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We call them crabs, but Atlantic horseshoe crabs (Limulus polyphemus) are more closely related to spiders and scorpions than true crabs. The blood of Limulus polyphemus is used in biomedical research to test pharmaceuticals and medical devices for potentially deadly endotoxins. This test, called Limulus Amebocyte Lysate (LAL), takes advantage of the horseshoe crab’s primitive immune system, which causes its blood to clot when it encounters potentially harmful gram-negative bacteria.

Fun Facts:

• The horseshoe crab is a “living fossil” that been on Earth more than 445 million years—longer than dinosaurs. Today, it is still almost identical to its ancient ancestors.
• It has blue blood because it has the protein hemocyanin, which is copper-rich vs. humans’ red blood, which has the iron-based hemoglobin protein.
• Its blood contains amebocytes, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens.