Horseshoe Crab

Description

Horseshoe crabs have been around for more than 300 million years, making them even older than dinosaurs. They look like prehistoric crabs, but are actually more closely related to scorpions and spiders. The horseshoe crab has a hard exoskeleton and 10 legs, which it uses for walking along the seafloor.

The body of the horseshoe crab is divided into three sections. The first section is the prosoma, or head. The name “horseshoe crab” originates from the rounded shape of the head, because just like the shoe on a horse’s foot, the head is round and U-shaped. It's the largest part of the body and contains much of the nervous and biological organs. The head has the brain, heart, mouth, nervous system, and glands—all protected by a large plate. The head also protects the largest set of eyes. Horseshoe crabs have nine eyes scattered throughout the body and several more light receptors near the tail. The two largest eyes are compound and useful for finding mates. The other eyes and light receptors are useful for determining movement and changes in moonlight.

The middle section of the body is the abdomen, or opisthosoma. It looks like a triangle with spines on the sides and a ridge in the center. The spines are movable and help protect the horseshoe crab. On the underside of the abdomen are muscles, used for movement, and gills for breathing.

The third section, the horseshoe crab’s tail, is called the telson. It's long and pointed, and although it looks intimidating, it is not dangerous, poisonous, or used to sting. Horseshoe crabs use the telson to flip themselves over if they happen to be pushed on their backs.

Female horseshoe crab are about one-third larger than the males. They can grow to be 18 to 19 inches (46 to 48 centimeters) from head to tail, while the males are approximately 14 to 15 inches (36 to 38 centimeters).

Range

The horseshoe crab species found around the United States (Limulus polyphemus) lives in the Atlantic Ocean along the North American coastline. Horseshoe crabs can also be seen along the East and Gulf coasts of the United States and Mexico. There are three other species of horseshoe crab worldwide, which are located in the Indian Ocean and in the Pacific Ocean along the coast of Asia.

Horseshoe crabs utilize different habitats depending on their stage of development. The eggs are laid on coastal beaches in late spring and summer. After hatching, the juvenile horseshoe crabs can be found offshore on the sandy ocean floor of tidal flats. Adult horseshoe crabs feed deeper in the ocean until they return to the beach to spawn. Many shorebirds, migratory birds, turtles, and fish use horseshoe crab eggs as an important part of their diet. Horseshoe crabs are a keystone species within the Delaware Bay ecosystem.

Diet

Horseshoe crabs like to dine at night on worms and clams, and may also eat algae. A horseshoe crab picks up food with appendages located in front of its mouth. Because it has no mandible or teeth, the horseshoe crab crushes food between its legs before passing it to the mouth.

Life History

During the late spring and early summer, adult horseshoe crabs travel from deep ocean waters to beaches along the East and Gulf coasts to breed. The males arrive first and wait for the females. When the females come to shore, they release natural chemicals called pheromones that attract the males and send a signal that it's time to mate. Horseshoe crabs prefer to breed at night during high tides and new and full moons. The males grasp onto the females and together they head to the shoreline. On the beach, the females dig small nests and deposit eggs, then the males fertilize the eggs. The process can be repeated multiple times with tens of thousands of eggs.

Horseshoe crab eggs are a food source for numerous birds, reptiles, and fish. Most horseshoe crabs will not even make it to the larval stage before being eaten. If the egg survives, the larval horseshoe crab will hatch from the egg after about two weeks or more. The larva looks like a tiny version of an adult horseshoe crab, but without a tail. Larval horseshoe crabs travel into the ocean water and settle on the sandy bottom of tidal flats for a year or more. As they develop, they will move into deeper waters and begin to eat more adult food. Over the next 10 years or so, the juvenile horseshoe crabs will molt and grow. The molting process requires shedding small exoskeletons in exchange for larger shells. Horseshoe crabs go through 16 or 17 molts during their development. At around 10 years of age, horseshoe crabs reach adulthood. They are ready to start breeding and will migrate to coastal beaches in the spring. A horseshoe crab can live for more than 20 years.

Conservation

Threats to horseshoe crabs include habitat loss and overharvesting. Beach developments hinder horseshoe crab breeding. Limulus polyphemus is internationally listed as vulnerable.

Fun Fact

During full moons, new moons, and high tides in May and June, hundreds of thousands of horseshoe crabs converge on the Delaware Bay to breed.

Sources

Florida Fish and Wildlife Conservation Commission

Maryland Department of Natural Resources

National Oceanic and Atmospheric Administration

The Horseshoe Crab, Ecological Research and Development Group

The IUCN Red List of Threatened Species

U.S. Fish & Wildlife Service