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Horseshoe crabs are marine and
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...
water
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or
crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean g ...
: they are chelicerates, most closely related to arachnids, such as spiders 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 Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
and overharvesting. Tetrodotoxin may be present in one horseshoe crab species, ''
Carcinoscorpius rotundicauda The mangrove horseshoe crab (''Carcinoscorpius rotundicauda''), also known as the round-tailed horseshoe crab, is a chelicerate arthropod found in tropical marine and brackish waters in India, Bangladesh, Indonesia, Malaysia, Brunei, Singapore, T ...
''. Fossil records for horseshoe crabs extend back as far as 480 million years ago, with extant forms being living fossils. A 2019 molecular analysis places them as the sister group of
Ricinulei Ricinulei is a small order of arachnids. Like most arachnids, they are predatory, eating small arthropods. They occur today in west-central Africa ('' Ricinoides'') and the Neotropics ('' Cryptocellus'' and '' Pseudocellus'') as far north as Texa ...
within Arachnida.


Taxonomy

Horseshoe crabs resemble
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...
s but belong to a separate subphylum of the
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s,
Chelicerata The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...
. 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 Xiphosura () is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appeared in the Hirnantian (Late Ordovician). Current ...
. The enigmatic Chasmataspidids are also thought to be closely related to the horseshoe crabs. The earliest horseshoe crab
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s are found in strata from the Lower Ordovician period, roughly 480 million years ago. The Limulidae are the only recent family of the
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
Xiphosura, and contains all four living
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
of horseshoe crabs: * ''
Carcinoscorpius rotundicauda The mangrove horseshoe crab (''Carcinoscorpius rotundicauda''), also known as the round-tailed horseshoe crab, is a chelicerate arthropod found in tropical marine and brackish waters in India, Bangladesh, Indonesia, Malaysia, Brunei, Singapore, T ...
'', 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 ''Tachypleus tridentatus'', commonly known as the Chinese horseshoe crab, Japanese horseshoe crab, or tri-spine horseshoe crab, is a species of horseshoe crab found in Southeast and East Asia, with records from China, Indonesia, Japan, South Kor ...
'', 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'' ** †'' Albalimulus'' Bicknell & Pates, 2019
Ballagan Formation The Ballagan Formation is a geologic formation in Scotland and England. It preserves fossils dating back to the early part of the Carboniferous period (Tournaisian – early Visean). Its name comes from the "Ballagan Beds" of Ballagan Glen, nea ...
, Scotland, Early Carboniferous ( Tournaisian) (Considered Xiphosura ''incertae sedis'' by Lamsdell, 2020) **†'' Casterolimulus'' Holland, Erickson & O'Brien, 1975 Late Cretaceous ( Maastrichtian) Fox Hills Formation, North Dakota, USA (Inconsistently placed in this family) ** †'' Heterolimulus'' gadeai Vıa & Villalta, 1966 Alcover Limestone Formation, Spain,
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma an ...
(
Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic) ...
) ** †'' Limulitella'' Størmer, 1952 Middle-Upper Triassic, France, Germany, Tunisia, Russia ** †'' Sloveniolimulus'' Bicknell et al., 2019 Strelovec Formation, Slovenia Middle Triassic (
Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic ...
) ** †'' Tarracolimulus'' Romero & Vıa Boada, 1977 Alcover Limestone Formation, Spain, Middle Triassic (Ladinian) ** †''
Victalimulus ''Victalimulus'' is an extinct genus of horseshoe crab, from Early Cretaceous (Aptian) Koonwarra Fossil Beds in eastern Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, ...
'' Riek & Gill, 1971
Lower Cretaceous Lower may refer to: * Lower (surname) * Lower Township, New Jersey *Lower Receiver (firearms) * Lower Wick Gloucestershire, England See also *Nizhny Nizhny (russian: Ни́жний; masculine), Nizhnyaya (; feminine), or Nizhneye (russian: Н� ...
( Aptian) Korumburra Group, NSW, Australia ** †'' Yunnanolimulus'' Zhang et al., 2009 Middle Triassic (Anisian),
Guanling Formation The Guanling Formation is a Middle Triassic (Anisian or Pelsonian in the regional chronostratigraphy) geologic formation in southwestern China. Description The formation encompasses two members. The first member is primarily calcareous mudst ...
, Yunnan, China ** †''
Mesolimulus ''Mesolimulus'' is an extinct genus of horseshoe crab. The best known examples are found in Solnhofen limestone near Solnhofen, Bavaria, Germany. Originally assigned to the living genus '' Limulus'', they are related to and look virtually ident ...
'' Middle Triassic-Late Cretaceous England, Spain, Siberia, Germany, Morocco **†'' Ostenolimulus'' Lamsdell et al. 2021 Early Jurassic ( Sinemurian) Moltrasio Limestone, Italy **†'' Volanalimulus'' Lamsdell, 2020 Early Triassic, Madagascar. * Subfamily Limulinae Leach, 1819 ** †'' Crenatolimulus'' Feldmann et al., 2011 Upper Jurassic (upper Tithonian)
Kcynia Formation Kcynia (german: Exin) is a town in Nakło County, Kuyavian-Pomeranian Voivodeship, Poland, with 4,712 inhabitants (2004). It is located in the Pałuki ethnographic region in the northern part of historic Greater Poland. History Kcynia started ...
, Poland. Lower Cretaceous ( Albian) Glen Rose Formation, Texas, USA ** '' Limulus'' O. F. Müller, 1785 Pierre Shale,
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
, Late Cretaceous ( Maastrichtian), Atlantic North America, Recent * Subfamily Tachypleinae Pocock, 1902 ** '' Carcinoscorpius'' Pocock, 1902, Asia, Recent ** ''
Tachypleus ''Tachypleus'' is a genus of south, southeast and east Asian horseshoe crabs in the family Limulidae. Species There are two extant (living) species: * '' Tachypleus gigas'' ( Müller, 1785) * '' Tachypleus tridentatus'' ( Leach, 1819) And two ...
'' Leach, 1819 Upper Cretaceous ( Cenomanian) Haqel and Hjoula Konservat-Lagerstatten, Lebanon, Upper
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...
Domsen Sands, Germany, Asia, Recent Cladogram after Lasmdell 2020.


