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Horseshoe crab

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Horseshoe crabsarearthropodsof the familyLimulidaeand are the only survivingxiphosurans.Despite their name, they are not truecrabsor evencrustaceans.Rather, they arechelicerates.This makes them more closely related toarachnidslikespiders,ticks,andscorpions.The body of a horseshoe crab is divided into three main parts: thecephalothorax,abdomen,andtelson.The largest of these, the cephalothorax, houses most of the animal'seyes,limbs, andinternal organs.It is also where the animal gets its name, as its shape somewhat resembles that of ahorseshoe.Horseshoe crabs have changed little in appearance since they first evolved in theTriassic,earning them the title of "living fossil".

Limulidae
Temporal range:Early Triassic–Present
Tachypleus gigas,one of the four extant species
Jurassic-aged limulids.Crenatolimulus(A,B),Limulus(C),Mesolimulus(D,E)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: Xiphosura
Superfamily: Limuloidea
Family: Limulidae
Leach,1819[1][2]
Genera

See text

Only four species of horseshoe crab are extant today. Most aremarine,though themangrove horseshoe crabis often found in brackish water. Additionally, certain extinct species transitioned to living infreshwater.Horseshoe crabs primarily live at thewater's bottombut they can swim if needed. In the modern day, their distribution is limited, only found along the east coasts ofNorth AmericaandSouth Asia.

Horseshoe crabs are often caught for theirblood,which containsLimulus amebocyte lysate,a chemical used to detect bacterialendotoxins.Additionally, the animals are used as fishing bait in theUnited Statesand eaten as a delicacy in some parts ofAsia.In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastalhabitat destructionand overharvesting. To ensure their continuous existence, many areas have enacted regulations on harvesting and established captive breeding programs.

External videos
video iconRendezvous with a Horseshoe Crab,August 2011, 4:34,NewsWorks
video iconThe Horseshoe Crab Spawn,June 2010, 5:08, HostOurCoast
video iconHorseshoe Crabs Mate in Massive Beach "Orgy",June 2014, 3:29, National Geographic

Phylogeny and evolution

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The fossil record ofxiphosuransextends back to theLate Ordovician,or around 445 million years ago.[3]For modern horseshoe crabs, their earliest appearance was approximately 250 million years ago during theEarly Triassic.[4]Because they have seen little morphological change since then, extant (surviving) forms have been described as "living fossils".[5]

Horseshoe crabs resemblecrustaceansbut belong to a separate subphylum of thearthropods,Chelicerata.Horseshoe crabs are closely related to the extincteurypterids(sea scorpions), which include some of the largest arthropods to have ever existed, and the two may besister groups.[6]The difficult-to-classifychasmataspididsare also thought to be closely related to horseshoe crabs.[6][7]

Theradiationof horseshoe crabs resulted in 22 known species, of which only 4 remain.[8]TheAtlanticspecies is sister to the three Asian species, the latter of which are likely the result of two divergences relatively close in time.[9]The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in theCretaceous.[10]

Limulidae is the only extantfamilyof theorderXiphosura,and contains all four livingspeciesof horseshoe crabs:[11][12]

  • Carcinoscorpius rotundicauda,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,the Chinese, Japanese, or tri-spine horseshoe crab, found in Southeast and East Asia

Genera

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After Bicknell et al. 2021 and Lamsdell et al. 2020[13][14]

Phylogeny

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The horseshoe crab's position withinCheliceratais complicated. However, most morphological analyses have placed them outside theArachnida.[17][18][19][6][7]This assumption was challenged when a genetics-basedphylogenyfound horseshoe crabs to be the sister group to thericinuleids,thereby making them anarachnid.[20]In response, a more recent paper has again placed horseshoe crabs as separate from the arachnids. This new study utilized both new and more completesequencing datawhile also sampling a larger number oftaxa.[21]

Below is acladogramshowing the internal relationships of Limulidae (modern horseshoe crabs) based on morphology. It contains both extant and extinct members.[14]

