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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. They can grow up to 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.

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 shellThe hard, rigid outer calcium carbonate covering of certain animals is called a shell . While many animals, particularly those that live in the sea, produce exoskeletons, usually only those of mollusks are considered to be shells. It is sometimes said tha of these animals consists of three parts. The carapaceThe carapace is the dorsal, convex part of the shell structure of the tortoise, consisting of an external layer of horny material, divided into large plates known as scutes which overlie a layer of interlocking bones. This construction allows for a sturdy is the smooth frontmost part of the crab; it has on it the eyeThis article refers to the sight organ. See Eye (disambiguation) for other usages. human eye. Note that not all eyes have the same anatomy as a human eye. An eye is an organ that detects light. Different kinds of light-sensitive organ are found in a varies, the walking legs, the chelicera (pincers), the mouth, the brainFor other articles about other subjects named brain see brain (disambiguation). In the anatomy of animals, the brain or encephalon is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate, and the heartThis page is about the muscular organ, the Heart . For other meanings of the word, see Heart (disambiguation). The heart ( Latin cor is a hollow, muscular organ that pumps blood through the blood vessels by repeated, rhythmic contractions. The term cardia. The abdomenThe abdomen (from the Latin word meaning "belly") is the part of the body between the pelvis and the thorax. The front of the abdomen is the abdominal cavity, which is separated from the thoracic cavity by the diaphragm. The lining of the abdomen is calle 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 visionVision can refer to: Visual perception is one of the senses. vision (religion) "Vision" can mean an imagined picturing originating with not-so-divine sources: politicians, business planners and change gurus play on this sense of the word. A company vision. It has a compound eyedragonfly A compound eye is a visual organ found in certain arthropods (some insects and crustaceans). The compound eye consists of between 12 and 1,000 ommatidia, little dark/bright sensors. The image perceived by the arthropod is "recalculated" from the, 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, behavioural 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 recognising potential mates.

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.

Underside of a female with legs and book lungs apparent.

Horseshoe crabs can live for sixteen to seventeen 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 15-20cm 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 split and new membrane, secreted by the embryo, forms a transparent spherical capsule" (Sturtevant). The larvae form and these larvae swim for about five to seven days. After swimming they settle, and begin the first molt: this happens approximately twenty days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters. During this time, molting still takes place for two to three years. They reach sexual maturity in five to seven years. The main cause of death during these days is the fish bait industry, which collects horseshoe crabs to make bait for lobsters and other catches.

Some arthropods have mandibles, however the horseshoe crab is jawless.

Horsehoe crabs are distant relatives of the spider 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.

No living species of horseshoe crab is endangered yet, but harvesting and habitat destruction have reduced their numbers at some locations and caused some concern for these animals' future.