The rock crab varies in color from red-orange to yellowish-green.
The rock crab varies in color from red-orange to yellowish-green.
Courtesy: University of Charleston, S.C.

Rock Crab (Cancer irroratus)

Color: Shell is yellow to red orange with darker red mottling on top; the underside is whitish to a cream yellow.
Size: Up to 5 inches wide and 3.5 inches long at maturity.
Habitat: Rocky marine environments, jetties and tide pools, under and around rocks.
Seasonal appearance: All year.

Description

Rock crabs are among the most common subtidal crabs in Rhode Island. Like other crabs, they possess a hard exoskeleton, which enables them to live successfully in the harsh rocky tidal environment. Rock crabs have relatively smooth oval or fan-shaped carapace with a rounded front border. Antennae used for taste and smell and two movable eyestalks are located at the front of the shell. Between the eyestalks are three spines and nine smooth spines run along the outside edge of the carapace.

A similar crab is the Jonah crab (Cancer borealis), which differs only slightly by having jagged spines along the shell and by being more common in deeper waters.

Life History and Behavior

During the mating season, the female releases a hormone into the water to attract a male. The male crab will encircle the female with his claws, protecting her during molting. Mating can only occur during molting, with the male providing protection while the female is soft-shelled and defenseless. Once the female's shell has hardened, in two or three days, the male releases her. Juvenile rock crabs can be found in shallow, brackish, intertidal zones, while adults prefer deeper, saltier waters.

The rock crab has two short front claws that are quite powerful but heavy and slow. It is a crawling crab and tends to move very little. Its short walking legs are covered in hair-like structures that function as sensory organs. Rock crabs are scavengers; their primary prey includes worms, clams, mussels, other crabs, and many other invertebrates. Fish, crabs, gulls, and humans eat rock crabs.

Special Notes


Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.