Spider Crab

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: EntireBay bottom rocky shores, harbors, pilings

Seasonal Appearance: All year.

Sensitivity to Human Action                                          







Distinguishing Features and Behaviors:

     The spider crab is one of the most widely recognized of all the Bay 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. These crabs attach bits of algae, shell, and seaweed to many fine, sticky hairs all over their bodies for camouflage.

     This crab’s 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 Bay 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. The ends of the claws are used 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 of their walking legs. This allows them to identify food in the water or in the mud as they walk over it.

     Like all 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.

Relationship to People:

     One of the Bay’s “living fossils,” the spider crab’s remarkable adaptability has enabled it to thrive practically unchanged for hundreds of millions of years. Spider crabs are highly tolerant of pollution and can live in harbors where there is oil and other pollutants. They can also tolerate low oxygen, or eutrophic, environments where there are generally few inhabitants.

    When startled, spider crabs will wave their pincers over their heads in a beckoning gesture to warn off potential predators.