Atlantic horseshoe crab

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Atlantic horseshoe crab
St. Lucie County Aquarium, Florida

Secure (NatureServe)[2]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: Xiphosura
Family: Limulidae
Genus: Limulus
L. polyphemus
Binomial name
Limulus polyphemus

Monoculus polyphemus Linnaeus, 1758
Cancer polyphemus Linnaeus, 1758

The Atlantic horseshoe crab (Limulus polyphemus), also known as the American horseshoe crab, is a species of horseshoe crab, a kind of marine and brackish chelicerate arthropod.[1] It is found in the Gulf of Mexico and along the Atlantic coast of North America.[1] The main area of annual migration is Delaware Bay along the South Jersey Delaware Bayshore.[3]

Their eggs were eaten by Native Americans,[4] but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine (especially for Limulus amebocyte lysate) and science.[1] They play a major role in the local ecosystems, with their eggs providing an important food source for shorebirds, and the juveniles and adults being eaten by sea turtles.[1][3]

The other three extant (living) species in the family Limulidae are also called horseshoe crabs, but they are restricted to Asia.[5] Despite the name, horseshoe crabs are more closely related to arachnids like spiders and scorpions than they are to crabs (or any crustacean).[6]

Names and classification[edit]

Atlantic horseshoe crab with attached Crepidula shells on the Delaware Bay beach in Villas, New Jersey.
Underside of living horseshoe crab
Underside view of a living male crab, showing the mouth, gills and legs

This group of animals is also known as horsefoot, or saucepan. Some people call the horseshoe crab a "helmet crab", but this common name is more frequently applied to a true crab, a malacostracan, of the species Telmessus cheiragonus. The term "king crab" is sometimes used for horseshoe crabs, but it is more usually applied to a group of decapod crustaceans.

Limulus means "askew"[7] and polyphemus refers to Polyphemus, a giant in Greek mythology.[7] It is based on the misconception that the animal had a single eye.

Former scientific names include Limulus cyclops, Xiphosura americana, and Polyphemus occidentalis.

It is the tail that earns this order its name Xiphosura, which derives from the Greek for "sword tail".

Studies of microsatellite DNA have revealed several distinct geographic groups in the Atlantic horseshoe crab. While there is extensive mixing between neighbouring populations, primarily due to movements by males, there is little or no mixing between the US and isolated Yucatán Peninsula horseshoe crabs, leading some to suggest that a taxonomic review is necessary.[4][8]

Anatomy and physiology[edit]

Underside view: The mouth opening is between the legs, and the gills are visible below.
Underside of a female showing the legs and book gills.
Underside of a male, showing the first leg modified for grasping the female during copulation.

Horseshoe crabs have three main parts to the body: the head region, known as the "prosoma", the abdominal region or "opisthosoma", and the spine-like tail or "telson". The smooth shell or carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. The sexes are similar in appearance, but females are typically 25 to 30% larger than the male in length and width, and can reach more than twice the weight.[9][10] Females can grow up to 60 cm (24 in) in length, including tail,[11] and 4.8 kg (11 lb) in weight.[10] There are also geographic differences: In the United States, there is a north–south cline in the size. Central animals, between Georgia and Cape Cod, are the largest. North of Cape Cod and south of Georgia they gradually become smaller.[12][13] In Delaware Bay, females and males have an average carapace width of about 25.5 cm (10.0 in) and 20 cm (7.9 in), respectively. In Florida, females average about 21.5 cm (8.5 in) and males 16 cm (6.3 in).[4] However, this north-south pattern does not exist in the Yucatán Peninsula, the southernmost population of the species, where some subpopulations are intermediate between the Delaware Bay and Florida horseshoe crabs, and others are smaller, averaging about 34 the size of Florida horseshoe crabs.[4]

Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.[14]

A wide range of marine species become attached to the carapace, including algae, flat worms, mollusks, barnacles, and bryozoans, and horseshoe crabs have been described as 'walking museums' due to the number of organisms they can support.[15] In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.

