The green crab, Carcinus maenas, is native to Europe. It has become a prominent component of shallow marine and estuarine waters of the northwest Atlantic and many other locations around the world. Green crabs can reach >100 mm width (approximately 4 inches) and are notorious for their high shellfish consumption rates. Nevertheless, they have generalist, opportunistic diets that can affect the structure of the surrounding communities. Green crabs have wide tolerance for salinities and water temperature and can adapt quickly to a new environment as long as its basic habitat requirements are met.
Recent research at Cape Cod National Seashore indicates that the green crabs found in East Harbor have very wide diets and that they are capable of consuming most of the shellfish species that are re-colonizing East Harbor after re-establishment of the tidal connection with Cape Cod Bay. Despite its name, the best way to identify a green crab is by looking at the shape of the carapace (shell), not its color. Marine animals have remarkable flexibility in their color and routinely change colors throughout their life history. Green crabs can be red, brown, mottled, calico etc, but the best identifying characteristic is the roughly triangular carapace shape and the five notches on each edge of the carapace.