Written by Willie Heard
On the cutting edge…
There are only four species of horseshoe crabs in existence in the world today.
Limulus polyphemus, Tachypleus gigas, Tachypleus tridentatus, and
These creatures are
sometimes called “living
fossils” because they have changed little from their fossilized relatives; the
earliest species identified is approximately 450 million years old.
crabs are a valuable resource, commercially as a fertilizer and as a source of
calcium for enriching fowl grains and medicinally in identifying endotoxins.
most persistent study on these animals has focused on the properties of their
blue blood. In 1977, The Food and Drug Administration of the United States
approved a new test for identifying endotoxins using
(LAL) purified from horseshoe crab blood.
In spite of their commercial and
medical importance, horseshoe crabs are threatened by the loss of living and
This habitat degradation has resulted in a rapid population
decline over the last few decades.
Students will be able to do the following:
Describe the difference between a true crab and a horseshoe crab
Briefly discuss the life history of the horseshoe crab
Give some medical and commercial uses of the horseshoe crab
Key Concepts: chitin,
Amoebocyte Lysate (LAL), chitosan, exoskeleton,
pedipalps, molt, arthropod, decapod, chemoreceptors, pheromone
What is a Horseshoe Crab?
Even though the
horseshoe crab has
a hard shell and
claws, it is not really
crabs belong to the
phylum, Arthropoda, along with
crabs, insects, and other
invertebrates with jointed legs, but
their closest living relatives are
spiders and scorpions.
True crabs have two pairs of
antennae and a pair of mandibles, or
jaws; horseshoe crabs lack these
structures. Further, comparing the
legs of a true crab with the legs of a
horseshoe crab reveals another
have five pairs of legs, which include
a pair of claws.
have seven pairs of legs under their
helmet-like shells; five of these
seven pairs of legs are equipped