Horseshoe crab

Limulidae Temporal range: Early Triassic–Present PreꞒ Ꞓ O S D C P T J K Pg N
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

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

and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.

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

Phylogeny and evolution

The fossil record of

The radiation of horseshoe crabs resulted in 22 known species, of which only 4 remain.^[8] 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.^[9] The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in the Cretaceous.^[10]

Limulidae is the only extant family of the order Xiphosura, and contains all four living species of 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

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

• Uncertain Placement
  • †Albalimulus? Bicknell & Pates, 2019^[15] Ballagan Formation, Scotland, Early Carboniferous (Tournaisian) (Considered Xiphosura incertae sedis by Lamsdell, 2020^[14])
  • †Casterolimulus Holland, Erickson & O'Brien, 1975 Late Cretaceous (Maastrichtian) Fox Hills Formation, North Dakota, USA (Inconsistently placed in this family)
  • †
    Heterolimulus gadeai Vıa & Villalta, 1966 Alcover Limestone Formation, Spain, Middle Triassic (Ladinian
    )
  • †
    Limulitella
    ? Størmer, 1952 Middle-Upper Triassic, France, Germany, Tunisia, Russia
  • †Sloveniolimulus Bicknell et al., 2019 Strelovec Formation, Slovenia Middle Triassic (Anisian)
  • †Tarracolimulus Romero & Vıa Boada, 1977 Alcover Limestone Formation, Spain, Middle Triassic (Ladinian)
  • †Victalimulus Riek & Gill, 1971 Lower Cretaceous (Aptian) Korumburra Group, NSW, Australia
  • †Yunnanolimulus Zhang et al., 2009 Middle Triassic (Anisian), Guanling Formation, Yunnan, China
  • †Mesolimulus Middle Triassic-Late Cretaceous England, Spain, Siberia, Germany, Morocco
  • †Ostenolimulus Lamsdell et al. 2021^[16] Early Jurassic (Sinemurian) Moltrasio Limestone, Italy
  • †Volanalimulus Lamsdell, 2020^[14] Early Triassic, Madagascar.
• Subfamily Limulinae Leach, 1819
  • †Crenatolimulus Feldmann et al., 2011 Upper Jurassic (upper Tithonian) Kcynia Formation, Poland. Lower Cretaceous (Albian) Glen Rose Formation, Texas, USA
  • Limulus O. F. Müller, 1785 Pierre Shale, United States, Late Cretaceous (Maastrichtian), Atlantic North America, Recent
• Subfamily Tachypleinae Pocock, 1902
  • Carcinoscorpius
    Pocock, 1902, Asia, Recent
  • Tachypleus Leach, 1819 Upper Cretaceous (Cenomanian) Haqel and Hjoula Konservat-Lagerstatten, Lebanon, Upper Eocene Domsen Sands, Germany, Asia, Recent

The horseshoe crab's position within Chelicerata is complicated. However, most morphological analyses have placed them outside the Arachnida.^{[17][18][19][6][7]} 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.^[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 complete sequencing data while also sampling a larger number of taxa.^[21]

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

The

Several hypotheses have been given as possible reasons why a size difference exists between male and female horseshoe crabs^[24] This phenomenon is known as sexual size dimorphism and 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]

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.^{[25]: 518–522}

Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts (

The

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.^[28][29] 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.^[28] 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.^[28]

A horseshoe crab's

At the front of the animal along the cardiac ridge are a pair of eyes known as median ocelli.^[28][29] 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.^[28] 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.^[28][29]

Other, more rudimentary eyes in horseshoe crabs include the endoparietal ocelli, the two lateral ocelli, two ventral ocelli, and a cluster of

Like all

Horseshoe crabs first break up their food using

The midgut is composed of a short stomach a 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 the cephalothoracic and abdominal hemocoel and are where most digestion and nutrient absorption takes place.^[25]: 557 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.^[25]: 557

Horseshoe crabs excrete waste through both their book gills and hindgut.

In the modern day, horseshoe crabs have a relatively limited distribution.

Horseshoe crabs primarily eat worms and mollusks living on the ocean floor. They may also feed on crustaceans and even small fish.^[37] Foraging usually takes place at night.^[38][39]

Horseshoe crabs live a primarily

Baby horseshoe crabs begin their lives as a "trilobite larva", a name given due to their resemblance to a

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 4 cm (1+1⁄2 in) 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 the mangrove horseshoe crab (C. rotundicauda) and the largest is the tri-spine horseshoe crab (T. tridentatus).^[44]

On average, males of C. rotundicauda are about 30 centimeters (12 inches) long, including a telson that is about 15 cm (6 in), and a carapace about 15 cm (6 in) wide.^[45] Some southern populations (in the Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise, this species is larger.^[43] In the largest species, T. tridentatus, females can reach as much as 79.5 cm (31+1⁄4 in) 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 of L. polyphemus and T. gigas, but roughly twice the weight.^[47][48]

Horseshoe crabs mating

Horseshoe crab eggs

During the breeding season (spring and summer in the

When mating, the smaller male clings to the back or opisthosoma of the larger female using specialized pedipalps.^[49] This typically leaves scars, allowing younger females to be easily identified.^[51] In the meantime, the female digs a hole in the sediment and lays between 2,000 and 30,000 large eggs.^[25]: 559 Unusual for arthropods, 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] In L. 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 the sand or mud in where horseshoe crab eggs have 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 insemination and induced spawning have been done since the 1980s.^[53] Additionally, eggs and juveniles collected from the wild can easily be raised to adulthood in a captive environment.^[54][55]

Though they have little meat, horseshoe crabs are valued as a delicacy in some parts of

While horseshoe crab meat is commonly prepared by grilling or stewing, it can also be pickled in vinegar or stir-fried with vegetables.^[56] Many recipes involve the use of various spices, herbs, and chilies to give the dish more flavor.^[56]

In addition to their

In the United States, horseshoe crabs are used as bait to fish for eels, whelk, or conch.^[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 in New Jersey in 2008 with a moratorium on harvesting to protect the red knot, a shorebird that eats the crabs' eggs.^[59] A ban on catching female crabs was put in place in Delaware, and a permanent moratorium is in effect in South Carolina.^[60][61]

The blood of a horseshoe crab contains cells known as amebocytes.^[62] These play a similar role to the white blood cells of vertebrates in defending the organism against pathogens.^[63] 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.^[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 500,000 Limulus are harvested annually for this purpose.^[70] Bleeding may prevent female horseshoe crabs from being able to

The harvesting of horseshoe crab blood in the pharmaceutical industry is in decline.[73] 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.^[73] 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.^[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 from Eli 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]

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.^[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 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.^[83]

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.^[84] It is estimated to take around 12 years before they are suitable for consumption.^[84]

A low horseshoe crab population in