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Species Name: Armases cinereum Bosc, 1802
Common Name: Squareback Marsh Crab
Gray Marsh Crab
Synonymy: Sesarma cinereum Bosc, 1802
  1. TAXONOMY

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Animalia Arthropoda Malacostraca Decapoda Sesarmidae Armases

    Species Description

    The squareback marsh crab, Armases cinereum, is one a several semi-terrestrial crab species occupying the intertidal habitats of the IRL. The carapace is brown to olive and the top margin of the upper finger on the claw bears small tubercles or bumps (Kaplan 1988). The last segment of the 4th walking leg is equipped with black spines on the dorsal and ventral surface. A characteristic tooth or spine behind the eye socket, which is found in several similar species, is absent in A. cinereum (Gosner 1978, Kaplan 1988, Voss 1980). The squareback marsh crab is sexually dimorphic, with male claws growing relatively larger than those of females (Buck et al. 2003).

    Potentially Misidentified Species

    Of the several species of marsh crabs found in Florida waters, two species may be commonly mistaken for A. cinereum: the mangrove marsh crab, Sesarma curacaoense; and the humic or marbled marsh crab, Armases (Sesarma) ricordi. The carapace of A. ricordi is orange to reddish-brown, also lacks the spine behind the eye socket, the legs are marbled, and the top of the upper finger on the claw is nearly smooth (Kaplan 1988). The mangrove marsh crab bears a deeply cut tooth behind the eye.

  2. HABITAT AND DISTRIBUTION

    Regional Occurrence & Habitat Preference

    The range of A. cinereum extends from Maryland to Florida, Gulf of Mexico, and the Caribbean (Gosner 1978, Kaplan 1988). Individuals are commonly seen above the high water line among rocks, on dock pilings, under debris on beaches and in high elevations of salt marshes and mangrove forests (Gosner 1978, Kaplan 1988, Rupert & Fox 1988, Teal 1958). However, this is a highly motile species that has been found up to 100 m inland of the nearest marsh (Pennings et al. 1998). The squareback marsh crab seems to prefer sandier sediments than similar species, and is a common stowaway on ships (Kaplan 1988, Ruppert & Fox 1988, Seiple 1979).

    IRL Distribution

    The squareback marsh crab can be found throughout the IRL among debris sheltered beaches, in mangrove forests and salt marshes.

  3. LIFE HISTORY AND POPULATION BIOLOGY

    Age, Size, Lifespan

    Little information exists on the maximum age and average lifespan of A. cinereum. The maximum size of the carapace is about 1.8 cm long x 2.2 cm wide (Buck et al. 2003, Kaplan 1988), though the carapace width of most specimens examined in the field is between 1.3 and 1.8 cm (Buck et al. 2003).

    Reproduction

    As with most decapod crustaceans, fertilization occurs during copulation. The male transfers sperm-filled cases, called spermatophores, to the female. After the eggs are fertilized, the female broods them on her abdomen until hatching. Ovigerous females are found in the IRL from April to November (Figueiredo et al. 2008).

    Embryology / Larval Development

    Each female lays approximately 2,000 to 12,000 eggs per brood, each measuring about 0.4 mm in diameter (Figueiredo et al. 2008). When water is located and a suitable release site is chosen, the female begins the degradation of the egg membranes via hormonal signaling (De Vries & Forward 1991). This process keeps larvae protected until an appropriate water body is found. Females then descend into the water to release planktonic larvae, usually in response to the lunar rhythm (eg. Seiple 1979). Larvae prey on a variety of plankton while in the water column. In captivity, zoeae have been raised on the brine shrimp, Artemia sp., and eggs of the sea urchin, Arbacia sp. (eg. Costlow & Bookhout 1960). Over 20-40 days, larvae pass through four zoeal stages and one megalopa before settling to the benthos and metamorphosing into juvenile crabs (Costlow & Bookhout 1960, Costlow et al. 1960).

  4. PHYSICAL TOLERANCES

    Temperature

    The squareback marsh crab is found in temperate to tropical locations, but most populations are likely found in and near warmer waters. Individuals have been successfully held in captivity at water temperatures between 25 and 30°C (Costlow et al. 1960).

    Salinity

    The squareback marsh crab prefers salinities around 30 ppt (Ruppert & Fox 1988), but individuals can be found in fresh to saltwater (Gosner 1978). Though adult crabs can tolerate this wide range in salinity, studies have shown that the final megalopa stage (postlarva) of the larvae can only survive at estuarine salinities, faring best at 26.7 ppt (Costlow et al. 1960). This small salinity tolerance in the postlarval stage of the crab likely has a significant bearing on the distribution of adult populations.

