Armases cinereum

Distinguished from Sesarma and Sarmatium by laterally parallel carapace margins creating square shape. Distinguished from other Armases species by combination of equatorial smooth carapace, hair row on lower margin, and lack of pubescence on second walking legs. Sex can be determined by abdomen shape: males subtriangular with equilateral telson, females subcircular with telson wider than long.

Small crab with dark brown to muddy coloration. Carapace is vertically compressed and square-shaped due to laterally parallel margins, slightly wider than long or vice versa. Distinct row of hair present on lower margin of carapace. Second walking legs lack pubescence. Eyes positioned on stalks at top of carapace. Abdomen and thorax positioned under carapace. Four walking legs per side with one claw per side positioned anterior to legs. Sexual dimorphism present: males slightly larger with more prominent palm; male abdomen subtriangular with telson equal in width and length, female abdomen subcircular with telson wider than long. Mature males 10.0–18.0 mm carapace length; mature females 11.0–17.2 mm.

Intertidal zones of coastal environments, from subtidal to supratidal elevations. Found in Spartina salt marshes and Rhizophora mangrove swamps. Occurs among and under rocks, debris, and wrack deposited by tides. Can be found up to 50 meters inland from shore. Tolerates range of salinity conditions across marine-terrestrial ecotone.

Western Atlantic Ocean: Chesapeake Bay south along Atlantic coast through Florida, west along Gulf of Mexico coast to Veracruz, Mexico. Present in temperate to tropical coastal zones.

Spawning occurs March through July. Active year-round in suitable climates with peak activity in warmer months.

Highly omnivorous. Consumes partially decomposed leaves (especially Avicennia and other mangrove vegetation), fresh leaves of softer plants, insects, aphids, snails, other crabs, marsh sediments, and mammal feces. Prioritizes animal prey when available; prefers softer plant material over tougher vegetation when feeding on plants. Diet composition varies with local food availability.

Ovigerous: females carry eggs on abdomen during embryonic development. Brood size 2,000–12,000 eggs, proportional to female size. Eggs hatch into planktonic zoea larvae with characteristic dorsal spine. Zoea undergo multiple molts, then transition to megalopa stage with eyes on stalks, formed carapace, and outward-positioned abdomen. Final molt produces juvenile crab resembling adult form.

Highly mobile with bi-directional movement between salt marsh and upland forest habitats. Females exhibit larger activity ranges spanning both marsh and forest; males prefer forest with more restricted ranges. Females feed preferentially in marshes and may transport energy landward as biotic vectors. Males possess larger bodies and claws enabling consumption of larger and more diverse prey. Limb loss via injury is common.

Key omnivore in mangrove and salt marsh food webs, functioning as carnivore, herbivore, and detritivore. Links marsh and forest habitats through movement, mediating energy fluxes along marine-terrestrial ecotone. Potential vector for spatial subsidies of energy from salt marsh to upland forest, particularly by females, though high metabolic rates limit net energy transfer. High local abundance makes it useful for studying trophic dynamics and as indicator of habitat conditions.

Used as indicator species for coastal habitat conditions due to high population density and sensitivity to environmental changes. Serves as research organism for studying trophic dynamics, metabolic ecology, and marine-terrestrial connectivity in coastal ecosystems.

• Sesarma spp.Similar family and habitat; distinguished by non-parallel, more rounded carapace margins lacking square shape
• Sarmatium spp.Similar family; distinguished by carapace shape without parallel lateral margins
• Other Armases speciesSame genus; distinguished by specific combination of carapace proportions, hair patterns, and leg pubescence

• crab
• marsh
• mangrove
• omnivore
• intertidal
• coastal
• western atlantic

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Metabolic rates in the squareback marsh crab Armases cinereum