Horseshoe crab

Horseshoe crab
Limulus polyphemus from many angles
Conservation status
Near Threatened (IUCN2.3)
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Chelicerata Class: Merostomata or Xiphosura Order: Xiphosura or Xiphosurida Family: Limulidae Species: Limulus polyphemus Tachypleus gigas Tachypleus tridentatus Carcinoscorpius rotundicauda

Horseshoe crab is the common name for various marine chelicerate arthropods of the family Limulidae, and in particular the extant species Limulus polyphemus of the Atlantic of North America (and sometimes Europe). The other extant species known as horseshoe crabs are the Indo-Pacific species Tachypleus gigas, Tachypleus tridentatus, and Carcinoscorpius rotundicauda. Horseshoe crabs are characterized by a heavy, domed, horseshoe-shaped carapace (exoskeleton), a pointed, spiky telson ("tail"), a jawless mouth, and the presence of compound eyes. Despite the name, horseshoe crabs are more closely related to spiders, ticks, and scorpions than to crabs, and are placed in the arthropod subphylum Chelicerata. Beneath the carapace, they look quite similar to a large spider.

Horseshoe crabs have a long fossil record and the group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera, such as the Jurassic Mesolimulus. Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years (Angier 2008).

Ecologically, horseshoe crabs, which can swim but are usually found on the ocean floor, are important in marine food chains. They consume invertebrates such as mollusks, worms, crustaceans, and small fish, and as larva, juveniles, and adults they are preyed upon by many species of fish and birds, as well as sea turtles and sharks. Horseshoe crab eggs are the primary source of fat for at least 20 species of migratory shore birds (MRRI).

For humans, horseshoe crabs are uniquely valuable.

Limulus has been extensively used in research into the physiology of vision. Horseshoe crabs are valuable as a species to the medical research community. The horseshoe crab has a simple but effective immune system.

have been used in bait for fishing for eel and whelk

captured for use in biomedicine, since a clotting agent in the blood, known as Limulus Amoebocyte Lysate (LAL), is used to detect microbial pathogens in medical intravenous fluids, injectable drugs, and supplies. (They are released alive after being bled.) Horseshoe crabs have also been used in eye research and the development of wound dressings and surgical sutures. (MRRI).

Physical description

File:Limuluskils4.jpg

The mouth opening is between the legs, the gills are visible below

Underside of a female showing the legs and book gills.

Horseshoe crabs differ from true crabs in that they lack antennae and jaws (true crabs have two pairs of antennae and a pair of jaws) and they have seven pairs of legs, including a pair of chelicerae (true crabs have five pairs of legs).

The following is a description of the well-known Atlantic horseshoe crab, Limulus polyphemus. However, all four extant species are similar in form and behavior.

Horseshoe crabs can be divided externally into three parts. The frontmost part, the prosoma (cephalothorax), is covered by smooth carapace on the dorsal surface, which is roughly shaped like a horse's shoe. This section contains the eyes (five pairs), one pair of small pincers/chelicerae used to move food towards the mouth, five pairs of walking legs (the first four with claws, the last with a leaf-like structure used for pushing) (DNR 2005), the mouth (located in between the legs), the brain, and the heart. The middle section is the abdomen or opisthosoma, which connects to the prosoma via a hinge. It is this portion where the gills are attached as well as the genital operculum. The last section is the telson (i.e., tail or caudal spine), which attaches to the abdomen and is used to steer in the water, as well as to flip the horseshoe crab over if stuck upside down.

The horseshoe crab can grow up to 60 centimeters (24 inches) in length (including tail); the female is typically 25 to 30 percent larger than the male (Angier 2008). The hard carapace that protects the body of the horseshoe crab is dark brown.

Horseshoe crabs possess five pairs of book gills, located just behind their appendages. These allow them both to breathe underwater, and for short periods of time even on land, provided the gills remain moist. In addition to the exchange of respiratory gases, the book gills are occasionally used for swimming.

Although most arthropods have mandibles, the horseshoe crab is jawless. The mouth is located in the middle of the underside of the cephalothorax, with chelicerae located at each side of the mouth.

