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Range / Habitat

  • Occurs along the coast of the Western Atlantic from Maine to the Yucatan peninsula, with the largest population concentrated in the Delaware Bay region.
  • Found in different habitats at different life stages. Beaches provide essential habitat for the adult horseshoe crab to spawn, while protected, shallow and intertidal areas are essential nursery habitats for the juvenile.
  • Specific habitat requirements for adults are unknown, although they have been found from intertidal areas to depths of more than 656 feet (200 m). Adults are believed to prefer depths less than 98 feet (30 m).

Physical Characteristics

  • Horseshoe crab has three divisions of its body: the prosoma (cephlathorax), the opisthosoma (abdomen) and the telson (tail).
  • A female can reach a width of 12 inches (30 cm).The male is somewhat smaller.
  • Has ten eyes: two compound eyes visible on top of their shell that help locate mates. In addition, each animal has eight other “eyes” located elsewhere on the prosoma, tail and underside of the body. These eyes sense visible and ultraviolet light, which are believed to allow the animal to respond to day-night and lunar cycles.
  • Uses its tail to plow through muck and sand, to act as a rudder when swimming, and to right the horseshoe crab when it flips on its back.
  • The mouth is located at the center of the underside of its body among its five pairs of walking legs.
  • Male can be distinguished from the female by the first pair of legs, which terminate in a hook-shaped structure used to grasp the shell of the female during mating. On the female, these legs terminate in a pincer structure. The female is also significantly larger than the male at maturity.

Diet / Feeding

  • Consumes mollusks, worms, dead fish and algae.
  • Feeds by locating prey by smell, crawling over it and using its legs to push food into horseshoe’s mouth.
  • Uses the stiff bristles around its mouth to tear and shred food.

Reproduction / Growth

  • Adults move into shallow water in the spring to spawn. The female will emerge onto the beach where the male grasps the back edges of her shell with his front legs. The pair moves together up into the intertidal beach zone where the female deposits up to 4,000 eggs in a shallow nest she digs in the sand. The male fertilizes the eggs before she covers them with sand. The female may make several trips up the beach during successive tides to lay four to five clutches of eggs with each visit.
  • Males will often attach themselves to each other behind a lead female during the spawning activities, forming a “chain” of four or five individuals as they move up the beach.
  • Eggs hatch in one to three weeks and the larvae emerge into the water. They later settle to the bottom in shallow, protected intertidal areas, which serve as a nursery area for the juvenile horseshoe crab for the next two to three years.    
  • Grows in size by molting. The process requires the animal to grow a new pliable shell under its existing hard one. When the new shell is ready, the horseshoe crab absorbs water through its gills and expands. The old hardened shell cannot expand and splits horizontally along the front edge where the top meets the bottom. The animal struggles out of its old shell and is vulnerable for about 24 hours while the new shell hardens.
  • Undergoes about 16 molts until it is fully grown (at 9 to 12 years). Most molting occurs during its first three years and then about once a year thereafter. Once mature, individual does not molt again.

Conservation Status

  • “Near Threatened” on the IUCN Red List.
  • Populations in some areas of the U.S. coast are declining.

Additional Information

  • Horseshoe crab is not a crab: it is more closely related to scorpions, ticks and land spiders. It also is related to trilobites that existed more than 500 million years ago. 
  • Likely has inhabited the world ocean for more than 350 million years.
  • Can survive extreme temperatures, go for long periods without food, and its hard shell protects it from most predators other than sharks.
  • Horseshoe crab blood is blue because it contains copper. The blood also contains a number of compounds that bind to and inactivate bacteria, fungi, and viruses, functioning as a primitive "immune" system. An extract of horseshoe crab blood is used by the pharmaceutical and medical device industries to test their products for contamination. 
  • Plays an important ecological role for many shorebirds that migrate through the U.S. Mid-Atlantic coast each spring on their annual migration from South America to their summer grounds in the Arctic. Eleven species of birds depend on horseshoe crab eggs as their primary food source during their 2 to 3 week stop-over.
  • Harmless to humans.

Sources

www.horseshoecrab.org/index.html 
www.ocean.udel.edu/horseshoecrab 
www.ceoe.udel.edu/horseshoecrab/History/lifestages.html