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
crab
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
s or
crustaceans: 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
The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...
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 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, Th ...
''.
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
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
of Ricinulei within Arachnida.
Taxonomy
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 group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
s. 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 fossils are found in strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
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 d ...
Xiphosura, and contains all four living species 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, Th ...
'', 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 Kore ...
'', 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, near ...
, Scotland, Early Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
(Tournaisian
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Ma. It is preceded by the Famennian (the uppermost stage ...
) (Considered Xiphosura ''incertae sedis'' by Lamsdell, 2020)
**†''Casterolimulus
''Casterolimulus'' is an extinct genus of xiphosuran. It is known from the Late Cretaceous Fox Hills Formation of North America, and it lived in freshwater environments. It is a member of the family Limulidae, and is placed as a close relative of ...
'' Holland, Erickson & O'Brien, 1975 Late Cretaceous ( Maastrichtian) Fox Hills Formation, North Dakota, USA (Inconsistently placed in this family)
** †''Heterolimulus
''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 ext ...
'' gadeai Vıa & Villalta, 1966 Alcover Limestone
Alcover is a municipality in the ''comarca'' of Alt Camp, Tarragona, Catalonia, Spain.
The Prades Mountains are located in the vicinity of this municipality.
It is the birthplace of the footballer Kiko Casilla
Francisco "Kiko" Casilla Cort� ...
Formation, Spain, Middle Triassic ( Ladinian)
** †'' Limulitella'' Størmer, 1952 Middle-Upper Triassic, France, Germany, Tunisia, Russia
** †'' Sloveniolimulus'' Bicknell et al., 2019 Strelovec Formation, Slovenia Middle Triassic ( Anisian)
** †'' 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, Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country c ...
'' Riek & Gill, 1971 Lower Cretaceous (Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
) Korumburra Group
Korumburra is a town in the Australian state of Victoria. It is located on the South Gippsland Highway, south-east of Melbourne, in the South Gippsland Shire local government area. At the Korumburra had an urban population of 3,639.
Surround ...
, 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 mudston ...
, 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 identica ...
'' Middle Triassic-Late Cretaceous England, Spain, Siberia, Germany, Morocco
**†'' Ostenolimulus'' Lamsdell et al. 2021 Early Jurassic ( Sinemurian) Moltrasio Limestone
Moltrasio ( lmo, label=Comasco, Moltras ) is a (municipality) in the Province of Como in the Italian region Lombardy, located about north of Milan and about north of Como, near the border with Switzerland, on the western shore of Lake Como.
...
, Italy
**†'' Volanalimulus'' Lamsdell, 2020 Early Triassic, Madagascar.
* Subfamily Limulinae Leach, 1819
** †'' Crenatolimulus'' Feldmann et al., 2011 Upper Jurassic (upper Tithonian
In the geological timescale, the Tithonian is the latest age of the Late Jurassic Epoch and the uppermost stage of the Upper Jurassic Series. It spans the time between 152.1 ± 4 Ma and 145.0 ± 4 Ma (million years ago). It is preceded by the K ...
) 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 i ...
, Poland. Lower Cretaceous ( Albian) Glen Rose Formation, Texas, USA
** '' Limulus'' O. F. Müller
O is the fifteenth letter of the modern Latin alphabet.
O may also refer to:
Letters
* Օ օ, (Unicode: U+0555, U+0585) a letter in the Armenian alphabet
* Ο ο, Omicron, (Greek), a letter in the Greek alphabet
* O (Cyrillic), a letter of the ...
, 1785 Pierre Shale
The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico.
The Pierre Shale was described by Meek and Hayden in 1862 in th ...
, United States, Late Cretaceous ( Maastrichtian), Atlantic North America, Recent
* Subfamily Tachypleinae Pocock, 1902
** ''Carcinoscorpius
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, Th ...
'' 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 ext ...
'' Leach, 1819 Upper Cretaceous (Cenomanian
The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...
) Haqel and Hjoula Konservat-Lagerstatten, Lebanon, Upper Eocene 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 ou ...
, plus a pair of median eyes that are able to detect both visible light and ultraviolet 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
A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...
( prosoma) 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, Malaysia 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 blood c ...
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 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
Limulus clotting factor overbar C (, ''factor C'', ''limulus factor C'') is an enzyme. This enzyme catalyses the following chemical reaction
: Selective cleavage of -Arg103-Ser- and -Ile124-Ile- bonds in limulus clotting factor B to form ''fact ...
(rFC), an enzyme found in LAL, as by using LAL itself. Jeak Ling Ding, a National University of Singapore
The National University of Singapore (NUS) is a national public research university in Singapore. Founded in 1905 as the Straits Settlements and Federated Malay States Government Medical School, NUS is the oldest autonomous university in the c ...
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 ...
. 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
Eli Lilly and Company is an American pharmaceutical company headquartered in Indianapolis, Indiana, with offices in 18 countries. Its products are sold in approximately 125 countries. The company was founded in 1876 by, and named after, Colonel ...
, 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 to "establish processes for evaluating alternative pyrogenicity tests and report back o the Senate
O, or o, is the fifteenth Letter (alphabet), letter and the fourth vowel letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in ...
on steps taken to increase their use" was released; PETA
Peta or PETA may refer to:
Acronym
* Pembela Tanah Air, a militia established by the occupying Japanese in Indonesia in 1943
* People for the Ethical Treatment of Animals, an American animal rights organization
* People Eating Tasty Animals, an ...
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 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
Conch () is a common name of a number of different medium-to-large-sized sea snails. Conch shells typically have a high spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point at both ends).
In North Am ...
. 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 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, and a permanent moratorium is in effect in South Carolina. The eggs are eaten in parts of Southeast Asia, Johor 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