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Horseshoe crab

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Template:Short description Template:Use dmy dates Template:Automatic taxobox

Template:Good article Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic.

Only four species of horseshoe crab are extant today. Most are marine, though the mangrove horseshoe crab is often found in brackish water. Additionally, certain extinct species transitioned to living in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed. In the modern day, their distribution is limited, only found along the east coasts of North America and South Asia.

Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.

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Phylogeny and evolutionEdit

The fossil record of xiphosurans extends back to the Late Ordovician, or around 445 million years ago.<ref name="Rudkin32">Template:Cite journal</ref> For modern horseshoe crabs, their earliest appearance was approximately 250 million years ago during the Early Triassic.<ref name="McKenzie20153">Template:Cite journal</ref> Because they have seen little morphological change since then, extant (surviving) forms have been described as "living fossils".<ref>Template:Cite journal</ref>

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 groups.<ref name="Garw3">Template:Cite journal</ref> The difficult-to-classify chasmataspidids are also thought to be closely related to horseshoe crabs.<ref name="Garw3" /><ref name="GarwoodDunlop20173">Template:Cite journal</ref>

The radiation of horseshoe crabs resulted in 22 known species, of which only 4 remain.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in the Cretaceous.<ref>Template:Cite journal</ref>

Limulidae is the only extant family of the order Xiphosura, and contains all four living species of horseshoe crabs:<ref name="Sekiguchi3">Template:Cite book</ref><ref name="Vestbo20183">Template:Cite journal</ref>

  • 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

GeneraEdit

After Bicknell et al. 2021 and Lamsdell et al. 2020<ref>Template:Cite journal</ref><ref name=":0" />

PhylogenyEdit

The horseshoe crab's position within Chelicerata is complicated. However, most morphological analyses have placed them outside the Arachnida.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Garw3" /><ref name="GarwoodDunlop20173" /> This assumption was challenged when a genetics-based phylogeny found horseshoe crabs to be the sister group to the ricinuleids, thereby making them an arachnid.<ref name=":6">Template:Cite journal</ref> In response, a more recent paper has again placed horseshoe crabs as separate from the arachnids. This new study utilized both new and more complete sequencing data while also sampling a larger number of taxa.<ref>Template:Cite journal</ref>

Below is a cladogram showing the internal relationships of Limulidae (modern horseshoe crabs) based on morphology. It contains both extant and extinct members.<ref name=":0" />Template:Clade

Whole genome duplicationEdit

The common ancestor of arachnids and xiphosurans (the group that includes horseshoe crabs) underwent a whole-genome duplication (WGD) event.<ref name=":8">Template:Cite journal</ref><ref name=":13">Template:Cite journal</ref> This was followed by at least two, possibly three, WGDs in a common ancestor of the living horseshoe crabs.<ref name=":13" /> This gives them unusually large genomes for invertebrates (the genomes of C. rotundicauda and T. tridentatus being approximately 1.72 Gb each).<ref name=":13" /> Evidence for the duplication events includes similarity in structure between chromosomes (synteny), and clustering of homeobox genes. Over time, many of the duplicated genes have changed through processes of neofunctionalization or subfunctionalization, meaning their functions are different from what they originally were.<ref name=":13" />

Evolution of sexual size dimorphismEdit

Several hypotheses have been given as possible reasons why a size difference exists between male and female horseshoe crabs<ref name=":5">Template:Cite journal</ref> This phenomenon is known as sexual size dimorphism and results in the females having a larger average size than males.<ref name=":5" /> 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.<ref name=":5" />
  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.<ref name=":5" />

Anatomy and physiologyEdit

General body planEdit

File:Horseshoe Crab Dorsal Anatomy Cut Telson.png
Generalized dorsal anatomy of a horseshoe crab

Like all arthropods, horseshoe crabs have segmented bodies with jointed limbs, which are covered by a protective cuticle made of chitin. They have heads composed of several segments, which eventually fuse as an embryo.<ref name=":42" />Template:Rp

Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts (tagma): the cephalothorax and the opisthosoma. The first tagma, the cephalothorax or prosoma, is a fusion of the head and thorax.<ref name=":42">Template:Cite book</ref>Template:Rp This tagma is also covered by a large, semicircular, carapace that acts like a shield around the animal's body. It is shaped like the hoof of a horse, giving this animal its common name.<ref name=":42" />Template:Rp In addition to the two main tagmata, the horseshoe crab also possesses a long tail-like section known as the telson.<ref name=":42" />Template:Rp

