Seaweeds are good for many native marine species, of that there is no doubt, but sometimes invasive species that can overcome wild native populations and change the dynamic of the shore use these same seaweed habitats too well. I hold in my hand a European green crab carapace—the top shell—which I picked up at the bay yesterday. It was lying on windrows of seaweeds pulled loose and thrown ashore by the high tides and summer storms. The shell has baked in the sun and turned a soft orange color. Five sharp incurved points run along each edge to the eye sockets. The eyestalk stems are neatly folded into the sockets, and between them a rostrum, like a narrow visor on a cap, projects in three lobes. This carapace was shed, and the new, larger crab now meanders somewhere out in the bay—hungry, intrepid and, so far, invincible.
We know the stories of how our cosmopolitan lives can fracture a delicate balance that native species have made with one another and with the places where they live. We move around a lot, trailing species from one location to another. Between climate change, extinctions, and international trade, we erase or imperil thousands of years of evolutionary adaptation and set off new bouts of survival of the fittest. One of the fittest invaders we’ve brought along with us is this crab, Carcinus maenas.
Seaweeds, especially Ascophyllum and the Fucus species, play a role in its success. When the floating green crab larva first falls out of the water column, sinks to the bottom, and takes on the shape of a tiny, translucent new crab, it can be gobbled up by almost any predator. Its safety lies in the long, sheltering fronds of the rockweeds, where it nudges itself into the crevices of the rocks and is draped within the protective seaweed blades. The crab’s enablers, they offer it shelter and food.
Here on the Maine coast, the green crab is the inshore crab. Two endemic species, the rock crab and the Jonah crab, are larger and live in deeper water, both of them regularly caught and sold for food. The great spider crab, Hyas araneus, also endemic, lives in between the range of the European green grab and the rock and Jonah crabs. And then there’s the little hermit crab, tugging around a snail shell, its borrowed home.
Masters of the intertidal zone, the young green crabs keep close to the safety of the nearby seaweeds, but as they grow they become bold predators, dancing and digging their way over and into mud, across sand and marsh and cobble—always hungry. This isn’t a big crab, but it has had an outsize impact, diminishing our abundance of native species. It has changed this big, complicated coastline, marginalizing the lives of coastal people who make a living digging clams and bait worms, harvesting mussels, and fishing for scallops. Until we figure out how to fight it, or manage it, or decide how we’d like to cook and eat it, we are stuck with it.
The first published report of a new species of crab in the Massachusetts intertidal zone was written in 1817, but the crabs had probably arrived years before. This was the era of sleek wooden trading ships racing west across the Atlantic to pick up American white pine and beaver pelts, rum and salt cod, corn and block ice packed in sawdust. The stowaways most likely embarked somewhere along the British coast. At that time, ships that were not carrying cargo, crossing to buy rather than to sell, used dry ballast to steady the hulls and to counteract the weight of the masts, which made the ships top-heavy. Dry ballast would have been stones.
I imagine sailors somewhere at a British port pulling stones off the shore and out of the upper tide, carrying them up a gangplank and into a ship’s hold. Perhaps a number of the stones had seaweeds growing on them, and within those wet handfuls of algae crouched a young green crab or two. While sheltering in their home cove, they were lifted and taken away. Green crabs can live two months out of water as long as they have enough moisture to keep their gills wet, and the holds of those ships were perfect redoubts for a crab’s transatlantic voyage: damp, briny, and dark.
The native range of the green crab runs from the coast of Norway, down the eastern Atlantic, out to the British Isles, and south to North Africa, where it lives in competition with a number of other species of inshore crab. It possesses an array of talents that make it a gifted adapter to new environments. One of these is high fecundity. The females can brood and release 180,000 eggs, sometimes twice a year. A prolonged larval stage means it can also float for days in the ballast water carried in metal-hulled ships, which quickly became far more efficient crab transports than the old wooden-hulled trading ships. It eats voraciously, omnivorously, and it can tolerate a range of salinity and cold.
By the late 1940s the European green crab was a fixture in the Gulf of Maine, its numbers waxing and waning in response to the severity and length of the winters. It spread south and has been halted along the Maryland and Virginia coasts, probably by the presence of blue crabs.
On a day in early July, out on Wass Island, I accompanied Brian Beal as he drove his truck down a dirt road to a bluff above Molly Cove. We had come to visit a project he has organized, with the help of the Beals Shellfish Conservation Committee, for students in the seventh and eighth grades of the local elementary school.
“We’re teaching kids how important the resources are here. They grow up in families harvesting lobsters and clams, but where those clams come from or where those lobsters come from and how they live aren’t usually conversations within a family. It’s just, ‘How many did you get today?’”
Not only is he teaching his regular classes at the University of Maine at Machias, but the Maine Economic Improvement Fund granted him $200,000 to continue his soft-shell clam research out on the flats in coastal towns around Casco Bay, focusing on bringing back clam populations and stabilizing that fishery for the local diggers. This means that Brian is spending a lot of time thinking about green crabs and how to minimize their impact.
