[3] Their eggs were eaten by Native Americans,[4] but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine (especially for Limulus amebocyte lysate) and science.

[1] 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.

[14] 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.

[citation needed] The Nobel Prize in Medicine was awarded in 1967 in part for research performed on the horseshoe crab eye.

The simple eyes are probably important during the embryonic or larval stages of the organism,[18] and even unhatched embryos seem to be able to sense light levels from within their buried eggs.

This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena such as lateral inhibition working at the cellular level.

Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by males when recognizing potential mates.

The retinula (literally, "small retina") cells of the ommatidium of the compound eye contain areas from which membranous organelles of conceivable size (rhabdomeres) extend.

[20] The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper-containing protein hemocyanin at concentrations of about 50 g per liter.

[1] Outside the United States, the only breeding population is in Mexico's Yucatán Peninsula, where it is found on the western, northern and eastern coasts.

[4] Individuals rarely appear outside the breeding range, with a few records from the Atlantic coast of Canada (Lahave Island on Nova Scotia), the Bahamas, the Turks and Caicos, Cuba and the western Gulf of Mexico (Texas and Veracruz).

Historic claims of horseshoe crabs on Jamaica and Hispaniola (on the Dominican Republic's southeast coast) have not resulted in any discoveries after expeditions there.

[1] Atlantic horseshoe crabs range from shallow coastal habitats such as lagoons, estuaries and mangrove to depths of more than 200 m (660 ft) up to 56 km (35 mi) offshore.

[1] In Massachusetts, horseshoe crabs spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn, with the males arriving first.

The smaller male grabs on to the back of a female with a "boxing glove" like structure on his front claws, often holding on for months at a time.

[31] Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule.

The oldest known horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.

[39] The oldest member of the subfamily Limulinae are known from the Late Jurassic (Tithonian), belonging to the species Crenatolimulus darwini from Poland.

[30] Procuring the raw materials for LAL testing involves collecting and bleeding horseshoe crabs from wild populations.

[43] Animals that survive the process may be more lethargic when released and less likely to mate – which has prompted concerns about the longer-term impacts of harvesting horseshoe crabs.

[1] Early in the 20th century and possibly before, there was the mistaken belief in some areas that horseshoe crabs were destructive to fisheries, folklore held that they used their long spines to drill into some shellfish.

The Atlantic horseshoe crab is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for this animal's future.

According to the Horseshoe Crab Benchmark Stock Assessment Peer Review Report published by the Atlantic States Marine Fisheries Commission (ASMFC), the population continues to remain stable where biomedical is present in the Northeast and thrive and grow in the Southeast due to protection efforts – a trend spanning decades.

[49] Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on the horseshoe crabs' eggs for food during their spring migration.

[50] In 1991, the species was provided legislated protection from bait fishing in South Carolina by calling on the management and regulation of the horseshoe crab fisheries, allowing only hand-collecting for biomedical applications and marine biological research.

[51] Without the need for LAL in biomedical use, the legal protection of the horseshoe crab is not guaranteed in the future, and they would again fall prey to overfishing and use as bait.

In 1995, the nonprofit Ecological Research and Development Group (ERDG) was founded with the aim of preserving the four remaining species of horseshoe crab.

ERDG founder Glenn Gauvry designed a mesh bag for whelk/conch traps, to prevent other species from removing the bait.

[56] Every year, about 10% of the horseshoe crab breeding population dies when rough surf flips the creatures onto their backs, a position from which they often cannot right themselves.

[57] New Jersey beaches campaign "ReTURN the Favor", trains volunteers to rescue impinged and overturned horseshoe crabs while collecting data on natural and man-made hazards.