The highly successful marine invasive species, the European green crab (Carcinus maenas) is native to the Atlantic coast of Europe. This marine predator was thought to have been introduced to the U.S. Atlantic coast in the early 19th century. The crab had spread to the north all the way along coast of Maine and up into Nova Scotia by the mid 1900’s. By the 1990’s, the crab had reached San Francisco Bay and started to spread along the west coast, and in 1996, the crab inhabited over 500 km in California. Although the European green crab has successfully spread to various locations, it has not been able to permanently establish itself anywhere. Aggressive control efforts and immediate response to their introduction could be attributed to this. The exact means of introduction of the European green crab is unknown. The general agreement among researchers in the field is that the crabs were introduced via ballast water or packing material for shipping live organisms. European green crabs have been so successful as invasive species due to their feeding habits and wide range of habitats they can tolerate.
The most distinctive morphologic feature of the European green crab is its five spines on either side of its shell. This feature can be used to distinguish it from other small crabs that it could be confused with, such as helmet crabs or hairy shore crabs. Another feature that can be used to distinguish the crab are three rounded lobes that can be found between its eyes.
Despite its name, the crab is not always green. The top carapace, or shell, can vary from brown to a dark, spotted green, with yellow patches. The bottom surface of the crab often changes color during molting, switching from green to orange to red. The carapace is wider than it is long, and rarely exceeds between 3.5 and 4 inches across. The crab’s last pair of hind walking legs is flatter than its other pairs.
The European green crab can be found in a wide variety of beach environments, including protected rocky shores, cobble beaches, sandflats, and tidal marshes. They also have the ability to tolerate a fairly wide range of salinities and temperatures.
Feeding and Predation
Many researchers have studied this voracious predator’s devastating impact upon various species in the marine environment. The European green crab has the ability to learn and modify its prey-handling skills during the process of foraging for its meal. It is also more dexterous and quicker than other similar sized species of crabs.
European green crabs have been shown to alter the populations of other smaller shore crabs, clams, and small oysters. Although they aren’t able to break the shell of a mature oyster, they can easily rip apart the shells of young oysters. The crab will also dig down deep in the sand to find a meal of little clams. The European green crab has been pegged responsible for the destruction of soft-shell clam (Mya arenaria) fisheries in New England and Canada in the 1950’s. Lab studies have shown that European green crabs can also actively prey upon Dungeness crabs (Cancer magister) of equal or smaller size. Although the populations of these aforementioned species are decreased by the presence of European green crabs, the crabs can also increase the populations of some species by consuming their predators.
Mating usually occurs between the spring and fall, though it can occur outside that window as well. In order to reproduce, a larger male holds a smaller female underneath him, carrying her around until she molts. After molting occurs, the mating takes place. The female then carries the mass of eggs underneath her abdomen. With favorable conditions, females are capable of spawning up to 200,000 eggs at a time!
In order to minimize the potential for the European green crab to spread further, the Washington Department of Fish and Wildlife has put into place several regulations controlling the movement of shellfish and other aquatic invertebrates within the state.
One of the many measures enlisted was to place more restrictions on out-of-state imports, including hour-long chlorine dips for shellfish coming from European green crab infested areas. Once European green crabs were discovered on the Washington coast, a stricter restriction was enacted, declaring it illegal to transport any live European green crabs within the state without a permit.
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Hoagland, P. and Jin, D. 2006. Science and economics in the management of an invasive species. BioScience 56(11): 931 – 935.
Laferty, K.D. and Kuris, A.M. 1996. Biological control of marine pests. Ecology 77(7): 1989 – 2000.