He’s invented “Beal boxes,” devices to prevent crab predation on shellfish. They are used by students in high school science classes to study the relationship between the crabs and the clams on their neighborhood clam flats. And here, at the Downeast Institute on the island, he teaches kids about using the scientific method to help protect the marine environment around them, the environment he hopes they will inherit someday and care for.
As we drive, I ask him about shellfish and seaweeds—whether, for instance, clams can siphon seaweed detritus into their bodies and use it as food.
“Clams are filter feeders,” he explains. “They have a certain size particle they can ingest. Most of it comes in the form of microalgae—phytoplankton—in the range of five microns to a hundred microns. Very, very small. Mussels have the ability to ingest larger particles, so it is possible that seaweeds can contribute directly to the diet of mussels through detritus.
“When seaweeds break down and die, they are sloughing off and coming apart into smaller and smaller particles, and those nutrients contained in them are being put back into the ocean, and they help the phytoplankton to grow. It’s all a part of recycling. The macroalgae (the seaweeds) break down to feed the microalgae (the phytoplankton), which feeds the filter feeders like clams and mussels.”
Molly Cove, a cup of mud-and-cobble shore, looks out across Eastern Bay. It is bracketed by ledges from which hang wet sheaves of Ascophyllum and bladder wrack, and behind the ledges lift groves of spruce. As we get out of the truck and walk down to the low-tide line, Brian tells me of an experiment he designed for a class of elementary school children to discover whether green crabs of a markedly small size were able to eat soft-shell clams without breaking their shells, by slipping their little pincers between the two shells and attacking live tissue.
Each student was given a Tupperware container with a screen cutout in it for water circulation. Inside they placed a young crab and a clam, and then they dropped the containers into fifty-gallon tanks. When they returned for the next class and pulled up their Tupperware cages, they found that the crabs, even those no larger than a Jacob’s cattle bean, had managed to pluck between the shells of the clams and eat them clean. This experiment proves that crabs, even small ones, are efficient predators of bivalves—two-shelled mollusks—but it doesn’t prove that green crabs can dramatically alter a mudflat, eating their way through masses of clams and worms, mussels, and snails, until what is left is just unfiltered mud.
“People make statements that they think are factual, but we need to do the critical tests. A lot of silliness is still being described as science. To be wrong about something isn’t something you should shun, it just means you haven’t approached the problem correctly. We start with an observation and we make a prediction. That is a hypothesis. There’s no way to prove a hypothesis. You can only disprove it. So you have to do everything in your power to disprove what you think is true.”
Ahead of us, lying in the low tide, bobbing in the mild chop of water no more than a few inches deep, are four squares of netting, fourteen feet by fourteen feet. They are made of polypropylene, and each is securely tucked into the mud at its edges like a well-made bed. Five toggles are set inside each net, lifting them above the sediment and keeping the nets aloft so that the soft-shell clams beneath them can raise their siphons through the mud and into the water to feed on the microalgae the water brings shoreward during a rising tide. The clams are protected from green crab predation by the nets, and the nets are kept free from the growth of algae by the periwinkles that slide over the tops of them, feeding.
“Most years there are few wild clams in this cove,” Brian tells me, “but there are always some nice big ones in the mud.” He begins digging with his hoe, which he carried down from the truck, and uncovers a big, fat clam. “Look at that!” he exclaims. “That’s a great clam! I’ll put it right back.”
As we walk in the tide, he tells me to look across the beach and points out what he calls pockmarks in the wet mud—round indentations the circumference of tether balls. They cover the intertidal shore. I assumed a clammer had been down with a hoe a tide or two ago, and this was what was left of his or her digging.
“They’re made by green crabs,” Brian says.
“What were the crabs doing down in the mud?” I ask.
“Clamming!”
Now as he digs, his hand latches onto something deep and he yanks it up. It’s a large male green crab, pincers flailing. All the proof he needs.
Green crabs can move up and down in the mud, Brian explains, but they can’t move sideways in it, so the clams that have been seeded under the nets here are safe, unless the nets happen to be placed over a crab already in the mud that comes up into them. That hasn’t happened, and the elementary school students have planted three thousand hatchery-raised juvenile clams under each net. In November they’ll be back for the harvest.
When I think of the range of work Brian has taken on, stretching up and down the coast, engaging adults, other scientists, and children in the issues of the inshore waters, and also raising seed clams and mussels and oysters and surf clams back at the institute to revive the coastal life of the state, I think of him as a sort of Catcher in the Rye character, trying to keep others from running off a cliff, and the cliff in question is this diminished coast that used to be, within his lifetime, a rough but giving Eden.
No one involved in digging clams for a living during the past couple of decades would argue with the statement that the rate of harvest was unsustainable, despite the fact that they themselves were doing it. But today, there’s a change of heart in some people who make their livelihoods out on the flats, and they are becoming dedicated stewards. That seems to be a common human response to wild resource collapse. We need to get knocked off our horse before we begin to see the light.
For the past four years, Brian has been driving to the Midcoast, conducting his experiments with help from Friends of Casco Bay and the Maine Clammers Association’s volunteers to reestablish the former abundance of their clam flats. Not only is research expensive, but it takes time. It takes patience. The years pile up, and careful science begins to accumulate as the clammers wait for a solution to their problem.
