Horseshoe crab
Overview
The Atlantic horseshoe crab, Limulus polyphemus, is a marine
Marine biology
Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...

 chelicerate arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

. Despite its name, it is more closely related to spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s, tick
Tick
Ticks are small arachnids in the order Ixodida, along with mites, constitute the subclass Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians...

s, and scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

s than to crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s. Horseshoe crabs are most commonly found in the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 and along the northern Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 coast of North America. A main area of annual migration
Animal migration
Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is a ubiquitous phenomenon, found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local...

 is Delaware Bay
Delaware Bay
Delaware Bay is a major estuary outlet of the Delaware River on the Northeast seaboard of the United States whose fresh water mixes for many miles with the waters of the Atlantic Ocean. It is in area. The bay is bordered by the State of New Jersey and the State of Delaware...

, although stray individuals are occasionally found in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

.

The other three species in the family Limulidae
Limulidae
Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating...

 are also called horseshoe crabs.
Encyclopedia
The Atlantic horseshoe crab, Limulus polyphemus, is a marine
Marine biology
Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...

 chelicerate arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

. Despite its name, it is more closely related to spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s, tick
Tick
Ticks are small arachnids in the order Ixodida, along with mites, constitute the subclass Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians...

s, and scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

s than to crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s. Horseshoe crabs are most commonly found in the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 and along the northern Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 coast of North America. A main area of annual migration
Animal migration
Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is a ubiquitous phenomenon, found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local...

 is Delaware Bay
Delaware Bay
Delaware Bay is a major estuary outlet of the Delaware River on the Northeast seaboard of the United States whose fresh water mixes for many miles with the waters of the Atlantic Ocean. It is in area. The bay is bordered by the State of New Jersey and the State of Delaware...

, although stray individuals are occasionally found in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

.

The other three species in the family Limulidae
Limulidae
Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating...

 are also called horseshoe crabs. The Japanese horseshoe crab (Tachypleus tridentatus
Tachypleus tridentatus
Tachypleus tridentatus is a species of horseshoe crab found in China, Indonesia, Japan, Malaysia, the Philippines, Taiwan, and Vietnam....

) 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
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

: Tachypleus gigas
Tachypleus gigas
Tachypleus gigas is one of the four extant species of horseshoe crab. It is found in shallow water in South East Asia at depths of up to . It grows up to long and wide, and is covered by a sturdy carapace, with a long caudal spine .-Description:...

and Carcinoscorpius rotundicauda
Carcinoscorpius rotundicauda
The mangrove horseshoe crab, Carcinoscorpius rotundicauda, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders and scorpions than to crabs. It is the only species in the genus Carcinoscorpius...

. All four are quite similar in form and behavior.

Names and classification

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
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 crustaceans.

Limulus means "askew" and polyphemus refers to the giant in Greek mythology
Polyphemus
Polyphemus is the gigantic one-eyed son of Poseidon and Thoosa in Greek mythology, one of the Cyclopes. His name means "much spoken of" or "famous". Polyphemus plays a pivotal role in Homer's Odyssey.-In Homer's Odyssey:...

. It is based on the misleading idea that the animal had a single eye.

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

It is the tail that earns this order its name Xiphosura, which derives from the Greek for 'sword tail'.

Despite their common name, they are not crabs but are related to arachnids (spiders, scorpions, ticks and mites), and are presumably the closest living relatives of the now extinct trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s.

Anatomy and physiology

Horseshoe crabs have three main parts to the body: the head region, known as the 'prosoma', the abdominal
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 region or 'opisthosoma
Opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...

', and the spine-like tail or 'telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

'. The smooth shell or carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 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 percent larger than the male and can grow up to 60 centimetres (23.6 in) in length (including tail).

Horseshoe crabs possess the rare ability to regrow lost limbs
Regeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...

, in a manner similar to sea stars.
A wide range of marine species become attached to the carapace, including algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, flat worms, mollusks, barnacle
Barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s, and bryozoans, and horseshoe crabs have been described as 'living museums' due to the number of organisms that they can support. In areas where Limulus is common, the shells, exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

s or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.

The brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 and the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 are located in the prosoma. On the underside of the prosoma there are six pairs of appendages, the first of which (the small pincer
Pincer (biology)
A pincer is the part of an arthropod that enables it to carry loads, to defend against other creatures, or to attack prey. In insects, the pincers are usually part of the creature's mandible, and often venom or acid can be injected through the pincer into an enemy during a pincer strike.Some...

s or chelicerae
Chelicerae
The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...

) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax
Cephalothorax
The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

, between the chilicerae. Although most arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s have mandibles, the horseshoe crab is jawless.

The second pair of appendages, the pedipalps, are used as walking legs; in males they are tipped with 'claspers' which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The first four pairs of legs have claws, the last pair has a leaflike structure used for pushing.

The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist.

The telson (i.e., tail or caudal spine) is used to steer in the water and also to flip itself over if stuck upside down.
Among other senses, they have a small chemoreceptor organ which 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
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...

. The Nobel prize in Medicine was awarded in 1967 in part for research performed on the horseshoe eye.

There is a large compound eye on each side of the prosoma with monochromatic vision,Peak absorption is at 525 nm, five simple eyes on the carapace, and two simple eyes on the underside, just in front of the mouth, making a total of nine eyes. The simple eyes are probably important during the embryonic or larval stages of the organism, and even unhatched embryos seem to be able to sense light levels from within their buried eggs. The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood.

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, complex structures consisting of upwards of 300 cells. The ommatidia are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods. Each ommatidium feeds into a single nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

 fiber. Furthermore, the nerves are large and relatively accessible. This made it possible for electrophysiologists
Electrophysiology
Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...

 to record the nervous response to light stimulation easily, and to observe visual phenomena like lateral inhibition
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...

 working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus. Habituation
Habituation
Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...

 and classical conditioning
Classical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...

 to light stimuli have been demonstrated, as has the use of brightness and shape information by males when recognizing potential mates.

The retinula (literally, "small retina") cells of the ommatidium of the compound eye contain areas from which membranous organelles of conceivable size (rhabdomeres) extend. Rhabdomeres have tiny microvilli (tiny tubes extending out of the retinula) that interlock with neighboring retinular cells. This forms the rhabdom, which contains the dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

 of the eccentric cell, and may also contribute some microvilli. The only other species with an eccentric cell is the silkworm moth
Bombyx mori
The silkworm is the larva or caterpillar of the domesticated silkmoth, Bombyx mori . It is an economically important insect, being a primary producer of silk...

. Microvilli are composed of a double layer, 7 nm each and with 3.5 nm space of two electron-deficient boundaries in between. Where the microvilli meet, these outer borders fuse and yield five membranes about 15 nm thick. In all arthropods, there is always a rhabdom below a crystalline cone, on or near the center of the ommatidium, and always aligned with the path of light. At right angles to the length of the rhabdome are the length of the microvilli, which are in line with each other. The microvilli are about 40-150 nm in diameter.

Blood

The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

-containing protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...

 at concentrations of about 50 g per liter. These creatures do not have hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 (iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-containing protein), which is the basis of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 transport in vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s. Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. The blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 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. Hemocyanin carries oxygen in extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...

 fluid, which is in contrast to the intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...

 oxygen transport in vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s by hemoglobin in red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

s.

The blood of horseshoe crabs contains one type of blood cell, the amebocyte
Amebocyte
An amebocyte or amoebocyte is a mobile cell in the body of invertebrates such as echinoderms, mollusks or sponges. They move by pseudopodia. Similarly to some of the white blood cells of vertebrates, in many species amebocytes are found in the blood or body fluid and play a role in the defense of...

s. These play an important role in the defense against pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s. Amebocytes contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial endotoxin
Endotoxin
Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...

 is encountered. The resulting coagulation
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...

 is thought to contain bacterial infections in the animal's semiclosed circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...

.

Life cycle and behavior

The crabs feed on mollusks, annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

 worms, other benthic invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s, and bits of fish. Lacking jaws, it grinds up the food with bristles on its legs and a gizzard
Gizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

 that contains sand and gravel.

They spend the winters on the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 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. Often several males will hold on to a single female. Females reach the beach at high tide. After the female has laid a batch of eggs in a nest at a depth of 15–20 cm in the sand, the male or males fertilize them with their sperm. Egg quantity is dependent on the female's 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. Before becoming sexually mature around age 9, they have to shed their shells some 17 times. Longevity
Longevity
The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....

 is difficult to assess, but the average lifespan is thought to be 20–40 years.