In total, horseshoe crabs have 6 pairs of appendages on their cephalothorax. The first of these are the chelicerae, which give chelicerates their name. In horseshoe crabs, these look like tiny pincers in front of the mouth.<ref name=":42" />Template:Rp Behind the chelicerae are the pedipalps, 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.<ref name=":42" />Template:Rp Following the pedipalps are three pairs of walking legs and a set of pusher legs for moving through soft sediment.<ref name=":42" />Template:Rp Each of these pusher legs is biramous or 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 a petal.<ref name=":42" />Template:Rp 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 as chilaria.<ref name=":42" />Template:Rp If severed from the body, lost legs or the telson may slowly regenerate, and cracks in the body shell can heal.<ref>Template:Cite thesis</ref>

File:Limulus polyphemus ventral male-female.png
Difference between the pedipalps of male (right) and female (left) horseshoe crabs. Pedipalps are highlighted in grey.

The opisthosoma or abdomen of a horseshoe crab is composed of several fused segments.<ref name=":42" />Template:Rp Similar to a trilobite, the abdomen is made up of three lobes: a medial lobe in the middle, and a pleural lobe on either side.<ref name=":72">Template:Cite journal</ref> 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.<ref name=":72" /> Along the line where these lobes meet are six sets of indentations known as apodeme. Each of these serves as a muscle attachment point for the animal's twelve movable spines.<ref name=":72" />

On the underside of the abdomen are several biramous limbs. The branches closest to the outside are flat and broad, while the ones on the inside are more narrow.<ref name=":42" />Template:Rp 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 their reproductive organs.<ref name=":42" />Template:Rp Following the operculum are five pairs of book gills. While mainly used for breathing, horseshoe crabs can also use their book gills to swim.<ref name=":42" />Template:Rp At the end of a horseshoe crab's abdomen is a long, tail-like spine known as a telson. It is highly mobile and serves a variety of functions.<ref name=":42" />Template:Rp

Nervous systemEdit

EyesEdit

File:Horseshoe crab eyes.jpg
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 of eyes that provide them with useful visual information. The most obvious of these are two large compound eyes found on top of the carapace. This feature is unusual, as all other living chelicerates have lost them in their evolution.<ref name=":03">Template:Cite journal</ref><ref name=":2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In adult horseshoe crabs, the compound eyes comprise around 1,000 individual units known as ommatidia. Each ommatidium is made up of a ring of retinal and pigment cells that surround something known as the eccentric cell.<ref name=":03" /> This secondary visual cell gets its name from the way it behaves. The eccentric cell is coupled with the dendrites of normal retinal cells so that when a normal cell depolarizes in the presence of light, the eccentric cell does too.<ref name=":03" />

A horseshoe crab's compound eyes are less complex and organized than those of most other arthropods. Ommatidia are arranged messily in what's been deemed an "imperfect hexagonal array" and have a highly variable number of photoreceptors (between 4 and 20) in their retina.<ref name=":03" /> Although each ommatidium typically has one eccentric cell, there are sometimes two, and occasionally more.<ref name=":03" /> All the eye's photoreceptors, both rods and cones, have a single visual pigment with a peak absorption of around 525 nanometers. This differs from insects or decapod crustaceans, as their photoreceptors are sensitive to different spectrums of light.<ref name=":03" /> 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'.<ref name=":2" /><ref name=":3">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Furthermore, their eyes are a million times more sensitive to light at night than during the day.<ref>Template:Cite book</ref>

At the front of the animal along the cardiac ridge are a pair of eyes known as median ocelli.<ref name=":03" /><ref name=":2" /> Their retina is even less organized than those of the compound eyes having between 5 and 11 photoreceptors paired with one or two secondary visual cells called arhabdomeric cells. Arhabdomeric cells are equivalent to eccentric cells as they function identically.<ref name=":03" /> The median ocelli are unique due to having two distinct visual 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 see ultraviolet light.<ref name=":03" /><ref name=":2" />

Other, more rudimentary eyes in horseshoe crabs include the endoparietal ocelli, the two lateral ocelli, two ventral ocelli, and a cluster of photoreceptors on the abdomen and telson.<ref name=":03" /><ref name=":2" /> 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.<ref name=":03" /> The endoparietal eye further differs due to being a fusion of two separate ocelli.<ref name=":03" /> This eye is found not far behind the median eyes and sits directly on the cardiac ridge.<ref name=":72" /> The two ventral ocelli are located on the underside of the cephalothorax near the mouth and likely help to orient the animal when walking around or swimming.<ref name=":2" /> The lateral eyes can be found directly behind the compound eyes and become functional just before a horseshoe crab larvae hatch.<ref name=":2" /> The telson's photoreceptors are unique as they're spaced throughout the structure rather than located in a fixed spot. Together with UV-seeing median ocelli, these photoreceptors have been found to influence the animal's circadian rhythm.<ref name=":03" />