Limulidae

Whole genome duplication

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Thecommon ancestorofarachnidsandxiphosurans(the group that includes horseshoe crabs) underwent awhole-genome duplication(WGD) event.[22][23]This was followed by at least two, possibly three, WGDs in a common ancestor of the living horseshoe crabs.[23]This gives them unusually largegenomesforinvertebrates(the genomes ofC. rotundicaudaandT. tridentatusbeing approximately 1.72 Gb each).[23]Evidence for the duplication events includes similarity in structure between chromosomes (synteny), and clustering ofhomeobox genes.Over time, many of the duplicatedgeneshave changed through processes ofneofunctionalizationorsubfunctionalization,meaning their functions are different from what they originally were.[23]

Evolution of sexual size dimorphism

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Severalhypotheseshave been given as possible reasons why a size difference exists between male and female horseshoe crabs[24]This phenomenon is known assexual size dimorphismand results in the females having a larger average size than males.[24]The existence of this trend is likely due to a combination of two things:

  1. First, females take a year longer to mature and undergo an additional molt, giving them a larger average body size.[24]
  2. Second, larger female horseshoe crabs can house more eggs within their bodies. This lets them pass on more genetic material than smaller females during each mating cycle, making larger females more prevalent.[24]

Anatomy and physiology

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General body plan

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Generalized dorsal anatomy of a horseshoe crab

Like allarthropods,horseshoe crabs have segmented bodies with jointed limbs, which are covered by a protectivecuticlemade ofchitin.They have heads composed of several segments, which eventually fuse as an embryo.[25]: 518–522 

Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts (tagma): thecephalothoraxand theopisthosoma.The first tagma, the cephalothorax orprosoma,is a fusion of theheadandthorax.[25]: 555 This tagma is also covered by a large, semicircular,carapacethat acts like a shield around the animal's body. It is shaped like the hoof of a horse, giving this animal its common name.[25]: 555 In addition to the two main tagmata, the horseshoe crab also possesses a long tail-like section known as thetelson.[25]: 555 

In total, horseshoe crabs have 6 pairs of appendages on theircephalothorax.The first of these are thechelicerae,which givecheliceratestheir name. In horseshoe crabs, these look like tiny pincers in front of the mouth.[25]: 555 Behind the chelicerae are thepedipalps,which are primarily used as legs. In the final molt of males, the ends of the pedipalps are modified into specialized, grasping claws used in mating.[25]: 555 Following the pedipalps are three pairs of walking legs and a set of pusher legs for moving through soft sediment.[25]: 555 Each of these pusher legs isbiramousor divided into two separate branches. The branch closest to the front bears a flat end that looks like a leaf. This end is called the flabellum. The branch towards the back is far longer and looks similar to a walking leg. However, rather than ending in just a claw, the back branch has four leaf-like ends that are arranged like apetal.[25]: 555 The final segment of the cephalothorax was originally part of the abdomen but fused in the embryo. On it are two flap-like appendages known aschilaria.[25]: 556 If severed from the body, lost legs or the telson may slowly regenerate, and cracks in the body shell can heal.[26]

Difference between the pedipalps of male (right) and female (left) horseshoe crabs. Pedipalps are highlighted in grey.

Theopisthosomaor abdomen of a horseshoe crab is composed of several fused segments.[25]: 556 Similar to atrilobite,the abdomen is made up of three lobes: a medial lobe in the middle, and a pleural lobe on either side.[27]Attached to the perimeter of each pleural lobe is a flat, serrated structure known as the flange. The flange on either side is connected by the telson embayment, which itself is attached to the medial lobe.[27]Along the line where these lobes meet are six sets of indentations known asapodeme.Each of these serves as a muscle attachment point for the animal's twelve movable spines.[27]

On the underside of the abdomen are severalbiramouslimbs. The branches closest to the outside are flat and broad, while the ones on the inside are more narrow.[25]: 556 Closest to the front is a plate-like structure made of two fused appendages. This is the genital operculum and is where horseshoe crabs keep theirreproductive organs.[25]: 556 Following the operculum are five pairs ofbook gills.While mainly used for breathing, horseshoe crabs can also use their book gills to swim.[25]: 556 At the end of a horseshoe crab's abdomen is a long, tail-like spine known as atelson.It is highly mobile and serves a variety of functions.[25]: 556 

Nervous system

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Eyes

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Horseshoe crabs have two primary compound eyes and seven secondary simple eyes. Two of the secondary eyes are on the underside.

