Horseshoe crabs are marine and brackish water arthropods of the family 'Limulidae' and the only living members of the order 'Xiphosura'. Their popular name is a misnomer, as they are not true crabs, nor even crustaceans, as crabs are, but a different order of arthropod. They live primarily in and around shallow coastal waters on soft, sandy, or muddy bottoms.

The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000 ommatidia (compound eyes), plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, and a pair of rudimentary lateral eyes on the top.

During the breeding season, horseshoe crabs migrate to shallow coastal waters. Natural breeding of horseshoe crabs in captivity has proven to be difficult. Some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk, or conch.

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