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Carcinus maenas is a common littoral crab, and an important invasive species. It is listed among the 100 "world's worst invasive alien species".

C. maenas is known by different names around the world. In the British Isles, it is generally referred to simply as the shore crab. In North America and South Africa, it bears the name green crab or European green crab. In Australia and New Zealand, it is referred to as either the European green crab or European shore crab.

C. maenas has a carapace up to 60 mm long and 90 mm wide, with five short teeth along the rim behind each eye, and three undulations between the eyes. The undulations, which do not protrude beyond the eyes are the simplest means of distinguishing C. maenas from the closely-related C. aestuarii, which can also be an invasive species. In C. aestuarii, the carapace lacks any bumps and extends forward beyond the eyes. The other character for distinguishing the two species is the form of the first and second pleopods (collectively the gonopods), which are straight and parallel in C. aestuarii, but curve outwards in C. maenas.

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