Story posted November 05, 2003
Clamming has been a part of life on the coast of Maine for centuries, but the steamer clam's journey from mudflat to dinner plate is tougher than it once was because of an armored invader — the green crab.
At a recent faculty seminar, Lindsay Whitlow, visiting assistant professor of biology and environmental studies tackled the tale of clams and crabs in "The Coast of Maine; Beautiful Coves, Tasty Steamers, and the Attack of the Crab Monster."
In the 1800s in Portland, families would gather together for clamming and clambakes, and to this day you can still find clamdiggers lining Maine mudflats at low tide. Some dig clams recreationally and others depend upon it for their livelihood, but either way, clamming produces revenue in the form of license fees, taxes, and sales.
"It's a viable part of Maine's coastal economy," Whitlow said. The value of the clam population is now about $10 million a year.
Soft-shell, or steamer, clams are bivalve mollusks. They live in the mud, but take oxygen and nutrients from the water through a siphon stretching from inside their shells to the surface of the mud. They grow their shells, and the rings on a shell function similar to the rings in a tree trunk, in terms of telling us their age.
Because the phytoplankton they eat aren't as plentiful in the winter, the shell grows more slowly, forming a thick band of rings. In summer, when the sun increases the population of phytoplankton, the shell grows more rapidly, and the rings are farther apart. By counting the number of thick bands, you can determine how many winters the clam has lived.
Clam larvae form in the water and float out to sea for 10 days to a month. When they're about a millimeter long they begin to form a shell, then return to shore and their permanent home in the mudflats. (Any scientists in search of a project might look to the larvae's time spent floating in the ocean, Whitlow said. "Nobody really understands just how far away these clams move from shore.")
In a clam's fantasy world, Whitlow said, it might be able to fight back against predators, but that's not really an option, and its shell doesn't provide much defense either.
"So the safety for the clams exists down in the mud," Whitlow said. So they burrow to escape predators, though for decades, there wasn't much need for the clams to burrow at all.
Cape Cod forms a biogeographic barrier in the Atlantic, and water north of it is significantly colder than the water south. While the blue crab preyed upon clams south of Cape Cod, it didn't live in the colder northern waters, so Maine clams lacked a predator and were more plentiful.
That all changed with the arrival of the green crab, brought to Maine's shore through changes in shipping and trade during the past 100 years.
"There's a massive difference [now] in both the size of the ships, the speed those ships travel and what they carry along with them," Whitlow said.
Because the cargo of many of these ships is heavy, a great deal of ballast water is used to balance the ship. Along with water, these ships take on any organisms in the water that can fit through the ballast hole. While these stowaways might have been in ballast water in the past, ships are much faster than they used to be, so more of the organisms are surviving the trip across the ocean. When they arrive they are dumped out with the ballast water in a new port.
"So that's how you get a lot of invasive species," Whitlow said. The green crab is just such a species. It's native to Northern Europe, but is now found in shipping ports all over the world. A 1955 survey found that green crabs were first seen on the east coast in about 1850 in New Jersey. By 1903 they were in southern Maine and by 1953 they had spread across the coast all the way to Eastport.
Since Maine's steamers weren't used to the presence of these predators and had only their soft shells for protection, they were especially vulnerable to the green crab.
"You've got a scantily clad native species...and a new predator that can take advantage of that resource," Whitlow said.
When the green crab population began to explode in Maine, the clam population crashed. That was followed by a crash in the crab population, which allowed the clams to come back. (Whitlow said scientists aren't sure if the crab crash came because the crabs had eaten so many clams that they couldn't find food or because the water got to cold for them to survive.)
Now, the green crab population is on the rise again, and because clamming is an important part of Maine's economy, communities and researchers are trying to do something to protect the fishery.
"One way we might be able to sustain our natural resource is to raise larvae ourselves," Whitlow said.
Some towns and clammers are using a process to speed the development of larvae into what they call seed clams. When the seed clams are about one centimeter long, they are sown onto the mudflats where, hopefully, they will grow into a crop of clams. But that's no guarantee of more clams at harvest time.
"One of the big problems is that if you have a lot of green crabs around, you're going to have a problem," he said. "Because green crabs love to eat little clams...they're like popcorn."
Whitlow and his students are researching various ways of protecting the clams. One possibility some clamdiggers are trying is covering clam beds with mesh tents that allow water and air to get through, but not green crabs.
Whitlow and several Bowdoin students have set up areas on the mudflats with lots of small tents so that they can change conditions under the tents and compare the results. One set of tents was set up normally, to protect the clams. In another area, tents weren't set up, so the clams were vulnerable. In another area, they put a crab under the tent to learn how the clams respond when in the constant presence of a crab. They performed their experiments with both seed clams and adult clams.
Shallow clams — both big and small — are the most vulnerable when crabs are present.
"Depth in the mud is really the refuge for those clams," Whitlow said. "It's all about how deep it is." So, humans realize that, but "do the clams know that, are they paying any attention?"
The answer, perhaps not surprisingly, is yes. When clams are exposed to crabs, they use their energy differently than when they aren't. Instead of putting all of their energy into growth, they put some of their energy into digging deeper into the mud and extending their siphon. (By examining old clam shells, Whitlow has confirmed a sudden shift to deeper burrowing when crabs appeared on the coast.)
If you put a tent up and crabs can't get in, the clams are happy, but they also feel safe, so they don't burrow down as deep.
"But the problem is, in this part of the world, in large parts of the year, snow falls," Whitlow said. The harsher winter weather destroys the tents, and the clams, lulled into a false sense of security, have lost their incentive to dig deeper, so they are even more vulnerable when the crab returns.
"So the crabs actually benefit in some ways from the tents if the tents can't be maintained," Whitlow said.
He began exploring what the clams were responding too, and he discovered that if clams are exposed to the smell of a crab (and especially the smell of a crab eating clams) they will burrow deeper, even if there is no crab around. So, if the clams are covered by a tent, but they are exposed to a crab cue, they'll burrow into the mud despite the protection of the tent. Then they'll be less vulnerable even if they lose that protection.
The problem with burrowing for protection (at least as far as we humans are concerned) is that a clam doesn't feed as efficiently when it's deeper in the mud, so it takes longer for it to grow. Since clams can't be harvested until they're two inches, this puts off the harvest.
"So, that's the trade off," Whitlow said.
There are other variables to consider and much research yet to do: Clams are also affected by other environmental conditions, such as red tide and the deterioration of the mud flats.
A Bowdoin student, Sara Ficke, is now looking at different mud flats to see how fast clams are growing in flats where they are harvested and in flats where they aren't . And Whitlow is looking at clamshells from shell middens found on the property of the Coastal Studies Center, to learn more about the size distribution of clams before widespread clamdigging. The results of both could contribute to management of the clam fishery in the future.
"When you sit down to a plate of these, sometime in the future," Whitlow concluded, "you can think about what's going on."