Evolution

Horseshoe crabs were traditionally grouped with the extinct eurypterids
Eurypterus
Eurypterus is an extinct genus of sea scorpions. They existed during the Silurian Period, from around 432 to 418 million years ago.There are fifteen species belonging to the genus Eurypterus, the most common of which is Eurypterus remipes, the first eurypterid fossil discovered and the state...

 (sea scorpions) as the Merostomata
Merostomata
Merostomata is the name given to a grouping of the extinct Eurypterida and the Xiphosura . The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups....

. They may have evolved in the shallow seas of the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 Era (570-248 million years ago) with other primitive arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s like the trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s. The four species of horseshoe crab are the only remaining members of the Xiphosura
Xiphosura
Xiphosura is an order 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...

, one of the oldest classes of marine arthropods.

The extinct diminutive horseshoe crab, Lunataspis aurora
Lunataspis
Lunataspis aurora is the oldest known xiphosuran and an ancestor of the modern horseshoe crab. It was first formally described by David Rudkin, Graham Young and Godfrey Nolan, from fossils found in northern Manitoba, Canada in 2005; the deposit dates from the Late Ordovician, ....

, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 strata in Manitoba
Manitoba
Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...

.

Horseshoe crabs are often referred to as living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...

s, as they have changed little in the last 445 million years. Forms almost identical to this species were present during the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period 230 million years ago, and similar species were present in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, 400 million years ago. However, the Atlantic horseshoe crab itself has no fossil record at all, and the genus Limulus "ranges back only some 20 million years, not 200 million."

Medical uses

Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. The above-mentioned clotting reaction of the animal's blood is used in the widely used Limulus Amebocyte Lysate
Limulus Amebocyte Lysate
Limulus amebocyte lysate is an aqueous extract of blood cells from the horseshoe crab, Limulus polyphemus. LAL reacts with bacterial endotoxin or lipopolysaccharide , which is a membrane component of Gram negative bacteria...

 (LAL) test to detect bacterial endotoxin
Endotoxin
Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...

s in pharmaceuticals and to test for several bacterial diseases.

Enzymes from horseshoe crab blood are used by astronauts in the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...

 to test surfaces for unwanted bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and fungi.

A protein from horseshoe crab blood is also under investigation as a novel antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

.

LAL is obtained from the animals' blood. Horseshoe crabs are returned to the ocean after bleeding, although some 3% die during the process. 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.

Conservation and management

Limulus polyphemus is not presently endangered
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

, but harvesting and habitat destruction
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...

 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
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

, whelk
Whelk
Whelk, also spelled welk or even "wilks", is a common name used to mean one or more kinds of sea snail. The species, genera and families referred to using this common name vary a great deal from one geographic area to another...

 and conch
Conch
A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

 trapping.

Conservationists have also voiced concerns about the declining population of shorebirds
Wader
Waders, called shorebirds in North America , are members of the order Charadriiformes, excluding the more marine web-footed seabird groups. The latter are the skuas , gulls , terns , skimmers , and auks...

, such as red knot
Red Knot
The Red Knot, Calidris canutus , is a medium sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia. It is a large member of the Calidris sandpipers, second only to the Great Knot...

s, which rely heavily on the horseshoe crabs' eggs for food during their spring migration
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

. 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 http://www.horseshoecrab.org/misc/erdg.html (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
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...

, these mesh bags are mandatory in whelk/conch fishery. The Atlantic States Marine Fisheries Commission
Atlantic States Marine Fisheries Commission
The Atlantic States Marine Fisheries Commission is a commission of U.S. states formed to coordinate and manage fishery resources — including marine fish, shellfish, and anadromous fish - along the Atlantic coast of the United States.The Commission was formed by the 15 Atlantic coast states in...

 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
Delaware Bay
Delaware Bay is a major estuary outlet of the Delaware River on the Northeast seaboard of the United States whose fresh water mixes for many miles with the waters of the Atlantic Ocean. It is in area. The bay is bordered by the State of New Jersey and the State of Delaware...

. 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 March 25, 2008.

Every year, about 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.
Due to the lack of information and knowledge regarding horseshoe crab populations, the management policies lack any abundance of rules and regulations. To implement management policies for the species, more population information needs to be attained.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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