European shore crab

    • Common names

    European shore crab, shore crab (Eng.); gancu (Xitsonga)

    Introduction

    Carcinus maenas, commonly called the European shore crab, is an omnivore and can feed on bivalves and clams of as big as its own shell length when available (Squires 1990; Smith 2004).

    How to recognise a European shore crab

    Carcinus maenas adult colouration varies from green, yellow to red, with a black pattern present on the upper surface of the carapace (head shield) and upper side and tips of the walking legs. The female crab grows to a size of 65 mm across the carapace; while males reach 80 mm. They grow by means of moulting, blowing itself up to a much larger size and then hardening the new cuticle or exoskeleton (Crothers, 1967).

    Males also differ from females by having fairly larger claws which are used to compete for females. Female crabs show a broad seven segmented abdomen. This is dark in colour serves as a brood chamber in which the eggs are attached to the four limbs. The male abdomen has five segments with 3, 4 and 5 fused together. This is narrower than in females and is not as dark in colour; also the two first swimming limbs on each side are present.

    The carapace has 5 teeth (both anterior and lateral) about equal in size. Carcinus maenas breathe through gills and use its 4 senses (i.e. touch, smell, taste, sight) to make their way around their surroundings. They have mechanoreceptors on their antennae (ears) that can detect changes in water currents and therefore enable them to sense objects or movement which cause disturbance in the water (Crothers, 1967), as well as chemoreceptors that allow the detection of objects at a distance. For example the chemoreceptors found on their legs allow the crab to taste with their legs (Crothers, 1967).

    Getting around

    The body of the shore crab is more suited to walking than swimming (Crowthers, 1967).

    Communicating

    No particular sounds or calls have been reported for C. maenas. However, chemical communication in the form of pheromones and other chemical signals have been recorded during mating (Ekerholm 2005).

    Distribution

    Carcinus maenas is naturally distributed in Europe (Walton et al., 2002). Successfully established reproductive populations of C. maenas have been documented in Australia, including Tasmania, South Africa and on both coasts of North America (Edgell & Hollander, 2011; Roman & Palumbi, 2004). In 1989, a self-spread population of C. maenas was discovered in San Francisco Bay. In South Africa, Table Bay and Hout Bay both have large subtidal populations (Hampton & Griffiths, 2007; Peters et al., 2014).

    Habitat

    It is found intertidally and subtidally on both hard-and-soft-bottom substrata (Smith, 2004). Although primarily an inshore intertidal species, the shore crab is also recognised as an intertidal migrant (Hunter & Naylor, 1993). In South Africa, vast populations of C. maenas are found in two harbours (Table and Hout Bay Harbours) as they are wave-protected (Hampton & Griffiths, 2007; Peters et al., 2014; Robinson et al., 2004).

    European shore crab, female abdomen

    Food

    Carcinus maenas is an omnivore and can feed on clams, mussels, and other species in natural settings and in aquaculture. (Squires, 1990; Smith, 2004; Behrens Yamada et al., 2008).

    SEX and LIFE CYCLES

    Sex

    The life cycle of the European shore crab alternates between benthic (bottom dwelling) adult and planktonic (living in the water column) larval stages. One or two clutches of eggs are produced annually. Females can spawn up to 185,000 eggs at a time (Cohen & Carlton, 1995; Squires, 1990). Females are reported to live about 3 years and males up to 5 years (Squires, 1990). Carcinus maenas has been estimated to moult approximately 18 times during its life cycle, after attaining the first crab stage (Crothers, 1967). Fertilization occurs internally and the eggs are later pushed out and attached to the pleopods.

    The male displays a pose raising high up on walking legs 2 to 4, and holds the fifth pair of legs at or above the level of the carapace when it encounters a female. He then walks around posing and finally grabs the female with his legs (but without using its claws). The male holds the female in a pairing stance referred to as a pre-copulatory embrace, or cradle-carrying. This embrace lasts until the female moults, which can take as long as one week.

