"Shore crab" redirects here. This may also be used for crabs in the superfamily Grapsoidea.

Carcinus maenas is a common littoral crab, and an important invasive species, listed among the 100 "world's worst alien invasive species". It is native to the north-east Atlantic Ocean and Baltic Sea, but has colonised similar habitats in Australia, South Africa, South America and both Atlantic and Pacific coasts of North America. It grows to a carapace width of 90 millimetres (3.5 in), and feeds on a variety of molluscs, worms and small crustaceans, potentially impacting a number of fisheries. Its successful dispersion has occurred via a variety of mechanisms, such as on ships' hulls, packing materials, bivalves moved for aquaculture, and rafting.