Green Crab  

            One of the most commonly sighted critters in Westport River is the green crab (Carcinus maenas), also know as the shore crab. Although this species of crab has recently come to call the east coast of North America its home over the last two centuries, it is not an animal that has been welcomed in this area. The green crab is listed among the top 100 world's worst invasive species. It is also one of the most successful invasive predators in coastal aquatic habitats, having expanded its population to cover five out of the seven continents.  

Map of the Green Crab range.  Blue areas indicate their native range, red areas

are their introduced or invasive range, and green areas are the potential

range of the species. Black dotsindicate sightings that did not lead to invasion.

            The green crab originated in the north-east Atlantic Ocean along the European and North African coasts and as far east as the Baltic Sea. Its first recorded sighting outside of its original habitat was on the Massachusetts shore in 1817. The species made its way across the vast Atlantic Ocean in the nineteenth century most likely by hitching a ride in the ballast tanks of shipping vessels. A ballast tank is a compartment on the very bottom of a ships hull that is filled with water when the ship is carrying a light load to ensure stability. This tank is emptied when the ship is carrying a heavier load that is able to stabilize the ship on its own. In the case of the green crab, a ship would most likely have taken on water from the European shores that included this specific species of crab and then emptied the water, green crabs included, from the ballast tank in the coastal waters of New England.

          One of the reasons why green crabs are able to spread so rapidly throughout the world's coastal environments is that they are able to tolerate a wide range of temperatures and salinity. They are also adaptable to various types of habitats, such as intertidal or sub-tidal zones as deep as 180 feet, unstructured sandy or muddy bottoms, and salt-marshes or sea-grass beds. In its North American east coast habitats, green crabs tend to favor sheltered rocky bottoms and are many times the most abundant crab species in estuaries and soft sediment bays in Australia, Europe, and both coasts of North America.  

          On the New England coast, green crabs pose a serious threat to all sorts of native species along the shores. The green crab has shown to have significant negative effects on the populations of commercial shellfish species such as scallops, quahogs, and both soft and hard shell clams. There have been state and regional campaigns in recent decades to control and eventually diminish green crab populations, but so far there hasn't been much documented success with these efforts.