At one time, there were many species of horseshoe crabs that are believed to have occupied all ancient seas, however only four have survived, three of which live along the shores of Southeast Asia and nearby islands. In North and South America, they range intermittently from Yucatan peninsula to northern Maine. Along the Atlantic coast, horseshoe crabs are most abundent between Virginia and New Jersey with Delaware Bay at the center of the species distribution. Horseshoe crabs molt or shed their shell to grow. Molting occurs several times during the first two to three years and about once a year afterwards. Once reaching sexual maturity it is believed they no longer molt. Mature horseshoe crabs repeat an annual spring migration to inshore spawning areas. If a horseshoe crab can survive the rigors of spawning, it may live to greater than 20 years of age.

Horseshoe crabs play an important role in the food web. A decline in the number of crabs will impact other species, particularly shorebirds and sea turtles. Therefore, adequate spawning densities must be maintained to ensure availability of horseshoe crab eggs for shorebirds. Sea turtles feed on adult horseshoe crabs, but their diet depends on relative abundance of prey species.

Horseshoe crabs mature slowly, requiring nine to eleven years to attain sexual maturity and are thus vulnerable to multiple forms of environmental pollution and human exploitation. It is belived that the combination of such pressures as shoreline development and habitat loss, bait harvesting, biomedical uses and pollution all play a role in effecting the population structure along the East Coast. Estimates of population structure and numbers are under going some debate although everyone agrees that horseshoe crabs were far more numerous only 40 or more years ago.