Appearance

The marsh crab has a small, square carapace (shell) that grows to one inch wide and varies in color from warm brown to dark olive. Its eyes are on stalks located at the front corners of its body. It has two teeth toward the front on either side of the carapace and a notch along the middle of the carapace. It has furry terminal joints on its first three walking legs.

Feeding

Marsh crabs eat the outermost leaves of marsh plants, especially cordgrass, and will sometimes prey upon fiddler crabs. 

Predators

Black-crowned night herons are known predators. 

Voice

Although the marsh crab does not produce vocal sounds, they do exhibit acoustic behavior. Male marsh craps produce a tapping sound to defend their burrows and to engage in mating with females. Males produce this sound by striking two of their legs together. According to a study published in the journal Crustaceana, “the sounds recorded on a tape recorder in the laboratory consisted of six to 29 taps (tongs contacts) and there was an interval of about a third of a second between taps.”

Reproduction and Life Cycle

Little is known about the marsh crab life cycle. Females may carry 5,000 to 13,000 eggs at a time and breed up to five times in her lifetime. Their mating behavior includes a courtship tapping ritual, in which the males will tap the female and create drumming sounds with their walking feet. Mating ends with an indirect sperm transfer. 

Did You Know?

• The marsh crab is also known as the purple marsh crab, the square-backed marsh crab and the heavy marsh crab.
• Males are known for making “rapping” sounds when defending their burrow.
• They have been linked to large losses of cordgrass in marshes on Cape Cod, Massachusetts, due to overeating.

Sources and Additional Information

• A Field Guide to the Atlantic Seashore by Kenneth L. Gosner
• Life in the Chesapeake Bay by Alice Jane Lippson and Robert L. Lippson
• Crustacea of New Jersey from the 1911 Annual Report of the New Jersey State Museum
• Animals of the Seashore by Horace G. Richards
• Beachcomber’s Guide to Gulf Coast Marine Life by Susan B. Rothschild
• Sesarma reticulatum – Sea Life Base
• Comparative Analysis of the Mating Strategies in Grapsid Crabs with Special References to the Intertidal Crabs Cyclograpsus Lavauxi and Helice Crassa (Decapoda: Grapsidae) from New Zealand by A. M. Brockerhoff and C. L. McLay
• Acoustic Behavior in the Purple Marsh Crab, Sesarma reticulatum Say (Decapoda, Grapsidae) by Richard E. Mulstay