thumb|Atlantic horseshoe crab on the shore at Brighton Beach, New York City
The Atlantic horseshoe crab (Limulus polyphemus), also known as the American horseshoe crab, is a species of horseshoe crab, a kind of marine and brackish chelicerate arthropod.
Their eggs were eaten by Native Americans, but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine (especially for Limulus amebocyte lysate) and science. Horseshoe crabs, despite their common name, are not crustaceans, and are more closely related to arachnids like spiders and scorpions.
Names and classification
thumb|upright=1.0|Atlantic horseshoe crab with attached [[Crepidula shells on the Delaware Bay beach in Villas, New Jersey.]]
alt=Underside of living horseshoe crab|thumb|Underside view of a living male crab, showing the mouth, gills and legs
This group of animals is also known as horsefoot, or saucepan. Some people call the horseshoe crab a "helmet crab", but this common name is more frequently applied to a true crab, a malacostracan, of the species Telmessus cheiragonus. The term "king crab" is sometimes used for horseshoe crabs, but it is more usually applied to a group of decapod crustaceans.
Limulus means "askew" and polyphemus refers to Polyphemus, a giant in Greek mythology.
Former scientific names include Limulus cyclops, Xiphosura americana, and Polyphemus occidentalis.
Studies of microsatellite DNA have revealed several discrete geographic populations in the Atlantic horseshoe crab. While there is extensive mixing between neighbouring populations, primarily due to movements by males, there is little or no mixing between the US and isolated Yucatán Peninsula horseshoe crabs, leading some to suggest that a taxonomic review is necessary.
Anatomy and physiology
thumb|upright|Underside view: The mouth opening is between the legs, and the gills are visible below.thumb|Underside of a female showing the legs and [[book gills.]]
thumb|Underside of a male, showing the first leg modified for grasping the female during copulation.
Horseshoe crabs have three main parts to the body: the head region, known as the "prosoma", the abdominal region or "opisthosoma", and the spine-like tail or "telson".
The smooth shell or carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. The sexes are similar in appearance, but females are typically 25 to 30% larger than the male in length and width, and can reach more than twice the weight. Females can grow up to in length, including tail, and in weight. In Delaware Bay, females and males have an average carapace width of about and , respectively. In Florida, females average about and males .
A wide range of marine species become attached to the carapace, including algae, flat worms, mollusks, barnacles, and bryozoans, and horseshoe crabs have been described as 'walking museums' due to the number of organisms they can support. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.
The brain and the heart are located in the prosoma. On the underside of the prosoma, six pairs of appendages occur, the first of which (the small pincers or chelicerae) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax, between the chelicerae. Although most arthropods have mandibles, the horseshoe crab is jawless, instead relying on the chelicerae to perform similar functions. The remaining ten additional limbs, the pedipalps, are used for locomotion on the sea floor. Males of the species develop clasping hooks on the first pair of pedipalps in order to hold on to the female during mating. The posterior pair end in structures used for burrowing into the sediment.
The telson (i.e., tail) is used by the horseshoe crab to steer during locomotion and to maintain stability while moving along the seafloor. The telson may also be used by L. polyphemus to right itself if flipped upside down. Horseshoe crabs are often flipped over by currents, and in this position they are vulnerable to suffocation.
Among other senses, they have a small chemoreceptor organ that senses smells on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.
Vision
Limulus has been extensively used in research into the physiology of vision. The Nobel Prize in Medicine was awarded in 1967 in part for research performed on the horseshoe crab eye.
A large compound eye with monochromatic vision is found on each side of the prosoma;
In addition, the tail bears a series of light-sensing organs along its length.
Each compound eye is composed of about 1000 receptors called ommatidia, The ommatidia are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods.
Blood
The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper-containing protein hemocyanin at concentrations of about 50 g per liter. and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed. The resulting coagulation is thought to contain bacterial infections in the animal's semiclosed circulatory system.
