Unit Five
COAST/Horseshoe Crabs
ï›™Project Oceanography Fall 2001 81
COAST
Written by Willie Heard
On the cutting edge…
There are only four species of horseshoe crabs in existence in the world today.
These are Limulus polyphemus, Tachypleus gigas, Tachypleus tridentatus, and
Carcinoscorpius rotundi-cauda. 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. Horseshoe
crabs are a valuable resource, commercially as a fertilizer and as a source of
calcium for enriching fowl grains and medicinally in identifying endotoxins. The
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 Limulus Amoebocyte Lysate
(LAL) purified from horseshoe crab blood. In spite of their commercial and
medical importance, horseshoe crabs are threatened by the loss of living and
breeding habitats. This habitat degradation has resulted in a rapid population
decline over the last few decades.
Horseshoe Crabs
Lesson Objectives: 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, Limulus 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
numerous
appendages with
claws, it is not really
a crab. Horseshoe
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 ha