Each spring, guided by the full moon, hundreds of thousands of horseshoe crabs clamber onto beaches across the U.S. mid-Atlantic to lay their eggs. For hungry birds, it’s a cornucopia. For drug companies, it’s a crucial resource for making human medicines safe.

That’s because these animals’ milky-blue blood provides the only known natural source of limulus amebocyte lysate, a substance that detects a contaminant called endotoxin. If even tiny amounts of endotoxin—a type of bacterial toxin—make their way into vaccines, injectable drugs, or other sterile pharmaceuticals such as artificial knees and hips, the results can be deadly.

“All pharmaceutical companies around the world rely on these crabs. When you think about it, your mind is boggled by the reliance that