Photographer:Brandon Woo Source: www.bugguide.org Copyright: Brandon Woo (used with permission)
The green crab has a hexagonal shaped carapace with five blunt spines behind each and three rounded bumps between the eyes. The carapace has a granular texture and is usually mottled dark green or brown with white to yellowish spots. The carapace is wider than it is broad; its width is 2.4-3.9 inches. The underside varies in color from green to yellow, orange, or red depending on the molt status. The second and third pairs of walking legs are the longest and are almost twice as long as carapace length. The fourth pair is the shortest; they are flat in comparison and bear hairs.
The feeding habits and tolerance of a wide variety of environmental conditions has enabled Carcinus maenas to spread far outside its native range from Australia, Japan, and South Africa to North America. What also makes this crab dangerous is that it preys on bivalves and other crustaceans like soft shell clams, scallops and is great at clearing populations of mussels. Carcinus maenas has been documented reducing populations of New England’s soft shell crab (Mya arenaria), northern quahog (Mercenaria mercenaria) and a scallop (Argopecten irradians). It is feared that the commercially important Dungeness crab (Cancer magister) could be similarly affect if the green crab were to expand its invasive range along the west coasts of the United States and Canada, since it could prey on juveniles.
Mating occurs after the females’ molt; which varies on geographical location but is most commonly between June and October. A male carries a smaller female underneath until she molts. After she does, she turns over and releases a mass of eggs onto her abdomen. The male then releases sperm and fertilizes them. The eggs are held by the female through the winter. When the larvae hatch, they become part of the plankton and drift in the water column for several weeks or months. During the final larval stage, the tides bring the larvae to shore, and they metamorphose and settle to the bottom as juvenile crabs and are sexually mature within 2-3 years. European green crabs may live up to 5 years.
Carcinus maenas arrived in the United States in 1817 in Cape Cod, Massachusetts. In 1929 they were caught off the coast of New Jersey and Virginia. By the 1950s their range expanded northward into Maine’s waters and by 1960s they were found in Nova Scotia’s waters. It was first reported on the West Coast in 1989 along California near Estero Americano and San Francisco Bay and by 1998 they were found off the coast of Washington and Oregon, thought to be carried northwards by the 1997-1998 el Nino phenomenon. Thankfully, West coast infestations are much smaller than on the East Coast. Genetic testing has shown that the crabs on the West Coast are more closely related to the crabs on the East Coast so it was not a direct introduction from Europe. The green crab might’ve reached the West Coast in algae used to pack New England bait worms.
Baltic and Atlantic Europe and North Africa
U.S. Habitat: Marine, estuaries, bays and any kind of seashore in shallow waters with sheltered areas
U.S. Present: North Atlantic Coast and Pacific Coast (from Alaska to California) and Hawaii
Fencing, trapping and poisoning were generally ineffective when tried on the East Coast. For the state of Washington they’re trying to make the green crab a deleterious exotic species and prohibiting the possession and transportation of the green crab. Importing of shellfish from out-of-state waters where the green crab occurs is also restricted. Other states should take the proactive steps that Washington is taking. Even though the crab has reached the state there are still major bays that contain commercially important species that haven’t been invaded because of their legal actions.
Berrill, M. 1982. The life cycle of the green crab Carcinus maenas at the northern end of its range. J. Crust. Biol. 2: 31-39.
Carlton, J.T. and A.N. Cohen. 2003. Episodic global dispersal in shallow water marine organisms: the case history of the European shore crabs Carcinus maenas and C. aestuarii. Journal of Biogeography 30(12):1809-1820.
Cohen, A.N., J.T. Carlton, and M.C. Fountain. 1995. Introduction, dispersal and potential impacts of the green crab Carcinus maenas in San Francisco Bay, California. Mar. Biol. 122:225-237.
Grosholz, E. and G. Ruiz. 2002. Management Plan for the European Green Crab (PDF | 517 KB) Submitted to the Aquatic Nuisance Species Task Force.
Woodward, Susan L., and Joyce Ann. Quinn. 2011. Green Crab. Encyclopedia of Invasive Species: From Africanized Honey Bees to Zebra Mussels. Santa Barbara, CA: Greenwood. 90-92. Print.