The brain and the heart are located in the prosoma. On the underside of the prosoma, six pairs of appendages occur, the first of which (the small pincers or chelicerae) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax, between the chelicerae. Although most arthropods have mandibles, the horseshoe crab is jawless.

The second pair of appendages, the pedipalps, are used as walking legs; in males they are tipped with 'claspers', which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The first four pairs of legs have claws, the last pair has a leaf-like structure used for pushing.[16]

The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist.

The telson (i.e., tail or caudal spine) is used to steer in the water and also to flip itself over if stuck upside down.

Among other senses, they have a small chemoreceptor organ that senses smells on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.[17]


Limulus has been extensively used in research into the physiology of vision.[citation needed] The Nobel Prize in Medicine was awarded in 1967 in part for research performed on the horseshoe crab eye.

A large compound eye with monochromatic vision is found on each side of the prosoma;[note 1][18] it has five simple eyes on the carapace, and two simple eyes on the underside, just in front of the mouth,[18] making a total of nine eyes. The simple eyes are probably important during the embryonic or larval stages of the organism,[18] and even unhatched embryos seem to be able to sense light levels from within their buried eggs.[19] The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood.[18]

In addition, the tail bears a series of light-sensing organs along its length.

Each compound eye is composed of about 1000 receptors called ommatidia,[16] complex structures consisting of upwards of 300 cells.[19] The ommatidia are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods.[18] Each ommatidium feeds into a single nerve fiber. Furthermore, the nerves are large and relatively accessible. This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena such as lateral inhibition working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus. Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by males when recognizing potential mates.

The retinula (literally, "small retina") cells of the ommatidium of the compound eye contain areas from which membranous organelles of conceivable size (rhabdomeres) extend. Rhabdomeres have tiny microvilli (tiny tubes extending out of the retinula) that interlock with neighboring retinular cells. This forms the rhabdom, which contains the dendrite of the eccentric cell, and may also contribute some microvilli. The only other species with an eccentric cell is the silkworm moth. Microvilli are composed of a double layer, 7 nm each and with 3.5 nm space of two electron-deficient boundaries in between. Where the microvilli meet, these outer borders fuse and yield five membranes about 15 nm thick. In all arthropods, there is always a rhabdom below a crystalline cone, on or near the center of the ommatidium, and always aligned with the path of light. At right angles to the length of the rhabdome are the length of the microvilli, which are in line with each other. The microvilli are about 40–150 nm in diameter.[20]


The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper-containing protein hemocyanin at concentrations of about 50 g per liter.[21] These creatures do not have hemoglobin (iron-containing protein), which is the basis of oxygen transport in vertebrates. Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow,[21] and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed.[21] Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in vertebrates by hemoglobin in red blood cells.[21]

The blood of horseshoe crabs contains one type of blood cell, the amebocytes. These play an important role in the defense against pathogens. Amebocytes contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial endotoxin is encountered.[22] The resulting coagulation is thought to contain bacterial infections in the animal's semiclosed circulatory system.[23]

Distribution and habitat[edit]

Atlantic horseshoe crabs in Mexico, such as this pair at Holbox Island, mostly breed in lagoons with mangrove and seagrass[4]

The Atlantic horseshoe crab is the only extant (living) species of horseshoe crab native to the Americas, although there are other extinct species only known from fossil remains from this region. The other living species of horseshoe crab are restricted to Asia, but all are quite similar in form and behavior. The Asian species are Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda.[5][24]

Most Atlantic horseshoe crabs are found along the Atlantic East Coast of the United States, ranging from Maine to Florida. In the Gulf Coast of the United States, they are found in Florida, Alabama, Mississippi and Louisiana.[1] Outside the United States, the only breeding population is in Mexico's Yucatán Peninsula, where it is found on the western, northern and eastern coasts.[4] Individuals rarely appear outside the breeding range, with a few records from the Atlantic coast of Canada (Lahave Island on Nova Scotia), the Bahamas, the Turks and Caicos, Cuba and the western Gulf of Mexico (Texas and Veracruz). Historic claims of horseshoe crabs on Jamaica and Hispaniola (on the Dominican Republic's southeast coast) have not resulted in any discoveries after expeditions there.[1][4] There have been attempts of introducing it to Texas, California and the southern North Sea, but these all failed to become established.[1] Records from Europe, Israel and Western Africa are considered releases/escapees of captives.[1]

