One of the most dominant crab species on the New England coast is actually an invader.
The Asian shore crab has been spreading swiftly along the Atlantic since it arrived in the 1980s.
And it has found a smorgasbord of goodies to munch on—including the native blue mussel.
But researchers at the University of New Hampshire have found the mussel is not accepting the crab invasion without a fight—with remarkable speed, it’s evolved to hold its ground.
New Hampshire Public Radio’s Kerry Grens has more.
…sounds waves…
On a clear August day, small waves lap the coastline at Odiorne State Park in Portsmouth.
…waves…
This tranquil scene belies the ecological and evolutionary bustle going on below the surface.
Jeb Byers, a professor at the University of New Hampshire, dips his hand into the water and pulls up an example.
Byers: One of our culprits just washed up, the Asian shore crab.
It’s a small, red and tan, striped crab husk from a molting juvenile.
The body is only the size of a thumbnail.
Byers: He is little and that’s one of the surprising things. It doesn’t seem like he’d be so menacing but they quite an strong appetite for juvenile mussels.
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 the mussels sense crabs are nearby they thicken their shells.
Freeman: 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.
It’s a drain on the mussel to thicken its shell, costing it energy that could go toward reproduction.
So when crabs aren’t around, the mussel doesn’t bother.
The ability for a mussel to thicken its shell in the face of a predator is not a new finding, but what scientists didn’t know was that the animals could evolve this defense against a new predator so quickly.
Freeman argues that the blue mussel’s thickening response to the Asian shore crab evolved in just the few decades since the crab arrived on the Atlantic coast.
Freeman found that only the mussels from Long Island Sound—which have lived among the Asian shore crab for the longest time—will thicken their shells when the crabs are present.
Mussels from Northern Maine—where the shore crab has not yet arrived—don’t change their shells in response to the shore crab.
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 good information that they don’t recognize that as a predator.
Freeman says the best explanation is a rapid evolution.
Although rapid evolution has happened in other animals, this study is the first to describe it resulting in the ability to turn on a physical change only when the environment calls for it.
Jeb Byers says the findings could hold a practical strategy for protecting the blue mussel industry in northern Maine from the advancing front of Asian shore crabs.
Byers: You could introduce some of these southern mussels to the gene pool in these northern populations in order to sort of grease the wheels and get these northern mussels ready to experience this wave, which is continuing to expand through the state of Maine.
Freeman and Byers’s study appears in this week’s issue of Science magazine.
For NHPR News, this is Kerry Grens.
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i heard this article with interest but do have a question. If mussels are successful in staving off the crabs what do the crabs do for food? Do they attack another organism, starve or what?
According to Jeb Byers, "the crabs are omnivorous...and the world is their buffet." Bourdeau and O'Connor, 2003, found the crabs eat mussels, snails, clams, and algae.