    The male then picks up the female and carries her with her abdomen facing up; this is referred to as the copulatory embrace. Both crabs then extend their abdomen and the male use his gonopods (modified pleopods) to transfer the spermatophores to the female. Copulation lasts between one and three days, followed by a period of post-copulatory cradle-carrying which lasts up to four days (Ekerholm, 2005).

    Family life

    There are no reports of C. maenas’ family life but as an invasive predator it is reported to feed on various shellfish and also outcompetes native crabs for food resources (Griffiths et al., 2009; Grosholz, E., Ruiz, 2002; Hampton & Griffiths, 2007).

    THE BIG PICTURE

    Friends and Foes

    C. maenas is reported to prey on and in some cases has caused severe declines and prevented the establishment in molluscs (Griffiths et al., 1992). C. maenas populations have been seen to decline in the presence of the grapsid shore crab Hemigrapsus sanguineus (Jensen et al., 2002).

    European shore crab, male abdomen

    Smart Strategies

    In South Africa the voracious predator, C. maenas, thrives in wave-protected areas and as a result dense populations are found in Table and Hout bay harbours, where they out-compete native species (Hampton & Griffiths, 2007).

    Poorer world without me

    Carcinus maenas in its native range is important as an indicator species for the monitoring of heavy metal contamination as this is associated with respiratory failure in crabs. It is reported also as a commercially fished crab (Klassen & Locke, 2007). In South Africa, C. maenas is considered an invasive marine alien species with high populations in wave-protected habitats (Hampton & Griffiths, 2007; Robinson et al., 2005).

    People & I

    Carcinus maenas is a well-documented invasive species that competes with native crab species and preys on clams, mussels and other species in natural settings as well as in aquaculture (Yamada & Gillespie, 2008). In its native range, such as Portugal, it is a commercially harvested species mainly used as food and bait (Cohen & Carlton, 1995).

    Conservation status and what the future holds

    Carcinus maenas is a well-known marine alien and invasive species, its global spread is from Australia, North America, South Africa and Tasmania (Behrens Yamada et al., 2008). There is a study in Hout Bay, Cape Town, South Africa that is looking at the manual removal of C. maenas using baited traps and also the efficacy of diver removal.

    European shore crab, female in berry

    Relatives

    Carcinus aestuarii is reported to be closely related to Carcinus maenas with which it hybridizes. Introduced populations in South Africa and Japan are comprised of the hybrid form (Geller et al., 1997).

    Scientific classification

    Kingdom: Animalia
    Phylum: Arthropoda
    Class: Malacostraca
    Order: Decapoda
    Family: Portunidae
    Genus: Carcinus
    Species: C. maenas (Linnaeus, 1758)