Horseshoe crabs have a variety ofeyesthat provide them with useful visual information. The most obvious of these are two largecompound eyesfound on top of thecarapace.This feature is unusual, as all other livingchelicerateshave lost them in theirevolution.[28][29]In adult horseshoe crabs, the compound eyes comprise around 1,000 individual units known asommatidia.Each ommatidium is made up of a ring ofretinalandpigment cellsthat surround something known as the eccentric cell.[28]This secondary visual cell gets its name from the way it behaves. The eccentric cell is coupled with thedendritesof normal retinal cells so that when a normal celldepolarizesin the presence oflight,the eccentric cell does too.[28]

A horseshoe crab'scompound eyesare less complex and organized than those of most otherarthropods.Ommatidiaare arranged messily in what's been deemed an "imperfect hexagonal array" and have a highly variable number ofphotoreceptors(between 4 and 20) in theirretina.[28]Although each ommatidium typically has one eccentric cell, there are sometimes two, and occasionally more.[28]All the eye's photoreceptors, bothrodsandcones,have a singlevisual pigmentwith a peak absorption of around 525 nanometers. This differs frominsectsordecapod crustaceans,as their photoreceptors are sensitive to different spectrums of light.[28]Horseshoe crabs have relatively poor vision, and to compensate for that, have the largest rods and cones of any known animal, about 100 times the size of humans'.[29][30]Furthermore, their eyes are a million times more sensitive to light at night than during the day.[31]

At the front of the animal along the cardiac ridge are a pair of eyes known asmedian ocelli.[28][29]Their retina is even less organized than those of the compound eyes having between 5 and 11photoreceptorspaired with one or two secondary visual cells called arhabdomeric cells. Arhabdomeric cells are equivalent to eccentric cells as they function identically.[28]The median ocelli are unique due to having two distinctvisual pigments.While the first functions similarly to the pigment in the compound eyes, the second has a peak absorption of around 360 nanometers, allowing the animal to seeultraviolet light.[28][29]

Other, more rudimentary eyes in horseshoe crabs include the endoparietal ocelli, the two lateral ocelli, two ventral ocelli, and a cluster ofphotoreceptorson the abdomen and telson.[28][29]The endoparietal, lateral, and ventral ocelli are very similar to the median ocelli, except like the compound eyes, they only see in visual light with a peak absorbance of around 525 nanometers.[28]The endoparietal eye further differs due to being a fusion of two separateocelli.[28]This eye is found not far behind the median eyes and sits directly on the cardiac ridge.[27]The two ventral ocelli are located on the underside of thecephalothoraxnear the mouth and likely help to orient the animal when walking around or swimming.[29]The lateral eyes can be found directly behind thecompound eyesand become functional just before a horseshoe crab larvae hatch.[29]The telson's photoreceptors are unique as they're spaced throughout the structure rather than located in a fixed spot. Together withUV-seeing median ocelli, these photoreceptors have been found to influence the animal'scircadian rhythm.[28]

Circulation and respiration

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Underside of two horseshoe crabs showing the legs and book gills

Like allarthropods,horseshoe crabs have anopen circulatory system.[25]: 558 This means that instead of using a system of closed-offveinsandarteries,gasses are transported through acavitycalled the hemocoel.[25]: 558 The hemocoel containshemolymph,a fluid that fills all parts of the cavity and serves as the animal's blood.[25]: 558 Rather than usingiron-basedhemoglobin,horseshoe crabs transportoxygenwith acopper-based protein calledhemocyanin,giving itsblooda bright blue color.[25]: 558 The blood also contains two types of cells:amebocytesthat are utilized in clotting, and cyanocytes that createhemocyanin.[25]: 558 