Circulation and respirationEdit

File:Tachypleus tridentatus Cat ba 2.JPG
Underside of two horseshoe crabs showing the legs and book gills

Like all arthropods, horseshoe crabs have an open circulatory system.<ref name=":42" />Template:Rp This means that instead of using a system of closed-off veins and arteries, gasses are transported through a cavity called the hemocoel.<ref name=":42" />Template:Rp The hemocoel contains hemolymph, a fluid that fills all parts of the cavity and serves as the animal's blood.<ref name=":42" />Template:Rp Rather than using iron-based hemoglobin, horseshoe crabs transport oxygen with a copper-based protein called hemocyanin, giving its blood a bright blue color.<ref name=":42" />Template:Rp The blood also contains two types of cells: amebocytes that are utilized in clotting, and cyanocytes that create hemocyanin.<ref name=":42" />Template:Rp

Horseshoe crabs pump blood with a long, tubular heart located in the middle of their body.<ref name=":42" />Template:Rp Like the hearts of vertebrates, the hearts of these animals have two separate states: a state of contraction known as systole, and a state of relaxation known as diastole.<ref name=":42" />Template:Rp At the beginning of systole, blood leaves the heart through a large artery known as the aorta and numerous arteries parallel to the heart.<ref name=":42" />Template:Rp Next, the arteries dump blood into large cavities of the hemocoel surrounding the animal's tissues. Larger cavities lead to smaller cavities, allowing the hemocoel to oxygenate all the animal's tissues.<ref name=":42" />Template:Rp 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.<ref name=":42" />Template:Rp

Horseshoe crabs breathe through modified swimming appendages beneath their abdomen known as book gills.<ref name=":42" />Template:Rp While they appear smooth on the outside, the insides of these book gills are lined with several thin "pages" called lamellae.<ref name=":42" />Template:Rp Each lamella is hollow and contains an extension of hemocoel, allowing gasses to diffuse between a Horseshoe crab's blood and external environment.<ref name=":42" />Template:Rp There are roughly 80–200 lamellae present in each gill, with all ten of them giving the animal with a total breathing surface area of about two square meters.<ref name=":42" />Template:Rp When underwater, the lamellae are routinely aerated by rhythmic movement of the book gills.<ref name=":42" />Template:Rp These movements create a current that enters through two gaps between the cephalothorax and abdomen and exits on either side of the telson.<ref name=":42" />Template:Rp

Feeding, digestion, and excretionEdit

File:Limulus polyphemus 397145247.jpg
Gnathobases, food groove, and chelicerae

Horseshoe crabs first break up their food using bristles known as gnathobases located at the coxa or base of their walking limbs.<ref name=":42" />Template:Rp 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's mouth.<ref name=":42" />Template:Rp The end of the groove is closed off by the animal's chilaria.<ref name=":42" />Template:Rp To break up any food, each pair of coxa moves in the opposite direction parallel to the ones in front of and behind it.<ref name=":4222">Template:Cite book</ref>Template:Rp This motion happens while feeding and walking, pushing food towards the mouth.<ref name=":42" />Template:Rp Horseshoe crabs catch soft prey with claws on their second to fifth legs and place them in the food groove to be ground up.<ref name=":4222" />Template:Rp For harder prey, Horseshoe crabs use a pair of stout, cuspid gnathobases (informally known as "nutcrackers") on the back of their sixth legs.<ref name=":4222" />Template:Rp After the food is sufficiently torn up, it is moved by the chelicerae into the mouth for further digestion.<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Horseshoe crabs are some of the only living chelicerates with guts that can process solid food.<ref name=":42" />Template:Rp Its digestive system is J-shaped, lined with a cuticle, and can be divided into three main sections: the foregut, midgut, and hindgut.<ref name=":42" />Template:Rp The foregut is contained in the animal's cephalothorax and comprises the esophagus, crop, and gizzard.<ref name=":42" />Template:Rp The esophagus moves food from the mouth to the crop where it is stored before entering the gizzard.<ref name=":42" />Template:Rp The gizzard is a muscular, toothed organ that serves to pulverize the food from the crop and regurgitate any indigestible particles.<ref name=":42" />Template:Rp The foregut terminates in the pyloric valve and sphincter, a muscular door of sorts that separates it from the midgut.<ref name=":42" />Template:Rp

