Limulidae Temporal range: | |
---|---|
Tachypleus gigas , one of the four extant species | |
Jurassic-aged limulids. Crenatolimulus (A,B), Limulus (C), Mesolimulus (D,E) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Subphylum: | Chelicerata |
Order: | Xiphosura |
Superfamily: | Limuloidea |
Family: | Limulidae Leach, 1819 [1] [2] |
Genera | |
See text |
Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. [2] [3] Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachnids such as spiders, ticks, and scorpions. [4] [5] [6]
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. [7] They are eaten in some parts of Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate, which is used for the detection and quantification of bacterial endotoxins). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. [3] Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda . [8]
The fossil record of Xiphosura goes back over 440 million years to the Ordovician period, with the oldest representatives of the modern family Limulidae dating to approximately 250 million years ago during the Early Triassic. As such, the extant forms have been described as "living fossils". [9] The last common ancestor of the four present species is estimated to have lived about 135 million years ago in the Cretaceous. [10] Some molecular analyses have placed Xiphosura within Arachnida, with a 2019 molecular analysis placing them as the sister group of Ricinulei. [11]
The family name Limulidae comes from the genus Limulus, from the word limulus in Latin meaning "askance", [12] or "a little askew". [13]
Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata. [14] 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. [14] [15] Other studies have placed eurypterids closer to the arachnids in a group called Merostomata. [16] The enigmatic Chasmataspidids are also thought to be closely related to the horseshoe crabs. [17] The earliest horseshoe crab fossils are found in strata from the Lower Ordovician period, roughly 480 million years ago. [18]
The Limulidae are the only recent family of the order Xiphosura, and contains all four living species of horseshoe crabs: [1] [3] [19]
After Bicknell et al. 2021 and Lamsdell et al. 2020 [20] [21]
Cladogram after Lasmdell 2020. [21]
Limulidae |
| ||||||||||||||||||
The entire body of the horseshoe crab is protected by a carapace, which like the rest of the exoskeleton is unmineralized. [24] It has two compound lateral eyes, each composed of about 1,000 ommatidia, 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 becomes 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', [25] [26] and their eyes are a million times more sensitive to light at night than during the day. [27]
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. [28] In extant species and other fossil xiphosurans (e.g. Alanops [29] ) their appendages are uniramous, but in other related non-xiphosurid euchelicerates such as Offacolus and Dibasterium the appendages are biramous. [30] 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 males and females are the main legs used for pushing when walking on the ocean floor. The remaining leg pairs have a weak claw at the tip. [31] Lost legs or the telson (tail) may slowly regenerate, and cracks in the body shell can heal. [32]
External videos | |
---|---|
Rendezvous with a Horseshoe Crab, August 2011, 4:34, NewsWorks | |
The Horseshoe Crab Spawn, June 2010, 5:08, HostOurCoast.com | |
Horseshoe Crabs Mate in Massive Beach "Orgy", June 2014, 3:29, National Geographic |
Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, and are also occasionally used for swimming. [33] 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.
Females are about 20–30% larger than males. [39] The smallest species is C. rotundicauda and the largest is T. tridentatus. [40] On average, males of C. rotundicauda are about 30 centimetres (12 inches) long, including a tail (telson) that is about 15 cm (6 in), and their carapace (prosoma) is about 15 cm (6 in) wide. [41] Some southern populations (in the Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise this species is larger. [39]
In the largest species, T. tridentatus, females can reach as much as 79.5 cm (31+1⁄4 in) long, including their tail, and up to 4 kg (9 lb) in weight. [42] This is only about 10–20 cm (4–8 in) longer than the largest females of L. polyphemus and T. gigas, but roughly twice the weight. [43] [44]
The juveniles grow about 33% larger with every molt until reaching adult size. [45] Atlantic horseshoe crabs molt in late July.
