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Clawing Their Way to the Top
INVASIVES WEEK: Ravenous and hardy, green crabs are eating up Maine’s softshell clams. How did a crustacean no wider than a credit card get an entire state fishery in its pincers?

This story is a part of OnEarth's Invasive Species Week.

In the chill of a New England April, Chad Coffin, a 12th-generation Mainer, raced against the dying of the light, the rising of the tide, and the prospect of his livelihood falling to pieces. Wearing chest waders, earmuffs, and multiple layers of hoodies, he muscled heavy wooden fence rails—strung with netting and aluminum flashing—into the soft muck at the mouth of Freeport’s Harraseeket River. He secured the sections with stakes, pounding them 2.5 feet into the seabed with a sledgehammer before moving on to the next plot. When the tide rose too high to work, he knocked off for a bit, returning for the next cycle. For three and a half weeks Coffin struggled—without complaint—in the tendon-ripping mud of the intertidal zone, battling hypothermia, frostbite, snow, and wind. By the end of that time, he and his clamming colleagues (along with some less rugged volunteers) had erected fourteen 30-by-30-feet, 18-inch-high aquatic corrals as part of the largest-ever investigation into how softshell clams might be protected from the ravages of European green crabs.

Coffin, who rocks a Yanomami haircut and has the build and physical intensity of a rugby player, has been clamming commercially his entire adult life, playing a prominent role in his state’s $17 million clam fishery (the third most lucrative, after lobsters and elvers). But he and his fellow Maine clammers have been watching with alarm in recent years as the population of green crabs has soared, and as the population of softshell clams, also known as steamers, has commensurately grown scarce. Introduced to various shores during the 1800s via the ballast water of European ships, Carcinus maenas now inhabits both coastlines of the United States, as well as those of South Africa, Australia, Tasmania, and Japan. Maine’s cold winters used to knock down crab populations, but water temperatures in the Gulf of Maine have been warming by roughly 0.5 degrees per year for the past decade—faster than almost any other ocean region in the world. And therein lies the problem.

Like many invasive species, green crabs have few natural predators, and softshell clams, which evolved without any intertidal crab species nipping at their umbos, have no defenses against them. (In Europe, the presence of a castrating barnacle in the green crab’s ecosystem is enough to hold the species in check, but Maine—for good reason—has no interest in importing this creature.) Aided by an unlimited supply of food and no competitors, green crabs have been marching up the Maine coast like Sherman through Georgia. The crabs were so thick last summer, one local told me, that “you could cross Biddeford Pool on their backs.” Armed with rakes and traps, angry citizens went after the crustaceans (which rarely exceed four inches from spike to spike, and which actually range in shade from red to brown to their eponymous green), eventually removing 14,000 pounds of them in Freeport alone. But it wasn’t enough. In Brunswick’s Buttermilk Channel, a former clam hot spot, “it wasn’t uncommon to find 500 crabs in one trap,” according to Dan Deveraux, Brunswick’s Marine Resources Officer. “We were inundated with them.” Meanwhile, Brunswick’s population of harvestable softshell clams had been falling precipitously—between 2011 and 2013, in fact, it had declined by nearly 38 percent.

Softshell clams have been on the menu in this part of the world for a very long time. Archeologists have found their shells tucked inside Native American middens that date back more than 5,000 years. “But we don’t know if we’re going to have softshell clams in another decade,” says Robert Steneck, a marine biologist at the University of Maine. Exaggeration isn’t unknown near town wharfs, but here’s one reason why even state biologists had, by this past spring, begun hinting at the possibility of an impending crab-pocalypse: green crabs actually appear to be expanding the range of salinity and temperature that they can tolerate. As a result, lobstermen are now catching larger crabs in deeper waters, where lobsters lurk. Last September, a fisherman was shocked to find that a bucket of green crabs had survived in his backyard for a month without food or water. A sea urchin fisher in the town of Lubbeck discovered that the green crabs he had captured the previous October—and abandoned in his fish totes during a bitterly cold winter—were still alive come April.

