Horseshoe Crab

Information from FAO Species Identification Guide Western Central Atlantic

Limulus polyphemus


horseshoe crab


Diagnostic Characteristics: Females are larger than males. Horseshoe crabs have a large, arched forebody covered by a horseshoe-shaped carapace, or upper shell, followed by a smooth abdomen with spines on the sides, and a thin tail. There are two pairs of simple eyes, or eyes with one lens, on top of the carapace and a pair of compound eyes, or eyes with multiple lenses, on ridges toward the sides. The mouthparts are made up of a pair of pincher-like mouthparts and a pair of clawed leg-like appendages. There are four pairs of clawed walking legs. The walking legs have seven segments, the last two form pinchers on the first four pairs of legs. The bases of the fourth pair of legs are fitted with special structures called flabella. The flabella are used to clean the book gills. The last pair of legs ends in four leaf-like structures. These legs are used to push through, and sweep away mud, silt, and sand as the horseshoe crab burrows through the sea bottom in search of food.The solid midsection, or abdomen, has six pairs of flap-like limbs. The first pair is joined together and protects the reproductive opening, through which the crab lays its eggs. The other five pairs form the gills, the organs through which the crab breathes underwater. They are called book gills because they resemble the pages of a book. Movable spines stick out on each side of the midsection. A long thin tail extends from the end of the midsection and is used for steering through the water and flipping over.

Size: 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.

Information from ASMFC and