There are four species of horseshoe crab, three on Asian coasts and one on the east coast of North America.
Horseshoe crabs are not crustaceans, nor are they crabs. But, like crustaceans, they are arthropods, a group that also includes lobsters, spiders, scorpions, and insects.
The creature's ancestors were on the earth about 500 million years ago and very similar species date back to more than 150 million years ago, so the horseshoe crab is sometimes called a living fossil.
An adult female horseshoe crab can grow to 2 feet in length, or a little more. The males are somewhat smaller.
The creature gets its name from the horseshoe shape of its shell, called the cephalothorax, the front part of its three-sectioned body. On the upper surface there are two compound eyes, along with a much smaller pair of eyes that respond to ultraviolet light.
Hinged to the cephalothorax is the abdomen, which is edged with spines. The third body section is a long spiked tail, called the telson, which is used to flip the horseshoe crab after it's been turned upside down by a wave.
Beneath the cephalothorax, the animal has a mouth and three sets of legs that have three very different uses.
The horseshoe crab feeds by moving along the water bottom on five pairs of legs that surround the mouth. A front pair of legs, called chelicerae, are used as feelers to detect food, including worms, thin-shelled mollusks, and dead fish.
When prey is found, the claws on the front legs are used to pick up and move it toward the walking legs. These walking legs are equipped with spiny projections that crush the food and roll it into the mouth.
Behind the walking legs is another pair of very small legs, the chilaria, which also move food in the general direction of the mouth.
Horseshoe crabs hibernate during the colder months, half-burying themselves in the bottom of bays or ocean shore areas. In the spring, they begin migrating to the shore to spawn.
The male clasps onto the female's abdomen with his front pair of walking legs. She tows him to the edge of the water, where she digs a shallow nest in the sand and deposits 200 to 300 eggs. Then she drags the male over the nest, and he fertilizes them.
After about two weeks, the eggs hatch, producing tiny creatures, about 2/10 of an inch long, called trilobite larvae, which live off a yolk sac until they've molted several times and developed tails.
This begins a second larval stage, during which a young horseshoe crab feeds on small organisms and winters in mud flats.
Like many creatures that have shells, the horseshoe crab has to molt periodically. In the molting process, the old shell splits around its front edge and the crab crawls out with a new, soft shell, which soon hardens. The animal is about one-quarter larger than it was before the molt.
The horseshoe crab becomes sexually mature after molting about 16 times over a period of 9 to 10 years. The lifespan is uncertain, but it's believed to be about 15 years.
It's been known for a long time that horseshoe crabs have blue blood. In 1956, Dr. Frederick Bang of Johns Hopkins University discovered that the blue blood contains a substance that causes clotting in the presence of several kinds of invading bacteria that are harmful to humans as well as to horseshoe crabs.
Bang and his colleagues isolated the chemical, known as LAL (for limulus amoebocyte lysate). It's now used to test drugs to ensure that they're free of harmful bacteria.
During summer months, blood is collected from thousands of horseshoe crabs so that the LAL can be extracted. The crabs are then returned to the water to go on with the rest of their lives, while the extract is used to save human lives.