Some 15 years ago, blue mussels knew their enemies and had a rather peaceful life in the New England waters.

But when an invasive crab species turned up, the mussels moved quickly to defend themselves against this new predator by thickening their shells.

Such rapid evolutionary response took place in a nanosecond compared with the thousands of years that it normally takes for a species to respond to a predator.

"It's the blending of ecological and evolutionary time," said study co-author Aaren Freeman, a graduate student at the University of New Hampshire. "It's an important development in the arms race between these crabs and these mollusks."

Crabs prey on blue mussels by crushing their shells.

The Asian shore crab, Hemigrapsus sanguineus, was introduced to North America in 1988. It arrived in New Jersey and never made it north of mid-coast Maine.

"We wanted to know, 'How is it that these mollusks can recognize a crab that is historically not present in North America?'" Freeman said.

Freeman and colleagues took mussels from points both north and south of the Asian shore crab's northernmost point of expansion, and exposed the mussels to the crabs in a controlled environment.

The southern mussels were the only ones to thicken their shells in response, probably because of their recent ancestors' previous contact with the invasive crab.

"The mussel's inducible response to H. sanguineus reflects natural selection favoring the recognition of this novel predator through rapid evolution of cue specificity or thresholds," the researchers write in the Aug. 11 issue of the journal Science.

The mussels most likely evolved quickly because they are used to being preyed upon by many species in the waters of southern Maine.

"When Hemigrapsus came along, the mussels' wheels were well-greased to respond," said co-author James Byers, associate professor of zoology at the university. "That's our best guess."

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