Atlantic Horseshoe Crab (Limulus polyphemus)

Atlantic Horseshoe Crab
Limulus polyphemus

Atlantic Horseshoe Crab:
King of the [Non-] Crabs

By Patricia B. Mitchell.

The greenish-brown Horseshoe Crab, which is not actually a crab but rather a creature more akin to the scorpion, has a horseshoe-shaped convex carapace. The Horseshoe Crab has a spike-like tail (or telson), which it does not use for defense but rather as a leverage device which it employs to turn itself over if it ends up on its back on land. Like a turtle, the Horseshoe Crab seems quite upset to be turned bottom up, although he does swim “upside-down,” usually at night. (Limulus also uses its telson as a brace to “hunker down” in the sand or mud on the ocean bottom.)

The Atlantic Horseshoe Crab “shell” may reach a length of over 24 inches, even though the actual creature inside is much smaller. It is not at all esteemed as a food for humans nowdays, although historically, Native Americans ate the meat of the Horseshoe Crab. (They also used the telson as a spear tip, and the carapace as a bowl to bail water, etc.) A bird called the Red Knot loves to eat Horseshoe Crab eggs.

The Horseshoe Crab was, at one time, called the King Crab. Nowadays it sometimes goes by the nickname “Big Brown.”

The creature has two pairs of compound eyes set atop the carapace and two simple eyes at the shell front, rather than eyes on stalks like true crabs. On the underside around the mouth are five pairs of dark brown legs. In front of the mouth is a pair of pinchers. Six spines stick out from each side of the part of the carapace over the abdomen.

During the spring mating season, the female releases a pheromone into the sea. When a male locates her, having been attracted by the chemical, he takes hold of her around her carapace. (The male is smaller than the female.) He uses his specialized strong front legs to accomplish this grasping. He hangs onto her in order to be on hand, so to speak, when she goes ashore to lay her 200-300 dark greenish eggs in a depression in the sand. The males then release sperm which fertilize the eggs. This mating process occurs in May or June during the high spring tides. (Sometimes as many as nine potential “daddies” hitchhike on a lady at once!)

Limulus polyphemus is of great value in the field of medicine because a clotting agent in its blue, copper-based blood is used to check drugs for dangerious toxins. (The Horseshoe Crab, after having a portion of blood extracted, can be returned to the sea unharmed.)

The eyes of the Horseshoe Crab are used in the medical study of human optic nerve structure.

Chitin from the “shell” of the Horseshoe Crab is non-toxic and biodegradable. When combined with chitosan, it can be used in cosmetics, hair spray, and contact lenses. Chitin can be used to purify drinking water and waste water. The substance can also be made into surgical suture and wound dressings.

Horseshoe Crabs are used in eel pots as bait.

Atlantic Horseshoe Crabs live in shallow water, and may be found anywhere from Maine to the Gulf of Mexico. A few other species of Horseshoe Crabs can be found in other seas.

Horseshoe Crabs molt, as do all of the crustaceans. The shell splits open and the crab emerges, now larger by a fourth than before. These empty “molts” may be found on the beach.

At one time Horseshoe Crabs were so abundant that farmers used them as fertilizer. Limulus polyphemus itself especially enjoys dining on clams.


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