Distribution and habitat
thumb|Atlantic horseshoe crabs in Mexico, such as this pair at [[Holbox Island, mostly breed in lagoons with mangrove and seagrass
Most Atlantic horseshoe crabs are found along the Atlantic East Coast of the United States, ranging from Maine to Florida. In the Gulf Coast of the United States, they are found in Florida, Alabama, Mississippi and Louisiana. The temperature preference varies depending on the population, with the northernmost being the most cold-resistant: in the Great Bay in New Hampshire they show increased activity above and in Delaware Bay they are active above .
A 2022 study of ancient (Early Pleistocene, 2 million years ago) environmental DNA from the Kap Kobenhavn Formation of northern Greenland identified preserved DNA fragments of horseshoe crabs, assigned to L. polyphemus. This suggests that horseshoe crabs ranged and spawned as far north as Greenland during these warmer conditions. Around this time, the sea surface temperature would have been 8 °C warmer than the present. These are among the oldest DNA fragments ever sequenced.
Life cycle and behavior
Horseshoe crabs feed on mollusks, annelid worms, other benthic invertebrates, and bits of fish. Lacking jaws, they grind food with bristles on their legs and a gizzard that contains sand and gravel.
thumb|Size comparison. This female has an unusual forked [[telson.]]
Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule. The larvae form and then swim for about five to seven days. After swimming, they settle, and begin the first molt. This occurs about 20 days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. Before becoming sexually mature around age 9, they have to shed their shells some 17 times.
Research from the University of New Hampshire gives insight into the circadian rhythm of Atlantic horseshoe crabs. For example, several studies have looked into the effect of a circa tidal rhythm on the locomotion of this species. While it has been known for a while that a circadian clock system controls eye sensitivity, scientists discovered a separate clock system for locomotion. When a sample of Atlantic horseshoe crabs were exposed to artificial tidal cycles in the lab, circa tidal rhythms were observed. The study found that light and dark cycles influence locomotion, but not as much as tidal activity.
Evolution
Despite the limited fossil record, many ancient specimens closely resemble modern horseshoe crabs, indicating that their overall body structure has remained largely unchanged over time.
This pattern of minimal structural change over long periods is often described as evolutionary stability.
Horseshoe crabs were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. However, recent studies suggest a relationship between the eurypterids and the arachnids, leaving Xiphosura in the clade Prosomapoda. They may have evolved in the shallow seas of the Paleozoic Era (541–252 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods.
The oldest known horseshoe crab, Lunataspis aurora, from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.. This fossil shows that early horseshoe crabs have a similar body structure to modern species. Its features suggest that the horseshoe crab body was established very early in their history. Discoveries like this help scientists understand how certain traits remained stable over a long period of time.
Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years. and the Atlantic horseshoe crab itself has no fossil record at all. Until recently it was thought that the genus Limulus "ranges back only some 20 million years, not 200 million".
The oldest member of the subfamily Limulinae are known from the Late Jurassic (Tithonian), belonging to the species Crenatolimulus darwini from Poland. The oldest and currently only other known species in the genus Limulus is Limulus coffini from the Late Cretaceous (Maastrichtian) of the United States.
Medical uses
Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. An extract of blood cells (amoebocytes) from Limulus polyphemus is a critical component in the widely used Limulus amebocyte lysate (LAL) test to detect and quantify bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases.
Procuring the raw materials for LAL testing involves collecting and bleeding horseshoe crabs from wild populations. Horseshoe crabs are returned to the ocean after bleeding, however, there is a level of mortality and sub-lethal impact involved. It is estimated that between 10 and 30 percent of horseshoe crabs die after bleeding. Released horseshoe crabs may also be more prone to disease and be less successful in reproducing.
Studies show the blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound. Animals that survive the process may be more lethargic when released and less likely to mate – which has prompted concerns about the longer-term impacts of harvesting horseshoe crabs.
The LAL test is a major source of animal product dependence in the biomedical industry, and a challenge to the Three Rs of science in relation to the use of animals in testing. There are efforts to reduce the dependence on horseshoe crabs, refine the process of collecting blood from the animals (including through aquaculture), and even replace the use of animal-derived assays utilizing synthetic approaches, such as the recombinant factor C (rFC) assay.
