Each summer, horseshoe crabs (Limulus polyphemus) from the Yucatan to Maine crawl up on beaches to mate and lay eggs. That makes June and July a good time of year for marine scientists like John Tanacredi, of Dowling College, to monitor their numbers. He's been tracking horseshoe crabs on Long Island, N.Y. for 10 years and has recently started a breeding program, with the aim of increasing their population size in the wild. En Español.