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 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.

Carcinus maenas
Scientific classification
Kingdom:
Animalia
Phylum:
Arthropoda
Subphylum:
Crustacea
Class:
Malacostraca
Order:
Decapoda
Infraorder:
Brachyura
Family:
Portunidae
Genus:
Carcinus
Species:
C. maenas
Binomial name
Carcinus maenas

C. maenas is a widespread invasive species, listed among the 100 "world's worst alien invasive species".[2] 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.

Description
A young C. maenas showing the common green colour

C. maenas has a carapace up to 60 mm (2.4 in) long and 90 mm (3.5 in) wide,[3] but can be larger outside its native range, reaching 100 mm (3.9 in) wide in British Columbia.[4] 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.[3]

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.[5] 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.[6] Juvenile crabs on average display greater patterning than adults.[7]

Native and introduced range
Rough map of the distribution of C. maenas: Blue areas are the native range, red areas are the introduced or invasive range, black dots represent single sightings that did not lead to invasion, and green areas are the potential range of the species.

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;[8] by 2007, this species had extended its range northwards to Placentia Bay, Newfoundland.[9] 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,[10][11] thus extending its range by 750 km (470 mi) in 10 years.[12] By 2003, C. maenas had extended to South America with specimens discovered in Patagonia.[13]

In Australia, C. maenas was first reported "in the late 1800s"[14] in Port Phillip Bay, Victoria, although the species was probably introduced as early as the 1850s.[15] 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.[14]

C. maenas first reached South Africa in 1983, in the Table Docks area near Cape Town.[16] 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.

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 C. aestuarii or a hybrid of C. aestuarii and C. maenas.[17]

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.[10] 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[18] in an effort to prevent colonisation by C. maenas.

Ecology
A female C. maenas carrying fertilised eggs

C. 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. C. maenas is euryhaline, meaning that it can tolerate a wide range of salinities (from 4 to 52 ‰), and survive in temperatures of 0 to 30 °C (32 to 86 °F).[19] 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. 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.[20]

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.[21] Young crabs live among seaweeds and seagrasses, such as Posidonia oceanica, until they reach adulthood.[22]

Argopecten irradians, a scallop which has been affected by the introduction of C. maenas

C. maenas has the ability to disperse by a variety of mechanisms,[21] including ballast water, ships' hulls, packing materials (seaweeds) 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.[14]

C. maenas is a predator, feeding on many organisms, particularly bivalve molluscs (such as clams, oysters, and mussels), polychaetes, and small crustaceans.[23][24] They are primarily diurnal, although activity also depends on the tide, and crabs can be active at any time of day.[25] In California, preferential predation of C. maenas on native clams (Nutricola spp.) resulted in the decline of the native clams and an increase of a previously introduced clam (the amethyst gem clam, Gemma gemma),[26] although it C. maenas also voraciously preys on introduced clams such as Potamocorbula amurensis.[27] The soft-shell clam (Mya arenaria) is a preferred prey species of C. maenas.[24] 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 (such as scallops, Argopecten irradians, and northern quahogs, Mercenaria mercenaria).[21] The prey of C. maenas includes the young of bivalves[28] and fish, although the effect of its predation on winter flounder, Pseudopleuronectes americanus is minimal.[29] 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.[30] Colder water temperatures reduce overall feeding rates of C. maenas.[24]

To protect itself against predators, C. maenas uses different camouflage strategies depending on their habitat: crabs in mudflats try to resemble their surroundings with colours similar to the mud while crabs in rock pool use disruptive coloration.[31]

Control
Cancer productus limits the spread of C. maenas in parts of North America.

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.[32]

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.[33] 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 (Romaleon antennarium and Cancer productus) and competition for shelter with a native shore crab, Hemigrapsus oregonensis.[34] Host specificity testing has recently been conducted on Sacculina carcini, a parasitic barnacle, as a potential biological control agent of C. maenas.[35] In the laboratory, Sacculina settled on, infected, and killed native California crabs, including the Dungeness crab, Metacarcinus magister (formerly Cancer 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.[35]

Use as a food

Researchers at the University of Maine have actively been developing value-added green crab products, with the goals of driving business interest, stimulating a commercial green crab fishery, and alleviating predation effects.[36] Specifically, one study evaluated the consumer acceptability of empanadas (fried, stuffed pastries) which contained varying amounts of green crab mince meat.[37] The empanadas were rated between "like slightly" and "like moderately" for overall acceptability by a consumer panel (n=87). Furthermore, about two-thirds of the panelists would "probably" or "definitely" buy the empanadas if available locally. Additionally, the same researchers developed a patty product made from green crab mince meat using restructuring additives (transglutaminase, dried egg white, isolated soy protein).[38] Although a successful green crab patty was developed, the restructuring additives may have had greater functionality in a raw crab meat system, as opposed to the fully cooked mince that was used in the present study. The results from both studies are considered promising, especially considering that these were initial rounds of green crab product development.

Legal Sea Foods, an East Coast restaurant chain has arranged to purchase green crabs in season from local fishermen, in the first known commercial use for the crabs in the US food industry. Legal plans to test crab and shrimp etouffee, crab risotto with spring vegetables, and crab minestrone, all made with green crab stock, in their test kitchen during the winter of 2015 and offer the dishes in their other restaurant locations during the crab season.[39]

In its native range, the green crab is mostly used as an ingredient in soups and sauces.

Anatomy

The C. maenas is a crustacean and has many typical crustacean traits, including, a hard shell, 8 legs, 2 claws (also known as chelae), and 2 eyes.[40] Almost all of these traits are shared throughout the crustacean family and it all applies to the C. maenas. Each of the traits is very useful for underwater living. The shell is used for protection if the crab is attacked. however this is not the only use of the shell, it can be used for deconstruction into different materials that are commonly used in waterway industries.[41] This shell is made up of the second most common biological material, Chitin. The 8 legs have a pretty obvious use, movement. C. maenas and other crabs move sideways through the water with their 8 legs. Their claws are used to fight off predators and attract mates.[42] Both of those uses are very important if a crab wants to stay alive and pass on its genes. They have 2 eyes, 8 legs, and 2 claw - hence they are known as decapods.

Fishery
Shore crabs in a bucket

C. maenas is fished on a small scale in the northeast Atlantic Ocean, with about 1200 tonnes being caught annually, mostly in France and the United Kingdom. In the northwest Atlantic, C. maenas was the subject of fishery in the 1960s, and again since 1996, with up to 86 tonnes being caught annually.[43]

Taxonomic history

Carcinus maenas was first given a binomial name, Cancer maenas, by Carl Linnaeus in his 1758 10th edition of Systema Naturae.[44] An earlier description was published by Georg Eberhard Rumphius in his 1705 work De Amboinsche Rariteitkamer, calling the species Cancer marinus sulcatus, but this antedates the starting point for zoological nomenclature.[44] A number of later synonyms have also been published:[44]

  • 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.[44] In 1814, writing for The Edinburgh Encyclopaedia, William Elford Leach erected a new genus, Carcinus to hold this species alone (making it the type species of the genus, by monotypy).[44] In 1847, Nardo described a distinct subspecies occurring in the Mediterranean Sea, which is now recognised as a distinct species, Carcinus aestuarii.[1]

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