Atlantic horseshoe crabs range from shallow coastal habitats such as lagoons, estuaries and mangrove to depths of more than 200 m (660 ft) up to 56 km (35 mi) offshore.[1] There are indications that they prefer depths shallower than 30 m (98 ft).[25] The temperature preference varies depending on the population, with the northernmost being the most cold-resistant: in the Great Bay in New Hampshire they show increased activity above 10.5 °C (51 °F) and in Delaware Bay they are active above 15 °C (59 °F).[1] In contrast, these northern populations do not tolerate as warm temperatures as southern populations of the species.[13] Atlantic horseshoe crabs can be seen in waters that range from brackish (almost fresh water) to hypersaline (almost twice the salinity of sea water), but their optimum growth is at salinities around or slightly below sea water (20–40‰).[1][26][27]

A 2022 study of ancient (Early Pleistocene, 2 million years ago) environmental DNA from the Kap Kobenhavn Formation of northern Greenland identified preserved DNA fragments of horseshoe crabs, assigned to L. polyphemus. This suggests that horseshoe crabs ranged and spawned as far north as Greenland during these warmer conditions. Around this time, the sea surface temperature would have been 8 °C warmer than the present. These are among the oldest DNA fragments ever sequenced.[28][29]

Life cycle and behavior[edit]

External video
video icon Rendezvous with a Horseshoe Crab, August 2011, 4:34, NewsWorks
video icon The Horseshoe Crab Spawn, June 2010, 5:08,
video icon Horseshoe Crabs Mate in Massive Beach "Orgy", June 2014, 3:29, National Geographic

The crab feeds on mollusks, annelid worms, other benthic invertebrates, and bits of fish. Lacking jaws, it grinds up the food with bristles on its legs and a gizzard that contains sand and gravel.[9]

Spawning tends to occur in the intertidal zone and to be correlated with spring tides (the highest tides of the month).[1] The breeding season varies. Northern populations (all in the United States, except those in southern Florida) generally breed from the spring to the autumn, while southern populations (southern Florida and Yucatán Peninsula) breed year-round.[1] In the north, breeding is triggered by a temperature increase, but this is reversed in the Yucatán Peninsula, the southernmost population, where decreasing temperatures stimulate breeding.[1] In Massachusetts, horseshoe crabs spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn, with the males arriving first. The smaller male grabs on to the back of a female with a "boxing glove" like structure on his front claws, often holding on for months at a time. Often several males will hold on to a single female.[30] Females reach the beach at high tide.[30] After the female has laid a batch of eggs in a nest at a depth of 15–20 cm (6–8 in) in the sand, the male or males fertilize them with their sperm.[30] Egg quantity is dependent on the female's body size, and ranges from 15,000 to 64,000 eggs per female.[31]

"Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule" (Sturtevant).[full citation needed] The larvae form and then swim for about five to seven days. After swimming, they settle, and begin the first molt. This occurs about 20 days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. Before becoming sexually mature around age 9, they have to shed their shells some 17 times.[9] In the first 2–3 years of their life, the juveniles stay in shallow coastal waters near the breeding beaches.[4] Longevity is difficult to assess, but the average lifespan is thought to be 20–40 years.[32]

Research from the University of New Hampshire gives insight into the circadian rhythm of Atlantic horseshoe crabs. For example, several studies have looked into the effect of a circa tidal rhythm on the locomotion of this species. While it has been known for a while that a circadian clock system controls eye sensitivity, scientists discovered a separate clock system for locomotion.[33] When a sample of Atlantic horseshoe crabs were exposed to artificial tidal cycles in the lab, circa tidal rhythms were observed. The study found that light and dark cycles influence locomotion, but not as much as tidal activity.[34]


Horseshoe crabs were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. However, recent studies suggest a relationship between the eurypterids and the arachnids, leaving Xiphosura in the clade Prosomapoda.[35] They may have evolved in the shallow seas of the Paleozoic Era (541–252 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods.

