There are four species of this ancient animal known to science now living. They are true survivors from millions of years ago. QI is their blood which uses copper rather than iron to convey oxygen and their eyes.
"Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, 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. Horseshoe crabs are considered living fossils." (Wiki)