Horseshoe crabs (Tachypleus gigas) live primarily in shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. Because of their origin 450 million years ago, horseshoe crabs are considered living fossils. Seen here at Chandipur-on-Sea on the Odisha coast on the shores of the Bay of Bengal. The blood of these creatures is used for high end diagnostics. The shallow sea is a tourist paradise. (Photo by Pallava Bagla/Corbis via Getty Images)