Atlantic Horseshoe Crab (Limulus polyphemus)
Color: Shell is light tan to dark brown. Legs
and gills are dark reddish brown with white markings.
Size: Averages 24 inches long and 12 inches wide. Females are larger than males.
Habitat: Shallow and deep water, over all types of bottom.
Seasonal appearance: All year; more visible in spring when breeding.
Horseshoe crabs are not true crabs as their name suggests. In fact, they are more closely related to spiders, ticks, and scorpions. The brown horseshoe-shaped helmet of the shell protects the crab from predation by sharks, turtles, and sea gulls. The spine-like tail of horseshoe crabs is not a weapon but helps them to turn over when they are upside down and acts as a rudder as they plow along the bottom. Horseshoe crabs walk with five pairs of legs, the last pair resembling a hand with five fingers. Unlike true crabs, the legs have long, slender claws that are not used for protection. The legs assist in eating by tearing and placing food into the centrally located mouth. Turning the crab over reveals the "book gills," gills that resemble folded leaves of paper, which the horseshoe crab uses for breathing and for moving when upside down.
Life History and Behavior
In the late spring, adult horseshoe crabs migrate from deep water to mate along the shore, where they gather at the water's edge in large groups. The male has a modified hook in place of the first pair of claws, that are used to grasp the female during mating. Atlantic horseshoe crabs grow larger by molting, shedding the old shell, and replacing it with a larger, soft shell from underneath that hardens in a few days. The shell left behind is often mistaken for a dead crab. Older horseshoe crabs molt less frequently than juveniles and often become covered in algae and mollusks. The crabs mature at 9 to 11 years with some living more than 30 years.
These creatures feed on small clams and other bivalves, worms, detritus, and other invertebrates.
Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.