“Green crabs are almost impossible to eradicate,” says Denis-Marc Nault, a biologist with Maine’s Department of Marine Resources. “They’re incredibly hardy, and their reproductive rate is through the roof.” Green crabs grow quickly and spawn early, producing up to 160,000 eggs in a single reproductive cycle. They are fast and agile; they eat voraciously (a single crab can gobble up to 40 clams a day); and, somewhat amazingly, they’ve learned how to steal bait from, and then escape from, traps that have been set for them. They thrive in all the different elements that make up the sub- and intertidal zone: in woody debris, in eelgrass beds, on rocky bottoms, and in mud, sand, and rock. Moving north, against the prevailing southwest currents in the Gulf of Maine, Carcinus has managed to wipe out at least two year-classes of clams in some areas. Giant steamers are still out there, observers note—but no one wants to eat the big ones. (Small clams taste sweeter, and look better on a restaurant plate.) More to the point, big clams can’t sustain a fishery if their offspring don’t survive. After closing the shellfish beds of Northern Bay a few years ago due to poor water quality, Nault returned in 2013 to find “no clams—just chipped-up shells over 900 acres of tidal flats,” he says. “It was frightening, a complete loss.” It wasn’t an anomalous finding: when Brian Beal, a professor of marine ecology at the University of Maine, surveyed 22 sites in the formerly rich clamming beds near Freeport, he found exactly zero live clams in the 110 core samples that he took.

The response of Mainers to hissing, clacking green crabs boiling out of peat banks and scrambling out of traps has been a kind of controlled panic. In August of 2013, the state held a one-day crab-trapping survey in an effort to help officials determine the crustacean’s relative abundance. Volunteers from 28 towns captured nearly 19,000 medium-to-large specimens (the smaller ones escaped). “There was just this sense of emergency,” Devereaux says. “We didn’t do a population estimate, because that would have meant trapping and releasing them”—which, naturally, no one wanted to do. (Most crabs were crushed or dropped into pots of hot water; some were delivered to a local farmer, who used them for compost.) Last winter, Governor Paul LePage held a Green Crab Summit, at which 600 scientists and citizens assembled to discuss the problem. Later LePage announced the formation of a special green-crab task force, which recently declared that commercial lobstermen no longer need to buy a license to harvest and sell the crabs, and that the green crab fishery will stay open this coming winter, even as other crab species will remain off limits. But in the absence of a booming market—green crabs, while edible, aren’t popular menu items in American seafood restaurants—opening up the fishery isn’t the same thing as ensuring a big haul. If no one wants to buy them, no one is going to want to catch them, either.

* * *

Since human beings first began burning fossil fuels in earnest, starting with the Industrial Revolution, the world’s oceans have become, on average, 30 percent more acidic. The more fossil fuels we combust, the more carbon dioxide settles in the oceans. Once there, it forms carbonic acid, which hinders the ability of bivalves like mussels and softshell clams (but not crustaceans), to build their protective shells. Nitrogen runoff from our coasts—from sewage, fertilizer, pesticides, and other manmade pollutants—only exacerbates the acidification process. Thinner shells make clams more vulnerable to predation. “They use up more energy to thicken their shells in acidic water,” explains Steneck. “Green crabs use their chela, or big claws, to shred the clams, and their mouth parts to chew them.”

Considering all of the crab’s advantages, then, can the humble softshell clam—not to mention the people who harvest it for a living—survive? The Freeport Softshell Clam Experiment, a $600,000 collaboration between the University of Maine and the Downeast Institute for Applied Marine Research and Education, may soon provide some answers.

At low tide on a sparkling June day, Brian Beal, a sizable man wearing hip waders and a cap advertising a brand of crab meat, and Sara Randall, the Freeport Softshell Clam Experiment’s scientific coordinator, putter away from a Freeport wharf with Chad Coffin at the wheel of their boat. At Staples Cove, Beal swings his legs over the gunwale and slogs across the muck toward the 14 fenced plots that Coffin laboriously erected two months ago, which are adjacent to fourteen different, unfenced plots whose dimensions are marked only with corner stakes. Each of the 28 plots contains four subplots: two with netting, and two without. Each of the 112 subplots has been planted with softshell clams—20 per square foot—that Beal grows by the hundreds of thousands in his homemade upweller, a bivalve nursery made up of two rows of connected plastic barrels through which seawater constantly flows.

To an outsider, the experiment looks fairly inauspicious. But it promises to answer an extremely important question: Which kind of barrier—Fencing only? Netting only? Fence plus netting?—best protects clams from their most lethal predator?

