Horseshoe crabs are more closely related to spiders, ticks and scorpions than they are to true crabs. Like other arthropods, they have a hard shell, or exoskeleton, a segmented body and jointed legs. Horseshoe crabs are in a class of their own - Merostomata - which means "legs attached to the mouth."
There are only four living species of horseshoe crabs, and ours is the only species native to the Atlantic Ocean. The other three species can be found in the Indian and Pacific Oceans.
The horseshoe crab is a living fossil. It has been on Earth some 220 million years, longer than dinosaurs. And it survives today almost identical to its ancient ancestors.
Horseshoe crabs get their name because their arc shaped carapace, or exoskeleton, has been compared to the shape of a horse's shoe. Their body is divided into three sections, the first of which contains their mouth, a pair of feeding pincers and five pairs of legs.
Their second body section is their abdomen, and it contains five sets of book gills -- flap-like structures that allow the horseshoe crab to breath under water. They sometimes use their book-gills to swim upside down.
They use their tail spike to steer and to right themselves if they get turned upside down while swimming.
Most horseshoe crabs spend most of the year in deep water. Every spring, they migrate to the shallows, emerging from the sea to mate along the beaches on moonlit nights, when the tide is high. Females can lay 90,000 or more eggs per season. Though only a small percentage of these eggs will survive into adulthood, they serve as a primary food supply for migratory birds on their way north.
One pair of legs is actually a pair of pincers near the mouth, and is smaller than the other pairs of legs. These pincers are used to grab the food and push it toward the mouth. They feed at night, primarily on mollusks, crustaceans and worms on the ocean floor.
Horseshoe crabs move by swimming or walking along the bottom. Because their exoskeleton doesn't grow with them, they must shed, or molt, in order to grow. They may molt 16 to 17 times before they reach their adult size. After this, adults rarely molt.
Horseshoe crab eggs and larvae are eaten by birds and many ocean animals. Their eggs are an important food source for at least 11 species of migratory shore birds, including the red knot, which relies strictly on horseshoe crab eggs for food during migration. Sea turtles also eat the eggs and larvae. Without the horseshoe crab eggs, the shoreline ecosystem, from North Carolina south, wouldn't be able to survive.
Adult horseshoe crabs are preyed upon by sharks, sea turtles, gulls and humans for use as bait or fertilizer.