The spider crab is a commonly seen crab in Rhode Island waters.
The spider crab is a commonly seen crab in Rhode Island waters.
Courtesy: University of Charleston, S.C.

Spider Crab (Libinia emarginata)

Color: Body is mud colored; claws are whitish-yellow and stand out from the rest of the crab.
Size: Carapace is up to 4 inches wide. Males grow larger than females and can be 9 inches from claw to claw when stretched out.
Habitat: Entire Bay bottom, rocky shores, harbors, eelgrass beds, and pilings.
Seasonal appearance: All year.


The spider crab is one of the most widely recognized of all Rhode Island marine inhabitants. This long-legged crustacean is in fact a crab and not a spider as its name suggests. The carapace of this crab is round and spiny, with nine small spines running down the center of the back. Its tapered snout and short eyestalks are located on the rostrum, or tip of the carapace, which extends out in a shallow V-shaped notch. Spider crabs range in size, with adult males growing larger than juveniles and females.

The legs and pincers of the male spider crabs can be nearly twice as long as those of the females. The spider crab's claws are different from those of other crabs. The claws are narrow, long pincers that are slow and not as strong as many other crabs; however, the larger males have big claws that can deliver quite a pinch.

Life History and Behavior

Spider crabs use the ends of their claws to scoop up bits of detritus and algae. Spider crabs are non-threatening and somewhat lethargic scavengers. They have poor eyesight; however, they do have sensitive tasting and sensing organs on the end of each walking leg. This allows them to identify food in the water or in the mud as they walk over it.

Like all other crabs, spider crabs molt to grow. The females stop molting after they become sexually mature and remain the same size for the rest of their lives. When molting, spider crabs will cling to the tops of eelgrass close to the water's surface.

Critter Fact Spider crabs attach bits of algae, shell, and seaweed to the many fine, sticky hairs covering their bodies for camouflage.

Special Notes

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