The oldest known horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.[36]

Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years.[9] Forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, 400 million years ago. However, horseshoe crab preservation in the fossil record is extremely rare[37] and the Atlantic horseshoe crab itself has no fossil record at all. Until recently it was thought that the genus Limulus "ranges back only some 20 million years, not 200 million".[38]

The oldest member of the subfamily Limulinae are known from the Late Jurassic (Tithonian), belonging to the species Crenatolimulus darwini from Poland. The oldest and currently only other known species in the genus Limulus is Limulus coffini from the Late Cretaceous (Maastrichtian) of the United States.[39]

Medical uses[edit]

Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. An extract of blood cells (amoebocytes) from Limulus polyphemus is a critical component in the widely used Limulus amebocyte lysate (LAL) test to detect and quantify bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases.[7] A protein from horseshoe crab blood is also under investigation as a novel antibiotic.[30]

Procuring the raw materials for LAL testing involves collecting and bleeding horseshoe crabs from wild populations. Horseshoe crabs are returned to the ocean after bleeding, however, there is a level of mortality and sub-lethal impact involved. It is estimated that between 10 and 30 percent of horseshoe crabs die after bleeding.[40][41]

Studies show the blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound.[42] Animals that survive the process may be more lethargic when released and less likely to mate – which has prompted concerns about the longer-term impacts of harvesting horseshoe crabs.[43]

The LAL test is a major source of animal product dependence in the biomedical industry, and a challenge to the Three Rs of science in relation to the use of animals in testing.[44] There are efforts to reduce the dependence on horseshoe crabs, refine the process of collecting blood from the animals (including through aquaculture),[45] and even replace the use of animal-derived assays utilizing synthetic approaches, such as the recombinant factor C (rFC) assay.[46]

Conservation and management[edit]

Overall the Atlantic horseshoe crab is recognized as vulnerable by the IUCN due primarily to overharvesting and habitat loss. There are, however, significant geographic differences with some populations increasing, some stable and some declining.[1]

United States[edit]

Horseshoe crab harvest, Delaware, circa 1926. Millions of crabs were harvested for fertilizer around Delaware Bay. Some were fed to hogs.[47]
The loggerhead sea turtle has suffered from the reduction in numbers of Atlantic horseshoe crabs

Early in the 20th century and possibly before, there was the mistaken belief in some areas that horseshoe crabs were destructive to fisheries, folklore held that they used their long spines to drill into some shellfish. Because of this mistaken belief and folklore, bounties were sometimes offered by authorities for them. On Cape Cod in the early 20th century five cents was offered for every dead horseshoe crab turned in.

The Atlantic horseshoe crab is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for this animal's future. Since the 1970s, the horseshoe crab population has been decreasing in some areas, due to several factors, including the use of the crab as bait in eel, whelk and conch trapping. According to the Horseshoe Crab Benchmark Stock Assessment Peer Review Report published by the Atlantic States Marine Fisheries Commission (ASMFC), the population continues to remain stable where biomedical is present in the Northeast and thrive and grow in the Southeast due to protection efforts – a trend spanning decades.[48]

Red knots feeding on horseshoe crab eggs in Delaware

Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on the horseshoe crabs' eggs for food during their spring migration. Precipitous declines in the population of the red knots have been observed in recent years. Predators of horseshoe crabs, such as the currently threatened Atlantic loggerhead turtle, have also suffered as crab populations diminish.[49]

