5 Animals Whose Blood Isn’t Red


Horseshoe Crabs

horseshoe, crab
Horseshoe crabs don't have white blood cells, but instead have amoebocytes, which help them fight off disease-carrying microorganisms. Digital Light Source/Getty Images

Mankind owes a great debt to the world's horseshoe crabs. Quite unwittingly, these marine invertebrates have become our allies in the endless fight against medical maladies. And it's all thanks to their wonderful blood. Despite the misleading common name, horseshoe crabs are not true crabs. They're more closely akin to spiders — and just like those arachnids, horseshoe crabs have blue blood filled with hemocyanin.

But there's something else in their blood, too. You see, horseshoe crabs don't have white blood cells, which should make them vulnerable to the many types of harmful bacteria and viruses roaming the ocean. Not to worry: Evolution's given the hard-shelled creatures a different way to fight off disease-carrying microorganisms.

Horseshoe crabs have moving cells inside their blood called amoebocytes. When one of these finds a bacterium, it secretes a rapidly-coagulating gel that encases the intruder. Known to scientists as coagulogen, the substance keeps unwanted bacteria from spreading.

To the medical community, it's a godsend. All experimental intravenous drugs are now required by U.S. law to pass a contamination test involving horseshoe crab blood. In it, a sample of the medicine is mingled with the invertebrate's azure blood. If any coagulogen clots appear within 45 minutes, then the researchers will know the drug contains (possibly harmful) bacteria. As such, it isn't ready to be used on human patients. To meet demand, some laboratories harvest these crabs and extract blood samples. Those who survive the ordeal are released back into the ocean.