xiphosurans

Xiphosurans are a small, ancient group of chelicerate arthropods that includes the living horseshoe crabs and numerous extinct lineages. They are more closely related to spiders and scorpions than to true crabs. The living representatives comprise four species in three genera: Limulus polyphemus, Tachypleus gigas, Tachypleus tridentatus, and Carcinoscorax rotundicauda. They inhabit shallow coastal waters, estuaries, and sandy or muddy bottoms in warm temperate to tropical regions, often spending time in intertidal zones or moving with the tides. They are primarily bottom-dwellers and feed on worms, mollusks, and other invertebrates, using their appendages to capture and macerate prey. Their gill surfaces are of the book-gill type, allowing gas exchange underwater.

Anatomically, xiphosurans have a broad, shield-like prosoma (the head and thorax) and a segmented opisthosoma (abdomen) ending in a long, rigid tail spine or telson. The body is armored with a hard carapace. They possess chelicerate mouthparts, including chelicerae and pedipalps, and they typically show six pairs of walking legs, some adapted for feeding rather than locomotion. Eyes include simple ocelli and, in many species, lateral compound eyes. A distinctive feature is their ability to right themselves using the telson if overturned.

Fossil records show xiphosurans dating back to the Ordovician period, around 450 million years ago. They have undergone relatively little gross change, leading scientists to describe them as living fossils. Human use of horseshoe crab blood for biomedical testing, along with habitat loss and harvesting, has affected some populations, prompting conservation concerns in parts of their range.