They managed to survive the asteroid that killed the dinosaurs. Surviving humans may prove more difficult. Like many marine animals, horseshoe crabs are overfished for food and bait, and coastal development has destroyed spawning sites. But they also are collected en masse for their blue blood, which contains a rare clotting agent critical for the development of safe vaccines. The blood may be lifesaving for humans, but its harvest often kills the animals—particularly in much of Asia, where they are drained of all their blood rather than just a portion of it.

Tri-spine horseshoe crabs have lost more than half their population in the past 60 years. But on the Philippine islet of Pangatalan, the species is