Anatomy and behavior

The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000
ommatidia The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The ...
, plus a pair of median eyes that are able to detect both visible light and
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
light, a single parietal eye, and a pair of rudimentary lateral eyes on the top. The latter become functional just before the embryo hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. Having relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans, and their eyes are a million times more sensitive to light at night than during the day. They use their chelicerae—a pair of small appendages—for moving food into the mouth. The next five pairs of appendages, the first of which are the pedipalps, are used for locomotion (ambulatory legs). The mouth is located in the center of the legs, whose bases are referred to as gnathobases, and have the same function as jaws and help grind up food. In extant species their appendages are uniramous, but the fossil genus '' Dibasterium'' had four pairs of branched walking legs. The pedipalps on a male change shape on their terminal molt, becoming boxing glove-like claspers that are used for grasping the female during mating. The last pair of legs for both male and female are the main legs used for pushing when walking on the ocean floor. The remaining leg pairs have a weak claw at the tip. Lost legs or the telson (tail) may slowly regenerate, and cracks in the body shell can heal. Behind its legs, the horseshoe crab has
book gills A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and conn ...
, which exchange respiratory gases, and are also occasionally used for swimming. As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. Horseshoe crabs are more often 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 about 20–30% larger than males. The smallest species is ''C. rotundicauda'' and the largest is ''T. tridentatus''. On average, males of ''C. rotundicauda'' are about long, including a tail ( telson) that is about , and their carapace (
prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
) is about wide. Some southern populations (in the Yucatán Peninsula) of ''L. polyphemus'' are somewhat smaller, but otherwise this species is larger. In the largest species, ''T. tridentatus'', females can reach as much as long, including their tail, and up to in weight. This is only about longer than the largest females of ''L. polyphemus'' and ''T. gigas'', but roughly twice the weight. The juveniles grow about 33% larger with every molt until reaching adult size. Atlantic horseshoe crabs molt in late July.