    References and further reading

    • Behrens Yamada, S., Gillespie, G.G.E., Yamada, S., Gillespie, G.G.E., 2008. Will the European green crab (Carcinus maenas) persist in the Pacific Northwest? ICES Journal of Marine Science 65, 725–729.
    • Cohen, A.N., Carlton, J.T., 1995. Nonindigenous aquatic species in a United States Estaury: A case study of the biological invasions of the San Fransisco Bay and Delta.
    • Crothers, J.., 1967. The biology of the shore crab Carcinus maenas (L.). The background- anatomy, growth and life history.
    • Edgell, T.C., Hollander, J., 2011. In the Wrong Place – Alien Marine Crustaceans: Distribution, Biology and Impacts. In the Wrong Place Alien Marine Crustaceans Distribution Biology and Impacts 641–659.
    • Ekerholm, M., 2005. Chemical communication in mating shore crabs Carcinus maenas. Doctoral Thesis. Lund University.
    • Geller, J.B., Walton, E.D., Grosholz, E.D., Ruiz, G.M., 1997. Cryptic invasions of the crab Carcinus detected by molecular phylogeography. Molecular ecology 6, 901–906. d
    • Griffiths, C.L., Hockey, P.A.R., Schurink, C.V.E., Le Roux, P.J., 1992. Marine invasive aliens on South African shores : implications for community structure and tropillc functioning. South African Journal of Marine Science 12, 713–722.
    • Griffiths, C.L., Robinson, T.B., Mead, A., 2009. The Status and Distribution of Marine Alien Species in South Africa, in: Rilov G, C.J. (Ed.), Biological Invasions in Marine Ecosystems. pp. 393–408.
    • Grosholz, E., Ruiz, G., 2002. Management Plan for the European Green Crab Submitted to the Aquatic Nuisance Species Task Force Green Crab Control Committee Frederick Kern , Chair Edited by Edwin Grosholz and Gregory Ruiz.
    • Hampton, S., Griffiths, C.C.L., 2007. Why Carcinus maenas cannot get a grip on South Africa’s wave-exposed coastline. African Journal of Marine Science 29, 123–126.
    • Hunter, E., Naylor, E., 1993. Intertidal migration by the shore crab Carcinus maenas. Marine Ecology-Progress Series 101, 131–138.
    • Jensen, G.C., McDonald, P.S., Armstrong, D.A., 2002. East meets west: Competitive interactions between green crab Carcinus maenas, and native and introduced shore crab Hemigrapsus spp. Marine Ecology Progress Series 225, 251–262.
    • Jurk, C., 2011. Fouled Yachts: A vector for marine invasive alien species to South African shores? Masters Thesis.
    • Kirkman, S., Hutchings, L., Leslie, R., Lamberth, S., Sink K, Holness S, Harris L, Majiedt P, Atkinson L, Robinson T, Kirkman S, Hutchings L, Leslie R, Lamberth S, Kerwath S, von der Heyden S, Lombard A, Attwood C, Branch G, Fairweather T, Taljaard S, Weerts S, Cowley P, Awad A, Halpern B, Grantham H, W.T., Kirkman, S., Hutchings, L., Leslie, R., Lamberth, S., 2011. Technical Report Volume 4 : Marine and Coastal Component Volume 4 : Marine and Coastal Component 4, 332.
    • Klassen, G., Locke, a, 2007. A Biological Synopsis of the European Green Crab, Carcinus maenas. Canadian Manuscript Report of Fisheries and Aquatic Sciences 2818, 1–82.
    • Peters, K., Griffiths, C., Robinson, T.B., 2014. Science Patterns and drivers of marine bioinvasions in eight Western Cape harbours , South Africa Patterns and drivers of marine bioinvasions in eight Western Cape harbours ,. African Journal of Marine Science 36, 49–57.
    • Robinson, T., Griffiths, C., McQuaid, C., Rius, M., 2005. Marine alien species of South Africa — status and impacts. African Journal of Marine Science 27, 297–306.
    • Robinson, T.B., Griffiths, C.L.L., Kruger, N., Robinson TB, Griffiths CL, K.N., 2004. Distribution and status of marine invasive species in and bordering the West Coast National Park. Koedoe 47, 79–87.
    • Roman, J., Palumbi, S.R., 2004. A global invader at home: Population structure of the green crab, Carcinus maenas, in Europe. Molecular Ecology 13, 2891–2898.
    • Smith, L., 2004. Biogeographic differences in claw size and performance in an introduced crab predator Carcinus maenas. Marine Ecology Progress Series 276, 209–222.
    • Squires, H.J., 1990. Decapod Crustacea of the Atlantic Coast of Canada. Canadian Journal of Fisheries and Aquatic Sciences 45, 1–531.
    • Walton, W.C., MacKinnon, C., Rodriguez, L.F., Proctor, C., Ruiz, G.M., 2002. Effect of an invasive crab upon a marine fishery: Green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia). Journal of Experimental Marine Biology and Ecology 272, 171–189.

    Author: Siyasanga Miza
    SANBI, Marine Programme
    August 2015

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