Horseshoe Crabs

According to the Columbia Electronic Encyclopedia (2009) horseshoe crabs are large primitive marine arthropods related to the spider. They are distinguished by a heavy dark brown exoskeleton or carapace, which is doomed and shaped like a horseshoe. Their body is divided into a broad flattened semicircular front part, the prosoma, a tapering middle part, the opisthoosma, and a pointed spiky tail like part, the telson. They have no jaws, and the mouth is flanked by a pair of pincer like chelicera that are used to crush worms and other invertebrates taken as food. Five pairs of walking legs attached to the prosoma enable the animals to swim awkwardly or burrow through the sand or mud. The respiratory organs are called book gills and are unique to horseshoe crabs. Each book gill is made of about 100 thin leaves, or plates; these are fitted like pages of a book onto one pair of flap like appendages on the opisthosoma. Rhythmic movement of the appendages circulates water over the gill surfaces and drives blood into and out of the gill leaves. They first appeared in the Upper Silurian period, and a number of fossil species have been described. Five species still survive; four of these are found along the Pacific coast of Asia. The American species, Limulus Polyphemus, is common along the Atlantic coast from Nova Scotia to Florida. They live in shallow water, preferring soft or sandy bottoms, and reach a maximum length of nearly 2 ft.

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National Geographic (1999) reports horseshoe crabs resemble fossilized trilobites and eurypterids of the Paleozoic era. They have changed little since an ancestor was fossilized 150 million years ago. Spawning crabs swarm Delaware Bay beaches in spring, fueling an egg-eating frenzy of gulls and migratory birds. Fishermen kill crabs for bait. Studies suggest that populations are dropping, leading some states to restrict harvests. By decoding their visual system, researchers have learned much about human sight. Crabs are also bled to extract a reagent used to test drugs and medical devices for contamination.

Horseshoe crabs are members of the phylum Arthropoda, arthro meaning joint and poda meaning foot. This is the most numerous phyla of all living organisms in both number of species and number of individuals. Conservative estimates are that well over one million species of insects exist. As far as individuals are concerned there are more ants than anything else, and there are more kinds of beetles than any other species. Between 40 and 50% of all insect species are beetles. There are more insects than all other animals and plants combined (Carter, 1997).

Arthropods have segmented bodies covered by an exoskeleton made from chitin and other chemicals. The exoskeleton serves as protection and provides places for muscle attachment. Because their exoskeleton doesn’t grow with them Arthropods must molt. Their circulatory system is open and consists of a dorsal heart which collects blood from the body cavity and pumps it back into the body cavity again. In insects, the anterior portion of the heart, located in the abdomen, is extended into a tube, the aorta, which directs the blood forward as it goes out into the body cavity. Arthropods have a well-developed, mesodermal, ventral, solid nerve cord and well-developed sense organs. The body feature from which the phylum takes its name is the jointed appendages, which include antennae and mouthparts as well as walking legs.

Arthropods early ancestors had a number of body segments, each with a pair of jointed appendages and looked similar to a centipede. From there, some of these segments became fused to form a head and some of the appendages became modified to form mouthparts or antennae. Early on, there was an evolutionary split which led to the various modern subphyla and classes. Currently, three living subphyla are recognized, with trilobites representing an extinct fourth subphylum (Carter, 1997).

Horseshoe Crabs belong to the subphylum Chelicerata, and are in the class Xiphosura, xipho meaning sword and ura meaning tail. Also in subphylum Chelicerata is class Arachnida, arachni meaning spider. This class includes scorpions, mites, ticks, daddy-long-legs, and spiders.

Subphylum Crustacea, crusta meaning crust or rind, includes crayfish, lobsters, crabs, pillbugs. They have gills, thus terrestrial pillbugs need to maintain a 100% humidity environment around their gills to be able to “breathe.” Crustaceans have the head and thorax combined into one body region, the cephalothorax, as well as an abdomen. They have two pairs of antennae, mandible-type mouthparts (of different evolutionary origin than mandibles in insects), and other mouthparts which include two pairs of maxillae and three pairs of maxillipeds, all of which are formed from modified appendages.

Subphylum Atelocerata includes insects, centipedes and millipedes. Members of this subphylum have one pair of antennae, mandibles, and two other pairs of mouthparts, either two pairs of maxillae or one pair of maxillae plus a labium, which are modified appendages.

Members of the fourth subphylum, Trilobita, are now all extinct. Their bodies were divided into three sections, lobes, a head and a body region.


Carter, J.S. (1997). Phylum Arhropoda. UC Clermont College Biology Home Page. Retrieved September 14, 2010, from

NAI. (1999, February). Horseshoe crabs. National geographic, Vol. 195, Issue 2. Retrieved September 14, 2010, from

NAI. (2009, December 1). Horseshoe crab. Columbia electronic encyclopedia, 6th Edition . Retrieved September 14, 2010, from