After surveying his square plots, Beal, who has studied clams for 30 years, trudges 50 feet to an array of 80 plastic flowerpots that have been sunk down into the muck, their tops level with the seabed. This section of the experiment, he explains, is testing whether crushed clamshells, which are alkaline, can sufficiently buffer acidic sediment, allowing baby clams that have been planted in the pots to settle and their shells to thicken as they mature. Some of the pots are controls, containing only baby clams (known as spat); some contain spat and broken clamshells; some have spat, broken shells, and netting for protection against crabs; some have spat and netting, but no broken shells. “And how do we know whether the clams are growing better because of buffering, or because the shards give them places to hide?” Beal asks. “That’s where the ‘magic plot’ comes in,” he answers himself. He points to a pot sprinkled not with broken clamshells but with shards of pink granite, which mimic the physical features of the shells without changing the pH of the water.

The response of Mainers to hissing, clacking green crabs boiling out of peat banks and scrambling out of traps has been a kind of controlled panic.

“This is a major, major experiment,” Beal says, surveying his watery kingdom of fencing, nets, stakes, traps, and pots—most of which can be seen only at low tide. “Now look over there,” he says, pivoting toward Spar Cove. “We’ve got netting over different densities of adult clams, and we’re testing whether wild spat preferentially settle in areas thick with adults. If they do settle out gregariously, and there’s a vast difference, a town might want to erect fencing” across its coves to protect clam beds. “You can’t just plant clams anymore,” Randall says, somewhat wistfully. “You have to protect them.” Indeed, if softshell clamming is to survive a warming world, it may eventually come to look more like farming than fishing.

Not long ago, Staples Cove was filled with lush and healthy beds of eelgrass: waving green fronds that serve as a nursery for half of Maine’s commercial fin- and shellfish populations, as well as for bryozoans, worms, hydroids (cousins to jellyfish), and countless other creatures. Eelgrass beds provide habitat and sustenance for wading birds, and they reduce water pollution by absorbing nutrients. The beds also dampen wave energy and slow currents, helping to buffer shorelines. But as green crabs dig through the mud in pursuit of their next meal, they loosen eelgrass roots and shred eelgrass blades; in this way they have gradually transformed tens of thousands of acres of Maine’s coves into moonscapes of sticky gray mud, peppered only with snails. Since 2001, eelgrass beds in Casco Bay, which sweeps from Cape Elizabeth to Bath, have declined 58 percent. Green crabs are the primary cause.

Upland, crabs have dug galleries into the peat banks, causing them to slump. “In the summertime, you can hear them clicking along the shore in the thatch grass,” Randall says. The eroded banks spew smothering sediment into the coves. Clams aren’t the only shellfish species to suffer; throughout the region, mussel populations have also been obliterated. “It’s not because their larvae have died,” Steneck, the University of Maine marine biologist, says. “I think they’re being eaten by green crabs. That’s the most parsimonious explanation.” Teasing out cause and effect in a dynamic ecosystem is notoriously complicated: overfishing, pollution, climate change and acidification may also play a role. “We’re seeing both biodiversity and ecosystem services degraded by the green crab,” he adds. “This is a big event, there’s no question about it.”

Steneck is quick to include economic benefits under the umbrella of ecosystem services provided. As he puts it, “We’re part of the ecosystem, too.” Maine has roughly 1,600 licensed clammers, and for every dollar they earn, the processors and other local businesses that serve them take in three more. But clams have grown scarce enough that this year the city of Brunswick cut 7 of its 57 commercial harvest licenses and closed two large growing areas. From a bench outside the Harraseeket Lunch and Lobster Company, which Chad Coffin’s family owns, Coffin tells me that “seafood restaurants and wholesaler-dealers are in decline.” He gestures toward the restaurant’s menu board. “Look,” he says. “The seafood basket now costs $28.95. Fish is just too expensive [for restaurants] to stay in business.” This past summer, Coffin earned $110 per bushel of clams that he brought in, compared with the $30 to $40 per bushel he earned 20 years ago. “You always get the highest price right before a collapse.”

