Horseshoe crab

The horseshoe crab (Limulus polyphemus), also known as king crab, 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. A Japanese variant (Tachypleus tridentatus) is found in the Seto Inland Sea, but is considered an endangered species through loss of habitat. They can grow up to 20 inches (51 cm), on a diet of mollusks, worms, and other invertebrates. They find this 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.

Related Topics:
Arthropod - Spider - Crab - Gulf of Mexico - Atlantic - Seto Inland Sea - Inches - Cm - Mollusk - Worm - Invertebrate - Squid - Shrimp - Cephalothorax - Pincer

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Horseshoe crabs possess five pairs of book lungs (sometimes known as 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; it has on it 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.

Related Topics:
Book lung - Appendage - Shell - Carapace - Eye - Brain - Heart - Abdomen - Telson

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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.

Related Topics:
Vision - Compound eyes - Ommatidium - Nerve - Electrophysiologists - Habituation - Classical conditioning

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Since 1964 a substance in 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 (except in Massachusetts, where their return is prohibited), so this is not necessarily a threat to the survival of horseshoe crabs. Horseshoe crabs have blue blood.

Related Topics:
1964 - Blood - Endotoxin - Massachusetts - Blue blood

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Horseshoe crabs can live for sixteen to nineteen years. They migrate into the shore in late spring, with the male arriving first. The female then arrives and makes a nest at a depth of 15-20 cm in the sand. In the nest, females deposit eggs which are subsequently fertilized by the male. Studies conducted in Delaware have revealed an average of 3,650 eggs laid per nest. "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 for two to three years. They reach sexual maturity in five to seven years. The main threat during this period is the fish bait industry, which collects horseshoe crabs to make bait for lobsters and other catches.

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

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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.

Related Topics:
Eurypterids - Paleozoic - Trilobite - Classes - Living fossil

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