The midgut is composed of a short stomach a long intestinal tube.<ref name=":42" />Template:Rp Connected to the stomach are a pair of large, sack-like digestive ceca known as hepatopancreases.<ref name=":42" />Template:Rp These ceca fill most of the cephalothoracic and abdominal hemocoel and are where most digestion and nutrient absorption takes place.<ref name=":42" />Template:Rp Before and following digestion, the midgut lining (epithelium) secretes a peritrophic membrane made of chitin and mucoproteins that surrounds the food and later the feces.<ref name=":42" />Template:Rp

Horseshoe crabs excrete waste through both their book gills and hindgut.<ref name=":42" />Template:Rp Similar to many aquatic animals, horseshoe crabs have an ammonotelic metabolism and eliminate ammonia and other small toxins through diffusion with their gills.<ref name=":42" />Template:Rp After being processed in the midgut, waste is passed into a muscular tube known as the hindgut or rectum and then excreted from a sphincter known as the anus.<ref name=":1" /><ref name=":42" />Template:Rp Externally, this opening is located on the bottom side of the animal right below its telson.<ref name=":42" />Template:Rp

Distribution and habitatEdit

File:Geographic distribution of modern and fossil horseshoe crabs.png
Known distribution of modern and fossil horseshoe crabs.

In the modern day, horseshoe crabs have a relatively limited distribution.<ref name=":02">Template:Cite journal</ref> The three Asian species mainly occur in South and Southeast Asia along the Bay of Bengal and the coasts of Indonesia. A notable exception is the tri-spine horseshoe crab, whose range extends northward to the coasts of China, Taiwan, and Southern Japan.<ref name=":02" /> The American species lives from the coast of Nova Scotia to the northern Gulf of Mexico, with another population residing around the Yucatán Peninsula.<ref name=":02" /> Extant horseshoe crabs generally live in salt water, though one species, the mangrove horseshoe crab (Carcinoscorpius) is often found in more brackish environments.<ref name="SeaLifeBase">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Past adaptation to freshwaterEdit

According to a phylogeny from 2015, now-extinct xiphosurans traveled to freshwater at least five times throughout history.<ref name=":4">Template:Cite journal</ref> This same transition happened twice in the horseshoe crabs Victalimulus and Limulitella, with both inhabiting environments such as swamps and rivers.<ref name=":4" />

Behavior and life historyEdit

DietEdit

Horseshoe crabs primarily eat worms and mollusks living on the ocean floor. They may also feed on crustaceans and even small fish.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Foraging usually takes place at night.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

LocomotionEdit

File:Limulus swimming.png
Horseshoe crab swimming position

Horseshoe crabs live a primarily benthic lifestyle, preferring to stay at the water's bottom. However, they're also known to swim.<ref name=":05">Template:Cite journal</ref> This behavior is widespread in young individuals or those traveling to the shore to breed.<ref name=":05" /> Horseshoe crabs swim upside-down with their bodies pointed downwards at an angle. They use their telson as a rudder, changing direction towards where it moved.<ref name=":05" /> To swim, the animal's retracted legs move to the front of its cephalothorax, extend, and stroke towards the back. This motion happens in unison with the genital operculum and the first three pairs of book gills.<ref name=":05" /> While the front appendages reset, the back two book gills perform a smaller stroke.<ref name=":05" />

Horseshoe crabs have a variety of ways to right or flip themselves over.<ref name=":05" /> The most common method involves the animal arching its opisthosoma towards the carapace and balancing its telson on the substrate. The animal then moves the telson while beating its legs and gills. This causes the animal to tilt and eventually flip over.<ref name=":05" /> 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.<ref name=":05" />

Growth and developmentEdit

File:Limulus polyphemus (YPM IZ 076994) 003.jpeg
Horseshoe crab "trilobite" larvae

Baby horseshoe crabs begin their lives as a "trilobite larva", a name given due to their resemblance to a trilobite.<ref name=":42" />Template:Rp Upon hatching, larva typically measure around Template:Convert long. Their telson is small, and they lack three pairs of book gills.<ref name=":42" />Template:Rp In all other respects, the larvae appear like minuscule adults.<ref name=":42" />Template:Rp Baby horseshoe crabs can swim and burrow in sediment after emerging from their egg.<ref name=":42" />Template:Rp

