For some, the coming of summer means wearing white, but for citizen scientists like Gina Mulhearn and Mark Cappiello, it means donning wellies and waders, then heading to the beach to count and tag horseshoe crabs as they spawn up and down the eastern seaboard, a phenomenon that peaks around the full moons in May and June.

During their 450-million-year span on the planet, horseshoe crabs—which resemble aquatic steam shovels and are more closely related to spiders than crabs—have survived mass extinctions, ice ages, and asteroids. As such, they hold a slot on evolution's time line that makes humans look like a vulnerable speck.

For decades, these animals have been invaluable to human health. If you've ever had an inoculation and