But if clams are disappearing, as Coffin suggests, then why have Maine’s annual clam landings hovered consistently at a weight of around ten million pounds for more than a decade? “I’m buying them from a wider area,” John Dennison, a clam wholesaler in Freeport, tells me. “There are no small clams nearby.” Coffin echoes Dennison. “I’m digging the last of the clams,” he says. “First, the clams in the low intertidal zone disappeared, maybe ten years ago. Then the mid-intertidal clams disappeared five years ago. Now, the only remaining clams are in the cobble, in the high intertidal.” Once those are gone, towns like Freeport, Brunswick, and Harpswell, just up the coast, will be far poorer places—not just economically but culturally and biologically as well. Clams filter and clean water, of course, and they’re a terrific and delicious source of protein. But in Maine, they’re also integral to the heritage of many coastal communities. Brunswick’s town seal tellingly features the images of a logger, a professor, and not one but two clammers. “Losing the clams would have a real social impact,” Coffinsays. “Type A guys like me, we need a place to rip mud.”

It would indeed be frightening to have type A guys like Coffin at loose ends. But what scares fisheries biologists most is the green crab’s potential impact on Maine’s lobster industry, which directly employs 5,900 people and generates $364 million annually. There would seem to be reason to worry. In 2013, lobstermen began discovering large green crabs in their deep-channel traps. Green crabs favor lobster bait, and in shallower water they even compete with lobsters for barnacles. But while lobsters can gain access to a barnacle’s meat only by crushing its shell, a crab can get to it by crushing the shell and also chipping at it and prying it open. In one 2006 study, crabs outraced lobsters to food resources in every single trial that pitted them against one another. And sometimes, lobsters are the food resource: when the study’s authors placed juvenile lobsters in front of green crabs, the crabs dominated in 6 out of 11 rounds—subduing, and then eating, their opponents. As the authors concluded: “It appears that green crabs have the potential to negatively impact native juvenile lobster.”

Over the years, entrepreneurs have proposed various schemes to convert Carcinus into cash. One pet-food manufacturer in North Carolina has expressed an interest in adding crab to her cat chow, but says she requires a steady supply of 25,000 pounds a week in order to do so—an amount that no one can guarantee, especially at the relatively low price (25 cents per pound) that crab buyers are willing to pay. Others have proposed using crabs as fish meal. “But green crabs contain too much limestone,” says Denis-Marc Nault of Maine’s Department of Marine Resources. “That’s good for cod or halibut, but we’re not growing those fish in pens here.”

Crabs as lobster bait? Maybe, but who’s going to pay enough for the crabs when they’re available just about everywhere? (Besides, Steneck says, “You crush a crab, and its extra-visceral fluids may act as an alarm to other crustaceans.”) Crabs as industrial ingredient? A businessman from Arundel, near Kennebunkport, is studying the feasibility of using green-crab chitin as an extract in food and pharmaceuticals. But, again, regularly catching, storing, and transporting a sufficient quantity of green crabs at the right price may prove problematic. Crabs as culinary sensation? They are, reportedly, tasty—but removing their tiny shards of meat is an awful lot of work, and if New Englanders want to eat crab, they have ready access to native Peaky Toe, blue, Jonah, and rock varieties.

Still, Ron Howse, of New Brunswick, has been talking for nearly a year about paying 30 to 35 cents a pound for green crab, then exporting the creatures, live, to the massive, seafood-crazy Asian market. If his plan ever comes to fruition (it’s been put on hold numerous times), Howse will be the latest participant in a growing culinary movement that seeks to reduce populations of invasive species—lionfish, wild boar, nutria, bullfrog, northern snakehead—by eating them. (One website,, even provides recipes online). Some caution that rebranding an invasive species as the next “it” food has the potential to exacerbate the problem by spreading these creatures farther afield as new markets are created. While that’s a valid concern, green crabs are already just about everywhere. Nault says he has no objections to Howse’s scheme, but he also doesn’t think that it would make a serious dent in green crab populations, given the creatures’ reproductive rate and other competitive advantages. Officially, Maine remains committed to eradicating Carcinus maenas, not to managing it as a sustainable resource.

Ten miles up the coast from Freeport, Brunswick is running its own experiments with fences and traps. But it’s also doing something different. “I spend a lot of time on the flats,” Dan Devereaux, the town’s marine warden, explains. “And four years ago, we started noticing an influx of quahog seed.” That was something of a surprise. Sixty years ago, quahogs (also known as a little-neck or hard clams) were the predominant industry in Brunswick’s resplendent Maquoit and Middle Bays; wagons would pull onto the beaches and cart away so many bivalves—destined for that evening’s chowder and clams casino—that the fishery was, by the late 1950s, exhausted. Now, however, quahogs seem to be thriving in places where softshell clams have been wiped out.

