Little Green Invaders

Contact: Julie Thompson, U.S. Fish and Wildlife Service | 410-573-4517

Little Green Invaders
Photo: John Haslam, foxypar4 on flickr.

ANNAPOLIS, MD (February 17, 2006) – A little green invader has established itself Maryland Coastal Bays and the mouth of the Chesapeake Bay. This invader is called the European green crab (Carcinus maenus), and has been chosen by the Maryland Invasive Species Council as the Invader of the Month for February. It is a voracious predator of many commercially important native species feeding on scallops, oysters, clams, mussels, snails, worms, juvenile fish, and other crustaceans. Extensive efforts to restore shellfish populations in the Maryland Coastal Bays and Chesapeake Bay are threatened by the presence of the green crab. This crab is a successful invader of coastal waters around the world. It has been introduced to South Africa, Australia, and both coasts of the United States. The green crab has been blamed for the demise of the soft shell clam industry in Maine. In California, it has caused losses as high as 50% in Manila clam stocks.

The European green crab is a small shore crab, ranging 2-3 inches in length. Despite its name, the green crab is actually not green. The color on its shell is variable, changing from green to orange and then red during the molting cycle. The abdomen has patches of yellow with five spines on each side. The green crab is a highly adaptive species, which makes it a prime candidate for becoming an invader. It can tolerate a wide range of temperatures (5-30°C) and salinities (5-30 ppt) and it grows fast and produces a large number of offspring. A female green crab can produce 200,000 eggs each year. Ocean currents disperse the larvae many miles up and down the coast. After a period of growth and development in the open sea, green crabs in the final larval stage aggregate at night in surface waters. Tides and currents sweep them back into coastal waters where they molt and settle out as juvenile crabs in the upper intertidal zone. Adult green crabs occupy protected rocky shores, sandflats, and tidal marshes.

The green crab’s native range includes the Atlantic coasts of Europe and northern Africa. The green crab lives up to five years in its native range. Green crabs first invaded the Atlantic coast of the United States in the mid-1800s. They likely arrived in dry ballast of wooden ships or from clinging to heavily fouled outer hulls. Those green crabs found suitable habitat in coastal embayments from New Jersey to Cape Cod. In the 1900s, they began spreading northwards up to Nova Scotia and south to the mouth of the Chesapeake Bay and the Delmarva Coastal Bays. The legal sale and use of green crabs as bait for sportfishing in Maryland may have contributed to the southern expansion of this species into Maryland waters.

At this time, there are no monitoring or control programs for green crabs in Maryland. This may be because it hasn’t had significant ecological or economic impacts in the Coastal Bays. Other states have utilized control methods to decrease predation of green crabs on commercially important shellfish. Some New England towns utilize a bounty system. Several states on the West Coast have enacted regulations that prohibit the transport or possession of green crabs and address the pathways in which green crabs are introduced. In Washington State, green crabs have been found in shellfish shipments packed with seaweed. That state has enacted regulations that prevent the transfer of shells and shellfish and prohibit the release of live animals and plants in state waters without written permission from the Washington Department of Fish and Wildlife. In Maryland, efforts to prevent the introduction and spread of green crab into new areas can be made by anglers disposing of unused live bait on land.

For more information on the Internet:

Smithsonian Environmental Research Center Green Crab Research

NEMESIS – National Exotic Marine and Estuarine Species Information System

National Invasive Species Information Center (NISIC)

For more information about other Invasive Species of Concern, visit the Maryland Invasive Species Council.