About the Species
Horseshoe crabs belong to the phylum of Arthropods, which consists of animals having an articulated body and limbs. The three major classes of Arthropods are Insects, Arachnids and Crustaceans.
The
horseshoe belongs to its own class called Merostomata, which means "legs attached to the mouth." Though they are called "crabs," a quick look at their taxonomy shows that they're not. In fact, they are most closely related to trilobites that existed 544 million years ago.
Horseshoe
crabs (Limulidae) are currently represented by four species including Limulus polyphemus (1), which is found along the eastern coast of North and Central America, and three IndoPacific species, Tachypleus gigas (4), Tachypleus tridentatus (3) and Carcinoscorpius rotundicauda (2). All four species are similar in terms of ecology, morphology, and serology. http://goo.gl/19gj1o
Morfology
Prosoma
Opisthosoma
Telson
Lifecyle
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Spawning
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When the Limuli head for shore, the males patrol along the foot of the beach, awaiting the females. The female horseshoes give off chemical attractants called pheromones, which the males can detect. Although there may be other means of identification, these attractants, the directional movement and the number of males involved (often several times the number of females) reduce the liklihood of a female reaching the beach unattended. Males, who are about 20% smaller than females, use a specially developed appendage to "clasp" themselves onto the back of the female. During peak spawning times, the horseshoes will form a dense "huddles" along the edge of the water, with 5 or 6 males grouped around one female.
By the beginning of the spawning season, each female will have developed about 80,000 eggs, which are located in dense masses near the front of her shell. She will return to the beach on successive tides, laying 45 clutches of eggs with each tide. Each cluster contains about 4,000 eggs and a female will lay about 20 egg clusters each year.
Ecological Importance
Horseshoe crabs play an important ecological role in the food web. A decline in the number of horseshoe crabs will impact other species, particularly shorebirds and sea turtles, a federally-listed threatened species that uses the Chesapeake Bay as a summer nursery area. Shorebirds primarily feed on horseshoe crab eggs exposed on the surface, but sufficient surface eggs are available only if horseshoe crabs are spawning at high densities.
Therefore, adequate spawning densities must be maintained to ensure availability of horseshoe crab eggs for shorebirds. Sea turtles feed on adult horseshoe crabs, but their diet depends on relative abundance of the prey species.