 Carcinus maenas is a common littoral crab , and a widespread 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 , and bivalves moved for aquaculture . 
