Horseshoe crab

Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, 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. Horseshoe crabs are considered living fossils.

Limulidae is the only recent family of the order Xiphosurida and contains all four living species of horseshoe crabs:
 * Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in Southeast Asia
 * Limulus polyphemus, the Atlantic horseshoe crab, found along the northwest Atlantic coast and in the Gulf of Mexico
 * Tachypleus gigas, found in South and Southeast Asia
 * Tachypleus tridentatus, found along East Asian coasts

Anatomy
The entire body of the horseshoe crab is protected by a hard shell. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are 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. While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, 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 long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.

One synapomorphy for the order Xiphosura is the fusion of opisthosomal tergites behind opercular tergite (free abdominal segments) to form a thoracetron. Fusion of opisthosomal tergites to form a thoracetron has previously been considered a characteristic of the xiphosuran superfamilies Euproopoidea and Limuloidea, but evidence was presented by Anderson & Selden that fusion also occurs in Bellinuroidea. Giribet et al. used the character in their matrix and found it to be in a synapomorphy for Limulus and Carcinoscorpius.

Breeding
During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. Many shore birds eat the eggs before they hatch. The eggs take about 2 weeks to hatch. The larvae molt six times during the first year.

It has proven to be difficult to raise horseshoe crabs in captivity. There is reason to believe that mating only takes place in the presence of the sand or mud in which the horseshoe crab eggs were hatched. It is not known with certainty what in the sand is being sensed by the crabs nor how they sense it.

Blood
Unlike mammals, 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 for 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.

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% to 15%.

Fishery
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.

It is hypothesized that low horseshoe crab populations in the Delaware Bay endangers the future of the red knot. Red knots are long distance migratory shorebirds that feed on the protein-rich eggs during their stopover on the beaches of New Jersey and Delaware. There is an ongoing effort to develop adaptive management plans to regulate horseshoe crab harvests in the Bay in a way that protects migrating shorebirds.