Temporal range: Ordovician-present
Leach, 1819 
They are invertebrates, meaning that they lack a spine. Horseshoe crabs 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, population declines have 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.
Horseshoe crabs superficially resemble crustaceans but belong to a separate subphylum, Chelicerata, and are closely related to arachnids. The earliest horseshoe crab fossils are found in strata from the late Ordovician period, roughly 450 million years ago.
The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, and a pair of rudimentary lateral eyes on the top. The latter become functional just before the embryo hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. The horseshoe crab has five additional eyes on top of its shell. Despite having relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans', and their eyes are a million times more sensitive to light at night than during the day. The mouth is located in the center of the legs, whose bases are referred to as gnathobases and have the same function as jaws and help grind up food. The horseshoe crab has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip, except for the last pair.
|Rendezvous with a Horseshoe Crab, August 2011, 4:34, NewsWorks|
|The Horseshoe Crab Spawn, June 2010, 5:08, HostOurCoast.com|
|Horseshoe Crabs Mate in Massive Beach "Orgy", June 2014, 3:29, National Geographic|
As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, and are also occasionally used for swimming. They are more often 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. Often, several males surround the female and all fertilize together, which makes it easy to spot and count females as they are the large center carapace surrounded by 3-5 smaller ones. The female digs a hole in the sand and lays her eggs while the male(s) fertilize them. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. The eggs take about two weeks to hatch; shore birds eat many of them before they hatch. The larvae molt six times during the first year. Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. It is not known with certainty what is in the sand that the crab can sense, nor how they sense it.
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 similar role to the 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. This requires the blood to be harvested, which 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%. Approximately 500,000 crabs are harvested annually.
In 2016, it was reported in Scientific American that the mortality rate of bled horseshoe crabs could be 29%, higher than the rate of 15% that was generally previously estimated. Bleeding may also prevent female horseshoe crabs from being able to spawn or decrease the number of eggs they are able to lay. Up to 30% of an individual's blood is removed, according to the biomedical industry, and the horseshoe crabs spend between one and three days away from the ocean before being returned. Some scientists are skeptical that certain companies return their horseshoe crabs to the ocean at all, instead suspecting them of selling the horseshoe crabs as fishing bait.
Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk, or conch. However, fishing with horseshoe crab is temporarily forbidden in New Jersey (moratorium on harvesting) and restricted to males in Delaware. A permanent moratorium is in effect in South Carolina. The eggs are eaten in parts of Southeast Asia and China.
A low horseshoe crab population in the 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.