Horseshoe crabs used to be everywhere. Millions of years before dinosaurs roamed the planet, each spring, the hard-shelled creatures gathered to mate in massive mounds along the beaches of the Atlantic coast. Later, migratory shorebirds like the robin-sized red knot learned to fly up from South America to join them for a feast. The crabs' eggs gave the birds the energy they needed to keep flying north to breed in the Arctic.
But humans began to want something from the crabs, too – their blood. In the 1960s, scientists discovered that the sky blue blood inside horseshoe crabs would clot when it detected bacterial toxins. Vaccines, drugs and medical devices have to be sterile before they're put inside people. A better toxin detection system meant less contamination risk for patients, so fishermen soon started collecting and selling the prehistoric animals to be bled.
