The Ancient Horseshoe Crab


Image courtesy of Delaware National Estuarine Research Reserve.

One of the most remarkable marine animals in our waters is the horseshoe crab (Limulus polyphemus), an ancient inhabitant of the sea. It survived the reign of the dinosaurs and continues to fascinate us today, 350 million years after it evolved. Found in tidal creeks, muddy shores, and the intertidal zone along the east coast, the horseshoe crab is an ecologically and economically important marine species.

Technically a horseshoe crab is not a true crab, but instead is more closely related to spiders, scorpions, and even the extinct trilobites. Its shell is called a carapace and may carry attached animals such as tube worms, boat snails, and barnacles. A horseshoe crab looks quite formidable at first glance, but they are harmless creatures. They feed on mollusks and worms by crushing food with the short spines at the base. Their tail, or telson, is not used in “sword fights,” but helps the animal right itself if it gets turned over.

Image Courtesy of Science News Journal


Horseshoe crabs have a complex nervous systems with a pair of compound eyes on its dorsal side. These eyes have been the subject of many studies on vision. In all, the animal has nine eyes that are located on other parts of the shell.

Other characteristics include the flaps of book gills on the abdomen for respiration, small pairs of pincers called chelicerae, and five pairs of legs. The first four pairs lift the animal up while the fifth pair projects it forward.

Image courtesy of Lillie Peterson-Wirtanen

The shell or carapace of the animal is actually an exoskeleton. As the animal grows larger it needs to shed its old shell, a process called molting. it accomplishes this by splitting the carapace and emerging head first. True crabs molt from the back. It’s not unusual to find large numbers of recently molted shells on the Cape’s beaches in late summer. I sometimes get phone calls alerting me to a “horseshoe crab kill” at Dowses Beach in Osterville or Loop beach in Cotuit in late summer and early fall. These always turn out to be recent molts. It takes about twelve hours for the soft new shell to harden.

Male and female crabs can be easily distinguished by looking at the claws. The males have a modified claw that has been compared to a boxing glove, which is used to grasp the female during mating. The female’s appendages are shaped as pincers. The female is often much larger than the male as well, though this is difficult to determine unless you have both specimens in front of you.

A female horseshoe crab will lay thousands of eggs during the spring full moon high tides. These are then fertilized by the smaller male and soon hatch out. In places such as Delaware Bay, horseshoe crabs are an important food source for thousands of migratory birds such as red knots and sanderlings. Without this essential food source many birds would not survive the long migrations to northern grounds.

Image courtesy of Lillie Peterson-Wirtanen

Horseshoe crabs are economically valuable in the medical field. The animal’s blood is blue due to the presence of copper and also contains a unique clotting agent in the presence of bacterial endotoxins. The substance is extracted from the animals and is used to test medical equipment and pharmaceuticals for these endotoxins. The substance is called LAL (Limulus amebocyte lysate) and its use has saved thousands of human lives.

Recently horseshoe crab numbers along the east coast have declined. Many causes are suspect including coastal pollution, the lysate industry, and even the taking of crabs for lobster traps. Several states have imposed restrictions on their harvesting. Some areas have been set aside as open space to conserve horseshoe crab populations. Close monitoring and continued research is necessary to protect this important species. It would be a shame to lose an animal that has survived for millions of years. Hopefully its presence in the waters of Barnstable will continue to amaze and delight those who encounter it.

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