Horseshoe crabs pump blood with a long, tubularheartlocated in the middle of their body.[25]: 558 Like the hearts ofvertebrates,the hearts of these animals have two separate states: a state of contraction known assystole,and a state of relaxation known asdiastole.[25]: 558 At the beginning of systole,bloodleaves the heart through a large artery known as the aorta and numerous arteries parallel to the heart.[25]: 558 Next, the arteries dump blood into largecavities of the hemocoelsurrounding the animal'stissues.Larger cavities lead to smaller cavities, allowing thehemocoelto oxygenate all the animal's tissues.[25]: 558 During diastole, blood flows from the hemocoel to a cavity known as the pericardial sinus. From there, blood re-enters the heart and the cycle begins again.[25]: 558 

Horseshoe crabs breathe through modified swimming appendages beneath their abdomen known asbook gills.[25]: 556 While they appear smooth on the outside, the insides of these book gills are lined with several thin "pages" called lamellae.[25]: 558 Each lamella is hollow and contains an extension ofhemocoel,allowing gasses to diffuse between a Horseshoe crab'sbloodand external environment.[25]: 558 There are roughly 80–200 lamellae are present in each gill, with all ten of them giving the animal with a total breathing surface area of about two square meters.[25]: 558 When underwater, the lamellae are routinely aerated by rhythmic movement of the book gills.[25]: 558 These movements create a current that enters through two gaps between thecephalothoraxand abdomen and exits on either side of thetelson.[25]: 558 

Feeding, digestion, and excretion

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Gnathobases, food groove, andchelicerae

Horseshoe crabs first break up their food usingbristlesknown as gnathobases located at the coxa or base of theirwalking limbs.[25]: 556 Gnathobases on the right and left legs form a cavity known as the food groove that begins near the pusher legs and extends to the animal'smouth.[25]: 556 The end of the groove is closed off by the animal'schilaria.[25]: 556 To break up anyfood,each pair of coxa moves in the opposite direction parallel to the ones in front of and behind it.[32]: 93 This motion happens while feeding and walking, pushing food towards the mouth.[25]: 556–557 Horseshoe crabs catch soft prey withclawson their second to fifth legs and place them in the food groove to be ground up.[32]: 94 For harder prey, Horseshoe crabs use a pair of stout, cuspid gnathobases (informally known as "nutcrackers" ) on the back of their sixth legs.[32]: 94 After the food is sufficiently torn up, it is moved by thecheliceraeinto themouthfor furtherdigestion.[33]

Horseshoe crabs are some of the only livingchelicerateswith guts that can process solid food.[25]: 556 Its digestive system is J-shaped, lined with acuticle,and can be divided into three main sections: the foregut, midgut, and hindgut.[25]: 556–557 The foregut is contained in the animal'scephalothoraxand comprises theesophagus,crop, andgizzard.[25]: 557 The esophagus moves food from the mouth to the crop where it is stored before entering the gizzard.[25]: 557 The gizzard is a muscular, toothed organ that serves to pulverize the food from the crop and regurgitate any indigestible particles.[25]: 557 The foregut terminates in thepyloric valve and sphincter,a muscular door of sorts that separates it from the midgut.[25]: 557 

The midgut is composed of a shortstomacha long intestinal tube.[25]: 557 Connected to the stomach are a pair of large, sack-like digestive ceca known as hepatopancreases.[25]: 557 These ceca fill most of thecephalothoracicand abdominal hemocoel and are where mostdigestionand nutrient absorption takes place.[25]: 557 Before and following digestion, the midgut lining (epithelium) secretes aperitrophic membranemade ofchitinandmucoproteinsthat surrounds the food and later thefeces.[25]: 557 

Horseshoe crabs excrete waste through both their book gills and hindgut.[25]: 558 Similar to many aquatic animals, horseshoe crabs have anammonotelic metabolismand eliminate ammonia and other small toxins through diffusion with their gills.[25]: 558 After being processed in the midgut, waste is passed into a muscular tube known as the hindgut orrectumand then excreted from asphincterknown as the anus.[33][25]: 558 Externally, this opening is located on the bottom side of the animal right below its telson.[25]: 558 

Distribution and habitat

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Known distribution of modern and fossil horseshoe crabs.

