horseshoe crab

Horseshoe crab
Limulus polyphemus
Conservation status
Near Threatened (IUCN 2.3)[1]
Scientific classification
Domain:	Eukarya
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Merostomata[2]
Order:	Xiphosura
Family:	Limulidae
Genus:	Limulus
Species:	L. polyphemus
Binomial name
Limulus polyphemus Linnaeus, 1758

The horseshoe crab or Atlantic horseshoe crab (Limulus polyphemus) is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs.^[3] Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is Delaware Bay, although stray individuals are occasionally found in Europe.^[4]

The other three species in the family Limulidae are also called horseshoe crabs.^[5] 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.^[6] All four are quite similar in form and behavior.

The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.^[7]

Names

This group of animals is also known as horsefoot, king crab, 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. King crab is also more usually applied to a group of decapod crustaceans.

Limulus means "odd"^[8] and polyphemus refers to the giant in Greek mythology.^[8] 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.

The horseshoe crab is a 'living fossil': forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, a staggering 400 million years ago. (However, as Gould points out, the Atlantic horseshoe crab has no fossil record at all, and the genus Limulus "ranges back only some 20 million years, not 200 million."^[9])

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 trilobites. 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'. It is the tail that earns this order its name Xiphosura, which derives from the Greek for 'sword tail'. The sexes are similar in appearance, but females are much larger than males. The carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour.

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 'living museums' due to the number of organisms that they can support. On the underside of the prosoma there are six pairs of appendages, the first of which (the chelicera) are used to pass food into the mouth. The second pair, 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 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 are used in breathing. There is a compound eye on each side of the prosoma, five eyes on the top of the carapace, and two eyes on the underside, close to the mouth, making a total of nine eyes. In addition, the tail bears a series of light-sensing organs along its length. A further unique and intriguing feature of this ancient species is that it has blue copper-based blood.^[10]

Shell description

Seashell topics
About mollusc shells:
snail shells
clam shells
tusk shells
chitons
nacre
conchology
About other seashells:
crustacean shells
horseshoe crabs
echinoderm tests
brachiopod shells
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Remains of a horseshoe crab on a beach

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 shell of these animals consists of three parts. The carapace is the smooth frontmost part of the crab which 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 leaflike structure used for pushing)^[11], the mouth in between the legs, the brain, and the heart. The abdomen is the middle 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 is used to steer in the water and also to flip itself over if stuck upside down.

The horseshoe crab can grow up to 60 centimetres (24 in) in length (including tail); the female is typically 25 to 30 percent larger than the male.^[12]

Anatomy and physiology

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

Underside of a female showing the legs and book gills.

Horseshoe crabs possess five pairs of book gills, located just behind their appendages, 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.

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. 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 bulbous claw that serves to lock the male to the female while she deposits the eggs and he waits to fertilize them.

The Horseshoe Crab has blue blood, as it uses copper rather than iron as the base of its system.

Limulus has been extensively used in research into the physiology of vision. It has four eyes, of which two are large compound eyes^[13] composed of about 1000 receptors called ommatidia^[14]. Each ommatidium feeds into a single nerve fibre. Furthermore the nerves are large and relatively accessible. This 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.

Limulus has two large compound eyes on the sides of its head, which have monochromatic vision.^{[note 1][15]} The individual ommatidia are complex, consisting of upwards of 300 cells;^[16] they number around a thousand,^[16] and are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods.^[15] An additional simple eye is positioned at the rear of each of these structures.^[15] In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils.^[15] A further simple eye is located beneath these, on the underside of the carapace.^[15] A further pair of simple eyes are positioned just in front of the mouth.^[15] The simple eyes are probably important during the embryonic or larval stages of the organism,^[15] and even unhatched embryos seem to be able to sense light levels from within their buried eggs.^[16] The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood.^[15]

The individual ommatidia of the compound eyes of Limulus Among other senses, they have a small scent organ which senses smells^{[citation needed]} on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.

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

Life cycle and behaviour

Horseshoe crab molting

Before becoming mature around age 9, they have to shed their shells some 17 times.^[12] They can live for as long as 20 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.^[12]

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 the female's body size and ranges from 15,000-64,000 eggs per female.^[17]

Evolution

Horseshoe crabs are stem group chelicerates,^[18] thus distant relatives of spiders. They were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. They may have evolved in the shallow seas of the Paleozoic Era (570-248 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. Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years.^[12]

Regeneration

Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.^[19]

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 litre.^[10] 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,^[10] and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed.^[10] Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in vertebrates by hemoglobin in red blood cells.^[10]

The blood of horseshoe crabs contains one type of blood cell, the amebocytes. These play an important role in the defense against pathogens. Amebocytes contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial endotoxin is encountered. The resulting coagulation is thought to contain bacterial infections in the animal's semi-closed circulatory system.^[20] This is known as the Limulus amebocyte lysate (LAL) test.

Enzymes from horseshoe crab blood are used by astronauts in the International Space Station to test surfaces for unwanted bacteria and fungi ^[21]

Medical research and testing

Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. The above-mentioned clotting reaction is used in the Limulus Amebocyte Lysate (LAL) test to detect bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases.^[8] LAL is obtained from the animals' blood.

Horseshoe crabs are also used in finding remedies for diseases that have developed resistances to penicillin and other drugs.

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.^[22] A single horseshoe crab can be worth US$2,500 over its lifetime for periodic blood extractions^{[citation needed]}.

Conservation & Management

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 eel, 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.^[23]

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.^[24] 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.^[25][26] Harvesting of the crabs was banned in New Jersey March 25, 2008.^[27]

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.^[12]

Horseshoe crab, top view

Horseshoe crab, bottom view

At Newport Aquarium

Due to the lack of information and knowledge regarding horseshoe crab populations, the management policies lack any abundance of rules and regulations. In order to implement management policies for the species, more population information needs to be attained.^[28]

Notes

1. ^ Peak absorption is at 525nm

References

External links

Wikimedia Commons has media related to: Limulus polyphemus

Wikispecies has information related to: Limulus polyphemus

• "The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research". Ecological Research and Development Group. 2003. http://www.horseshoecrab.org. Retrieved May 14 2006. 
• The Alarming Decrease in Population.
• Biomedical Eye Research
• Timeless Traveller - The Horseshoe Crab
• All about the horseshoe crab.
• Horseshoe crab anatomy
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