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horseshoe.JPG (233907 bytes) horseshoeb.JPG (251678 bytes) horseshoec.JPG (157562 bytes) horseshoed.JPG (144732 bytes) horseshoee.JPG (149623 bytes) horseshoeg.JPG (238862 bytes)
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KINGDOM : Animalia

PHYLUM : Arthropoda

SUBPHYLUM : Chelicerata

CLASS : Merostomata

ORDER : Xiphosura

FAMILY : Limulidae

GENUS : Limulus

SPECIES Limulus polyphemus

HORSESHOE CRAB - Limulus polyphemus

The Atlantic horseshoe crab (Limulus polyphemus) is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is Delaware Bay, although stray individuals are occasionally found in Europe.

The other three extant species in the family Limulidae are also called horseshoe crabs. These are Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda, which all are restricted to Asia. All four are quite similar in form and behavior.

This group of animals is also known as horsefoot, or saucepan. Some people call the horseshoe crab a "helmet crab", but this common name is more frequently applied to a true crab, a malacostracan, of the species Telmessus cheiragonus. The term "king crab" is sometimes used for horseshoe crabs, but it is more usually applied to a group of decapod crustaceans.

Limulus means "askew" and polyphemus refers to the giant in Greek mythology. It is based on the misconception that the animal had a single eye. Former scientific names include Limulus cyclops, Xiphosura americana, and Polyphemus occidentalis. It is the tail that earns this order its name Xiphosura, which derives from the Greek for 'sword tail'.

Horseshoe crabs were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. They may have evolved in the shallow seas of the Paleozoic Era (570-248 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods.

The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.

Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years. Forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, 400 million years ago. However, the Atlantic horseshoe crab itself has no fossil record at all, and the genus Limulus "ranges back only some 20 million years, not 200 million."

Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. The above-mentioned clotting reaction of the animal's blood is used in the widely used Limulus amebocyte lysate (LAL) test to detect bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases. Enzymes from horseshoe crab blood are used by astronauts in the International Space Station to test surfaces for unwanted bacteria and fungi. A protein from horseshoe crab blood is also under investigation as a novel antibiotic. LAL is obtained from the animals' blood. Horseshoe crabs are returned to the ocean after bleeding. Studies show the blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound.