Horseshoe crabs, those mysterious ancient mariners, lured me into the sea as a child along the beaches of New Jersey. Drawn to their shiny domed shells and spiked tails, I could not resist picking them up, turning them over and watching the wondrous mechanical movement of their glistening legs, articulating with one another as smoothly as the inner working of a clock. What was it like to be a horseshoe crab, I wondered? What did they eat? Did they...