Species Description: The squareback marsh crab, Armases cinereum, is one of 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.
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.
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 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).
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.
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.
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).
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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.
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