Up and down the U.S. Eastern Seaboard, horseshoe crabs, which have been around for some 450 million years (200 million years before dinosaurs), are spawning. The crabs live in the ocean year-round but come ashore like clockwork every year between the May and June full moons to mate and lay eggs.

On any night during the spawning season, volunteers troll eastern beaches to help conduct the population surveys managed by state fisheries departments and marine and conservation organizations.

Some coastal areas—in Massachusetts and New York, for instance—may yield as few as 20 crabs in a night. But along the shores of the Delaware Bay, where crabs have been counted since the early 1990s, they may number in the tens of thousands.