Essential habitat is defined as those waters and substrates necessary for horseshoe crab spawning, breeding, feeding, and growth to maturity. Horseshoe crabs use different habitats at different life stages. For example, protected beaches provide essential habitat for horseshoe crab spawning efforts, while nearshore shallow waters are essential nursery habitat.
Of all these habitats, the beaches are the most critical. Optimal spawning beaches may be a limiting reproductive factor for the horseshoe crab population.
Spawning adults prefer sandy beach areas within bays and coves that are protected from wave energy. Beach habitat also must include porous, well-oxygenated sediments to provide a suitable environment for egg survival and development. Differences between coarse- and fine-grained sand, as well as how rapidly the sand drains, affect nest-site selection and nesting synchrony. The preferred sites are usually located next to large intertidal sand flat areas, which provide protection from wave energy and an abundance of food for juveniles.
Although spawning as been observed as far north as Augustine Beach, in Delaware and Alder Cove in New Jersey, prime spawning beaches within the Delaware Bay consist of sand beaches between the Maurice River and the Cape May Canal in N.J. and between Bowers Beach and Lewes in DE. Prime spawning habitat is widely distributed throughout the Chesapeake and coastal bays and includes some tributaries, though the horseshoes are restricted to areas where the salinity exceeds seven parts per thousand. In the Chesapeake Bay, spawning habitat generally extends to the mouth of the Chester River but can occur farther north during years of above-normal salinity levels.
The shoalwater and shallow water areas of bays such as the Delaware Bay and the Chesapeake Bay are essential nursery areas. Juveniles usually spend their first two years on these intertidal sand flats. Salinity seems to play a role in determining which waters are suitable for juvenile development, with more than 99 percent of juveniles found in waters where the salinity was greater than 5 parts per thousand.
Older juveniles will migrate out of the intertidal sand flats to deeper bay waters, where they will remain until they have developed into adults and are ready to reproduce.
Specific habitat requirements for adult horseshoe crabs are not known. Although the animals have been found at depths greater than 200 meters, some studies suggest that adults prefer depths of less than 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 into the Atlantic Ocean to overwinter on the continental shelf.