Underside of a male, showing the first leg modified for grasping the female during copulation

Horseshoe crabs have five pairs of legs for walking, swimming, and moving food into the mouth, located just before the book gills. In the female, the four large legs are all alike, and end in pincers. In the male, the first of the four large legs is modified, with a bulbuous claw that serves to lock the male to the female while she deposits the eggs and he waits to fertilize them.

Horseshoe crabs have ten eyes. They have two large compound eyes, each with about 1,000 receptors or ommatidia, and with each ommatidium feeding into a single nerve fiber. There are five additional eyes on the top side of its prosoma, two ventral eyes located near the mouth, and photoreceptors located on the telson that constitute the last eye (DNR 2005).

Among other senses, they have a small sense organ on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.

Distribution and habitat

The Atlantic horseshoe crab (Limulus polyphemus) is most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is the Delaware Bay, although stray individuals are occasionally found in Europe (Hansson 1998).

The Japanese horseshoe crab (Tachypleus tridentatus) is found in the Seto Inland Sea, and is considered an endangered species because of loss of habitat. Two other species occur along the east coast of India: Tachypleus gigas and Carcinoscorpius rotundicauda (Tripathy 2006).

Life cycle and behaviour

Horseshoe crab molting

Before becoming mature around age 9, they have to shed their shells some 17 times. They can live for as long as 31 years.^{[citation needed]}

The crabs feed on mollusks, annelid worms, other benthic invertebrates, and bits of fish. Lacking jaws, it grinds up the food with bristles on its legs and a gizzard that contains sand and gravel.

While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, which are their main food. They may also feed on crustaceans and even small fish. annelids, nemertean, and polychete worms nematodes razor clam blue mussel arthropods, annelids, nemertean, and polychaete worms soft-shell clams

They spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn, with the males arriving first. The smaller male grabs on to the back of a female with a "boxing glove" like structure on his front claws, often holding on for months at a time. After the female has laid a batch of eggs in a nest at a depth of 15-20 cm in the sand, the male fertilizes them with his sperm. Egg quantity is dependent on female body size and ranges from 15,000-64,000 eggs per female.

"Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule" (Sturtevant). The larvae form and then swim for about five to seven days. After swimming they settle, and begin the first molt. This occurs approximately twenty days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. They reach sexual maturity in approximately eleven years and may live another 10-14 years beyond that.

Evolution and classification

Horseshoe crabs are distant relatives of spiders and are probably descended from the ancient eurypterids (sea scorpions). They evolved in the shallow seas of the Paleozoic Era (540-248 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the class Merostomata, one of the oldest classes of marine arthropods.

whose ancestors can first be seen in the Devonian period's fossil record.

The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimeters (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.

Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species known of the class Xiphosura, formerly called Merostomata. They are found along the northwestern Atlantic coast and in the Gulf of Mexico. Horseshoe crabs are found in shallow water on soft sandy bottoms.

Xiphosura is a class of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs.

Xiphosura are typically placed the class Merostomata, comprising also the eurypterids, but this traditional usage in the past reflected the acceptance of an unnatural (paraphyletic) group. Although the name Merostomata can be seen in some textbooks, without reference to the Eurypterida, some individuals have urged that this usage should be discouraged (e.g. Boudreaux, 1979).

Merostomata is a class of marine Chelicerata which includes horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. Some favor the abandonment of the term class Merostomata, in favour of splitting the class into two classes, Xiphosura and Eurypterida. This is because Merostomata is thought to be paraphyletic, with the Eurypterida more closely related to the Arachnida, forming the group Cryptopneustida (Boudreaux, 1979). This change has not been incorporated into most textbooks, which typically use the traditional "Merostomata" without reference to the Eurypterida.

The Merostomata are the only chelicerates to possess compound eyes; the arachnids have lost the ancestral arthropods' fully developed latero-anterior compound eyes.

The species is also known as horsefoot, king crab, or sauce-pan. Many people refer to the horseshoe crab as a "helmet crab", however this is a common misconception.

Limulus means "odd" and polyphemus refers to the giant in greek mythology.