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. [46] Foraging usually takes place at night. Gravel and sand particles are ingested to further grind up food in a gizzard before passing it into the stomach. [47] [48]
During the breeding season (spring and summer in the Northeast U.S.; year-round in warmer locations or when the full moon rises), [49] horseshoe crabs migrate to shallow coastal waters. The smaller male horseshoe crab clings to the back or opisthosoma 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. [50] Young female horseshoe crabs can be identified by the lack of mating scars. [51]
The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. The eggs may be inseminated within 20 to 30 minutes. [49] 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. [52] [53]
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. [54] 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. [55] [56]
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. [57]
Horseshoe crabs use hemocyanin to carry oxygen through their blood. [58] Their blood contains amebocytes, 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. [59] 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. [60] Estimates of mortality rates following blood harvesting vary from 3–15% [61] [62] to 10–30%. [63] [64] [65] Approximately 500,000 Limulus are harvested annually for this purpose. [66] Declining horseshoe crab populations on the East Coast of the United States endanger certain bird species which feed upon their eggs. [62]
Bleeding may also prevent female horseshoe crabs from being able to spawn [62] 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, although NPR reported that it "can deplete them of more than half their volume of blue blood." [62] 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. [67] 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. [68]
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. [69] 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. [70] 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. 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, [62] is projected to all but end the practice of blood harvesting from horseshoe crabs. [71]
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 [to the Senate] on steps taken to increase their use" was released; [72] PETA backed the report. [73]
In June 2020, it was reported that U.S. Pharmacopeia had declined to give rFC equal standing with horseshoe crab blood. [74] 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. [75]
Vaccine research and development during the COVID-19 pandemic has added additional "strain on the American horseshoe crab." [76]
In 2023, the U.S. Fish and Wildlife Service halted the harvesting of horseshoe crabs in the Cape Romain National Wildlife Refuge, South Carolina, from March 15 to July 15 to aid their reproduction. This decision was influenced by the importance of horseshoe crab eggs as a food source for migratory birds and the ongoing use of horseshoe crabs for bait and their blood in medical products. The ban supports the conservation goals of the refuge, spanning 66,000 acres (26,700 hectares) of marshes, beaches, and islands near Charleston. [77]
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 in 2008 with a moratorium on harvesting to protect the red knot, a shorebird which eats the crab's eggs. [78] A moratorium was restricted to male crabs in Delaware, and a permanent moratorium is in effect in South Carolina. [79] The eggs are eaten in parts of Southeast Asia, Johor and China. [80]
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. [81] An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds. [82]
The population of T. gigas in Indonesia and Malaysia has decreased dramatically in the past decade.[ when? ] The harvesting of T. gigas is largely used to supply Thailand with primarily female T. gigas, which is considered a local delicacy. This female harvesting has led to an unbalanced sex ratio in the wild, which also contributes to its declining population in the area. [83]
Horseshoe crabs are the sister group to ricinuleids, and are within the family Arachnida, classifying them as aquatic arachnids. Their aquatic nature is the result of a secondary shift to marine life, following the initial shift to land by the common ancestor of all arachnids. [84]
The radiation of horseshoe crabs occurred rapidly and resulted in 88 [85] total lineages, of which only 4 remain. The Atlantic species is sister to the three Asian species, the latter of which are likely the result of two divergences relatively close in time. [86]
Now-extinct horseshoe crabs traveled to land at least five times throughout history, two of which resulted in the formation of a major clade. These clades, Belinurina and Austrolimulidae both inhabited marginally marine environments, such as swamps and rivers, which are both distinct from the habitats of the extant species, which did not undergo the same transition to land. [87]
As generalists, horseshoe crabs can have a broad diet and live in diverse habitats, meaning they are more likely to survive and produce viable offspring in more places. Horseshoe crabs also have an incredibly efficient immune system, and can also be successful in areas with high concentrations of bacteria. Amoebocytes in the bloodstream attack bacterial cells, and in doing so act as coagulants around the foreign body, preventing them from multiplying. This trait is an adaptation to their often bacteria-rich environment. Their ability to succeed in many environments limits selective forces, as there are few, if any, mutations that would result in more beneficial alleles that would make horseshoe crabs more suited for survival.
In the Atlantic horseshoe crab, microRNA’s exist on 7 loci, comparatively high to the 2 loci in spiders and scorpions, meaning horseshoe crabs have comparatively high rates of gene regulation, which could contribute to their morphological status. Additionally, several other gene clusters are present in at least 6, and often 7 loci. [88]
The common ancestor to Arachnids underwent a whole genome duplication event, which was likely followed by another duplication event in the common ancestor to the 4 extant species of horseshoe crabs about 135 million years ago, and evidence points to two additional events having occurred since then. However, many of these ancient genes have likely undergone either neo-functionalization or sub-functionalization as a result of functional divergence, meaning their expression is not the same as it was following the WGD event, as seen in other chelicerate lineages. [89]
Several hypotheses have been proposed as possible mechanisms for the size difference between male and female horseshoe crabs. The sexual size dimorphism of horseshoe crabs that results in a larger average size in females than males is likely a result of the amalgamation of many different aspects of these hypotheses and more:
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. [92]
Because of the destruction of habitat and shoreline development, use in fishing, plastic pollution, 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 extirpated from 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 extirpated from 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. [83]
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.
Amplexus is a type of mating behavior exhibited by some externally fertilizing species in which a male grasps a female with his front legs as part of the mating process, and at the same time or with some time delay, he fertilizes the eggs, as they are released from the female's body. In amphibians, females may be grasped by the head, waist, or armpits, and the type of amplexus is characteristic of some taxonomic groups.
