Introduced Species Summary Project
European Green Crab (Carcinus maenas)
Scientific Name: Carcinus maenas
Phylum or Division: Arthropoda
Identification: Green crabs, although the name indicates otherwise, are not necessarily distinguished by their shell color. Adults, which measure approximately 3” across, have shells (carpace) that range from dark green with yellow markings to orange or red. The underside of the green crab is often bright red or yellow. Do not let these small creatures trick you though, they can wreak havoc on a grand scale.
Current Distribution: Has invaded numerous coastal shores including South Africa, Australia and both coasts of North America.
Site and Date of Introduction: Arrived on the eastern seaboard over 150 years ago, first finding amenable habitats in coastal areas from New Jersey to Cape Cod and down in the Chesapeake Bay (1879). In the early 1900s, they began spreading northwards, up through Maine and all the way to Canada (Cape Sable, Nova Scotia). First seen in San Francisco Bay in 1989 (Redwood Shores), the green crab moved northward to Bodega Bay (1993) and southward to Monterey Bay (1995) Tomales Bay and Humboldt Bay, before crossing over to Coos Bay and several other estuaries in Oregon (1996). Green crabs were then sighted in Willapa Bay and Grays Harbor, Washington and on the west coast of Vancouver Island, in 1998 and 1999.
Mode(s) of Introduction: There is no general consensus on the means by which the species was introduced, although there are a number of natural and human-driven causes. The natural spread of the species is predicated on the fact that the crab is almost protean in its ability to sustain itself in widely disparate environmental conditions. Green crab can survive as larvae up to 80 days and are very adept at establishing new populations as they flow up and down a coast, eventually spreading themselves over great expanses of shoreline. There are also a number of human interventions that have the effect of spreading the species to new areas. Among these is ballast water from incoming ships, seaweed packed with lobsters, commercial oysters, and bate (particularly from Maine), bait buckets or boat wells from recreational boaters and through availability from marine biology supply companies. The original East Coast invasion was thought to be caused by dry ballast on wooden ships and through the crabs clinging to mossy crevices of heavily fouled outer hulls, coming from Europe.
Reason(s) Why it has Become Established: The Green Crab is an effective forager, adept at opening bivalve shells. Studies have shown it to be quicker and more dexterous than other crabs and capable of improving its food gathering skills over time. It preys on a multitude of organisms, including clams, oysters, mussels, marine worms and small crustaceans, making it a major potential competitor of the native fish and bird species. It is highly adaptable and can survive in a wide range of temperatures and salinities. In addition, the green crab can produce an astounding 200,000 eggs in one reproductive cycle and can, under special circumstances, survive up to two months out of water. It is theorized that one major way the species spreads, for example between San Francisco Bay and Bodega Bay, is as crab larvae that can travel up to five miles a day with the current.
Ecological Role: They have the potential to restructure the crab population in ecosystems in which they establish themselves, as they feed on the larvae of other crab species devastating their near shore nurseries. They like shallow water, out of range of octopus and other natural predators. Their natural habitat is under rocks, and in disturbed areas, making it difficult for birds to capture them. In fact, they pose a direct threat to shorebirds, as they have similar diets. In total, green crabs eat hundreds of species and are a voracious predator difficult to stop once they have established a foothold in a biotope.
Threat(s): At the turn of the century, this species basically wiped out the soft clam industry of Maine and the surrounding waterways. More recently, it has taken partial blame as a culprit in the scallop population decline on Martha’s Vineyard. In California, it has been estimated to cause the loss of as much as 50 percent of Manila clam stocks and substantial decreases in other crab populations. In Washington, where there is a huge shellfish industry, the potential loss to the clam and oyster fisheries could be astronomical. It is additionally posited that they could compete with the native Dungeness Crab for resources and thus cause stress to that population. In addition, the green crab is an intermediate host to marine worms that could potentially be harmful to local shore birds.
Control Level Diagnosis: In Washington, they believe they cannot eliminate the species, but can control the population to the point where it is not disastrous to the surrounding ecosystem. In other areas, controlling the population has also been the primary strategy, rather than trying to eliminate it. Australia, however, is considering using another introduced species to have a more profound long-term impact on green crabs in their waters.
Control Method: On the east coast, they are captured in much the same way as blue crabs, using pyramid shaped wire mesh traps that are baited with fish and set in the water attached to buoys. In Washington, an Exotic Species work group was established in 1998 that submitted recommendations on controlling the species. Among the wide array of recommendations (over 50 in total) were educating boaters, increased regulation that includes inspecting arriving boats and setting up volunteer programs with citizens, other agencies and crab fisherman and oyster growers to catch the crabs with similar traps to those described above. Bounty programs on the east coast have helped to control the populations and mollify the affects on scallop fishing, but they have not eradicated the species. As globalization continues to broaden its reach, shipping traffic will only increase, introducing green crabs into more and more estuaries and disabling efforts to control populations where they are currently underway. In Australia, researchers have suggested introducing the green crab’s natural European nemesis, the Sacculina carcini barnacle– which pierces the crab’s exoskeleton and causes sterility. At last check, they were ensuring that the barnacles would not have adverse affects on native species before introducing it on a wide scale.
Blair, Tim, “Lateral Thought on the Sea Bed: The fix for a European pest may be its European foe,” Time, No. 13, March 30, 1998.
Fincham, Michael, “An Endless Invasion? Green Crabs, New England Intruders Move West,” Maryland Marine Notes, Volume 14: Number 2, March-April, 1996.
Interview with Scott Smith, Aquatic Mutant Species Coordinator, Washington Department of Fish & Wildlife, December 3, 1998.
Bassett, Zasha, “The European Green Crab....A New Invader,” Sustainable Ecosystems Institute (www.sei.org), 2000.
Macaulay, Craig, “Farmers Join War Against Seastars, Green Crabs,” Environmental News Network, December 23, 1998.
Smithsonian Environmental Research Center Web Site, 1998.
Washington Sea Grant Program, Office of Marine Environmental and Resource Programs (University of Washington) Web site. 2002.
Author: Richard Van Heertum
Last Edited: 02/28/2002
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Project Editor: James A. Danoff-Burg, Columbia University