The American horseshoe crab that comes ashore on the East Coast in vast numbers to mate and nest is much the same creature that haunted the coast before the time of the dinosaurs. It is among the world's most intensely studied marine invertebrates, critical to our understanding of many groups of organisms, both modern and extinct, and crucial to the ecology of large estuaries such as the Delaware Bay. Some stocks of this great survivor, whose ancestors made it through the mass extinction some 286 million years ago, have been severely depleted today because of overfishing and habitat...