Horseshoe crab

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{{Taxobox | color = pink | name = Horseshoe crab | image = horseshoe crab pair.jpg | image_width = 200px | image_caption = Pair of horseshoe crabs | regnum = Animalia | phylum = Arthropoda | classis = Xiphosura | ordo = Xiphosurida | familia = Limulidae | genus = Limulus | species = L. polyphemus | binomial = Limulus polyphemus | binomial_authority = Linnaeus, 1758 }}

The horseshoe crab (Limulus polyphemus) is an arthropod that is more closely related to spiders than crabs. They are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A Japanese variant (Tachypleus tridentatus) is found in the Seto Inland Sea, but is considered an endangered species because of loss of habitat. They can grow up to 20 inches (51 cm), on a diet of mollusks, annelid worms, and other invertebrates. They find these prey under the sand, where they spend most of their lives. In captivity, its diet should be supplemented with meaty items such as pieces of squid and shrimp (Foster and Smith, 2004). Its mouth is located in the middle of the underside of the cephalothorax. A pair of pincers (chelicerae) for seizing food are found on each side of the mouth.

1 Physical description
2 Regeneration
3 Medical Research
4 Conservation
5 External links

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Physical description

Image:Horseshoe crab female.jpg 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 lungs remain moist. The outer shell of these animals consists of three parts. The carapace is the smooth frontmost part of the crab which contains the eyes, the walking legs, the chelicera (pincers), the mouth, 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 which is used to flip itself over if stuck upside down.

Limulus has been extensively used in research into the physiology of vision. It has four compound eyes, and 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 male Limuli when recognizing potential mates.

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

Image:Limuluskils4.jpg

Horseshoe crabs can live for sixteen to nineteen years. They migrate into the shore in late spring, with the males arriving first. The females then arrive and make nests at a depth of 15-20 cm in the sand. In the nests, females deposit eggs which are subsequently fertilized by the male. Studies conducted in Delaware have revealed an average of 20,000 eggs per crab. "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.

Although most arthropods have mandibles, the horseshoe crab is jawless. The horseshoe crab's mouth is located in the center of the body. 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.

Their body also contains a cartilaginous tissue.

Horsehoe 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. Horseshoe crabs are one of the oldest classes of marine arthropods, and are often referred to as "living fossils", as they have not changed much in the last 350 to 400 million years. Image:Horseshoe crab male pedipalp.jpg

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Regeneration

Horseshoe crabs possess the rare ability to regrow limbs lost, in a manner similar to sea stars. This attribute was recently proven by Sue Shaller of the United States Fish and Wildlife Service.

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Medical Research

Horseshoe crabs are extremely valuable as a species to the medical research community. Since 1964 a substance made from their blood called Limulus Amebocyte Lysate (LAL) has also been used to test for bacterial endotoxins in pharmaceuticals and for several bacterial diseases. The animals can be returned to water after extraction of a portion of their blood, so this is not necessarily a threat to the survival of horseshoe crabs. A single horseshoe crab can be worth $2,500 over its lifetime for periodically drawing its blood for this extract. Horseshoe crabs have blue blood.

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Conservation

Limilus polyphemus is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for these animals' future. Since the 1970s, the horseshoe crab population has been decreasing, owing to several factors, including the use of the crab as bait in conch trapping.

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 conch traps, to prevent other species from taking off with the bait. This has led to the amount of bait needed being decreased by approximately 50%. In the state of Virginia, these mesh bags are now mandatory in conch fishery.

Every year, around 10% of the horseshoe crab's 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 (Ecological Resource and Development Group) launched a "Just Flip 'Em" campaign, in the hopes that beachgoers will simply turn the crabs back over.

Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on horseshoe crabs for food during their migration. Declines in the populations of these birds have been observed recently. Other predators of horseshoe crabs, such as the currently threatened Atlantic loggerhead turtle, have also suffered as crab populations diminish. [1]

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