On a June afternoon, during high tide at Jamaica Bay in New York, a crowd gathers to watch the mating ritual of one of Earth’s most ancient creatures. Undeterred by onlookers, American horseshoe crabs scoot around like bumper cars in the surf until they collide with a mate. A male latches onto a larger female, holding tight in the roiling waves. She finds a suitable place to dig into the sand and, as she lays thousands of eggs, he fertilizes them. These helmet-shaped arthropods—not really crabs, they’re more closely related to spiders—have survived 5 mass extinctions and for more than 400 million years. And yet, today, across much of their range they face the threat of being wiped out. Habitat destruction has taken a toll, but so has overharvesting. Commercial fishermen argue that the crabs, which inhabit the Atlantic coast from Maine to Mexico, are the only viable bait for conch, whelk, and eel. Biomedical companies rely on a compound extracted from horseshoe crab...