Carcinus maenas, the shore crab or green crab, is a common littoral crab, and an important invasive species, listed among the 100 "world's worst alien invasive species". Although native to the Baltic Sea, North Sea and north-east Atlantic Ocean, it has since become established on both east and west coasts of North America, in Argentina, South Africa and Australia. C. maenas is a predator feeding on many organisms, particularly bivalve molluscs (such as clams, oysters, and mussels), polychaetes and small crustaceans. It is primarily nocturnal, although activity also depends on the tide, and crabs can be active at any time of day.