It’s a warm June night and a full moon is painting a silvery path across the gentle ripples on Delaware Bay.I’m on the beach, southeast of Dover. And from the darkness of the bay, I watch what looks like an invading force of army helmets with eyes on them emerge from the murk to crowd, clatter and scrape against each other along the shoreline.These are horseshoe crabs – prehistoric creatures that have been summoned by full moons and high tides like this for hundreds of millions of years to perform this springtime mass mating ritual on the beach.