Horseshoe crabs, also known as king crabs, are named for the thick, horseshoe-shaped shells covering their heads and thoraxes. They can mostly be found on sandy or muddy bottoms in shallow ocean waters.There are four species of horseshoe crabs. One, Limulus polyphemus, lives around the Atlantic and Gulf coasts from Maine to Mexico. The other three species are all found in southeast Asia.

1. Horseshoe crabs aren't crabs. They are marine arthropods that are more closely related to spiders and scorpions than crabs and lobsters.

2. Horseshoe crabs are ancient. Horseshoe crabs are the only living members of a group of animals that first appeared about 450 million years ago. They are the closest living relative of the now extinct trilobites.

3. Horseshoe crabs can reach the size of a large serving dish. While Carcinoscorpius rotundicauda is only about the size of a human hand, L. polyphemus can grow to 24 inches long, including the tail.

4. Their bodies are divided into three sections. The first section is the head, or prosoma. It is the largest part, and contains the brain, heart, mouth, and nervous system and is covered by a hard exoskeleton. The middle section is called the abdomen or opisthosoma, and contains muscles and gills on the bottom and spines on the side. The last section is the tail, or telson, which is long and rigid. The horseshoe crab uses it to right itself if it is flipped over.

Horseshoe crabs have five pairs of legs for walking, swimming, and placing food into their mouths. Behind the legs are the book gills, which are used for breathing and propulsion through the water. The first pair of book gills is the operculum, and it acts as a cover for the other five pairs as well as housing the opening of the genital pores through which eggs and sperm are released.

5. Their mouths are located in the middle of their abdomens. Horseshoe crabs dig for food in the mud and sediment, looking for mollusks, worms, and algae. Their small front appendages, called chelicera, are used to place food into their mouths, which are located in the center of their bodies where the legs attach.

6. They swim upside down. Horseshoes crabs usually walk along the bottom of shallow water, but they can also swim awkwardly on their backs by using their gills as paddles. Swimming this way, they can move at about 0.22-0.34 miles per hour.

7. They have nine eyes. Horseshoe crabs have nine eyes on their bodies and several more light receptors near their tails. The largest eyes are a pair of compound eyes on either side. They also have a pair of median eyes that can detect both visible and ultraviolet light, a single endoparietal eye, a pair of simple eyes on top, and a pair of ventral eyes on their undersides near their mouths. Horseshoe crabs also possess a cluster of light receptors on their tails. Even though they have relatively poor eyesight, horseshoe crabs have the largest rods and cones of any known animal, about 100 times larger than those of humans.

8. Their breeding is tied to the lunar and tidal cycles. During spring and summer in the mid-Atlantic, horseshoe crabs arrive on beaches during high tides in the three days before and after a new or full moon. Males, which are about 20% smaller than females, use a special appendage to clasp onto the back of a female. The female digs a hole in the sand and lays her eggs while the male fertilizes them. She can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. As the tide goes out, the horseshoe crabs make their way back to the sea.

9. Horseshoe crabs are slow-growing. The eggs hatch in about two weeks. The larvae look like miniature versions of the adults. Over the next 10 years or so, the juvenile horseshoe crabs molt and grow, undergoing around 16 molts during development. They reach sexual maturity at about 10 years old, and can live more than 20 years.

10. Horseshoe crabs' blue blood is important to the biomedical industry. Horseshoe crabs have blue blood because, unlike vertebrates, they do not have hemoglobin. Instead, they use hemocyanin to carry oxygen, which contains copper and makes the blood blue. Their blood contains a substance called Limulus amebocyte lysate (LAL),which coagulates when exposed to bacterial toxins. Researchers harvest horseshoe crab blood to use as a simple, reliable test for screening for bacterial contamination of medical equipment and injectable medicines like vaccines. Harvesting the blood involved collecting and bleeding the animals and then releasing them back into the ocean. Most of the animals, about 85% or more, survive this process. 

References and Other Resources:

The Horseshoe Crab. Accessed August 20, 2015 at http://www.horseshoecrab.org.

Horseshoe crab (Limulus polyphemus), Arkive. Accessed August 20, 2015 at http://www.arkive.org/horseshoe-crab/limulus-polyphemus/.

Horseshoe Crab, National Wildlife Federation. Accessed August 20, 2015 at https://www.nwf.org/Wildlife/Wildlife-Library/Invertebrates/Horseshoe-Crab.aspx.Lamerato, A. (2001). Limulus polyphemus, Animal Diversity Web. Accessed August 20, 2015 at http://animaldiversity.org/accounts/Limulus_polyphemus/.

Limulus polyphemus (Horseshoe Crab), Encyclopedia of Life. Accessed November 15, 2015 at http://eol.org/pages/393278/overview.