Cracking the secret of green crabs

Marine biologist Carolyn Tepolt kept a keen eye out for baby green crabs along a rocky beach in Woods Hole. (Photo by John Tlumacki/Globe Staff)/Globe Staff

WOODS HOLE — It’s hot where Carolyn Tepolt goes to work on this summer day, but she has no complaints as she crouches over her task, her dark ponytail splayed on her back.

Tepolt’s work space — on this and some other days — is the shoreline. In jeans, T-shirt, and sandals, she’s toiling on a hidden stretch of sand on Eel Pond, a saltwater inlet where breezes crinkle the surface and small pleasure boats bob.

The midday sun beats down as Tepolt gently pokes a tangle of seaweed. “You see movement and go for the movement,” she explains.

Soon, something almost invisible stirs, and Tepolt pounces. She stands up, cupping in her hands two of the day’s quarry, no bigger than spiders: baby green crabs.

They are an invasive species; each will grow to about three inches. A marine biologist at the Woods Hole Oceanographic Institution, Tepolt wants to know what has enabled them to survive on shores far from their origins in Europe and North Africa. The answer may hide in their genes.

Green crabs arrived in the Western Hemisphere in the early 1800s, stowaways on ships. They are arguably one of the world’s most successful invasive species, now scuttling along the east and west coasts of North America, the east coast of South America, and the shorelines of Japan, South Africa, Australia, and Tasmania.

Green crabs have been blamed for eating young soft-shelled clams in the Gulf of Maine and damaging the eel grass where lobsters and scallops breed in Canada.

Tepolt drops several of the rascals into a test tube. She will take them back to her lab, extract pieces of DNA from their cells, and send the pieces to a lab for sequencing.

In their larval form, green crabs float in the water, drifting on currents. They adapt quickly to the new habitats where they land. But humans have taken them farther than nature ever intended, especially when ships started using water as ballast, sucking up the invisible larvae on one shore and depositing them on another.

Even on far-flung coastlines, they continue to thrive, benefiting from innate adaptability.

In their larval stage they look nothing like crabs, more like minuscule horned shrimp. After several moltings, they finally emerge on land as eight-legged specks that freakily resemble ticks.

And it’s those littlest ones — a week or two postlarva — that Tepolt is looking for. She wants to compare their genetic makeup with that of older green crabs that have survived many seasons. She’s also comparing local green crabs with ones she recently collected in Nova Scotia.

Green crabs actually came to this continent in two invasions — the second one happening in the late 20th century. The later-arriving crabs seem to have come from Northern Europe, possibly carrying genes that enable them to survive cold waters.

As they commingled with the earlier invaders, over the past few decades the green crab population has exploded northward, reaching as far as Newfoundland.

Tepolt is trying to identify the genes that enable the crabs to tolerate those cold waters.

Meanwhile, can anything be done to control them? How about tapping into humans’ craving for seafood? In Europe the green crabs’ relatives are used to flavor bouillabaisse. The Portuguese enjoy eating them over a long, slow evening.

“They taste good,” says Tepolt, who once served green crabs as an appetizer at a dinner party. But each shell holds just a shred of meat, and it’s hard to extract it. She doesn’t see much immediate potential for green crabs in the gourmet marketplace.

Other possibilities are to use them as fertilizer or bait.

Tepolt’s mission is not to get rid of green crabs, but to learn the genetic secret to their adaptability. Such understanding can inform preparations for global climate change. Which species will make it, which will perish?

“Traditionally we thought of evolution as a nice, long, slow process,” Tepolt said. But increasingly, scientists are finding animals that evolve rapidly, in decades. Animals that possess the genes for faster evolution — genes that endow the flexibility to adapt, as green crabs have — are more likely to survive climate change.

Tepolt, 38, has been working at Woods Hole since 2017, after earning a doctorate in marine biology at Stanford and doing postdoctoral work at the Smithsonian.

She came to studying invasive crabs by a circuitous route. As a youngster growing up in New Hampshire, she loved observing sea creatures and collecting seashells, but never envisioned those pleasures as conduits to a career.

Instead, in college she got interested in genetics. She considered medical school but abandoned the idea after earning a master’s degree in New Zealand on bird conservation.

Later, she worked on invasive species research as a contractor for the Environmental Protection Agency.

That’s when Tepolt first started studying crabs, and when, she says, “I decided to become a marine biologist, going back to my roots as a kid playing on the beach.”