Temporal range: Ordovician–Recent
Leach, 1819 
Horseshoe crabs (family Limulidae, order Xiphosura or Xiphosurida) are arthropods that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, a decline in the number of individuals has occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand. Horseshoe crabs are considered living fossils.
Horseshoe crabs resemble crustaceans, but belong to a separate subphylum, Chelicerata, and are closely related to arachnids, e.g. spiders and scorpions. The earliest horseshoe crab fossils are found in strata from the late Ordovician period, roughly .
- Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in Southeast Asia
- Limulus polyphemus, the Atlantic horseshoe crab, found along the northwest Atlantic coast and in the Gulf of Mexico
- Tachypleus gigas, found in South and Southeast Asia
- Tachypleus tridentatus, found along East Asian coasts
Anatomy and behavior 
The entire body of the horseshoe crab is protected by a hard shell. It has two primary compound eyes and seven secondary simple eyes, two of which are on the underside. Beneath the carapace, it looks quite similar to a large spider. It has five pairs of legs for walking, swimming, and moving food into the mouth. The long, straight, rigid tail can be used to flip itself over if turned upside down, so a horseshoe crab with a broken tail is more susceptible to desiccation or predation.
Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. Horseshoe crabs normally swim upside down, inclined at about 30° to the horizontal and moving at about 10–15 cm/s (0.22–0.34 mph). Despite this, they usually are found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish.
Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 cm (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.
During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. Shore birds eat many of the eggs before they hatch. The eggs take about two weeks to hatch. The larvae molt six times during the first year.
Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates mating only takes place in the presence of the sand or mud in which the horseshoe crab eggs were hatched. Neither what in the sand is being sensed by the crabs nor how they sense it is known with certainty . 
Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a role similar to white blood cells for vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins.
Harvesting horseshoe crab blood involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation. Estimates of mortality rates following blood harvesting vary from 3 to 15%.
Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk. However, fishing horseshoe crab is temporarily forbidden in New Jersey (moratorium on harvesting) and restricted to only males in Delaware. A permanent moratorium is in effect in South Carolina.
Low horseshoe crab populations in the Delaware Bay is hypothesized to endanger the future of the red knot. Red knots, long-distance migratory shorebirds, feed on the protein-rich eggs during their stopovers on the beaches of New Jersey and Delaware. An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.
See also 
- Kōichi Sekiguchi (1988). Biology of Horseshoe Crabs. Science House. ISBN 978-4-915572-25-8.
- Attaya Kungsuwan, Yuji Nagashima & Tamao Noguchi (1987). "Tetrodoxin in the horseshoe crab Carcinoscorpius rotundicauda inhabiting Thailand" (PDF). Nippon Suisan Gakkaishi 53: 261–266.
- David Sadava, H. Craig Heller, David M. Hillis & May Berenbaum (2009). Life: the Science of Biology (9th ed.). W. H. Freeman. p. 683. ISBN 978-1-4292-1962-4.
- B.-A. Battelle (2006). "The eyes of Limulus polyphemus (Xiphosura, Chelicerata) and their afferent and efferent projections". Arthropod Structure & Development 35 (4): 261–274. doi:10.1016/j.asd.2006.07.002. PMID 18089075.
- Robert B. Barlow (2009). "Vision in horseshoe crabs". In John T. Tanacredi, Mark L. Botton & David Smith. Biology and Conservation of Horseshoe Crabs. Springer. pp. 223–235. ISBN 978-0-387-89958-9.
- S. M. Manton (1977). The Arthropoda: Habits, Functional Morphology, and Evolution. Clarendon Press. p. 57.
- Carl N. Shuster, Jr. & Lyall I. Anderson (2003). "A history of skeletal structure: clues to relationships among species". In Carl Nathaniel Shuster, Robert B. Barlow & H. Jane Brockmann. Harvard University Press. pp. 154–188. ISBN 978-0-674-01159-5.
- E. D. Vosatka (1970). "Observations on the swimming, righting, and burrowing movements of young horseshoe crabs, Limulus polyphemus". Ohio Journal of Science 70 (5): 276–283.
- Lesley Cartwright-Taylor, Julian Lee & Chia Chi Hsu (2009). "Population structure and breeding pattern of the mangrove horseshoe crab Carcinoscorpius rotundicauda in Singapore". Aquatic Biology 8: 61–69. doi:10.3354/ab00206.
- David Funkhouser (April 15, 2011). "Crab love nest". Scientific American 304 (4): 29. doi:10.1038/scientificamerican0411-29.
- Lenka Hurton (2003). Reducing post-bleeding mortality of horseshoe crabs (Limulus polyphemus) used in the biomedical industry (M.Sc. thesis). Virginia Polytechnic Institute and State University. http://scholar.lib.vt.edu/theses/available/etd-12172003-122534/unrestricted/HurtonThesis.pdf.
- "Crash: A Tale of Two Species – The Benefits of Blue Blood", PBS
- "Horseshoe crab". SC DNR species gallery. Retrieved 6 June 2011.
- "Red knots get to feast on horseshoe crab eggs". Environment News Service. March 26, 2008. Retrieved January 19, 2011.