In 1991, the species was provided legislated protection from bait fishing in South Carolina by calling on the management and regulation of the horseshoe crab fisheries, allowing only hand-collecting for biomedical applications and marine biological research.[50] Without the need for LAL in biomedical use, the legal protection of the horseshoe crab is not guaranteed in the future, and they would again fall prey to overfishing and use as bait. In 1995, the nonprofit Ecological Research and Development Group (ERDG) was founded with the aim of preserving the four remaining species of horseshoe crab.[51] Since its inception, the ERDG has made significant contributions to horseshoe crab conservation. ERDG founder Glenn Gauvry designed a mesh bag for whelk/conch traps, to prevent other species from removing the bait. This has led to a decrease in the amount of bait needed by approximately 50%. In the state of Virginia, these mesh bags are mandatory in whelk/conch fishery. The Atlantic States Marine Fisheries Commission in 2006 considered several conservation options, among them being a two-year ban on harvesting the animals, affecting both Delaware and New Jersey shores of Delaware Bay.[52] In June 2007, Delaware Superior Court Judge Richard Stokes has allowed limited harvesting of 100,000 males. He ruled that while the crab population was seriously depleted by overharvesting through 1998, it has since stabilized, and that this limited take of males will not adversely affect either horseshoe crab or red knot populations. In opposition, Delaware environmental secretary John Hughes concluded that a decline in the red knot bird population was so significant that extreme measures were needed to ensure a supply of crab eggs when the birds arrived.[53][54] Harvesting of the crabs was banned in New Jersey on March 25, 2008.[55]

Every year, about 10% of the horseshoe crab breeding population dies when rough surf flips the creatures onto their backs, a position from which they often cannot right themselves. In response, the ERDG launched a "Just Flip 'Em" campaign, in the hopes that beachgoers will simply turn the crabs back over.[56] New Jersey beaches campaign "ReTURN the Favor," trains volunteers to rescue impinged and overturned horseshoe crabs while collecting data on natural and man-made hazards.[57]

A large-scale project to tag and count horseshoe crabs along the North American coast was started in 2008, termed Project Limulus.[9] Due to the lack of information and knowledge regarding horseshoe crab populations, the management policies lack any abundance of rules and regulations. To implement management policies for the species, more population information needs to be obtained.[58]


Since 1994, the populations in the Yucatán Peninsula have been recognized as endangered under Mexican law. They have declined since the 1960s and remaining significant Yucatán populations are mostly within protected areas.[4]


  1. ^ Peak absorption is at 525 nm


  1. ^ a b c d e f g h i j k l m n o p Smith, D.R.; Beekey, M.A.; Brockmann, H.J.; King, T.L.; Millard, M.J.; Zaldívar-Rae, J.A. (2016). "Limulus polyphemus". IUCN Red List of Threatened Species. 2016: e.T11987A80159830. doi:10.2305/IUCN.UK.2016-1.RLTS.T11987A80159830.en. Retrieved 19 November 2021.
  2. ^ "Limulus polyphemus. NatureServe Explorer 2.0". Retrieved 7 July 2023.
  3. ^ a b "Horseshoe crab spawning and red knot migration". Delaware Audubon. Archived from the original on 26 February 2017. Retrieved 16 January 2017.
  4. ^ a b c d e f g h i Zaldívar-Rae, J.; Sapién-Silva, R.E.; Rosales-Raya, M.; Brockmann, H. J. (2009). "American horseshoe crabs, Limulus polyphemus, in México: open possibilities". In Tanacredi, J.T.; Botton, M.L.; Smith, D.R. (eds.). Biology and Conservation of Horseshoe Crabs. Springer. pp. 97–113. ISBN 9780387899589.
  5. ^ a b "The Horseshoe Crab Natural History: Crab Species". Archived from the original on 12 February 2007. Retrieved 2007-03-01.
  6. ^ Krisfaulsi-Gannon, Jordan (2018). "The Role of Horseshoe Crabs in the Biomedical Industry and Recent Trends Impacting Species Sustainability". Frontiers in Marine Science. doi:10.3389/fmars.2018.00185.
  7. ^ a b c Heard, Willie (Fall 2001). "Coast" (PDF). Project Oceanography: 81–91. Archived from the original (PDF) on 19 February 2017. Retrieved 5 July 2008.
  8. ^ King, T.L.; Eackles, M.S.; Spidle, A.P.; Brockmann, H.J. (2005). "Regional differentiation and sex-biased dispersal among populations of the horseshoe crab Limulus polyphemus". Transactions of the American Fisheries Society. 134 (2): 441–465. doi:10.1577/T04-023.1.
  9. ^ a b c d e Angier, Natalie (June 10, 2008). "Tallying the Toll on an Elder of the Sea". The New York Times. ISSN 0362-4331. Archived from the original on September 29, 2013. Retrieved June 11, 2008.
  10. ^ a b Brad Floyd; Larry DeLancey (1 March 2012). "South Carolina Horseshoe Crab Fishery and Management Program Compliance Report for the Year 2011" (PDF). South Carolina Department of Natural Resources. Retrieved 26 June 2018.
  11. ^ "Horseshoe Crab (Limulus polyphemus)". WAZA. Archived from the original on 3 July 2017. Retrieved 26 June 2018.
  12. ^ Bruce Riska (1981). "Morphological Variation in the Horseshoe Crab Limulus polyphemus". Evolution. 35 (4): 647–658. doi:10.2307/2408237. JSTOR 2408237. PMID 28563146.
  13. ^ a b Koichi Sekiguchi; Carl N. Shuster, Jr (2009). "Limits on the Global Distribution of Horseshoe Crabs (Limulacea): Lessons Learned from Two Lifetimes of Observations: Asia and America". In Tanacredi, John T.; Botton, Mark L.; Smith, David (eds.). Biology and Conservation of Horseshoe Crabs. Springer. pp. 5–24. ISBN 978-0-387-89959-6.
  14. ^ Misty Edgecomb (June 21, 2002). "Horseshoe Crabs Remain Mysteries to Biologists". Bangor Daily News (Maine), repr. National Geographic News. p. 2.
  15. ^ Susan M. Wells; Robert M. Pyle; N. Mark Collins (1983). The IUCN Invertebrate Red Data Book. IUCN. p. 235.
  16. ^ a b Anatomy of the Horseshoe Crab, Maryland Department of Natural Resources. Retrieved 12 August 2008.
  17. ^ Elizabeth Quinn; Kristen Paradise & Jelle Atema (1998). "Juvenile Limulus polyphemus generate two water currents that contact one proven and one putative chemoreceptor organ". The Biological Bulletin. 195 (2): 185–187. doi:10.2307/1542829. JSTOR 1542829. PMID 28570188.
  18. ^ a b c d e Battelle, B.A. (December 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.
  19. ^ a b Harzsch, S.; Hafner, G. (2006). "Evolution of eye development in arthropods: phylogenetic aspects". Arthropod Structure & Development. 35 (4): 319–340. doi:10.1016/j.asd.2006.08.009. hdl:11858/00-001M-0000-0012-A87C-4. PMID 18089079.
  20. ^ "Photoreception" McGraw-Hill Encyclopedia of Science & Technology, vol. 13, p. 461 2007
  21. ^ a b c d Shuster, Carl N (2004). "Chapter 11: A blue blood: the circulatory system". In Shuster, Carl N Jr; Barlow, Robert B; Brockmann, H. Jane (eds.). The American horseshoe crab. Harvard University Press. pp. 276–277. ISBN 978-0-674-01159-5.
  22. ^ Mürer, EH; Levin, J; Holme, R (December 1975). "Isolation and studies of the granules of the amebocytes of Limulus polyphemus, the horseshoe crab". Journal of Cellular Physiology. 86 (3 Pt 1): 533–42. doi:10.1002/jcp.1040860310. PMID 413. S2CID 24546343.
  23. ^ "The Rabbit and the Horse Shoe Crab". 23 September 2014. Retrieved 7 March 2015.
  24. ^ Basudev Tripathy (2006). "In-House Research Seminar: The status of horseshoe crab in east coast of India". Wildlife Institute of India: 5. Archived from the original on 2011-07-21.
  25. ^ "Habitat Considerations". Retrieved 26 June 2018.
  26. ^ G. Ehlinger (25 July 2001). "Indian River Lagoon Species Inventory, Limulus polyphemus". Smithsonian Marine Station at Fort Pierce. Retrieved 27 June 2018.
  27. ^ "Horseshoe crabs in captivity". 8 October 2010. Retrieved 26 June 2018.
  28. ^ Kjær, Kurt H.; Winther Pedersen, Mikkel; De Sanctis, Bianca; De Cahsan, Binia; Korneliussen, Thorfinn S.; Michelsen, Christian S.; Sand, Karina K.; Jelavić, Stanislav; Ruter, Anthony H.; Schmidt, Astrid M. A.; Kjeldsen, Kristian K.; Tesakov, Alexey S.; Snowball, Ian; Gosse, John C.; Alsos, Inger G. (December 2022). "A 2-million-year-old ecosystem in Greenland uncovered by environmental DNA". Nature. 612 (7939): 283–291. doi:10.1038/s41586-022-05453-y. ISSN 1476-4687. PMC 9729109. PMID 36477129.
  29. ^ Pappas, Stephanie. "World's Oldest DNA Discovered, Revealing Ancient Arctic Forest Full of Mastodons". Scientific American. Retrieved 2022-12-08.
  30. ^ a b c d Sandy Huff (April 11, 2004), "Crab Watch", Tampa Tribune
  31. ^ Leschen, A. S.; et al. (2006). "Fecundity and spawning of the Atlantic horseshoe crab, Limulus polyphemus, in Pleasant Bay, Cape Cod, Massachusetts, USA". Marine Ecology. 27 (1): 54–65. Bibcode:2006MarEc..27...54L. doi:10.1111/j.1439-0485.2005.00053.x. hdl:1912/906.
  32. ^ "Horseshoe crab (Limulus polyphemus)". ARKive. Archived from the original on 2009-09-28. Retrieved March 28, 2010.
  33. ^ Chabot, CC; Watson, Win III (2010). "Circatidal rhythms of locomotion in the American horseshoe crab, Limulus polyphemus: Underlying mechanisms and cues that influence them". Current Zoology. 56 (5): 499–517. doi:10.1093/czoolo/56.5.499.
  34. ^ Chabot, CC; Watson, Win III (2008). "Artificial tides synchronize circatidal rhythms of locomotion in the American horseshoe crab, Limulus polyphemus". The Biological Bulletin. 215 (1): 24–25. doi:10.2307/25470681. JSTOR 25470681. PMID 18723635. S2CID 4934311.
  35. ^ Lamsdell, James C. (2012-12-18). "Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura". Zoological Journal of the Linnean Society. 167 (1): 1–27. doi:10.1111/j.1096-3642.2012.00874.x. ISSN 0024-4082.
  36. ^ "Ancient horseshoe crabs get even older". Fox News. January 30, 2008. Archived from the original on February 3, 2008. Retrieved January 31, 2008.
  37. ^ Kin, Adrian; Błażejowski, Błażej (2014-10-02). "The Horseshoe Crab of the Genus Limulus: Living Fossil or Stabilomorph?". PLOS ONE. 9 (10): e108036. Bibcode:2014PLoSO...9j8036K. doi:10.1371/journal.pone.0108036. ISSN 1932-6203. PMC 4183490. PMID 25275563.
  38. ^ Stephen Jay Gould (1989). Wonderful life: The Burgess Shale and the nature of history. New York: W. W. Norton & Co. pp. 43. ISBN 978-0-393-02705-1.
  39. ^ Bicknell, Russell D. C.; Błażejowski, Błażej; Wings, Oliver; Hitij, Tomaž; Botton, Mark L. (2021-03-02). Zhang, Xi‐Guang (ed.). "Critical re‐evaluation of Limulidae uncovers limited Limulus diversity". Papers in Palaeontology. 7 (3): 1525–1556. doi:10.1002/spp2.1352. ISSN 2056-2799. S2CID 233783546.
