Carcinus maenas
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 European green crab. The European Green Crab is native to Europe and North Africa.
Carcinus maenas is a widespread invasive species, listed among the 100 of the World's Worst Invasive Alien 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, and feeds on a variety of mollusks, worms, and small crustaceans, 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.
Description
Carcinus maenas has a carapace up to long and wide, but can be larger outside its native range, reaching 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, 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.
Native and introduced range
Carcinus 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 Mediterranean green crab species C. aestuarii.Carcinus 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. The green crab did not begin to expand its range until 1993, before rapidly expanding northward, reaching Oregon in 1997, Washington in 1998, and British Columbia in 1999, extending its total range by in 10 years. Invasive green crabs were first discovered in Alaska in 2022 by Metlakatla Indian Community Department of Fish and Wildlife. 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 southeastern and southwestern 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.
Carcinus 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 apart.
Appearances of C. maenas have been recorded in Brazil, Panama, Hawaii, Madagascar, the Red Sea, Pakistan, Sri Lanka, and Myanmar; however, these have not resulted in invasions, but remain isolated findings. Japan has been invaded by a related crab, either Carcinus aestuarii or a hybrid of C. aestuarii and C. maenas.
Based on the ecological conditions, C. maenas could eventually extend its range to colonise the Pacific Coast of North America from Baja California to Alaska. Similar ecological conditions are to be found on many of the world's coasts, with the only large potential area not to have been invaded yet being New Zealand; the New Zealand government has taken action, including the release of a Marine Pest Guide in an effort to prevent colonisation by C. maenas.
In 2019, C. maenas was first found in Lummi Bay, Lummi Indian Reservation, Whatcom County, Washington, United States. The Lummi Nation began trapping and removing the crabs in an effort to get rid of them. Then in 2020, hundreds were found in traps, and more intensive trapping clearly will be necessary to keep their numbers down. Eradication will not be possible.
Over a 19-year study concluding in 2020, Oregon's Coos Bay was found to have an established and increasing population.
While in 2020, fewer than 3,000 were trapped, more than 79,000 were caught in 2021. This led the Lummi Indian Business Council to declare a disaster in November 2021 and the Washington Department of Fish and Wildlife to request emergency funding from the governor.
In 2025, the complete genome of Carcinus maenas was sequenced, providing a valuable foundation for investigating the genetic factors that contribute to this crab's global invasion success.
Ecology
Carcinus maenas can live in all types of protected and semiprotected marine and estuarine habitats, including those with mud, sand, or rock substrates, submerged aquatic vegetation, and emergent marsh, although soft bottoms are preferred. Adult specimens of C. maenas are euryhaline, meaning they can tolerate a wide range in salinity, and survive in temperatures of. The wide salinity range allows C. maenas to survive in the lower salinities found in estuaries, and the wide temperature range allows it to survive in extremely cold climates beneath the ice in winter. Early life stages are more constrained in their tolerances; for example, larvae survive from hatch to metamorphosis to megalopa at temperatures between 12 and 27 °C. 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 vs. those elsewhere. This suggests that C. maenas is unable to cross deeper water. Different native populations of C. maenas differ in traits such as larval and adult thermal tolerance, larval salinity tolerance, and body mass at hatching and at metamorphosis.Females can produce more than 400,000 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.
Carcinus 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. C. maenas dispersed in Australia mainly by rare long-distance events, possibly caused by human actions.
Carcinus maenas is a predator, feeding on many organisms, particularly bivalve molluscs, polychaetes, and small crustaceans – including other crabs up to their own size. They are primarily diurnal, 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, although C. maenas also voraciously preys on introduced clams such as Potamocorbula amurensis. The soft-shell clam is a preferred prey species of C. maenas. Consequently, it 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 and eating the Zostera marina that Dungeness and juvenile salmon depend upon for habitat. Colder water temperatures reduce overall feeding rates of C. maenas.
To protect itself against predators, C. maenas uses different camouflage strategies depending on its habitat; crabs in mudflats try to resemble their surroundings with colours similar to the mud, while crabs in rock pools use disruptive coloration.
Control
Due to its potentially harmful effects on ecosystems, various efforts have been made to control introduced populations of C. maenas around the world. In Edgartown, Massachusetts, a bounty was levied in 1995 for catching C. maenas, to protect local shellfish, and 10 tons were caught.Some evidence shows that the native blue crab in eastern North America, Callinectes sapidus, is able to control populations of C. maenas; numbers of the two species are negatively correlated, and C. maenas is not found in the Chesapeake Bay, where C. sapidus is most frequent. On the west coast of North America, C. maenas appears to be limited to upper estuarine habitats, in part because of predation by native rock crabs and competition for shelter with a native shore crab, Hemigrapsus oregonensis. Host specificity testing has recently been conducted on Sacculina carcini, a parasitic barnacle, as a potential biological control agent of C. maenas. In the laboratory, Sacculina settled on, infected, and killed native California crabs, including the Dungeness crab, Metacarcinus magister, and the shore crabs Hemigrapsus nudus, Hemigrapsus oregonensis and Pachygrapsus crassipes. Dungeness crabs were the most vulnerable of the tested native species to settlement and infection by the parasite. Although Sacculina did not mature in any of the native crabs, developing reproductive sacs were observed inside a few M. magister and H. oregonensis crabs. Any potential benefits of using Sacculina to control C. maenas on the west coast of North America would need to be weighed against these potential nontarget impacts.