Horseshoe crab are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, a decline in the number of individuals has occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand. Because of their origin 450 million years ago, Horseshoe crabs are considered living fossils.

Horseshoe crabs resemble crustaceans, but belong to a separate subphylum, Chelicerata, and are closely related to arachnids, e.g., spiders and scorpions. The earliest horseshoe crab fossils are found in strata from the late Ordovician period, roughly 450 million years ago.

The Limulidae are the only recent family of the order Xiphosura, and contain all four living species of horseshoe crab:

Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in Southeast Asia
Limulus polyphemus, the Atlantic horseshoe crab, found along the American Atlantic coast and in the Gulf of Mexico
Tachypleus gigas, found in Southeast and East Asia
Tachypleus tridentatus, found in Southeast and East Asia

The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1000 ommatidia, plus a pair of median eyes that are able to detect both visible light and UV-light, a single endoparietal eye and a pair of rudimentary lateral eyes on the top. The latter becomes functional just before the embryo hatches. There is also a pair of ventral eyes located near the mouth, as well as a cluster of photoreceptors on the telson. Despite having a relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans'. The mouth is located in the center of the legs, where their base have the same function as jaws and helps grinding up food. It has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip except the last pair. The long, straight, rigid tail can be used to flip itself over if turned upside down, so a horseshoe crab with a broken tail is more susceptible to desiccation or predation.

Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that supports the book gills. Horseshoe crab normally swim upside down, inclined at about 30? to the horizontal and moving at about 10?15 cm/s (0.22?0.34 mph). Despite this, they usually are found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish.

Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 cm (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.

During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. Shore birds eat many of the eggs before they hatch. The eggs take about two weeks to hatch. The larvae molt six times during the first year.

Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates mating only takes place in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. Neither what is in the sand that the crab can sense nor how they sense it is known with certainty.

Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a role similar to white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins in medical applications. The blood of horseshoe crabs is harvested from living horseshoe crabs for this purpose.

Harvesting horseshoe crab blood involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation. Estimates of mortality rates following blood harvesting vary from 3-15% to 10-30%.

Horseshoe crab are used as bait to fish for eels (mostly in the United States) and whelk. However, fishing horseshoe crab is temporarily forbidden in New Jersey (moratorium on harvesting) and restricted to only males in Delaware. A permanent moratorium is in effect in South Carolina.

A low horseshoe crab population in Delaware Bay is hypothesized to endanger the future of the red knot. Red knots, long-distance migratory shorebirds, feed on the protein-rich eggs during their stopovers on the beaches of New Jersey and Delaware. An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.