As the larvae molt into juveniles, their telson gets longer and they gain their missing book gills. Juveniles can attain a carapace width of around Template:Convert in their first year. For each molt, the juvenile will grow about 33% larger.<ref name=":07">Template:Cite journal</ref> This process continues until the animal reaches its adult size.<ref name=":07" />

When mature, female horseshoe crabs are typically 20–30% larger than males.<ref name="Zaldivar20092">Template:Cite book</ref> The smallest species is the mangrove horseshoe crab (C. rotundicauda) and the largest is the tri-spine horseshoe crab (T. tridentatus).<ref name="AboutTheSpecies2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

On average, males of C. rotundicauda are about Template:Convert long, including a telson that is about Template:Convert, and a carapace about Template:Convert wide.<ref name="Srijaya20102">Template:Cite journal</ref> Some southern populations (in the Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise, this species is larger.<ref name="Zaldivar20092" /> In the largest species, T. tridentatus, females can reach as much as Template:Convert long, including their telson, and up to Template:Convert in weight.<ref name="Manca20172">Template:Cite journal</ref> This is only about Template:Convert longer than the largest females of L. polyphemus and T. gigas, but roughly twice the weight.<ref name="WAZA2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Jawahir20172">Template:Cite journal</ref>

ReproductionEdit

Template:Multiple image During the breeding season (spring and summer in the Northeast US, year-round in warmer locations) horseshoe crabs migrate to shallow coastal waters.<ref name=":062">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Nesting typically happens at high tides around full or new moons.<ref name=":062" /> When nesting, they spawn on beaches and salt marshes.<ref>Template:Cite journal</ref>

When mating, the smaller male clings to the back or opisthosoma of the larger female using specialized pedipalps.<ref name=":062" /> This typically leaves scars, allowing younger females to be easily identified.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In the meantime, the female digs a hole in the sediment and lays between 2,000 and 30,000 large eggs.<ref name=":42" />Template:Rp Unusual for arthropods, fertilization is done externally.<ref name=":42" />Template:Rp In most species, procreation is done by both the main and additional "satellite males".<ref name=":062" /> Satellite males surround the main pair and may have some success fertilizing eggs.<ref name=":062" /> In L. polyphemus, the eggs take about two weeks to hatch with shore birds eating many of them in the process.<ref name=":062" />

Natural breeding of horseshoe crabs in captivity has proven to be difficult.<ref name=":922">Template:Cite journal</ref> Some evidence indicates that mating takes place only in the presence of the sand or mud in where horseshoe crab eggs have previously hatched.<ref name=":922" /> 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.<ref name=":922" /> In contrast, artificial insemination and induced spawning have been done since the 1980s.<ref name=":422">Template:Cite book</ref> Additionally, eggs and juveniles collected from the wild can easily be raised to adulthood in a captive environment.<ref name="Chen201022">Template:Cite journal</ref><ref>Template:Cite journal</ref>

Relationship with humansEdit

ConsumptionEdit

File:Horseshoe Crab in Si Racha.jpg
Horseshoe crab being served in Thailand (Si Racha, Chonburi Province, 2007).

Though they have little meat, horseshoe crabs are valued as a delicacy in some parts of East and Southeast Asia.<ref name=":22" /> The meat is white, has a rubbery texture similar to lobster, and possesses a slightly salty aftertaste.<ref name=":22" /> Horseshoe crab can be eaten both raw and cooked, but must be properly prepared to prevent food poisoning.<ref name=":22">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Furthermore, only certain species can be eaten. There have been numerous reports of poisonings after consuming mangrove horseshoe crabs (Carcinoscorpius rotundicauda) as its meat contains tetrodotoxin.<ref>Template:Cite journal</ref>

While horseshoe crab meat is commonly prepared by grilling or stewing, it can also be pickled in vinegar or stir-fried with vegetables.<ref name=":22" /> Many recipes involve the use of various spices, herbs, and chilies to give the dish more flavor.<ref name=":22" />

In addition to their meat, horseshoe crabs are valued for their eggs.<ref name=":22" /> Much like the meat, only the eggs of specific species can be eaten. The eggs of mangrove horseshoe crabs contain tetrodotoxin and will result in food poisoning if consumed.<ref>Template:Cite journal</ref>

