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Horseshoe crabs are of the family Limulidae and the only surviving . Despite their name, they are not or even crustaceans; they are Chelicerata, more closely related to like , , and . The body of a horseshoe crab is divided into three main parts: the cephalothorax, Opisthosoma, and telson. The largest of these, the cephalothorax, houses most of the animal's , 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 "", having changed little since they first appeared in the Triassic around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.
Only four species of horseshoe crab are extant today, the Atlantic horseshoe crab ( Limulus polyphemus), native to the eastern coast of North and Central America, as well as the mangrove horseshoe crab ( Carcinoscorpius rotundicauda), tri-spine horseshoe crab ( Tachypleus tridentatus) and Indo-Pacific horseshoe crab ( Tachypleus gigas), which are native to South, South East, and East Asia.
Most horseshoe crabs are Marine life, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems such as the Delaware Bay and Chesapeake Bay bays. Additionally, certain extinct species transitioned to living solely in Fresh water. Horseshoe crabs primarily live at the Benthic zone but they can swim if needed.
Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial . 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.
Horseshoe crabs resemble but belong to a separate subphylum of the , Chelicerata. Horseshoe crabs are closely related to the extinct (sea scorpions), which include some of the largest arthropods ever to have existed, and the two may be . The difficult-to-classify are also thought to be closely related to horseshoe crabs.
The radiation of horseshoe crabs resulted in 22 known species, of which only 4 remain. The Atlantic Ocean species is sister to the three Asian species, the latter of which are likely the result of two divergences relatively close in time. The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in the Cretaceous.
Limulidae is the only extant family of the order Xiphosura, and contains all four living species of horseshoe crabs: (1988). 9784915572258, Science House. ISBN 9784915572258
Below is a cladogram showing the internal relationships of Limulidae (modern horseshoe crabs) based on morphology. It contains both extant and extinct members.
Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts (tagmata): the cephalothorax and the opisthosoma. The first tagma, the cephalothorax or prosoma, is a fusion of the head and thorax. This tagma is also covered by a large, semicircular carapace that acts as a shield around the animal's body. It is shaped like the hoof of a horse, giving this animal its common name. In addition to the two main tagmata, the horseshoe crab also possesses a long tail-like section known as the telson.
In total, horseshoe crabs have 6 pairs of appendages on their cephalothorax. The first of these are the chelicerae, which give Chelicerata their name. In horseshoe crabs, these look like tiny pincers in front of the mouth. Behind the chelicerae are the , 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. Following the pedipalps are three pairs of walking legs and a set of pusher legs for moving through soft sediment. 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. 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. If severed from the body, lost legs or the telson may slowly regenerate, and cracks in the body shell can heal. The opisthosoma or abdomen of a horseshoe crab is composed of several fused segments. 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. 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. 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.
On the underside of the abdomen are several Arthropod leg limbs. The branches closest to the outside are flat and broad, while the ones on the inside are more narrow. 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 Gonad. Following the operculum are five pairs of . While mainly used for breathing, horseshoe crabs can also use their book gills to swim. 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.
A horseshoe crab's are less complex and organized than those of most other . Ommatidium 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. Although each ommatidium typically has one eccentric cell, there are sometimes two, and occasionally more. All the eye's photoreceptors, both Rod cell and Cone cell, have a single visual pigment with a peak absorption of around 525 nanometers. This differs from insects or Decapoda, as their photoreceptors are sensitive to different spectrums of light. 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'. Furthermore, their eyes are a million times more sensitive to light at night than during the day.
At the front of the animal along the cardiac ridge are a pair of eyes known as median ocelli. 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. 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.
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. 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. The endoparietal eye further differs due to being a fusion of two separate ocelli. This eye is found not far behind the median eyes and sits directly on the cardiac ridge. 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. The lateral eyes can be found directly behind the and become functional just before a horseshoe crab larvae hatch. The telson's photoreceptors are unique as they're spaced throughout the structure rather than located in a fixed spot. Together with Ultraviolet-seeing median ocelli, these photoreceptors have been found to influence the animal's circadian rhythm.
Horseshoe crabs pump blood with a long, tubular heart located in the middle of their body. Like the hearts of , the hearts of these animals have two separate states: a state of contraction known as systole, and a state of relaxation known as diastole. At the beginning of systole, blood leaves the heart through a large artery known as the aorta and numerous arteries parallel to the heart. 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. 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.