The Atlantic horseshoe crab, also known as the American horseshoe crab, is a species of horseshoe crab, a kind of marine and brackish chelicerate arthropod. It is found in the Gulf of Mexico and along the Atlantic coast of North America. The main area of annual migration is Delaware Bay along the South Jersey Delaware Bayshore.
Xiphosura is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs. They first appeared in the Hirnantian. Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.
Limulus is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab. One fossil species is currently assigned to the genus though several other species have been named, which have since been assigned to other genera.
Limulus amebocyte lysate (LAL) is an aqueous extract of motile blood cells (amebocytes) from the Atlantic horseshoe crab Limulus polyphemus. LAL reacts with bacterial endotoxins such as lipopolysaccharides (LPS), which are components of the bacterial capsule, the outermost membrane of cell envelope of gram-negative bacteria. This reaction is the basis of the LAL test, which is widely used for the detection and quantification of bacterial endotoxins.
The mangrove horseshoe crab, also known as the round-tailed horseshoe crab, is a species of horseshoe crab, a chelicerate arthropod found in tropical marine and brackish waters of India, Bangladesh, and Southeast Asia. It may also occur in Sri Lanka, Myanmar and the Philippines, but confirmed records are lacking. It is the only species in the genus Carcinoscorpius.
Tachypleus is a genus of south, southeast and east Asian horseshoe crabs in the family Limulidae.
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 Korea, Malaysia, the Philippines, Taiwan, and Vietnam. It is found in coastal marine and brackish waters, and tolerates colder temperatures than the other Asian horseshoe crabs, although juveniles still need water warmer than 22 °C (72 °F) to moult.
An amebocyte or amoebocyte is a motile cell in the bodies of invertebrates including cnidaria, echinoderms, molluscs, tunicates, sponges, and some chelicerates.
Bunodes is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Bunodes was regarded as part of the clade Planaterga. Fossils of the single and type species, B. lunula, have been discovered in deposits of the Silurian period in Ludlow, England. Bunodes is the type genus of the family Bunodidae, the other genera of the same family being Limuloides. There are 64 direct children of Bunodes.
Legrandella is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Legrandella was regarded as part of the clade Prosomapoda. Fossils of the single and type species, L. lombardii, have been discovered in deposits of the Devonian period in Cochabamba, Bolivia.
Pseudoniscus is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Pseudoniscus was regarded as part of the clade Planaterga. Fossils of the genus have been discovered in deposits of the Silurian period in the United Kingdom, the United States and Estonia. Pseudoniscus is one of the two members of the family Pseudoniscidae, the other being Cyamocephalus.
Weinbergina is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Fossils of the single and type species, W. opitzi, have been discovered in deposits of the Devonian period in the Hunsrück Slate, Germany.
Synziphosurina is a paraphyletic group of chelicerate arthropods previously thought to be basal horseshoe crabs (Xiphosura). It was later identified as a grade composed of various basal euchelicerates, eventually excluded form the monophyletic Xiphosura sensu stricto and only regarded as horseshoe crabs under a broader sense. Synziphosurines survived at least since early Ordovician to early Carboniferous in ages, with most species are known from the in-between Silurian strata.
Tachypleus gigas, commonly known as the Indo-Pacific horseshoe crab, Indonesian horseshoe crab, Indian horseshoe crab, or southern horseshoe crab, is one of the four extant (living) species of horseshoe crab. It is found in coastal water in South and Southeast Asia at depths to 40 m (130 ft).
Coagulin is a gel-forming protein of hemolymph that hinders the spread of bacterial and fungal invaders by immobilizing them. It is produced in the coagulogen form before being cleaved into the active form through a serine proteinase cascade. It has been most extensively studied in horseshoe crabs. It has also been produced by other organisms, such as Bacillus coagulans I4 in a plasmid location. In human medicine, coagulation of coagulin is the basis of detection of bacterial endotoxin through the Limulus amebocyte lysate test for parenteral medications.
Limulus clotting factor B is an enzyme. This enzyme catalyses the following chemical reaction
Offacolus is an extinct genus of euchelicerate, a group of chelicerate arthropods. Its only species, O. kingi, has been found in deposits from the Silurian period in the Wenlock Series Lagerstätte of Herefordshire, England. It is the only member of the monotypic family Offacolidae, and classified as a basal ("primitive") genus in the clade Euchelicerata, along with Dibasterium, the closely related Setapedites and Prosomapoda. The genus is named after Offa, a king from the ancient kingdom of Mercia, and colus, a person who dwelled among the Offa's Dyke. The species name honors Robert Joseph King, a British mineralogist who found the fossils of Offacolus.
Camanchia is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. Camanchia was regarded as part of the clade Prosomapoda. Fossils of the single and type species, C. grovensis, have been discovered in deposits of the Silurian period in Iowa, in the United States. Alongside Venustulus, Camanchia is one of the only Silurian synziphosurine with fossil showing evidence of appendages.