About the Species
Horseshoe crabs belong to the phylum of Arthropods, which consists of animals having an articulated body and limbs. The three major classes of Arthropods are Insects, Arachnids and Crustaceans. The horseshoe belongs to its own class called Merostomata, which means "legs attached to the mouth." (See Anatomy) Though they are called "crabs," a quick look at their taxonomy shows that they're not. In fact, they are most closely related to trilobites that existed 544 million years ago. (See Evolution)
Horseshoe crabs (Limulidae) are currently represented by four species including Limulus polyphemus (1), which is found along the eastern coast of North and Central America, and three Indo-Pacific species, Tachypleus gigas (4), Tachypleus tridentatus (3) and Carcinoscorpius rotundicauda (2). All four species are similar in terms of ecology, morphology, and serology.