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]
Taxonomy
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 entire body of the horseshoe crab is protected by a hard carapace. It has two
compound lateral eyes, each composed of about 1,000
ommatidia, 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,[24][25] and their eyes are a million times more sensitive to light at night than during the day.[26]
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.[27] In extant species and other fossil xiphosurans (e.g. Alanops[28]) their appendages are
uniramous, but in other related non-xiphosurid euchelicerates such as Offacolus and Dibasterium the appendages are biramous.[29] 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.[30] Lost legs or the telson (tail) may slowly regenerate, and cracks in the body shell can heal.[31]
Behind its legs, the horseshoe crab has
book gills, which exchange respiratory gases, and are also occasionally used for swimming.[32]
As in other arthropods, a true
endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills.
Horseshoe crabs normally swim upside down, inclined at about 30° to the horizontal and moving at about 10–15 cm/s.[33][34][35]
Horseshoe crabs have two primary compound eyes and seven secondary simple eyes. Two of the secondary eyes are on the underside.[36][37]
Females are about 20–30% larger than males.[38] The smallest species is C. rotundicauda and the largest is T. tridentatus.[39] 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.[40] Some southern populations (in the
Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise this species is larger.[38]
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.[41] 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.[42][43]
The juveniles grow about 33% larger with every
molt until reaching adult size.[44] Atlantic horseshoe crabs molt in late July.
Diet
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.[45]
Breeding
During the breeding season (spring and summer in the Northeast U.S.; year-round in warmer locations or when the
full moon rises),[46] 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.[47] Young female horseshoe crabs can be identified by the lack of mating scars.[48]
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.[46] 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.[49][50]
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.[51]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.[52][53]
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.[54]
Relationships with humans
Blood harvesting
Horseshoe crabs use
hemocyanin to carry oxygen through their blood.[55] 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.[56] 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.[57] Estimates of mortality rates following blood harvesting vary from 3–15%[58][59] to 10–30%.[60][61][62] Approximately 500,000 Limulus are harvested annually for this purpose.[63] Declining horseshoe crab populations on the East Coast of the United States endanger certain bird species which feed upon their eggs.[59]
Bleeding may also prevent female horseshoe crabs from being able to
spawn[59] 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."[59] 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.[64] 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.[65]
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.[66] 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.[67] 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,[59] is projected to all but end the practice of blood harvesting from horseshoe crabs.[68]
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;[69]PETA backed the report.[70]
In June 2020, it was reported that
U.S. Pharmacopeia had declined to give rFC equal standing with horseshoe crab blood.[71] 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.[72]
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.[74]
Fishery
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.[75] A moratorium was restricted to male crabs in
Delaware, and a permanent moratorium is in effect in
South Carolina.[76] The eggs are eaten in parts of Southeast Asia,
Johor and China.[77]
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.[78] An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.[79]
Culinary use
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 biased harvesting has led to an unbalanced sex ratio in the wild, which also contributes to its declining population in the area.[80]
Conservation status
Development along shorelines is dangerous to horseshoe crab spawning, limiting available space and degrading habitat.
Bulkheads can block access to intertidal spawning regions as well.[81]
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.[80]
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RedKnot.orgArchived 2014-04-15 at the
Wayback Machine links to shorebird recovery sites, movies, events & other info on Red Knot rufa & horseshoe crabs.
Crab Bleeders Article about the men who bleed horseshoe crabs for science.