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  Which Coastal Crab is Known as a Living Fossil?

13 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.


Technically, the horseshoe crab is not a crab at all. The horseshoe crab belongs in the large group of animals called Arthropoda, which includes lobsters, crabs, and insects.

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Horseshoe crabs have a robust and highly adaptable physiology.  These living fossils can live in high and low salinity waters, and they are usually the last to leave polluted and oxygen starved areas.


Even though it looks crab-like, the horseshoe crab 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, they 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 top called the cephalothorax, and a triangular abdomen on the bottom from which a long tail extends. Beneath the abdomen are gills arranged in five sections called "books," while underneath the cephalothorax are the animal's legs.


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. Since the horseshoe crab has no jaws, it uses these bristles to crush the food as it moves its legs. This means that a crab can only eat while it walks along the ocean floor.


Horseshoe crabs also use their appendages to crawl ashore each spring to participate in a mating ritual that is millions of years old. Males attach themselves to the backs of the larger, stronger females who then drag their mates 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. In two weeks, the baby crabs emerge from their shells looking like miniature, tail-less versions of the adults. They make their way to the ocean where they will remain until they reach sexual maturity in 9-11 years. At this point, they will migrate back to the same beaches from which they hatched to mate.


Despite their large, armored bodies and menacing looking tails, these living fossils are harmless. In fact, they are quite beneficial to man. Because of their unique physiology, horseshoe crabs are 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. And, their large eyes, which have structures that are 100 times bigger than a human's, have been used extensively in eye research.