War on Invasive Green Crabs

“They just boil out of the water”- Clammer Clint Goodenow

In 2012, clammers from the Maine Clammers Association sounded the alarm about the dramatic increase in populations of invasive green crabs, Carcinus maenas, in southern Maine and the corresponding decrease in soft-shell “steamer” clams, Mya arenaria. Immediate action was needed to safeguard this economically and culturally important resource.

MCA has been the voice from the clam flats yelling, “crab!” and has brought the invasive green crab problem to light with the hopes that Maine’s coastal communities can begin to take steps to address it. The 2013 Green Crab Summit was the result of the MCA’s advocacy work. Despite scientists, policymakers, and the media starting to wake up to the green crab problem, the MCA sees that local authorities can’t transition fast enough to stop the total devastation of our coast.

MCA offers education and coordination for municipal shellfish protection efforts. In 2013, the MCA partnered with the town of Freeport to built the biggest invasive green crab fencing project in the history of the state.

Background on the Green Crab Invasion

Invasive species, also called “exotics” or “non-native”, are species that have been introduced by humans to an ecosystem that they did not evolve in. Invasive species, like green crabs, adversely affect the habitats and bioregions they invade because they often have no natural predators so their population is not held in check. Invasive species can change habitats and alter ecosystem function and ecosystem services, destroy biodiversity, crowd out or eat native species, and cause immeasurable economic damages to human activities.

One of the top 100 “World’s worst alien invasive species”

European Green Crabs  are native to the coast of the North and Baltic Seas. They first entered the US in the mid 1800’s, coming by sailing ship to the east coast. By the 1870’s these invasive green crabs were found all the way from the Chesapeake to Cape Cod. Unfortunately these predators found their way to Maine by the early 1900’s. The spread of the species is predicated on the crab’s adaptability – the crab can sustain itself in widely disparate environmental conditions, including a wide range of temperatures (30-88 degrees Fahrenheit) and salinities.

Without natural predators, the invasive green crab has an incredible ability to reproduce, which allows it to spread at an alarming rate. Invasive green crabs can produce an astounding 200,000 eggs in one reproductive cycle. They have a reproductive season ranging from May to August (July to October in some areas) and can have more than one reproductive event a year. These crabs live about 3 years and reach sexual maturity at 1-2 years, so each crab can leave behind at least 370,000 offspring. Green crab larvae can float up to 80 days in the water stream. In Maine, these crabs use the deep-water channels as “highways” for traveling. Horrifyingly, under certain circumstances, invasive green crabs can survive up to two months out of water. Found in the subtidal zone and all levels of the intertidal zone, the green crab can make its home virtually anywhere. Green crabs may prefer woody debris, grass beds, or rocky bottoms, but are also found in mud, sand, rock or other vegetation.


A green crab.

Despite their name, green crabs can be found in colors besides green, and  can be identified by their 5 “horns” on each side of their carapace. They are not very big crabs, on average tending to be around 40mm in carapace width. The largest sizes range in the 80mm carapace range.

Population Explosion Fueled by Climate Change

Though invasive green crabs entered Maine over 100 years ago, Maine’s long cold, icy winters kept invasive green crab populations in check. When ocean temperatures began to rise in the 1990’s, green crab populations rose too and Maine’s mussel beds disappeared as the crabs devoured them. Milder winters have meant that the clamflats haven’t iced over in a few years, and this lack of ice allows the crabs to proliferate.

Eating it’s Way through our Ecosystem

Green crabs are omnivores and will eat anything from zooplankton, worms, bivalves and other crustaceans, making it a major competitor of the native fish and bird species. Studies have shown the invasive green crab to be quicker and more dexterous than other crabs and capable of improving its food gathering skills over time. Juvenile invasive green crabs feeds on clam spat or larvae, while the adult crab can easily crack into adult softshell clams. On the ground clammer observations and scientific studies have shown that the crabs preferred food are scallops, mussels and clams. Clammers have watched invasive green crabs eat the species that is most accessible first. Thus it was Maine’s mussels, which live in dense beds on top of the clam flats, that became the first large scale casualty of the crabs. Since the green crabs have now devastated most of Maine’s mussel beds, the invasive green crabs have moved on to feasting on relatively assessable soft shell clams that live burrowed in the mud. A 2009 study by Lindsey Whitlow that was published in Marine Ecology, found that invasive green crab predation reduced clam density, and surviving clams were found deeper in the mud, with longer siphons. Traditionally clammers would routinely harvest the lower intertidal gradient (the section closest to the ocean waters at low tide) of the clamflats. Over the last decade, the clams in those areas have all but disappeared. The few clammers left have been pushed to the mid and upper intertidal (closest to shore) clamflats to compete with the crabs for the last remaining shellfish resources. Clammers are now observing that the flats are void of spat, indicating the future is bleak for shellfish and whatever else remains in stock on the green crab menu.

It is based on these frightening observations that the Maine Clammers Association is calling on the state and towns to direct and transition all available resources to meet the modern day challenges presented by the invasive green crabs. Our ability to maintain our once renewable shellfish resources and the biodiversity of our native habitats is under attack by these invasive green crabs.

It’s now very clear that if we continue to do nothing we can expect the loss of everything that we treasure and value about coastal Maine and our working waterfront culture to disappear. Without question a way of life has already certainly disappeared because we can no longer expect mother nature to replenish our marine resources. Instead, now fishermen and municipal programs are required to implement stewardship measures. Thanks to you, the Maine Clammers Association continues its dedication to fighting to protect this important food resource and way of life.