  5. COMMUNITY ECOLOGY

    Trophic Mode

    The diet of A. cinereum is considered "truly omnivorous", with crabs consuming both plant and animal tissue (Ho & Pennings 2008). Individuals eat fresh plant material, leaf litter and fungi (Ho & Pennings 2008, Seiple & Salmon 1982, Pennings et al. 1998). When offerred animal tissue, the consumption of vegetation usually decreases, and crabs grow faster on mixed diets (Buck et al. 2003). Common prey items of A. cinereum include: small fiddler crabs of the genus Uca; aphids and spiders; the amphipod, Orchestia grillus; the isopod, Venezillo parvus; the eastern melampus snail, Melampus bidentatus; and the marsh periwinkle, Littorina irrorata (Buck et al. 2003, Ho & Pennings 2008).

    Predators

    Little is known about the predators of A. cinereum. However, individuals are likely consumed by a variety of birds, mammals and larger crabs, as well as several species of fishes when crabs venture below the water line. Larvae of the marsh crab are preyed upon by filter feeders, fishes and other zooplankton.

    Associated Species

    No known obligate associations exist for A. cinereum. However, marsh crabs are associated with several organisms common to mangroves, salt marshes and other sheltered estuarine intertidal areas. For extensive lists of other species found throughout the ecosystems in which A. cinereum occurs, please refer to the "Habitats of the IRL" link at the left of this page.

  6. ADDITIONAL INFORMATION

    Special Status

    None

    Ecological Importance

    The squareback marsh crab is commonly found in salt marshes, where it consumes vegetation and insects. This omnivorous diet has been found to benefit the shrub, Iva frutescens, by reducing herbivory. Aphids that consume Iva have been reduced by approximately 70% in some Georgia marshes, yielding a 124% increase in Iva leaf number (Ho & Pennings 2008).

  7. REFERENCES

    Buck, TL, Breed, GA, Pennings, SC, Chase, ME, Zimmer, M & TH Carefoot. 2003. Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity. J. Exp. Mar. Biol. Ecol. 292: 103-116.

    Costlow, JD & CG Bookhout. 1960. The complete larval development of Sesarma cinereum (Bosc) reared in the laboratory. Biol. Bull. 118: 203-214.

    Costlow, JD, Bookhout, CG & R Monroe. 1960. The effect of salinity and temperature on larval development of Sesarma cinereum (Bosc) reared in the laboratory. Biol. Bull. 118: 183-202.

    De Vries, MC & RB Forward, Jr. 1991. Control of egg-hatching times in crabs from different tidal heights. J. Crust. Biol. 11: 29-39.

    Figueiredo, J, Penha-Lopes, G, Anto, J, Narciso, L & J Lin. 2008. Fecundity, brood loss and egg development through embryogenesis of Armases cinereum (Decapoda: Grapsidae). Mar. Biol. 154: 287-294.

    Gosner, KL. 1978. A field guide to the Atlantic seashore: Invertebrates and seaweeds of the Atlantic coast from the Bay of Fundy to Cape Hatteras. Houghton Mifflin Co. Boston, MA. USA. 329 pp.

    Ho, C-K & SC Pennings. 2008. Consequences of omnivory for trophic interactions on a salt marsh shrub. Ecology. 89: 1714-1722.

    Pennings, S, Carefoot, TH, Siska, EL, Chase, ME & TA Page. 1998. Feeding preferences of a generalist salt-marsh crab: relative importance of multiple plant traits. Ecology. 79: 1968-1979.

    Ruppert, EE. & RS Fox. 1988. Seashore animals of the Southeast: A guide to common shallow-water invertebrates of the southeastern Atlantic coast. University of SC Press. Columbia, SC. USA. 429 pp.

    Seiple, W. 1979. Distribution, habitat preferences and breeding periods in the crustaceans Sesarma cinereum and S. reticulatum (Brachyura: Decapoda: Grapsidae). Mar. Biol. 52: 77-86.

    Seiple, W & M Salmon. 1982. Comparative social behavior of two grapsid crabs, Sesarma reticulatum (Say) and S. cinereum (Bosc). J. Exp. Mar. Biol. Ecol. 62: 1-24.

    Seiple, W & M Salmon. 1987. Reproductive, growth and life-history contrasts between two species of grapsid crabs, Sesarma cinereum and S. reticulatum. Mar. Biol. 94: 1-6.

    Staton, JL & SD Sulkin. 1991. Nutritional requirements and starvation resistance in larvae of the brachyuran crabs Sesarma cinereum (Bosc) and S. reticulatum (Say). J. Exp. Mar. Biol. Ecol. 152: 271-284.

    Teal, JM. 1958. Distribution of fiddler crabs in Georgia salt marshes. Ecology. 39: 185-193.

    Voss, GL. Seashore life of Florida and the Caribbean. Dover Publications, Inc. Mineola, NY. USA. 199 pp.

    Williams, AB. 1984. Shrimps, lobsters, and crabs of the Atlantic coast of the eastern United States, Maine to Florida. Smithsonian Institution Press. Washington, DC. USA.

Report by: LH Sweat, Smithsonian Marine Station at Fort Pierce
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Page last updated: 20 August 2009

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