28. Horseshoe Crab Blood

Every spring, thousands of horseshoe crabs swarm beaches along the U.S. Atlantic coast for their mating ritual, a behavior that has persisted for about 450 million years through mass extinctions. These ancient arthropods—more closely related to spiders and scorpions than true crabs—now draw humans who harvest up to a third of their distinctive blue blood, used to test pharmaceuticals for deadly bacterial toxins and valued at around $60,000 per gallon.

The Blood's Bacterial Defense

Horseshoe crabs evolved a hypersensitive blood response to endotoxins, components of certain bacteria's cell walls. If a crab's shell cracks, amoebocytes in its blood detect the intruders and clot around them, forming a protective barrier. This mechanism caught the attention of infectious disease expert Frederick Bang in the 1960s. Before his work, drug safety testing relied on injecting rabbits and observing illness. Bang and colleague Jack Levin refined the crab's blood into Limulus Amebocyte Lysate (LAL), a gel that clots in the presence of endotoxins when mixed with a test sample. Approved in 1977, LAL now runs over 80 million tests annually for injectables like insulin, flu shots, COVID vaccines, and implantable devices.