Breeding

During the breeding season (spring and summer in the Northeast U.S.; year-round in warmer locations), horseshoe crabs migrate to shallow coastal waters. The smaller male horseshoe crab clings to the back of the larger female using specialized front claws and fertilizes the eggs as they are laid in the sand. Additional males called "satellite males" which are not attached to the female may surround the pair and have some success in fertilizing eggs. Young female horseshoe crabs can be identified by the lack of mating scars. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. In ''L. polyphemus'', the eggs take about two weeks to hatch; shore birds eat many of them before they hatch. The larvae molt six times during the first year and annually after the first 3 or 4 years. Natural breeding of horseshoe crabs in captivity has proven to be difficult. Some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. It is not known with certainty what is in the sand that the crabs can sense or how they sense it. Artificial insemination and induced spawning have been done on a relatively large scale in captivity, and eggs and juveniles collected from the wild are often raised to adulthood in captivity. In order to preserve and ensure the continuous supply of the horseshoe crab, a breeding centre was built in
Johor Johor (; ), also spelled as Johore, is a state of Malaysia in the south of the Malay Peninsula. Johor has land borders with the Malaysian states of Pahang to the north and Malacca and Negeri Sembilan to the northwest. Johor shares mariti ...
,
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federal constitutional monarchy consists of thirteen states and three federal territories, separated by the South China Sea into two regions: Peninsular Malaysia and Borneo's East Mal ...
where the crabs are bred and released back into the ocean in thousands once every two years. It is estimated to take around 12 years before they are suitable for consumption.


Relationships with humans


Blood harvesting

Horseshoe crabs use hemocyanin to carry oxygen through their blood. Because of the copper present in hemocyanin, their blood is blue. Their blood contains
amebocyte An amebocyte or amoebocyte () is a mobile cell (moving like an amoeba) in the body of invertebrates including cnidaria, echinoderms, molluscs, tunicates, sponges and some chelicerates. They move by pseudopodia. Similarly to some of the white bloo ...
s, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of ''L. polyphemus'' are used to make ''Limulus'' amebocyte lysate (LAL), which is used for the detection of bacterial endotoxins in medical applications. There is a high demand for the blood, the harvest of which 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 3–15% to 10–30%. Approximately 500,000 ''Limulus'' are harvested annually for this purpose. Bleeding may also prevent female horseshoe crabs from being able to
spawn Spawn or spawning may refer to: * Spawn (biology), the eggs and sperm of aquatic animals Arts, entertainment, and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** '' Spawn: A ...
or decrease the number of eggs they are able to lay. Up to 30% of an individual's blood is removed, according to the biomedical industry, and the horseshoe crabs spend between one and three days away from the ocean before being returned. As long as the gills stay moist, they can survive on land for four days. Some scientists are skeptical that certain companies return their horseshoe crabs to the ocean at all, instead suspecting them of selling the horseshoe crabs as fishing bait. The harvesting of horseshoe crab blood in the pharmaceutical industry is in decline. In 1986, Kyushu University researchers discovered that the same test could be achieved by using isolated Limulus clotting factor C (rFC), an enzyme found in LAL, as by using LAL itself. Jeak Ling Ding, a National University of Singapore researcher, patented a process for manufacturing rFC; on 8 May 2003, synthetic isolated rFC made via her patented process became available for the first time. Industry at first took little interest in the new product, however, as it was patent-encumbered, not yet approved by regulators, and sold by a single manufacturer,
Lonza Group Lonza Group is a Swiss multinational manufacturing company for the pharmaceutical, biotechnology and nutrition sectors, headquartered in Basel, with major facilities in Europe, North America and South Asia. Lonza was established under that name ...
. In 2013, however, Hyglos GmbH also began manufacturing its own rFC product. This, combined with the acceptance of rFC by European regulators, the comparable cost between LAL and rFC, and support from Eli Lilly and Company, which has committed to use rFC in lieu of LAL, is projected to all but end the practice of blood harvesting from horseshoe crabs. In December 2019, a report of the US Senate which encouraged the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
to "establish processes for evaluating alternative pyrogenicity tests and report back o the Senateon steps taken to increase their use" was released; PETA backed the report. In June 2020, it was reported that U.S. Pharmacopeia had declined to give rFC equal standing with horseshoe crab blood. Without the approval for the classification as an industry standard testing material, U.S. companies will have to overcome the scrutiny of showing that rFC is safe and effective for their desired uses, which may serve as a deterrent for usage of the horseshoe crab blood substitute. Vaccine research and development during the
COVID-19 pandemic The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing global pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The novel virus was first identi ...
has added additional "strain on the American horseshoe crab."


Fishery

Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk, or conch. Nearly 1 million (1,000,000) crabs a year are harvested for bait in the United States, dwarfing the biomedical mortality. However, fishing with horseshoe crab was banned indefinitely in
New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delawa ...
in 2008 with a moratorium on harvesting to protect the red knot, a shorebird which eats the crab's eggs. A moratorium was restricted to male crabs in
Delaware Delaware ( ) is a state in the Mid-Atlantic region of the United States, bordering Maryland to its south and west; Pennsylvania to its north; and New Jersey and the Atlantic Ocean to its east. The state takes its name from the adjacent ...
, and a permanent moratorium is in effect in
South Carolina )''Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = " Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = ...
. The eggs are eaten in parts of Southeast Asia,
Johor Johor (; ), also spelled as Johore, is a state of Malaysia in the south of the Malay Peninsula. Johor has land borders with the Malaysian states of Pahang to the north and Malacca and Negeri Sembilan to the northwest. Johor shares mariti ...
and China. A low horseshoe crab population 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.


Culinary use

The population of T. gigas in Indonesia and Malaysia has decreased dramatically in the past decade. The harvesting of T. gigas is largely used to supply Thailand with primarily female T. gigas, which is considered a local delicacy. This female biased harvesting has led to an unbalanced sex ratio in the wild, which also contributes to its declining population in the area.


Conservation status

Development along shorelines is dangerous to horseshoe crab spawning, limiting available space and degrading habitat. Bulkheads can block access to intertidal spawning regions as well. Because of the destruction of habitat and shoreline development, use in fishing, status as a culinary delicacy in some areas, and use for scientific research and advancements, the horseshoe crab is facing down endangered and extinct statuses. One species, ''T. tridentatus'', has already been declared extinct in one area of Taiwan. Facing a greater than 90% population decrease in ''T. tridentatus'' juveniles, it is suspected that Hong Kong will be the next to declare the horseshoe crab extinct in its area. The species is listed as endangered on the IUCN Red List, specifically because of overexploitation and loss of critical habitat leading to a steep decline in population size.


References


Further reading

*


External links

*
LAL Update

Science Friday Video: horseshoe crab season

Horseshoe crab at the Smithsonian Ocean Portal


* ttp://redknot.org/ RedKnot.orglinks to shorebird recovery sites, movies, events & other info on Red Knot rufa & horseshoe crabs.
Crab Bleeders
Article about the men who bleed horseshoe crabs for science.
Day time mating of horseshoe crabs in Maine

Sarah Zhang, The Last Days of the Blue-Blood Harvest, The Atlantic, May 9, 2018
{{Taxonbar, from=Q1329239 Extant Ordovician first appearances Xiphosura