||Armases cinereum Bosc, 1802
||Squareback Marsh Crab
Gray Marsh Crab
||Sesarma cinereum Bosc, 1802
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.
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).
The squareback marsh crab can be found throughout the IRL among
debris sheltered beaches, in mangrove forests and salt marshes.
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).
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).
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).
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.
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
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
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.
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).
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.
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.
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.
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.
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.
Report by: LH
Sweat, Smithsonian Marine Station at Fort Pierce
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Page last updated: 20 August 2009