Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachnids such as spiders, ticks, and scorpions. Horseshoe crabs live primarily in and around shallow coastal waters on soft, sandy or muddy bottoms. They are generally found in the intertidal zone at spring high tides. They are eaten in some parts of Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda. The fossil record of Xiphosura goes back over 440 million years to the Ordovician period, with the oldest representatives of the modern family Limulidae dating to approximately 250 million years ago during the Early Triassic. As such, the extant forms have been described as "living fossils". Some molecular analyses have placed Xiphosura within Arachnida, with a 2019 molecular analysis placing them as the sister group of Ricinulei. Taxonomy The family name Limulidae comes from the genus Limulus, from the word limulus in Latin meaning "askance", or "a little askew". Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata. Horseshoe crabs are closely related to the extinct eurypterids (sea scorpions), which include some of the largest arthropods to have ever existed, and the two may be sister groups. Other studies have placed eurypterids closer to the arachnids in a group called Merostomata. The enigmatic Chasmataspidids are also thought to be closely related to the horseshoe crabs. The earliest horseshoe crab fossils are found in strata from the Lower Ordovician period, roughly 480 million years ago. The Limulidae are the only recent family of the order Xiphosura, and contains all four living species of horseshoe crabs: Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in South and Southeast Asia Limulus polyphemus, the Atlantic or American horseshoe crab, found along the Atlantic coast of the United States and the Southeast Gulf of Mexico Tachypleus gigas, the Indo-Pacific, Indonesian, Indian or southern horseshoe crab, found in South and Southeast Asia Tachypleus tridentatus, the Chinese, Japanese or tri-spine horseshoe crab, found in Southeast and East Asia Genera After Bicknell et al. 2021 and Lamsdell et al. 2020 Incertae sedis