Evolving over 200 million years ago, horseshoe crabs have been pretty successful throughout the test of time. Currently, there are 4 extant species of horseshoe crab, and if you live on the east coast like me, you might have seen a member of the species Limulus polyphemus before (perhaps you've held one at the Baltimore Aquarium). While these animals may look intimidating, they are completely harmless to humans. These chelicerate (which refers to their mouth structure, shared with arachnids) arthropods are the closest living ancestors of the ancient trilobites, and are closely related to spiders, ticks and scorpions. There are a couple of unique characteristics which have allowed these animals, above others, to be so successful. They have a unique ability to re-grow limbs similar to sea-stars, they have a tough exoskeleton like all arthropods, and have special blood. Instead of hemoglobin, horseshoe crabs use copper-containing hemocyanin to carry oxygen. But it is their "amebocytes", specialized blood cells, which make them particularly interesting. These cells contain a special coagulant which forms a clot whenever a bacterial endotoxin is encountered. This ability is exploited by humans in purity testing of medicines by collecting their blood:
Unfortunately, horseshoe crab populations are in decline due to over-harvesting in the 90's for use in fertilizers. This trend could limit our ability for medical testing because alternative methods are much more costly. Their decline has also lead to a downfall in the red knot, which feeds on the horseshoe crabs during a stop made during their long migration.
The horseshoe crab has also been used extensively in vision research.
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