The Horseshoe Crab
The horseshoe crab is a living fossil which can be traced back about 300 million years, when water scorpion ancestors dominated the seas. Not a true crab at all, it is more closely related to scorpions and spiders.
The horseshoe crab sweeps across muddy and sandy seabeds hunting for prey. Its jointed limbs can crush shells of mollusks, and spikes at the base of its legs tear up prey. This familiar species of the shore is a survivor from a group that dominated the world’s seas more than 300 million years ago.
The horseshoe crab lives in coastal waters on the Atlantic coast of North America. It is common off New Jersey, but may live on any sand or mud flat from the Gulf of Maine to the Gulf of Mexico, particularly in more remote areas. The horseshoe crab needs a sandy or muddy seabed on which to hunt and scavenge and burrow into when in danger. The crab’s smooth, horseshoe shaped shell helps safeguard it from predators above and provides protection for its legs and soft underparts. The streamlined outline of the shell also helps the horseshoe crab slip under the loose surface as it burrows.
Horseshoe crabs feed on worms, mollusks and other small organisms including algae. The stout bases of the crab’s fifth pair of legs act like a nutcracker to open the shells of bivalves such as clams. When the crab picks a clam, it uses the leg bases to crack it open. It then manipulates the prey forward to spikes at the base of the walking legs and uses these to crush and slice the flesh, mashing it to an edible pulp. The crab passes pieces of prey forward to its mouth, and then into the gizzard. Large, indigestible particles are ejected from the mouth, while useful food passes to the stomach for digestion.
Horseshoe crabs are more closely related to spiders and scorpions than to true crabs, classed in a group of animals called chelicerates. Chelicerates lack antennae and have a specialized first pair of legs, known as the chelicerae. The horseshoe crab’s two compound eyes on the sides of the shell detect movement but can not produce an image. A predator probably casts a shadow over the compound eyes, prompting the crab to escape into the sand. The crab burrows by pushing the front half of its body into the mud with its tail and shoveling with the fifth pair of walking legs. The tail looks dangerous but is never used for defense.
Mating and egg laying takes place in the tidal zone, during the high tides of full and new moons in spring and summer. Adults gather in large numbers to mate. The smaller male climbs onto the back of the female, gripping her shell with his first pair of walking legs. Once the male is in position, the female digs several hole up to 5” deep in the sand and deposits clutches of 2,000 to 30,000 eggs. As she lays the eggs, the male fertilizes them. The pair separate with the fertilized eggs left buried in the sand. After about six weeks, the eggs hatch. The larva molts its shell several times as it grows to reach a shell width of about 1.5” after one year. Sexual maturity is reached in 9-12 years.
All of the living species of horseshoe crabs are listed by the World Conservation Union. No species of horseshoe crab is known, or suspected, to have become extinct within recorded history.