Common shore crab

Carcinus maenas is a common littoral crab. It is known by different names around the world. In the British Isles, it is generally referred to as the shore crab, or green 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 is 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 mm (0.090 m), and feeds on a variety of mollusks, worms, and small crustaceans, potentially affecting a number of fisheries. Its successful dispersal has occurred by a variety of mechanisms, such as on ships' hulls, sea planes, packing materials, and bivalves moved for aquaculture. C. maenas has a carapace up to 90 mm (9.0 cm) long and 210 mm (8.3 in) wide, but can be larger outside its native range, reaching 713 mm (28.1 in) wide in British Columbia. The carapace has five short teeth along the rim behind each eye, and three undulations between the eyes. The undulations, which 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. Another characteristic 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. The colour of C. maenas varies greatly, from green to brown, grey, or red. This variation has a genetic component, but is largely due to local environmental factors. In particular, individuals which delay moulting become red-coloured rather than green. Red individuals are stronger and more aggressive, but are less tolerant of environmental stresses, such as low salinity or hypoxia. Juvenile crabs on average display greater patterning than adults. C. maenas is native to European and North African coasts as far as the Baltic Sea in the east, and Iceland and Central Norway in the north, and is one of the most common crabs throughout much of its range. In the Mediterranean Sea, it is replaced by the closely related species C. aestuarii. C. maenas was first observed on the east coast of North America in Massachusetts in 1817, and may now be found from South Carolina northwards; by 2007, this species had extended its range northwards to Placentia Bay, Newfoundland. In 1989, the species was found in San Francisco Bay, California, on the Pacific Coast of the United States. Until 1993, it was not able to extend its range, but reached Oregon in 1997, Washington in 1998, and British Columbia in 1999, thus extending its range by 750 km (470 mi) in 10 years. By 2003, C. maenas had extended to South America with specimens discovered in Patagonia. In Australia, C. maenas was first reported 'in the late 1800s' in Port Phillip Bay, Victoria, although the species was probably introduced as early as the 1850s. It has since spread along the south-eastern and south-western seaboards, reaching New South Wales in 1971, South Australia in 1976 and Tasmania in 1993. One specimen was found in Western Australia in 1965, but no further discoveries have been reported in the area since. C. maenas first reached South Africa in 1983, in the Table Docks area near Cape Town. Since then, it has spread at least as far as Saldanha Bay in the north and Camps Bay in the south, over 100 km (62 mi) apart.