In the modern day, horseshoe crabs have a relatively limited distribution.[34]The threeAsianspecies mainly occur inSouthandSoutheast Asiaalong theBay of Bengaland the coasts ofIndonesia.A notable exception is thetri-spine horseshoe crab,whose range extends northward to the coasts ofChina,Taiwan,andSouthern Japan.[34]The American species lives from the coast ofNova Scotiato the northernGulf of Mexico,with another population residing around theYucatán Peninsula.[34]Extant horseshoe crabs generally live insalt water,though one species, themangrove horseshoe crab(Carcinoscorpius) is often found in morebrackishenvironments.[35]

Past adaptation to freshwater

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According to a phylogeny from 2015, now-extinctxiphosuranstraveled tofreshwaterat least five times throughout history.[36]This same transition happened twice in the horseshoe crabsVictalimulusandLimulitella,with both inhabiting environments such as swamps and rivers.[36]

Behavior and life history

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Diet

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Horseshoe crabs primarily eatwormsandmollusksliving on the ocean floor. They may also feed oncrustaceansand even smallfish.[37]Foraging usually takes place at night.[38][39]

Locomotion

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Horseshoe crab swimming position

Horseshoe crabs live a primarilybenthiclifestyle, preferring to stay at the water's bottom. However, they're also known to swim.[40]This behavior is widespread in young individuals or those traveling to the shore tobreed.[40]Horseshoe crabs swim upside-down with their bodies pointed downwards at an angle. They use theirtelsonas arudder,changing direction towards where it moved.[40]To swim, the animal's retracted legs move to the front of itscephalothorax,extend, and stroke towards the back. This motion happens in unison with the genital operculum and the first three pairs ofbook gills.[40]While the front appendages reset, the back two book gills perform a smaller stroke.[40]

Horseshoe crabs have a variety of ways to right or flip themselves over.[40]The most common method involves the animal arching itsopisthosomatowards thecarapaceand balancing itstelsonon the substrate. The animal then moves the telson while beating itslegsandgills.This causes the animal to tilt and eventually flip over.[40]Furthermore, horseshoe crabs can right themselves while swimming. This method involves the animal swimming to the bottom, rolling on its side, and touching the bottom with its pusher legs while still in the water column.[40]It has been found that harvesting blood from horshoecrabs drastically impacts their percent daily activity, decreasing their overall movement.[41]

Growth and development

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Horseshoe crab "trilobite" larvae

Baby horseshoe crabs begin their lives as a "trilobite larva", a name given due to their resemblance to atrilobite.[25]: 559 Upon hatching,larvatypically measure around1 cm (1⁄2in) long. Theirtelsonis small, and they lack three pairs ofbook gills.[25]: 559 In all other respects, the larvae appear like minuscule adults.[25]: 559 Baby horseshoe crabs can swim and burrow in sediment after emerging from theiregg.[25]: 559 

As the larvaemoltinto juveniles, their telson gets longer and they gain their missing book gills. Juveniles can attain acarapacewidth of around4 cm (1+1⁄2in) in their first year. For each molt, the juvenile will grow about 33% larger.[42]This process continues until the animal reaches its adult size.[42]

When mature, female horseshoe crabs are typically 20–30% larger than males.[43]The smallest species is themangrove horseshoe crab(C. rotundicauda) and the largest isthetri-spine horseshoe crab(T. tridentatus).[44]

On average, males ofC. rotundicaudaare about 30 centimeters (12 inches) long, including atelsonthat is about 15 cm (6 in), and acarapaceabout 15 cm (6 in) wide.[45]Some southern populations (in theYucatán Peninsula) ofL. polyphemusare somewhat smaller, but otherwise, this species is larger.[43]In the largest species,T. tridentatus,females can reach as much as79.5 cm (31+1⁄4in) long, including their telson, and up to 4 kg (9 lb) in weight.[46]This is only about 10–20 cm (4–8 in) longer than the largest females ofL. polyphemusandT. gigas,but roughly twice the weight.[47][48]

