page_title: Atlantic horseshoe crab
section_title: Anatomy and physiology
hierarchical_section_title: Atlantic horseshoe crab / Anatomy and physiology
caption_attribution_description: English: Dead female horseshoe crab. Location: Patuxent River, Maryland
context_page_description: The Atlantic horseshoe crab, also known as the American horseshoe crab, is a species of marine and brackish chelicerate arthropod. Despite their name, horseshoe crabs are more closely related to spiders, ticks, and scorpions than to crabs. This species is found in the Gulf of Mexico and along the Atlantic coast of North America. The main area of annual migration is Delaware Bay along the South Jersey Delaware Bayshore.
  Their eggs were eaten by Native Americans, but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine and science. They play a major role in the local ecosystems, with their eggs providing an important food source for shorebirds, and the juveniles and adults being eaten by sea turtles.
  The other three extant species in the family Limulidae are also called horseshoe crabs, but they are restricted to Asia.
context_section_description: Horseshoe crabs have three main parts to the body: the head region, known as the "prosoma", the abdominal region or "opisthosoma", and the spine-like tail or "telson". The smooth shell or carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. The sexes are similar in appearance, but females are typically 25 to 30% larger than the male in length and width, and can reach more than twice the weight. Females can grow up to 60 cm (24 in) in length, including tail, and 4.8 kg (11 lb) in weight. There are also geographic differences: In the United States, there is a north–south cline in the size. Central animals, between Georgia and Cape Cod, are the largest. North from Cape Cod they gradually become smaller and south from Georgia they gradually become smaller. In Delaware Bay, females and males have an average carapace width of about 25.5 cm (10.0 in) and 20 cm (7.9 in), respectively. In Florida, females average about 21.5 cm (8.5 in) and males 16 cm (6.3 in). However, this north-south pattern does not exist in the Yucatán Peninsula, the southernmost population of the species, where some subpopulations are intermediate between the Delaware Bay and Florida horseshoe crabs, and others are smaller, averaging about ​³⁄₄ the size of Florida horseshoe crabs.
  Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.
  A wide range of marine species become attached to the carapace, including algae, flat worms, mollusks, barnacles, and bryozoans, and horseshoe crabs have been described as 'walking museums' due to the number of organisms they can support. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.
  The brain and the heart are located in the prosoma. On the underside of the prosoma, six pairs of appendages occur, the first of which (the small pincers or chelicerae) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax, between the chelicerae. Although most arthropods have mandibles, the horseshoe crab is jawless.
  The second pair of appendages, the pedipalps, are used as walking legs; in males they are tipped with 'claspers', which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The first four pairs of legs have claws, the last pair has a leaf-like structure used for pushing.
  The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, 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.
  The telson (i.e., tail or caudal spine) is used to steer in the water and also to flip itself over if stuck upside down.
  Among other senses, they have a small chemoreceptor organ that senses smells on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.
caption: Underside of a female showing the legs and book gills.
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