Which Coastal Crab is Known as a Living Fossil?

May 1998

Because its basic body design has remained unchanged for millions of years, the horseshoe crab (Limulus polyphemus) is often called a living fossil. The genus Limulus dates back to the Triassic (the first period in the Age of Dinosaurs), but its earliest ancestors lived more than 350 million years ago.

Horseshoe crabs have a robust and highly adaptable physiology and can live in high and low salinity. They are usually the last to leave polluted and oxygen starved areas.

Technically, the horseshoe crab is not a crab at all. It belongs in the large group of animals called Arthropoda, which includes lobsters, crabs and insects, and is more closely related to spiders and scorpions.

The horseshoe crab’s only living relatives are found in the East Indies, China and Japan. In the United States, the crabs are found only along the Atlantic coast from Nova Scotia to Mexico’s Yucatan Peninsula.

The horseshoe crab gets its name from the “U” or horseshoe-shape of its shell, called a carapace. The carapace, usually the color of sand or mud to help the animal blend in with the muddy, sandy bottoms on which it lives, consists of two hinged parts; a rounded section on the front and a triangular abdomen in the rear from which a long tail extends. Its gills and legs on the underside are protected by the hard outer shell.

To find its favorite foods – worms, mollusks, and dead fish – the crab crawls along the ocean bottom, using its small, first pair of legs as feelers. The small claws pick up the prey and move it to the bristly area near the base of the walking legs. The horseshoe crab has no jaws and uses these bristles to crush the food.

Horseshoe crabs crawl into the shallows each spring for a mating ritual millions of years old. Males attach themselves to the backs of the larger, stronger females, who then drag them up the beach to nest. The female lays up to 20,000 eggs in a series of sand nests where they are fertilized by the male. Some two weeks late, baby crabs emerge looking like miniature, tail-less versions of their parents. They make their way to deeper water, where they remain until they reach sexual maturity in 9-11 years, before migrating back to mate on the same beaches from which they hatched.

Despite their large, armored bodies and menacing-looking tails, these living fossils are harmless. In fact, they are quite beneficial to man. Their unique physiology has rendered them one of the most studied animals in the world. Their blood is used in testing pharmaceutical products. Chitin, the material found in their shell, is used in the production of many food and medical products. Their large eyes, which have structures that are 100 times bigger than a human’s, have been used extensively in eye research.