At dead low tide, Devereaux and Darcie Couture, a local marine biologist who’s consulting for Brunswick, step into an airboat and roar over the 1,200-acre Maquoit Bay, whose bottom used to be a vivid green but is now—like the bottom of Staples Cove—a depressing shade of gray. “These changes are so dramatic,” Couture says. “The rapidity and scope are shocking.” When they’re a hundred yards out, Devereaux kills the engine next to a large rectangle of netting that’s been staked to the muck. Couture lifts a corner of the netting, reaches underneath, and extracts a one-inch quahog. “See its notch?” she asks, scrutinizing the outer edge of its gray-and-white shell. Couture planted this clam and 750 other hand-notched quahogs just like it into a pair of plots last May, and has been periodically checking in to weigh and measure them ever since. “These guys will reach market size in two years,” she says.

If softshell clamming is to survive a warming world, it may eventually come to look more like farming than fishing.

Quahogs could turn out to be one of those species that benefits, at least in the short term, from climate change, Couture suggests: they actually do well in warmer water. And, importantly, their shells are thick enough to stymie a green crab. “We’re studying quahogs to see if they could be a replacement industry for softshell clammers,” Devereaux says. “We need to have a solid adaption plan to climate change, or our softshell clammers will be the last of the Mohicans.” So far, quahog meat is more of a mid-Atlantic enthusiasm than a Gulf-of-Maine one, but Couture envisions introducing it—or, more accurately, reintroducing it—to New Englanders one day as a locavore specialty.

On the other side of the boat, which rests just outside of Couture’s plot, I reach my hand into the cool, soupy mud and feel around, like a farmer harvesting potatoes. Within seconds, I pull up a quahog nearly two inches across. “That would sell for $2.50 in the Oyster Bar at Grand Central,” Couture says. I plunge my arm back in to the water and pull up one quahog after another, feeling like a kid collecting a spill of someone else’s coins.

Through its universities and research centers, Maine is spending hundreds of thousands of dollars in its all-out effort to outwit the European green crab. So far these expenditures have shown that an intensive focus on trapping has the potential to reduce populations. But tending these traps is incredibly time consuming, and Maine has 4,000 miles of coastline to defend. Fencing can be effective at keeping crabs out of narrower inlets and bays, but such barriers require a permit from the Army Corps of Engineers before they can be installed. And maintaining them is a labor-intensive process: They must be cleared of debris daily, and they don’t hold up well to storms.

And so the prospects for protecting the softshell clam industry would appear to be bleak. Nevertheless, there is some hope. Last winter was unusually cold and long in Maine, and the number of green crabs that scuttled into Brunswick and Freeport traps this past summer was far lower than the numbers from 2013 and 2012. Crab hauls in Harpswell’s Quahog Bay were, by contrast, far higher—but notably, so were its populations of softshell clams: Quahog Bay has been closed to shellfishing for more than a decade. Many are asking: Were this year’s crabs simply lurking in deeper, colder water, waiting for the temperature to rise before launching their attacks on the intertidal zone? Or could it be that the especially cold winter served to knock their population back to a more tolerable level? Because scientists don’t have enough years of data, they’re unable to say whether last year’s higher numbers or this year’s lower numbers are the anomaly. Thanks to the efforts of trappers in Quahog Bay, Darcie Couture says, we do know that “there is a healthy, fairly large, deeper-water population—and now we have no excuse not to be ready for them the next time green crabs surge.”

As Steneck puts it, “The most important factor in limiting the green crab is water temperature. And the trends—this summer’s colder water notwithstanding—are not in the softshell clam’s favor.” He falls silent as he considers the future. “Unless the green crab population collapses, I think the softshell-clam fishery will be gone.”

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image of Elizabeth Royte
OnEarth contributing editor Elizabeth Royte also writes for the New York Times Book Review, which called her "no stranger to the pleasures and perils of chasing errant pieces of plastic and other castoffs to surprising (and often disgusting) places." She's the author of Bottlemania: How Water Went on Sale and Why We Bought It and Garbage Land: On the Secret Trail of Trash. MORE STORIES ➔