Horseshoe crabs breathe through modified swimming appendages beneath their abdomen known as book gills. While they appear smooth on the outside, the insides of these book gills are lined with several thin "pages" called lamellae. Each lamella is hollow and contains an extension of hemocoel, allowing gasses to diffuse between a horseshoe crab's blood and external environment. 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. When underwater, the lamellae are routinely aerated by rhythmic movement of the book gills. These movements create a current that enters through two gaps between the cephalothorax and abdomen and exits on either side of the telson.
Horseshoe crabs are some of the only living Chelicerata with guts that can process solid food. Its digestive system is J-shaped, lined with a cuticle, and can be divided into three main sections: the foregut, midgut, and hindgut. The foregut is contained in the animal's cephalothorax and comprises the esophagus, crop, and gizzard. The esophagus moves food from the mouth to the crop where it is stored before entering the gizzard. The gizzard is a muscular, toothed organ that serves to pulverize the food from the crop and regurgitate any indigestible particles. The foregut terminates in the pyloric valve and sphincter, a muscular door of sorts that separates it from the midgut.
The midgut is composed of a short stomach, a long intestinal tube. Connected to the stomach are a pair of large, sack-like digestive ceca known as hepatopancreases. These ceca fill most of the Cephalothorax and abdominal hemocoel and are where most digestion and nutrient absorption takes place. Before and following digestion, the midgut lining (epithelium) secretes a peritrophic membrane made of chitin and that surrounds the food and later the feces.
Horseshoe crabs excrete waste through both their book gills and hindgut. Similar to many aquatic animals, horseshoe crabs have an Ammonotelic and eliminate ammonia and other small toxins through diffusion with their gills. 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. Externally, this opening is located on the bottom side of the animal right below its telson.
As recently as the Early Pleistocene, 2 million years ago, Limulus polyphemus inhabited the Kap Kobenhavn Formation of northern Greenland, as demonstrated by environmental DNA. Around this time, the sea surface temperature would have been 8 °C warmer than the present.
Horseshoe crabs have a variety of ways to right or flip themselves over. 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 Arthropod leg and Book gill. This causes the animal to tilt and eventually flip over. 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.
As the larvae Moulting into juveniles, their telson gets longer and they gain their missing book gills. Juveniles can attain a carapace width of around in their first year. For each molt, the juvenile will grow about 33% larger. This process continues until the animal reaches its adult size.
When mature, female horseshoe crabs are typically 20–30% larger than males. The smallest species is the mangrove horseshoe crab ( C. rotundicauda) and the largest is the tri-spine horseshoe crab ( T. tridentatus).
On average, males of C. rotundicauda are about long, including a telson that is about , and a carapace about wide. Some southern populations (in the Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise, this species is larger. In the largest species, T. tridentatus, females can reach as much as long, including their telson, and up to in weight. This is only about longer than the largest females of L. polyphemus and T. gigas, but roughly twice the weight.
When mating, the smaller male clings to the back or opisthosoma of the larger female using specialized . This typically leaves scars, the absence of which allows younger females to be easily identified. In the meantime, the female digs a hole in the sediment and lays between 2,000 and 30,000 large . Unusual for , fertilization is done externally. In most species, procreation is done by both the main and additional "satellite males". Satellite males surround the main pair and may have some success fertilizing eggs. In Limulus, the eggs take about two weeks to hatch with shore birds eating many of them in the process.
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 horseshoe crab have previously hatched. However, it is not known with certainty what the animals sense in the sand, how they sense it, or why they mate only in its presence. In contrast, artificial insemination and induced spawning have been done since the 1980s. Additionally, and juveniles collected from the wild can easily be raised to adulthood in a captive environment.
While horseshoe crab meat is commonly prepared by grilling or , it can also be Pickling in vinegar or Stir frying with . Many recipes involve the use of various , , and Chili pepper to give the dish more flavor.
In addition to their meat, horseshoe crabs are valued for their . 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 poisioning if consumed.
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. 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.
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. 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. 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 committed to using rFC in lieu of LAL, is projected to all but end the practice of blood harvesting from horseshoe crabs.
Vaccine research and development during the COVID-19 pandemic has added an additional "strain on the American horseshoe crab." 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 on steps taken to increase their use" was released; PETA backed the report.
In June 2020, it was reported that U.S. Pharmacopeia had declined to give rFC equal standing with horseshoe crab blood. 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.
The population of Tachypleus gigas ( 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 . This method of harvesting has led to an unbalanced sex ratio in the wild, something that also contributes to the area's declining population.
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 Extinction 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.
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. It is estimated to take around 12 years before they are suitable for consumption.
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. An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds. 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. This decision was influenced by the importance of horseshoe crab eggs as a food source for , the ongoing use of horseshoe crabs for bait, and the use of 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.
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