Conservation and management
Overall the Atlantic horseshoe crab is recognized as vulnerable by the IUCN due primarily to overharvesting and habitat loss. There are, however, significant geographic differences with some populations increasing, some stable and some declining.
United States
thumb|upright=1.2|Horseshoe crab harvest, [[Delaware, circa 1926. Millions of crabs were harvested for fertilizer around Delaware Bay. Some were fed to hogs.]]
thumb|upright=1.2|The [[loggerhead sea turtle has suffered from the reduction in numbers of Atlantic horseshoe crabs]]
Early in the 20th century and possibly before, there was the mistaken belief in some areas that horseshoe crabs were destructive to fisheries, folklore held that they used their long spines to drill into some shellfish. Because of this mistaken belief and folklore, bounties were sometimes offered by authorities for them. On Cape Cod in the early 20th century five cents was offered for every dead horseshoe crab turned in.
The Atlantic horseshoe crab is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for this animal's future. Since the 1970s, the horseshoe crab population has been decreasing in some areas, due to several factors, including the use of the crab as bait in eel, whelk and conch trapping. According to the Horseshoe Crab Benchmark Stock Assessment Peer Review Report published by the Atlantic States Marine Fisheries Commission (ASMFC), the population continues to remain stable where biomedical is present in the Northeast and thrive and grow in the Southeast due to protection efforts – a trend spanning decades.
thumb|upright=1.2|Red knots feeding on horseshoe crab eggs in Delaware
Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on the horseshoe crabs' eggs for food during their spring migration. Precipitous declines in the population of the red knots have been observed in recent years. Predators of horseshoe crabs, such as the currently threatened Atlantic loggerhead turtle, have also suffered as crab populations diminish.
In 1991, the species was provided legislated protection from bait fishing in South Carolina by calling on the management and regulation of the horseshoe crab fisheries, allowing only hand-collecting for biomedical applications and marine biological research. Without the need for LAL in biomedical use, the legal protection of the horseshoe crab is not guaranteed in the future, and they would again fall prey to overfishing and use as bait. In 1995, the nonprofit Ecological Research and Development Group (ERDG) was founded with the aim of preserving the four remaining species of horseshoe crab. Since its inception, the ERDG has made significant contributions to horseshoe crab conservation. ERDG founder Glenn Gauvry designed a mesh bag for whelk/conch traps, to prevent other species from removing the bait. This has led to a decrease in the amount of bait needed by approximately 50%. In the state of Virginia, these mesh bags are mandatory in whelk/conch fishery. The Atlantic States Marine Fisheries Commission in 2006 considered several conservation options, among them being a two-year ban on harvesting the animals, affecting both Delaware and New Jersey shores of Delaware Bay. In June 2007, Delaware Superior Court Judge Richard Stokes has allowed limited harvesting of 100,000 males. He ruled that while the crab population was seriously depleted by overharvesting through 1998, it has since stabilized, and that this limited take of males will not adversely affect either horseshoe crab or red knot populations. In opposition, Delaware environmental secretary John Hughes concluded that a decline in the red knot bird population was so significant that extreme measures were needed to ensure a supply of crab eggs when the birds arrived. Harvesting of the crabs was banned in New Jersey on March 25, 2008.
<!-- Just Flip 'Em and Just Flip Em redirect here -->
Every year, about 10% of the horseshoe crab breeding population dies when rough surf flips them onto their backs, a position from which they often cannot right themselves. In response, the ERDG launched a "Just Flip 'Em" campaign, encouraging beachgoers to turn the crabs back over. New Jersey beaches campaign "ReTURN the Favor", trains volunteers to rescue impinged and overturned horseshoe crabs while collecting data on natural and man-made hazards.
A large-scale project to tag and count horseshoe crabs along the North American coast was started in 2008, termed Project Limulus.
Mexico
Since 1994, the populations in the Yucatán Peninsula have been recognized as endangered under Mexican law. They have declined since the 1960s and remaining significant Yucatán populations are mostly within protected areas.