Despite their name, horseshoe crabs are more closely related to spiders and scorpions (all are in the subphylum Chelicerata) than to crabs. Recent phylogenetic analysis suggests that horseshoe crabs may themselves be arachnids. The mangrove horseshoe crab is the only species in the genus "Carcinoscorpius". There are four extant (living) 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, but "C. rotundicauda" is considerably smaller than the others and the only species where the cross section of the tail (telson) is rounded instead of essentially triangular.