The other three species in the family Limulidae are also called horseshoe crabs. 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. All four are quite similar in form and behavior.
Limulus means "odd and polyphemus refers to the giant in greek mythology.
Former scientific names include Limulus cyclops, Xiphosura americana and Polyphemus occidentalis.
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), 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 in length (including tail); the female is typically 25 to 30 percent larger than the male.
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 bulbuous claw that serves to lock the male to the female while she deposits the eggs and he waits to fertilize them.
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 males when recognizing potential mates. It has also been said that it is able to see ultraviolet light.
Among other senses, they have a small sense organ on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.
Before becoming mature around age 9, they have to shed their shells some 17 times. They can live for as long as 31 years.
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 female 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. They reach sexual maturity in approximately eleven years and may live another 10-14 years beyond that.
Horseshoe crabs are stem group chelicerates, thus distant relatives of spiders. They 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. The four species of horseshoe crab are the only remaining members of the class Merostomata, 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.
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.
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. 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. A single horseshoe crab can be worth $2,500 over its lifetime for periodic blood extractions.
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.
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. 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. Harvesting of the crabs was banned in New Jersey March 25, 2008.
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