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Often referred to as living fossils, the ancestors of Atlantic horseshoe crabs can be traced back more than 400 million years! Since that time, not much about the horseshoe crab’s anatomy has changed. Its shell is horseshoe-shaped, rounded in the front and designed for plowing through bottom sand and mud in search of invertebrate prey. Horseshoe crabs have a dark brown, hinged shell and a long, pointed tail, called a telson. The spiked telson might look menacing but it’s harmless—the animal uses it as a rudder to steer through the water and flip itself over when toppled by a wave.
This species has 10 eyes scattered throughout its body. Its brain can be found in a thin circle around its mouth, which is located on its underside and is surrounded by bristles used to chew food.
Despite the common name “horseshoe crab,” it is not a crab or a crustacean but an arthropod. Their closest living relatives are spiders and scorpions and their closest ancestor is probably the trilobite that lived 400 million years ago. Horseshoe crabs have unique blood that turns blue when exposed to oxygen due to the presence of a copper-based protein. Cells in the blood can detect the presence of disease-causing bacteria. Pharmaceutical companies produce a biomedical product from horseshoe crab blood, which is used to screen for the presence of bacterial contamination in injectable medicines and implants.
There are four species of horseshoe crabs found in coastal waters of the western Atlantic and three species found in coastal waters off Southeast Asia in the Indo-Pacific.
A Note From the Caretaker
Horseshoe crabs do not bite or sting, and they can die if they strand on their backs on the shore in the hot sun. If you come across an upside-down horseshoe crab on the beach, never grab its telson (or tail) since it’s delicate and you could harm the animal. Instead, gently flip it over by holding either side of its shell.
Learn more about Atlantic horseshoe crabs! Did you know that in late spring and early summer, you may spot two or more horseshoe crabs hooked together. Special claws allow males to clasp onto a female’s shell and hitch a ride to a beach where the female lays her eggs in the sand. Each spring, up to a million migratory shorebirds arrive on the beaches of Delaware Bay to feed on horseshoe crab eggs and rebuild energy reserves before migrating north to their nesting habitats.
Horseshoe crabs are found all along the North American Atlantic coast, from Nova Scotia in the north to Mexico’s Yucatan Peninsula in the south.
Horseshoe crabs feed on marine worms, clams, snails, crustaceans and dead fish, supplemented by algae and detritus.
Female horseshoe crabs grow larger than males, reaching a maximum length of 24 inches (including the tail), while males reach total lengths of around 15 inches.
Horseshoe crab populations are decreasing and considered vulnerable, according to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. In addition to overharvesting, bycatch and habitat loss from shoreline development, their spawning beaches are threatened by climate change and the severe weather it causes.
Sharks, other fishes, sea turtles and birds all prey on adult and young horseshoe crabs. Many migrating shorebirds feed on horseshoe crab eggs during their spawning season. Humans are also a predator of horseshoe crabs, harvesting them for bait.
Meet the Expert Jack Cover
As the National Aquarium's general curator, Jack Cover ensures that all animals in our care thrive in healthy, beautiful habitats.