The mangrove horseshoe crab, Carcinoscorpius rotundicauda, is a chelicerate arthropod found in marine and brackish waters. Despite its name, it is more closely related to spiders and scorpions (all are in the subphylum Chelicerata) than to crabs. It is the only species in the genus Carcinoscorpius. The mangrove horseshoe crab occurs only in Asia and can be found in India, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Hong Kong. There are four extant species of horseshoe crab. The biology, ecology and breeding patterns of C. rotundicauda and the two other Asian horseshoe crab species, Tachypleus gigas and Tachypleus tridentatus, have not been as well documented as those of the North American species Limulus polyphemus. All four extant species of horseshoe crabs are anatomically very similar.