||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.
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
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
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.