These crabs probably saved your life. Can we save theirs?

Kneeling at low tide, the biologist dug up small shovelfuls of sand, scanning each scoop for tiny jewels. One yielded a cluster of jade-colored beads. Another, from a few feet away, contained a clutch the color of amethyst.

They were eggs. In a few weeks, they would hatch into horseshoe crabs, one of the most ancient and important animals in the United States. The crabs in the Delaware Bay are the stars of an annual ecological opera involving sex, binge eating and literal bloodlust.