Limulidae Leach, 1819
There are four extant species og horseshoe crabs distributed across the coastal zones of the continental shelfs in North America and South East Asia. The American horseshoe crab Limulus polyphemus (Linnaeus, 1758) has a distribution along the eastern coast of North America and a genetically and geographically isolated population on the northern east coast of the Yucatán Peninsula in Mexico. The three remaining species Tachypleus gigas (Müller, 1785), Tachypleus tridentatus (Leach, 1819), and Carcinoscorpius rotundicauda (Latreille, 1802) inhabit the shallow waters in the Indo-Pacific. In this area two species, Tachypleus gigas and C. rotundicauda have a largely overlapping distribution ranging from the Andaman Sea to the South China Sea, but inhabit different habitats (Sekiguchi, 1988). While T. gigas lives in sandy and shallow near-coast habitats, C. rotundicauda mostly inhabits estuaries and mangroves. The third Asian species T. tridentatus lives in shallow coastal zones of Southern China from Vietnam to Japan (Sekiguchi, 1988).