Carcinus maenas
Carcinus maenas, also known as the green crab or the European shore crab, can be found at a wide variety of shore types, water depths and salinities. This crab lives predominantly in the intertidal area, but populations can also reach high densities in estuaries and salt marshes (www.marlin.ac.uk ). This crab exhibits a broad carapace, usually colored green or dark brown. The front of the carapace has five serrations on each side of the eyes, sometimes referred to as a “string of pearls” formation. C. maenas uses its rear, pointed legs to hold itself on a rock and then feeds with its front claws (Lerman 1984). These crabs can grow up to 8 cm wide, but did not exceed 3 cm at the test site.
These crabs prefer to live under
the cover of a seaweed canopy, often Ascophyllum nodosum, for protection
from extreme temperatures and predation. Herring gulls, black-backed gulls and
purple sandpipers all feed on New England crabs, but C. maenas manage to
stave off the attacks through cryptic coloration and their tendency to remain
under the Ascophyllum canopy (Bertness 1999).
Carcinus maenas feeds mainly on mussels (Mytilus edulis) and snails (commonly Littorina sp. in New England). The green crab can also feed on barnacles, oysters, other crabs and sometimes algae, and has been known to destroy some mariculture species, particularly oysters and clams (Lerman 1986).
Carcinus maenas is indigenous to the North Eastern Atlantic, from Norway to North Africa (www.marlin.ac.uk). C. maenas, however, has proven to be a very hardy crab species. Beginning in the early nineteenth century, the green crab has been embarking on transcontinental journeys by way of ship’s rock ballast. It spread from southern New England to Cape Cod by 1872, Maine by the early 1900’s and Nova Scotia (its Northern limit on the American Atlantic coast) by 1954 (Bertness 1999). It can now be found as far south as Maryland. In the early 1900’s, it was found in Australia, and since then has been colonizing surrounding archipelagos. The green crab was found on USA’s West Coast in 1989 in San Francisco, CA. Recently, it has been recorded in Japan, South Africa, Madagascar and Sri Lanka (www.serc.si.edu).
Despite being an introduced species, Carcinus maenas has become very well established in New England’s rocky intertidal. Not only are they the most abundant intertidal crab in New England, but they are one of the most dominant omnivorous consumers in the intertidal from Canada to Chesapeake Bay (Glude 1955). They have become so dominant and influential in this ecosystem that they have already begun to induce substantial shell-thickening among snails. C. maenas predation is considered the “driving force” behind natural selection for Littorina obtusata with shell thickness increases of up to 47% (Trussell and Smith 2000).
A picture of Carcinus maenas drawn by Laran Kaplan, Clark University