Use in fisheriesEdit

In the United States, horseshoe crabs are used as bait to fish for eels, whelk, or conch.<ref name=":11" /> Nearly 1 million crabs are harvested yearly for bait in the United States, dwarfing the biomedical mortality.<ref name=":11" /> 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 that eats the crabs' eggs.<ref name=":11">Template:Cite news</ref> A ban on catching female crabs was put in place in Delaware, and a permanent moratorium is in effect in South Carolina.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Use in medicineEdit

The blood of a horseshoe crab contains cells known as amebocytes.<ref name=":04">Template:Cite book</ref> These play a similar role to the white blood cells of vertebrates in defending the organism against pathogens.<ref name=":7" /> Amebocytes from the blood of Limulus polyphemus are used to make Limulus amebocyte lysate (LAL), which is used for the detection of bacterial endotoxins in medical applications.<ref name=":7">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Hurton" /> 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.<ref name=Hurton>Template:Cite thesis</ref> Estimates of mortality rates following blood harvesting vary from 3–15%<ref name=pbsbenefits>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Eisner 2023">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> to 10–30%.<ref>The Blood Harvest Template:Webarchive The Atlantic, 2014.</ref><ref>Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Approximately 500,000 Limulus are harvested annually for this purpose.<ref>Template:Cite news</ref> According to the biomedical industry, up to 30% of an individual's blood is removed. NPR disagrees with this claim, reporting that it "can deplete them of more than half their volume of blue blood".<ref name="Eisner 2023"/> Bleeding may prevent female horseshoe crabs from being able to spawn or decrease the number of eggs they can lay.<ref name="Eisner 2023" /> It has been found that harvesting blood from horseshoe crabs drastically impacts their percent daily activity, decreasing their overall movement.<ref name="Anderson, Watson, & Chabot 2013 pp=137–151">Template:Cite journal</ref>

The horseshoe crabs spend between one and three days away from the ocean before being returned.<ref name=":9" /> As long as the gills stay moist, they can survive on land for four days.<ref name=":9">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite magazine</ref>

The harvesting of horseshoe crab blood in the pharmaceutical industry is in decline.<ref name=":10" /> 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.<ref name=":10">Template:Cite journal</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Eisner 2023" /> In 2013, however, Hyglos GmbH also began manufacturing its own rFC product.<ref name="Eisner 2023" /> 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 committed to using rFC in lieu of LAL,<ref name="Eisner 2023"/> is projected to all but end the practice of blood harvesting from horseshoe crabs.<ref>Template:Cite news</ref>

Vaccine research and development during the COVID-19 pandemic<ref name="Pavid 2020">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> has added an additional "strain on the American horseshoe crab."<ref name="Iovenko 2021">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Eisner 2023" /> 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;<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> PETA backed the report.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In June 2020, it was reported that U.S. Pharmacopeia had declined to give rFC equal standing with horseshoe crab blood.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite news</ref>

Conservation statusEdit

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The population of Indo-Pacific horseshoe crabs (Tachypleus gigas) in Malaysia and Indonesia has decreased dramatically since 2010. This is primarily due to overharvesting, as horseshoe crabs are considered a delicacy in countries like Thailand. The individuals most likely to be targeted are gravid females, as they can be sold for both their meat and eggs. This method of harvesting has led to an unbalanced sex ratio in the wild, something that also contributes to the area's declining population.<ref name=":12">Template:Cite journal</ref>

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 both endangered and extinct statuses. One species, the tri-spine horseshoe crab (Tachypleus tridentatus), has already been declared locally extinct in Taiwan. Facing a greater than 90% decrease in T. tridentatus juveniles, it is suspected that Hong Kong will be the next to declare tri-spine horseshoe crabs as extinct from the area. This species is listed as endangered on the IUCN Red List, specifically because of the overexploitation and loss of critical habitat.<ref name=":12" />

To preserve and ensure the continuous supply of horseshoe crabs, a breeding center was built in Johor, Malaysia where animals are bred and released back into the ocean in the thousands once every two years.<ref name=":732">Template:Cite news</ref> It is estimated to take around 12 years before they are suitable for consumption.<ref name=":732" />

A low horseshoe crab population in 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.<ref>Template:Cite news</ref> An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In 2023, the US 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.<ref name="Whittle 2023" /> This decision was influenced by the importance of horseshoe crab eggs as a food source for migratory birds, the ongoing use of horseshoe crabs for bait, and the use of their blood in medical products.<ref name="Whittle 2023" /> The ban supports the conservation goals of the refuge, spanning 66,000 acres (26,700 hectares) of marshes, beaches, and islands near Charleston.<ref name="Whittle 2023">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

ReferencesEdit

CitationsEdit

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Further readingEdit

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