  40. ^ Leschen, A.S.; Correia, S.J. (March 2010). "Mortality in female horseshoe crabs (Limulus polyphemus) from biomedical bleeding and handling: implications for fisheries management". Marine and Freshwater Behaviour and Physiology. 43 (2): 135–147. doi:10.1080/10236241003786873. ISSN 1023-6244. S2CID 35152647.
  41. ^ Hurton, L.; Berkson, J. (April 2006). "Potential causes of mortality for horseshoe crabs (Limulus polyphemus) during the biomedical bleeding process". Fishery Bulletin. ISSN 0090-0656.
  42. ^ "Medical Uses". Ecological Research and Development Group. Archived from the original on 28 February 2008. Retrieved February 21, 2008.
  43. ^ Anderson, Rebecca L.; Watson, Winsor H.; Chabot, Christopher C. (December 2013). "Sublethal Behavioral and Physiological Effects of the Biomedical Bleeding Process on the American Horseshoe Crab, Limulus polyphemus". The Biological Bulletin. 225 (3): 137–151. doi:10.1086/BBLv225n3p137. ISSN 0006-3185. PMC 4079546. PMID 24445440.
  44. ^ Gorman, Richard (2020-09-30). "Atlantic Horseshoe Crabs and Endotoxin Testing: Perspectives on Alternatives, Sustainable Methods, and the 3Rs (Replacement, Reduction, and Refinement)". Frontiers in Marine Science. 7: 582132. doi:10.3389/fmars.2020.582132. ISSN 2296-7745. PMC 7612741. PMID 35591980.
  45. ^ Tinker-Kulberg, Rachel; Dellinger, Kristen; Brady, Terry E.; Robertson, Lee; Levy, Jerrold H.; Abood, Sarah K.; LaDuca, Frank M.; Kepley, Christopher L.; Dellinger, Anthony L. (2020-04-01). "Horseshoe Crab Aquaculture as a Sustainable Endotoxin Testing Source". Frontiers in Marine Science. 7: 153. doi:10.3389/fmars.2020.00153. ISSN 2296-7745.
  46. ^ Bolden, Jay; Smith, Kelly (2017). "Application of Recombinant Factor C Reagent for the Detection of Bacterial Endotoxins in Pharmaceutical Products". PDA Journal of Pharmaceutical Science and Technology. 71 (5): 405–412. doi:10.5731/pdajpst.2017.007849. ISSN 1079-7440. PMID 28733334. S2CID 28882050.
  47. ^ The Blood Harvest, The Atlantic magazine, February 2014
  48. ^ "2019 Horseshoe Crab Benchmark Stock Assessment Peer Review Report" (PDF).
  49. ^ Juliet Eilperin (June 10, 2005). "Horseshoe Crabs' Decline Further Imperils Shorebirds (subtitle: Mid-Atlantic States Searching for Ways to Reverse Trend)". The Washington Post. p. A03. Retrieved May 14, 2006.
  50. ^ "2012 South Carolina Code of Laws. Title 50 - Fish, Game and Watercraft. Chapter 5 - MARINE RESOURCES ACT. Section 50-5-1330 - Horseshoe crab permits, taking and handling requirements; penalty".
  51. ^ "About ERDG". ERDG. Retrieved 31 January 2015.
  52. ^ Molly Murray (May 5, 2006). "Seafood dealer wants to harvest horseshoe crabs (subtitle: Regulators look at 2-year ban on both sides of Delaware Bay)". The News Journal. pp. B1, B6.
  53. ^ "Horseshoe Crabs in Political Pinch Over Bird's Future / Creature is Favored Bait On Shores of Delaware; Red Knot Loses in Court". The Wall Street Journal. June 11, 2007. pp. A1, A10.
  54. ^ "Judge dumps horseshoe crab protection". The Charlotte Observer. Associated Press.
  55. ^ "NJ to ban horseshoe crabbing..." Philly Burbs.Com. Associated Press. Archived from the original on 2009-01-16.
  56. ^ 'Just Flip 'Em': An initiative to Protect Horseshoe Crabs Across Delmarva, WBOC, May 25, 2018, retrieved 2020-06-07
  57. ^ Tomlinson, Phillip (January 17, 2019) [2014], Delaware Bay's stranded horseshoe crabs saved by volunteer effort,, retrieved 2020-06-07
  58. ^ "Atlantic States Marine Fisheries Commission: Horseshoe Crab". Archived from the original on April 27, 2004. Retrieved June 30, 2009.


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