Former scientific names include Limulus cyclops, Xiphosura americana and Polyphemus occidentalis.

Regeneration

Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.

Medical research

Horseshoe crabs are valuable as a species to the medical research community. The horseshoe crab has a simple but effective immune system. When a foreign object such as a bacterium enters through a wound in the animal's body, a substance called Limulus Amebocyte Lysate (LAL) almost immediately clots into a clear gel-like material, effectively trapping the foreign body. LAL is used to test for bacterial endotoxins in pharmaceuticals and for several bacterial diseases. If the bacterium is harmful, the blood will form a clot. Horseshoe crabs are helpful in finding remedies for diseases that have developed resistances to penicillin and other drugs. Horseshoe crabs are returned to the ocean after bleeding. Studies show that blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound. A single horseshoe crab can be worth $2,500 over its lifetime for periodic blood extractions.

Each compound eye has about 1,000 receptors or ommatidia. The cones and rods of the lateral eyes have a similar structure to those found in human eyes. Except that they are around 100 times larger in size. (DNR 2005).

Furthermore the nerves are large and relatively accessible. This has made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena like lateral inhibition working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus. Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by males when recognizing potential mates. It has also been said that it is able to see ultraviolet light.^{[citation needed]}

Hemocyanin

The blood of most molluscs, including cephalopods and gastropods, as well as some arthropods such as horseshoe crabs, contains the copper-containing protein hemocyanin at concentrations of about 50 g per litre. These creatures do not have hemoglobin (iron-containing protein) which is the basis of oxygen transport in vertebrates. Hemocyanin is colourless when deoxygenated and dark blue when oxygenated. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow, and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed. This is due to change in color of hemocyanin when it is oxidized. Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in mammals by hemoglobin in red blood cells.

Conservation

Limulus polyphemus 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 whelk and conch trapping.

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 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 over-harvesting 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 March 25, 2008.

Every year approximately 10% of the horseshoe crab breeding population dies when rough surf flips the creatures onto their backs, a position from which they often cannot right themselves. In response, the ERDG launched a "Just Flip 'Em" campaign, in the hopes that beachgoers will simply turn the crabs back over.

A large-scale project to tag and count horseshoe crabs along the north-American coast was underway in the spring and summer of 2008, termed projectlimulus.org.

References

ISBN links support NWE through referral fees

Botton, M. L. and J. W. Ropes. 1987. The horseshoe crab, Limulus polyphemus, fishery and resource in the United States. Marine Fisheries Review 49(3):57-61.

• Boudreaux H. B., 1979. Arthropod phylogeny with special reference to insects. John Willey & sons, New York, Chichester, Brisbane, Toronto. 320 pp.

• Department of Natural Resources (DNR). 2005. Anatomy of the Horseshoe Crab, Maryland Department of Natural Resources. Retrieved 12 August 2008.</ref>

• Hansson, H. G. (Comp.). 1998. NEAT (North East Atlantic Taxa): South Scandinavian marine and maritime Chelicerata and Uniramia. Checklist. Tjarno Marine Biological Laboratory. Retrieved September 20, 2008.

Marine Resources Research Institute (MRRI). n.d. Horseshoe crab. South Carolina Department of Natural Resources. Retrieved September 20, 2008.

World Conservation Monitoring Centre (1996). Limulus polyphemus. 2007 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on September 20, 2008.

External links

• The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research. Ecological Research and Development Group (2003). Retrieved May 14, 2006.
• http://www.saltwater-fish-tanks.com/fish/horseshoe-crab-conservation.php The Alarming Decrease in Population.
• http://www.ocean.udel.edu/horseshoecrab/Research/eye.html Biomedical Eye Research
• http://earthmattersfoundation.org/horse_shoe_crab.htm Timeless Traveller - The Horseshoe Crab
• http://animaldiversity.ummz.umich.edu/site/accounts/information/Limulus_polyphemus.html All about the horseshoe crab.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Horseshoe_crab  ^history
• Limulidae  ^history
• Xiphosura  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Horseshoe crab"

Note: Some restrictions may apply to use of individual images which are separately licensed.