Horseshoe crab is a large marine animal with a hard, shell-like carapace shaped something like a horse’s hoof. The horseshoe crab is sometimes called the king crab. However, it is not related to the widely eaten red king crab and other true crabs. Horseshoe crabs are the only surviving members of a large group of animals that appeared on Earth at least 445 million years ago. Their closest living relatives are scorpions and spiders.
There are four species (kinds) of horseshoe crabs. The largest reaches about 2 feet (61 centimeters) long. It lives along the eastern coast of North America from Maine to the Yucatán Peninsula in Mexico. The other species live in coastal waters of the Philippines and Southeast Asia.
The horseshoe crab’s body consists of two “hinged” parts, the prosoma and the abdomen. The prosoma is the part of the body covered by the carapace and includes the head. The carapace covers several pairs of light-sensitive eyes. The mouth and legs are located beneath the carapace on the lower surface of the prosoma. Horseshoe crabs have six pairs of legs. The front pair has pincers and is used to capture prey. The other five pairs of legs are used for walking. The last pair also cleans the gills.
The horseshoe crab’s abdomen has six pairs of flattened, platelike structures. The reproductive organs are under the front pair of these plates. The remaining plates each carry more than 100 soft gills by which the animal breathes. A jointed spine called the teslon sticks out from the end of the abdomen. The horseshoe crab uses this “tail” as a lever for turning over if it is flipped upside down.
The horseshoe crab feeds at night on worms and small mollusks, such as clams, that it digs up from sand and mud. Horseshoe crabs mate in the spring. The female carries the male on her back to shore, where she digs several holes in the sand. The female lays thousands of eggs in each of the holes, and the male then fertilizes the eggs.
People use horseshoe crabs as bait for fishing and as fertilizer. Also, the animal’s blue blood contains a substance that can detect contamination of medicine by harmful bacteria. The Asian species are fished for food. Horseshoe crabs are threatened by overfishing and habitat destruction in areas where they lay their eggs.
