Despite the name, horseshoe crabs are not crabs at all. They are closer to spiders and scorpions, and their body plan has barely changed in 450 million years, since long before the dinosaurs. The Atlantic horseshoe crab grows up to 24 inches (60 cm) long including its tail and weighs up to 10 lbs (4.5 kg); females are larger than males.
Under the smooth domed shell are ten legs, book-like gills and a mouth in the middle of the legs. The long tail spike is not a weapon; the animal uses it to steer and to flip itself upright when a wave turns it over. It has ten eyes scattered over its body, including two large compound eyes.
Horseshoe crab blood is blue because it carries oxygen with copper instead of iron. It clots instantly around bacteria, and a test made from it is used to check that vaccines and medical implants are free of contamination.
The Atlantic horseshoe crab lives along the east coast of North America from Maine to the Yucatan in Mexico, with the largest numbers in Delaware Bay. Three other species live in the coastal waters of southern and eastern Asia.
They spend most of the year on sandy or muddy seabeds in shallow water, and come ashore on sheltered beaches to breed.
Horseshoe crabs plough through the sediment for worms, clams and other small animals, along with some algae. They have no jaws, so they crush food between the spiny bases of their legs and push it into the mouth.
Each spring, around the high tides of the full and new moon, horseshoe crabs crawl onto beaches by the thousands to spawn. A female digs into the sand and lays clusters of tiny green eggs, up to 90,000 in a season, while one or more males cling to her shell to fertilise them.
Those eggs are a vital feast for migrating shorebirds such as the red knot, which time their flights to arrive as the crabs spawn. Young horseshoe crabs take 9-10 years to mature and can live for 20 years or more.
There are four living species in the family Limulidae: the Atlantic horseshoe crab (Limulus polyphemus) and three Asian species, the tri-spine, mangrove and coastal horseshoe crabs. The IUCN lists the Atlantic species as Vulnerable and the tri-spine horseshoe crab as Endangered.
Threats include harvesting for bait, the bleeding of crabs for the medical test (many are returned to the sea but some die), and the loss of the quiet beaches they need to breed. Horseshoe crabs are now protected or managed along much of the US coast.
