERS: Yeah, he is little and that's one of the surprising things. It doesn't seem like he'd be so menacing, but they have quite an appetite for small juvenile mussels.

GRENS: The Asian shore crab was first reported on the Atlantic coast in New Jersey in 1988. In the two decades since, the crabs have migrated north to Penobscot Bay in Maine and south to North Carolina. Byers and his student, Aaren Freeman, found that as the crabs have worked their way up and down the coast the mussels have evolved a way to fend them off - and they've done it in the blink of an evolutionary eye. Freeman says that when mussels sense crabs nearby they will thicken their shells. Mr. AAREN FREEMAN (Student, University of New Hampshire): And that then benefits the mussel by making them less susceptible to predation from the crab. The crab usually accesses its prey by crushing the shell.

GRENS: The thickening process takes a few months and it's a drain on the mussel, costing it energy that could go toward reproduction. To save energy, the thickening process is like a switch. When crabs are around, mussels flip the switch on. If the mussels don't sense crabs, they leave the switch off.

Scientists have long known that mussels could thicken their shells in response to other varieties of crabs, but what they didn't know was that the mussels could rapidly evolve the same response to a new crab predator. Freeman argues that it's taken the blue mussel just 15 years.

He found that only mussels from Long Island Sound will flip the thickening switch when the crabs are present. Their ancestors have lived among Asian shore crabs since the 1980s. But mussels from northern Maine - where the Asian shore crabs have not yet arrived - don't thicken their shell when they're placed in the same environment as the crabs.

Mr. FREEMAN: I knew that they would have the capacity to recognize a crab and they would have this capacity to respond. And the fact that they didn't respond to the Asian shore crab, it's pretty good information that they don't recognize that as a predator.

GRENS: Freeman says the best explanation is rapid evolution. Although rapid evolution has happened in other animals, this is the first time scientists have seen it result in a major physical change an animal can turn on and off.

Jeb Byers says shell fishermen could use the adaptation to their advantage by selectively breeding animals that can thwart Asian shore crabs. But mussel harvesters say their more concerned about starfish than crabs. Byers says mussels have also evolved a defense against starfish. Now he wants to know whether the mussels can fend off both predators at once. His study appears in the current issue of Science Magazine.

For NPR News, this is Kerry Grens.

CHADWICK: Photos of the fearsome Asian predator - actually it's kind of a cute crab - anyway, they're at our Web site, npr.org.

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