Xiphosura

Distinguished from other chelicerates by the combination of: semicircular carapace covering the prosoma; five pairs of walking legs with the fifth pair modified for gill cleaning rather than grasping; fused, unsegmented opisthosoma in adults (present species); and a long, sword-like telson. The four extant species are distinguished by geographic distribution and subtle morphological differences in carapace shape and telson structure. Xiphosura can be separated from fossil eurypterids (sea scorpions) by the presence of a true telson rather than a flattened, blade-like structure, and from chasmataspids by details of appendage and opisthosomal segmentation.

Body divided into two distinct tagmata: an anterior prosoma covered by a semicircular carapace, and a posterior opisthosoma. Prosoma bears five pairs of walking legs and one pair of chelicerae on the ventral surface; the first four leg pairs end in pincers with gnathobase spines for food processing, while the fifth pair lacks pincers and functions in gill cleaning and burrowing. A sixth pair of reduced appendages (chilaria) lies behind the walking legs. Opisthosoma consists of a mesosoma with flattened gill appendages and a metasoma lacking appendages; in modern forms, the entire opisthosoma is fused into a single unsegmented structure. Body terminates in a long, mobile caudal spine (telson) used for righting when overturned. Up to four eyes present: two lateral compound eyes and one or two median ocelli on the carapace, plus an additional chemoreceptive organ anterior to the ocelli. Exoskeleton composed of tough cuticle without crystalline biominerals; exhibits UV fluorescence in the hyaline exocuticular layer. Modern species reach maximum adult lengths of approximately 60 cm, though Paleozoic forms were often considerably smaller.

Marine and estuarine environments. Adults inhabit shallow coastal waters, typically on sandy or muddy substrates. Spawning occurs in intertidal zones, where females deposit eggs in excavated depressions. Larval stages are planktonic before settling to benthic habitats. Some extinct lineages, particularly the Belinuridae and Austrolimulidae, occupied freshwater environments during the Carboniferous through Triassic periods.

Extant species distributed in coastal marine waters of eastern North America (Limulus polyphemus) and Southeast Asia (three Tachypleus and Carcinoscorpius species). Fossil record indicates former presence in marine and freshwater environments across North America, Europe, Asia, and Australia from the Ordovician through Mesozoic.

Spawning activity concentrated during spring and early summer months in temperate populations, often synchronized with high tides and full or new moons. Activity patterns otherwise poorly documented for most populations.

Benthic omnivores and scavengers. Food is masticated by gnathobase spines on the first four pairs of walking legs before ingestion. Specific dietary components include polychaete worms, molluscs, and other small invertebrates found in sediment, though detailed dietary studies are limited.

Sexual reproduction with external fertilization. Females deposit 200–300 eggs in sand depressions; males release sperm to cover eggs. Embryo undergoes four molts within the egg before hatching as a lecithotrophic, planktonic larva lacking a telson (often termed 'trilobite larva' due to superficial resemblance to trilobites). Larva settles to bottom and molts, after which the telson appears. Development proceeds through successive molts with gradual addition of gill pairs, elongation of the caudal spine, and assumption of adult morphology. Sexual maturity reached after approximately three years in modern species.

Mating involves amplexus: male climbs onto female's back and grips her with modified first walking legs. Adults burrow in sediment, using the fifth leg pair to clear mud and the telson to push upright if overturned. Locomotion is primarily walking along the substrate; swimming occurs rarely and awkwardly, with the animal flipped upside-down.

Eggs and larvae serve as food sources for migratory shorebirds, fish, and other predators. Adults are benthic predators and scavengers that process sediment. Population declines in some areas have raised concern for dependent shorebird species.

Limulus amebocyte lysate (LAL), derived from horseshoe crab blood, is used to detect bacterial endotoxins in medical and pharmaceutical applications. Harvest for biomedical purposes poses conservation concerns. Fishery bait use has historically contributed to population declines. Eggs are ecotourism attractions during mass spawning events.

• Eurypterida (sea scorpions)Extinct chelicerates with superficially similar body plan, but possess flattened, blade-like telson rather than cylindrical spine, and show different opisthosomal segmentation. Historically grouped with Xiphosura in Merostomata, but now considered distant relatives.
• ChasmataspididaExtinct chelicerates with divided opisthosoma and different appendage morphology; telson structure and segmentation patterns distinguish them from Xiphosura.

Despite the common name 'horseshoe crab,' Xiphosura are not crustaceans (crabs) but chelicerate arthropods more closely related to arachnids. The name 'Merostomata' persists in some literature for this group, but this term is taxonomically problematic as it originally included eurypterids, which are not closely related to xiphosurans.

• living fossil
• marine
• chelicerate
• biomedical
• conservation concern
• ancient lineage

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