An abundance of soft-shell clam larvae circulates in the Casco Bay water column, Brian explains. Something happens, however, when they settle down on the flats to form shells.
“To think of the way clams come into a flat,” Brian begins, “you need to think of holding a saltshaker. Clams are swimming in the water as larvae. When they get to be about two or three weeks old, their developing shell is heavy, and their swimming organ can’t keep up, so they settle. I call it the saltshaker effect. Larvae falling out of the water column are just like salt crystals coming out of a saltshaker. They’re landing here and there and there. They settle. A stream of millions of clams. A clam that settles is one-fifth of a millimeter, one hundred-twenty-fifth of an inch. They can go right through the holes in the nets we’ve put down and settle on the mud beneath them.”
What Brian did in these towns was to set up experiments that tested two hypotheses: whether the mudflats had such dramatically low pH numbers, owing to the effects of acid rain, that the clams were unable to make their calcium-dependent shells, and whether green crabs were responsible for the lack of young clams growing into harvestable adults. After three years of work and a variety of testing methods, such as putting crushed clamshells onto the flats to introduce calcium to boost the pH numbers, what he has found is that the pH in the bays where his experiments took place had nothing to do with the loss of clams. Green crab predation did.
He set up test plots over and over, because, he says, “Nothing is definitive unless you’ve done it multiple times in multiple places. And there were always more clams under the nets. It was a pattern. The crushed clamshells didn’t matter.
“If you’re a community and you have the option of putting out crushed shell on a flat or not, don’t bother. It’s not going to do you any good. Rather, if you want to do something, you should put down netting. Netting doesn’t guarantee you’ll get clams—the saltshaker effect, again. But it does say that you’ll deter predators.”
Back at the institute, we walk to the shore of Black Duck Cove, and I ask what this shore would look like now if it had never met a single green crab. Most of the damage, of course, is underwater, but the crabs have eaten the green seaweeds along the shore, Brian explains, particularly the sea lettuce, Ulva lactuca, and there is none left here.
Only the brown seaweeds, the Ascophyllum and the Fucus species, fringe the land in a slow incoming tide. No doubt within them lithe crabs are moving, preying on almost everything they find, including crabs smaller than they are.
Although people who have tried green crabs say that their meat is sweet and they make a nice broth, the trouble with eating them is that they are small, their claws narrow, and picking them out claw by claw—which can be fun with Jonah and rock crabs—is an oversize amount of work for a tiny dab of meat. Another problem is that the soft-shell crab market, which works so well for blue crabs, can’t work for these: they don’t all shed at one time on this coast, and there are no visual clues that we know of to predict when a single crab is about to creep out of its shell to start a new one.
Companies that make compost would be glad to incorporate them into their products, but they want the crabs to be dead, dried, and delivered, and that is expensive work for what they’d pay. Furthermore, it would be difficult to develop a profitable market when populations of the crabs fluctuate, although as the Gulf of Maine warms, the numbers are bound to stabilize and increase.
A friend of mine, a farmer from the town of Gouldsboro, was told by the town’s clam warden that the clammers had caught a lot of green crabs in their traps out on the flats and would be happy to bring them over to the farm to use as compost.
He was delighted. “Sounds great!” he said.
“One more thing,” the warden said. “They run every which way out of the traps . . . and they’re hard to kill.”
“Well, maybe not,” my friend told him.
What remains the biggest setback to developing a green crab fishery of any sort is that the people who harvest native species of shellfish, snails, and worms, alarmed by the toll taken by these voracious invaders, would prefer a program of eradication. And you can’t blame them. If a green crab industry were established, we’d have to protect the crabs, the creatures that have flummoxed the greatest predator of all.
In summer, neighbors and I walk down to the bay we live on for a quick swim. The ledge that slants into the high tide offers a steep angle by the road. A few green crabs move diagonally across it. We step into the water, and they approach. Some are large, with four-inch carapace widths, and red backs rather than green. They are the alpha males that have grown thicker shells and larger claws than the green-colored males have, and they are armed to fight for access to females. After a brief toe check, they dance away.
Our bay has changed. We’ve lost mud snails and blue mussels and soft-shell clams. We’ve lost our eelgrass beds. Along the head of the bay, to the east of this small swimming cove, the Spartina grasses have almost disappeared. The crabs dig caves into what’s left of them and feed on their nutritious rhizomes. Chunks of dislodged Spartina banks riddled with crab burrows lie upturned against the cobble-and-gravel shore. These were once a gentle, grassy extension of the shore itself, softening the impact of incoming storm surges and high spring tides.
Mature green crabs head out into deeper water in winter to escape the cold. Here at the shore, the young crabs pack together in their sod Spartina caves until spring, fifty or more crammed together. Come March, the constellation of Cancer, the sign of the crab, rises directly to the south of this bay, a small group of faraway stars barely visible to the human eye. It shines dimly down on the snow crust and through the occasional squall. Where the incoming tides nudge the layers of rotting winter ice against the shore, juvenile green crabs hunkered down in their Spartina caves have not stirred. But they will.