What is "horseshoe crab"

Limulus \Lim"u*lus\ (l[i^]m"[-u]*l[u^]s), n.; pl. Limuli (-l[imac]). [L., dim. of limus sidelong, askance.] (Zo["o]l.) The only existing genus of Merostomata. It includes only a few species from the East Indies, and one ( Limulus polyphemus) from the Atlantic coast of North America. Called also Molucca crab, king crab, horseshoe crab, and horsefoot.

n. A type of chelicerate arthropod, of the genus (taxlink Limulus genus noshow=1), resembling a crab.

n. large marine arthropod of the Atlantic coast of North America having a domed carapace that is shaped like a horseshoe and a stiff pointed tail; a living fossil related to the wood louse [syn: king crab, Limulus polyphemus, Xiphosurus polyphemus]

Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida, that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, a decline in the population has occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand. Because of their origin 450 million years ago (Mya), horseshoe crabs are considered living fossils.