|Two atlantic horseshoe crab (Limulus polyphemus) mating. Female (larger) in front, male attached in back. Horseshoe Crabs are chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Refered to as living Fossils horseshoe crabs been around for over 350 million years. Horseshoe crabs are valuable as a species to the medical research community. The horseshoe crab has a substance called Limulus Amebocyte Lysate (LAL) is used to test for bacterial endotoxins in pharmaceuticals and for several bacterial diseases. I do research on Horseshoe crabs, contact me if you need more info on this species.