Reproduction

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Horseshoe crabs mating
Horseshoe crab eggs

During the breeding season (spring and summer in theNortheast US,year-round in warmer locations) horseshoe crabs migrate to shallow coastal waters.[49]Nesting typically happens athigh tidesaroundfullornew moons.[49]When nesting, they spawn on beaches andsalt marshes.[50]

When mating, the smaller male clings to the back oropisthosomaof the larger female using specializedpedipalps.[49]This typically leaves scars, allowing younger females to be easily identified.[51]In the meantime, the female digs a hole in thesedimentand lays between 2,000 and 30,000 largeeggs.[25]: 559 Unusual forarthropods,fertilization is done externally.[25]: 559 In most species, procreation is done by both the main and additional "satellite males".[49]Satellite males surround the main pair and may have some success fertilizing eggs.[49]InL. polyphemus,the eggs take about two weeks to hatch with shore birds eating many of them in the process.[49]

Natural breeding of horseshoe crabs in captivity has proven to be difficult.[52]Some evidence indicates that mating takes place only in the presence of thesandormudin where horseshoe crabeggshave previously hatched.[52]However, it is not known with certainty what the animals sense in the sand, how they sense it, or why they only mate in its presence.[52]In contrast,artificial inseminationand induced spawning have been done since the 1980s.[53]Additionally,eggsand juveniles collected from the wild can easily be raised to adulthood in a captive environment.[54][55]

Relationship with humans

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Consumption

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Horseshoe crab being served inThailand(Si Racha,Chonburi Province,2007).

Though they have little meat, horseshoe crabs are valued as a delicacy in some parts ofEastandSoutheast Asia.[56]The meat is white, has a rubbery texture similar tolobster,and possesses a slightly salty aftertaste.[56]Horseshoe crab can be eaten both raw andcooked,but must be properly prepared to preventfood poisoning.[56]Furthermore, only certain species can be eaten. There have been numerous reports of poisonings after consumingmangrove horseshoe crabs(Carcinoscorpius rotundicauda) as its meat containstetrodotoxin.[57]

While horseshoe crab meat is commonly prepared bygrillingorstewing,it can also bepickledin vinegar orstir-friedwithvegetables.[56]Many recipes involve the use of variousspices,herbs,andchiliesto give the dish more flavor.[56]

In addition to theirmeat,horseshoe crabs are valued for theireggs.[56]Much like the meat, only the eggs of specific species can be eaten. The eggs ofmangrove horseshoe crabscontaintetrodotoxinand will result infood poisoningif consumed.[58]

Use in fisheries

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In theUnited States,horseshoe crabs are used as bait to fish foreels,whelk,orconch.[59]Nearly 1 million crabs are harvested yearly for bait in the United States, dwarfing the biomedical mortality.[59]However, fishing with horseshoe crab was banned indefinitely inNew Jerseyin 2008 with a moratorium on harvesting to protect thered knot,a shorebird that eats the crabs' eggs.[59]A ban on catching female crabs was put in place inDelaware,and a permanent moratorium is in effect inSouth Carolina.[60][61]

Use in medicine

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The blood of a horseshoe crab contains cells known asamebocytes.[62]These play a similar role to thewhite blood cellsofvertebratesin defending the organism againstpathogens.[63]Amebocytes from thebloodofLimulus polyphemusare used to makeLimulus amebocyte lysate(LAL), which is used for the detection ofbacterialendotoxins in medical applications.[63]There is a high demand for blood, the harvest of which involves collecting the animals, bleeding them, and then releasing them back into the sea.[64]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.[64]Estimates of mortality rates following blood harvesting vary from 3–15%[65][66]to 10–30%.[67][68][69]Approximately 500,000Limulusare harvested annually for this purpose.[70] Bleeding may prevent female horseshoe crabs from being able tospawnor decrease the number of eggs they can lay.[66]According to the biomedical industry, up to 30% of an individual's blood is removed.NPRdisagrees with this claim, reporting that it "can deplete them of more than half their volume of blue blood".[66]The horseshoe crabs spend between one and three days away from the ocean before being returned.[71]As long as the gills stay moist, they can survive on land for four days.[71]Some scientists are skeptical that certain companies return their horseshoe crabs to the ocean at all, instead suspecting them of selling the horseshoe crabs asfishing bait.[72]

