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Horseshoe Crabs

Evolution, Physical Characteristics, BehaviorUses to humans

Often referred to as a living fossil, the horseshoe crab has changed very little in over 400 million years. Related to spiders, this animal is easily identified by the large greenish brown, helmet-like dorsal plate, called either the cephalothorax or prosoma. A separate plate covers its abdomen. A long tail spine, referred to as the caudal spine or telson, extends from its abdomen. Measured from the front of its dorsal plate to the tip of its tail spine, the horseshoe crab can reach a length of 60 cm. Its mouth and six body segments lie underneath its dorsal plate; a pair of limbs is attached to each segment. Today's horseshoe crab populations are rather sporadically distributed. One species—Limulus polyphemus—lives off the coast of the eastern United States, and four species live in the marine waters of southeast Asia.

The phylum Arthropoda is the largest phylum in the animal kingdom, containing more than one million species. Within this phylum, the subphylum Chelicerata includes spiders and their relatives. This subphylum can be broken down into three classes: (1) class Arachnida (Otherwise known as arachnids, this class includes true spiders and scorpions); (2) class Pantopoda (also known as sea spiders); and (3) class Merostomata (referred to as Merostomates). Within the Merostomata class, there are two orders. One extinct order, the order Eurypterida, contained sea scorpions; the other order, Xiphosura, includes only horseshoe crabs. There is one family, Limulidae, and three genera within this family—Limulus, Tachypleus, and Carcinoscorpius. In total, there are four species.

When a horseshoe crab is wounded, its blood cells release a special protein to clot the bleeding. The same thing happens when certain toxins are introduced to stop invading bacteria. (Horseshoe crabs are a favorite host of flatworms.) Thus, hospitals sometimes use extracts of their blood when diagnosing human bacterial diseases and checking for toxins in intravenous solutions.



Bonaventura, Joseph, Celia Bonaventura, and Shirley Tesh, eds. Physiology and Biology of Horseshoe Crabs: Studies on Normal and Environmentally Stressed Animals. New York: Alan R. Liss, Inc., 1982.

Grzimek, H. C. Bernard, ed. Grzimek's Animal Life Encyclopedia. New York: Van Nostrand Reinhold Company, 1993.

The New Larousse Encyclopedia of Animal Life. New York: Bonanza Books, 1987.

Pearl, Mary Corliss, Ph.D. Consultant. The Illustrated Encyclopedia of Wildlife. London: Grey Castle Press, 1991.

Pearse, John and Vicki, and Mildred and Ralph Buchsbaum. Living Invertebrates. Palo Alto, California: Blackwell Scientific Publications; Pacific Grove, California: The Boxwood Press, 1987.

Kathryn Snavely


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Caudal spine

—Also called the telson, this appendage extends from the crab's abdomen and resembles a tail; it is often as long as the crab's body. It is used by the crab to right itself if it falls on its back; the crab flaps it against the abdomen when it swims.


—The head and thorax (upper part of the body) combined.


—Pincers on the last pair of walking legs with which the crab grabs food from the sea floor.


—Feeding appendages.

Compound eyes

—Two large eyes appearing widely separated on the anterior of the dorsal plate. They are actually composed of numerous simple eyes clustered together.


—Attached to the legs, these spiny devices function like jaws, shredding and manipulating food before passing it to the mouth.


—Their second pair of legs, highly specialized, depending on the species.


—See cephalothorax.


—The amountdissolved salts in water.

Simple eyes

—Located fairly close to each other at the anterior of the crab's back. Easy to overlook.


—The area just below the head and neck; the chest.

Additional topics

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