European Green Crab Facts

Native to Europe, green crabs now span coastal waters around the globe

Green shore crab (Carcinus maenas), Scotland
Paul Kay/Oxford Scientific/Getty Images

Green crabs (Carcinus maenas) are relatively small, with a carapace of about four inches across. Their coloration varies from green to brown to reddish-orange. While commonly found in tide pools along the East Coast of the United States from Delaware to Nova Scotia, this now-abundant species is not native to America.

Fast Facts: Green Crab Classification

  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Subphylum: Crustacean
  • Class: Malacostraca
  • Order: Decapoda
  • Family: Portunidae
  • Genus: Carcinus
  • Species: maenas

Feeding

The green crab is a voracious predator, feeding primarily on other crustaceans and bivalves such as softshell clams, oysters, and scallops. The green crab moves quickly and is quite dexterous. It's also capable of adapting. Its prey-catching skills actually improve while foraging as it learns where the prime hunting areas are and how to best catch available prey.

Reproduction and Life Cycle

Green crabs are estimated to live up to five years. Females of the species can produce up to 185,000 eggs at a time. Females molt once a year and are very vulnerable until a new shell hardens. During this time, males guard females by pairing with them in "pre-molt cradling" to defend them from predators and other males.

Green crabs generally mate toward the end of summer. A few months after mating, the egg sac appears, which the females carry through winter and spring. In May or June, hatchlings are released in the form of free-swimming plankton larvae that move with the tides of the water column for 17 to 80 days before settling to the bottom.

Green crab larvae spend most of their first summer progressing through a series of stages until they reach megalopa—mini versions of adult crabs that still have a tail used for swimming. In a final molt, the larvae lose their tails and emerge as juvenile crabs with a carapace measuring about two millimeters across.

Why Are Green Crabs So Wide-Ranging?

Green crab populations have expanded rapidly since spreading out from their native range, which lies along the Atlantic coast of Europe and northern Africa. Once they are introduced, they compete with native shellfish and other animals for prey and habitat.

In the 1800s, the species was transported to Cape Cod, Massachusetts. It's thought they arrived in the ballast water of ships, or in seaweed that was used to pack seafood, although some have been transported for the purposes of aquaculture, while others may have made the trip on water currents.

Today, green crabs are plentiful along the eastern coast of the United States from the Gulf of Saint Lawrence to Delaware. In 1989, green crabs were also discovered in San Francisco Bay, and now populate waters of the West Coast as far north as British Columbia. Green crabs have also been recorded in Australia, Sri Lanka, South Africa, and Hawaii.

Impact of Global Warming on Green Crab Populations

Until recently, the proliferation of green crabs in American coastal waters has been offset by cold winters, but with the onset of warmer summers, their numbers are on the rise. Warmer climates have also been linked to an upswing in the green crab's growth cycle. 

Between 1979 and 1980, Michael Berrill, a professor (now emeritus) at Trent University in Peterborough, Ontario Canada—whose research involved the behavioral ecology, conservation, and the impact of environmental stresses on species survival—observed the growth rate and mating cycles of green crabs in the coastal waters off Maine. A comparison between findings from that study and more recent ones shows that green crabs are growing larger much sooner thanks to the prolonged growing season that results from having more months of warm water temperatures.

Since female green crabs become sexually mature not when they reach a certain age, but rather, a certain size, the increasing growth rate is also affecting the mating cycle. According to the 1980s research, females generally reproduced in their third year. It's believed that with warmer waters and faster growth cycles, some crabs now are reproducing as early as their second year. As a result, the burgeoning population of green crabs is likely putting certain prey species at risk.

According to a statement from Maine Community Science Investigations (CSI-Maine), this may prove devastating to some species on which green crabs prey—especially softshell clams. Research presented by Dr. Brian Beal and colleagues of the Downeast Institute indicates that at least along the coast of Maine, green crabs are responsible for a substantial decline in softshell clam populations.

Sources

  • MIT Sea Grant. 2009. Introduced Species. MIT Sea Grant Center for Coastal Resources.
  • National Heritage Trust. 2009. European Shore Crab (Carcinus maenas). National Introduced Marine Pest Information System, CRIMP No. 6275.
  • Perry, Harriet. 2009. Carcinus maenas. USGS Nonindigenous Aquatic Species Database, Gainesville, Florida
  • Prince William Sound Regional Citizens' Advisory Council. 2004. Green Crab (Carcinus maenas). Non-Indigenous Aquatic Species of Concern for Alaska.
  • The Green Crab Lifecycle. CSI-Maine.
  • Beal, B. F. (2006). Relative importance of predation and intraspecific competition in regulating growth and survival of juveniles of the soft-shell clam, Mya arenaria L., at several spatial scales. Journal of Experimental Marine Biology and Ecology336(1), 1–17.
  • Berrill, Michael. (1982). The Life Cycle of the Green Crab Carcinus maenas at the Northern End of Its Range. Journal of Crustacean Biology2(1), 31–39.