Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins.

Horseshoe crabs are closely related to the extinct eurypterids (sea scorpions), which include some of the largest arthropods to have ever existed, and the two may be sister groups.

[17][18][19][6][7] This assumption was challenged when a genetics-based phylogeny found horseshoe crabs to be the sister group to the ricinuleids, thereby making them an arachnid.

[24] The existence of this trend is likely due to a combination of two things: Like all arthropods, horseshoe crabs have segmented bodies with jointed limbs, which are covered by a protective cuticle made of chitin.

Ommatidia are arranged messily in what's been deemed an "imperfect hexagonal array" and have a highly variable number of photoreceptors (between 4 and 20) in their retina.

[28] Horseshoe crabs have relatively poor vision, and to compensate for that, have the largest rods and cones of any known animal, about 100 times the size of humans'.

While the first functions similarly to the pigment in the compound eyes, the second has a peak absorption of around 360 nanometers, allowing the animal to see ultraviolet light.

[25]: 558  Rather than using iron-based hemoglobin, horseshoe crabs transport oxygen with a copper-based protein called hemocyanin, giving its blood a bright blue color.

[25]: 556  While they appear smooth on the outside, the insides of these book gills are lined with several thin "pages" called lamellae.

[25]: 558  Each lamella is hollow and contains an extension of hemocoel, allowing gasses to diffuse between a Horseshoe crab's blood and external environment.

[25]: 558  There are roughly 80–200 lamellae are present in each gill, with all ten of them giving the animal with a total breathing surface area of about two square meters.

[32]: 94  For harder prey, Horseshoe crabs use a pair of stout, cuspid gnathobases (informally known as "nutcrackers") on the back of their sixth legs.

[25]: 557  The gizzard is a muscular, toothed organ that serves to pulverize the food from the crop and regurgitate any indigestible particles.

[25]: 557  Before and following digestion, the midgut lining (epithelium) secretes a peritrophic membrane made of chitin and mucoproteins that surrounds the food and later the feces.

[25]: 558  Similar to many aquatic animals, horseshoe crabs have an ammonotelic metabolism and eliminate ammonia and other small toxins through diffusion with their gills.

[25]: 558  After being processed in the midgut, waste is passed into a muscular tube known as the hindgut or rectum and then excreted from a sphincter known as the anus.

A notable exception is the tri-spine horseshoe crab, whose range extends northward to the coasts of China, Taiwan, and Southern Japan.

[34] The American species lives from the coast of Nova Scotia to the northern Gulf of Mexico, with another population residing around the Yucatán Peninsula.

[43] In the largest species, T. tridentatus, females can reach as much as 79.5 cm (31+1⁄4 in) long, including their telson, and up to 4 kg (9 lb) in weight.

[47][48] During the breeding season (spring and summer in the Northeast US, year-round in warmer locations) horseshoe crabs migrate to shallow coastal waters.

[52] Some evidence indicates that mating takes place only in the presence of the sand or mud in where horseshoe crab eggs have previously hatched.

There have been numerous reports of poisonings after consuming mangrove horseshoe crabs (Carcinoscorpius rotundicauda) as its meat contains tetrodotoxin.

[59] A ban on catching female crabs was put in place in Delaware, and a permanent moratorium is in effect in South Carolina.

[80] Without the approval for the classification as an industry standard testing material, U.S. companies will have to overcome the scrutiny of showing that rFC is safe and effective for their desired uses, which may serve as a deterrent for usage of the horseshoe crab blood substitute.

This method of harvesting has led to an unbalanced sex ratio in the wild, something that also contributes to the area's declining population.

Facing a greater than 90% decrease in T. tridentatus juveniles, it is suspected that Hong Kong will be the next to declare tri-spine horseshoe crabs as extinct from the area.

[83] To preserve and ensure the continuous supply of horseshoe crabs, a breeding center was built in Johor, Malaysia where animals are bred and released back into the ocean in the thousands once every two years.

Red knots, long-distance migratory shorebirds, feed on the protein-rich eggs during their stopovers on the beaches of New Jersey and Delaware.

[85] An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.

[87] The ban supports the conservation goals of the refuge, spanning 66,000 acres (26,700 hectares) of marshes, beaches, and islands near Charleston.