Common littoral crab

Appearance

"C. maenas" has a carapace up to 60 millimetres long and 90 mm wide, with 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, 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.

Habitat

"C. maenas" can live in all types of protected and semi-protected marine and estuarine habitats, including habitats with mud, sand, or rock substrates, submerged aquatic vegetation, and emergent marsh, although soft bottoms are preferred. "C. maenas" is euryhaline, meaning that it can tolerate a wide range of salinities, and survive in temperatures of 0 to 30 °C....hieroglyph snipped... The wide salinity range allows "C. maenas" to survive in the lower salinities found in estuaries. A molecular biological study using the COI gene found genetic differentiation between the North Sea and the Bay of Biscay, and even more strongly between the populations in Iceland and the Faroe Islands and those elsewhere. This suggests that "C. maenas" is unable to cross deeper water.

Females can produce up to 185,000 eggs, and larvae develop offshore in several stages before their final moult to juvenile crabs in the intertidal zone. Young crabs live among seaweeds and seagrasses, such as "Posidonia oceanica", until they reach adulthood.

"C. maenas" has the ability to disperse by a variety of mechanisms, including ballast water, ships' hulls, packing materials used to ship live marine organisms, bivalves moved for aquaculture, rafting, migration of crab larvae on ocean currents, and the movement of submerged aquatic vegetation for coastal zone management initiatives. Thresher "et al." found "C. maenas" dispersed in Australia mainly by rare long-distance events, possibly caused by human actions.

"C. maenas" is a predator, feeding on many organisms, particularly bivalve molluscs, polychaetes and small crustaceans. They are primarily nocturnal, although activity also depends on the tide, and crabs can be active at any time of day. In California, preferential predation of "C. maenas" on native clams resulted in the decline of the native clams and an increase of a previously introduced clam. "C. maenas" has been implicated in the destruction of the soft-shell clam fisheries on the east coast of the United States and Canada, and the reduction of populations of other commercially important bivalves. The prey of "C. maenas" includes the young of bivalves and fish, although the effect of its predation on winter flounder, "Pseudopleuronectes americanus" is minimal. "C. maenas" can, however, have substantial negative impacts on local commercial and recreational fisheries, by preying on the young of species, such as oysters and the Dungeness crab, or competing with them for resources.

Evolution

"Carcinus maenas" was first given a binomial name, "Cancer maenas", by Carl Linnaeus in his 1758 tenth edition of "Systema Naturae". An earlier description was published by Georg Eberhard Rumphius in his 1705 work "De Amboinsche Rariteitkamer", calling the species "Cancer marinus sulcatus", but this predates the starting point for zoological nomenclature. A number of later synonyms have also been published:
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⤷ "Monoculus taurus" Slabber, 1778
⤷ "Cancer granarius" Herbst, 1783
⤷ "Cancer viridis" Herbst, 1783
⤷ "Cancer pygmaeus" Fabricius, 1787
⤷ "Cancer rhomboidalis" Montagu, 1804
⤷ "Cancer granulatus" Nicholls, 1943
⤷ "Megalopa montagui" Leach, 1817
⤷ "Portunus menoides" Rafinesque-Schmaltz, 1817
⤷ "Portunus carcinoides" Kinahan, 1857
The lectotype chosen for the species came from Marstrand, Sweden, but it is assumed to have been lost. In 1814, writing for "The Edinburgh Encyclopaedia", William Elford Leach erected a new genus, "Carcinus" to hold this species alone. In 1847, Nardo described a distinct subspecies occurring in the Mediterranean Sea, which is now recognised as a distinct species, "Carcinus aestuarii".