Horseshoe crabs are arthropods in the family Limulidae. They are one of the best-known examples of “living fossils,” organisms whose extant representatives look very similar to their ancient ancestors. While a large number of extinct horseshoe crab species are known dating as far back as the early Triassic and possibly even earlier, there are four species alive today: the Atlantic horseshoe crab (Limulus polyphemus), the three-spine horseshoe crab (Tachypleus tridentatus), the southern horseshoe crab (Tachypleus gigas), and the mangrove horseshoe crab (Carcinoscorpius rotundicauda). Their closest living relatives are arachnids, particularly the hooded tickspiders of the order Ricinulei.
