Adult horseshoe crabs range in length from 3.5 to 33.5 inches (89 to 850 millimeters).
Habitat, biology, and fisheries
Adults migrate inshore to intertidal sandy beaches to spawn in the spring. In the fall, adults move to deep bay waters or migrate to the Atlantic continental shelf to overwinter. Spawning generally occurs on protected sandy beaches from March through July, with peak activity occurring on the evening new and full moon high tides in May and June. Delaware Bay has the largest concentration of spawning horseshoe crabs.The horseshoe crab is a benthic or bottom-dwelling arthropod that utilizes both estuarine and continental shelf habitats. Spawning adults prefer sandy beach areas within bays and coves that are protected from wave energy. Horseshoe crabs spawn multiple times per season. Egg development is dependent on temperature, moisture, and oxygen content of the nest environment. Spawning habitat varies throughout the horseshoe crab range. In Massachusetts, New Jersey, and Delaware beaches are typically coarse-grained and well-drained as opposed to Florida beaches, which are typically fine-grained and poorly drained. Optimal spawning beaches may be a limiting reproductive factor for horseshoe crabs because they typically select beaches based on geochemical criteria. For example, results from a geomorphology study conducted along the New Jersey side of the Delaware Bay estimated that only 10.6 percent of the New Jersey shore adjacent to Delaware Bay provided optimal horseshoe crab spawning habitat and only 21.1 percent provided suitable spawning habitat. Nursery Habitat - The shoal water and shallow water areas of bays (e.g., Delaware Bay and Chesapeake Bay) are important nursery areas. Juveniles usually spend their first two years on intertidal sand flats. Older juveniles move out of intertidal areas to a few miles offshore, except during breeding migrations. Adults are exclusively subtidal, except during spawning. Specific requirements for adult habitat are not known. Although horseshoe crabs have been taken at depths >200 meters, scientists suggest that adults prefer depths <30 meters. During the spawning season, adults typically inhabit bay areas adjacent to spawning beaches and feed on bivalves. In the fall, adults may remain in bay areas or migrate to the Atlantic Ocean to overwinter on the continental shelf. Deep water areas are used by larger juveniles and adults to forage for food. They play a vital ecological role in the migration of shorebirds along the entire Atlantic seaboard, as well as providing bait for the American eel and conch fisheries along the coast. Additionally, their unique blood is used by the biomedical industry to produce Limulus Amoebocyte Lysate (LAL), an important tool in the detection of contaminants in patients, drugs and other medical supplies.
Distribution
Maine to the Gulf of Mexico, but are most abundant from New Jersey to Virginia with their center of abundance around Delaware Bay.
