Yucatan Peninsula and the Gulf of Mexico.
Biology/Life History of the Horseshoe Crab Horseshoe crabs are among the world’s oldest and most fascinating creatures. The earliest horseshoe crab species had already inhabited Earth at least 200 million years before the dinosaurs arrived or about 400 million years ago. Chitin is a substance found in the shells, or exoskeleton, of horseshoe crabs, as well as other arthropods, such as lobsters, crabs, shrimp, spiders, beetles, and mosquitoes. Chitin has received a lot of attention from scientists because it is non-toxic and biodegradable. Horseshoe crabs don’t have antennae or jaws and have six pairs of legs. Horseshoe crabs are in a class of their own: Merastomata, which means “mouth surrounded by legs.Horseshoe crab fossils, similar in form to the ones you see in estuaries today, are found in rocks over 450 million years old. Introduction Atlantic horseshoe crabs are one of the many important organisms living in estuaries along the East and Gulf coasts of the United States. They are abundant from Virginia to New Jersey with the largest population of spawning horseshoe crabs found in the Delaware Bay, site of the Delaware NERR. Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft). About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the world's tropical and semi-tropical regions. Horseshoe Crab. Horseshoe crabs (Limulus polyphemus) are a marine arthropod found along the Atlantic coast from northern Maine to the Yucatan Peninsula and the Gulf of Mexico. The Delaware Bay supports the largest spawning population in the world. Horseshoe crabs provide the backdrop for one of the most interesting marine resource management issues along the Atlantic coast. In addition to their role as a food source for birds, horseshoe crabs provide bait for commercial American eel and conch fisheries along the coast. Life History. Horseshoe crabs (Limulus polyphemus) are a marine arthropod found along the Atlantic coast from northern Maine to the Yucatan Peninsula and the Gulf of Mexico. The Delaware Bay supports the largest spawning population in the world. Horseshoe crabs provide the backdrop for one of the most interesting marine resource management issues along the Atlantic coast. In addition to their role as a food source for birds, horseshoe crabs provide bait for commercial American eel and conch fisheries along the coast. World Range & Habitat. There are 4 species of horseshoe crabs found in the world today. Limulus is the only species found in the Atlantic Ocean. The remaining three are found in SE Asia where the eggs of some species are used for food. A protein found in the blood of horseshoe crabs is used to detect impurities in intravenous medications; the animals are apparently not harmed during blood extraction. Horseshoe crab blood has also been used in cancer therapy research, leukemia diagnosis and to detect vitamin B12 deficiency. Horseshoe crabs are often called living fossils because fossils of their ancestors date back almost 450 million years--that's 200 million years before dinosaurs existed. Despite inhabiting the planet for so long, horseshoe crab body forms have changed very little over all of those years. Ancestors of horseshoe crabs date back over 450 million years--long before the age of the dinosaurs. Four species of horseshoe crabs exist today. Only one species, Limulus polyphemus, is found in North America along the Atlantic and Gulf coasts from Maine to Mexico. The other three species are found in Southeast Asia Species Description: The horseshoe crab, Limulus polyphemus, is more closely related to chelicerates such as spiders, scorpions, ticks and mites than it is to true crabs and other crustaceans. Horseshoe crabs are considered to be living fossils that have evolved little in the past 250 million years. 
http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf http://estuaries.noaa.gov/Teachers/pdf/07_Horseshoe_Crab_TG.pdf https://en.wikipedia.org/wiki/Crab http://www.asmfc.org/species/horseshoe-crab http://www.asmfc.org/species/horseshoe-crab http://marinebio.org/species.asp?id=281 http://myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/facts/ http://www.sms.si.edu/IRLSpec/Limulu_polyph.htm
what type of marine environment are horseshoe crabs found in