The harvesting of horseshoe crab blood in the pharmaceutical industry is in decline.[73]In 1986,Kyushu Universityresearchers discovered that the same test could be achieved by using isolatedLimulus clotting factor C(rFC), an enzyme found in LAL, as by using LAL itself.[73]Jeak Ling Ding, aNational University of Singaporeresearcher, patented a process for manufacturing rFC; on 8 May 2003, synthetic isolated rFC made via her patented process became available for the first time.[74]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.[66]In 2013, however, Hyglos GmbH also began manufacturing its own rFC product.[66]This, combined with the acceptance of rFC by European regulators, the comparable cost between LAL and rFC, and support fromEli Lilly and Company,which committed to using rFC in lieu of LAL,[66]is projected to all but end the practice of blood harvesting from horseshoe crabs.[75]

Vaccine research and developmentduring theCOVID-19 pandemic[76]has added an additional "strain on the American horseshoe crab."[77][66]In December 2019, a report of theUS Senatewhich encouraged theFood and Drug Administrationto "establish processes for evaluating alternative pyrogenicity tests and report back [to the Senate] on steps taken to increase their use" was released;[78]PETAbacked the report.[79]

In June 2020, it was reported thatU.S. Pharmacopeiahad declined to give rFC equal standing with horseshoe crab blood.[80]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.[81]

Conservation status

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Development along shorelines is dangerous to horseshoe crabspawning,limiting available space and degrading habitat.Bulkheadscan block access to intertidal spawning regions as well.[82]

The population ofIndo-Pacific horseshoe crabs(Tachypleus gigas) inMalaysiaandIndonesiahas decreased dramatically since 2010. This is primarily due to overharvesting, as horseshoe crabs are considered a delicacy in countries likeThailand.The individuals most likely to be targeted aregravidfemales, as they can be sold for both theirmeatandeggs.This method of harvesting has led to an unbalanced sex ratio in the wild, something that also contributes to the area's declining population.[83]

Because of habitat destruction for shoreline development, use in fishing,plastic pollution,status as a culinary delicacy, and use in research and medicine, the horseshoe crab faces bothendangeredandextinctstatuses. One species, thetri-spine horseshoe crab(Tachypleus tridentatus), has already been declared locally extinct inTaiwan.Facing a greater than 90% decrease inT. tridentatusjuveniles, it is suspected thatHong Kongwill be the next to declare tri-spine horseshoe crabs as extinct from the area. This species is listed as endangered on theIUCN Red List,specifically because of theoverexploitationand loss of critical habitat.[83]

To preserve and ensure the continuous supply of horseshoe crabs, a breeding center was built inJohor,Malaysiawhere animals are bred and released back into the ocean in the thousands once every two years.[84]It is estimated to take around 12 years before they are suitable for consumption.[84]

A low horseshoe crab population inDelaware Bayis hypothesized to endanger the future of thered knot.Red knots, long-distancemigratoryshorebirds, feed on the protein-rich eggs during their stopovers on the beaches of New Jersey and Delaware.[85]An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.[86]In 2023, theUS Fish and Wildlife Servicehalted the harvesting of horseshoe crabs in theCape Romain National Wildlife Refuge,South Carolina,from March 15 to July 15 to aid their reproduction.[87]This decision was influenced by the importance of horseshoe crab eggs as a food source formigratory birds,the ongoing use of horseshoe crabs for bait, and the use of theirbloodin medical products.[87]The ban supports the conservation goals of the refuge, spanning 66,000 acres (26,700 hectares) of marshes, beaches, and islands near Charleston.[87]

Declining horseshoe crab populations are also a danger to the population stability of other species in shared ecosystems, such asbirdspecies on the East Coast of the USA which feed upon theireggs,for example.[66]

References

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Citations

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Further reading

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