Learn about horseshoe crabs in this video from Nature. Horseshoe crabs spend most of their time, and are safest, on the ocean floor, but when it is time to spawn, they take a risk by coming to the surface to lay eggs. With the current moratorium imposed on fisherman using crabs as bait, they now harvest crabs for medical purposes and are then required to return them alive to Chesapeake Bay. Horseshoe crabs have blue blood containing a coagulant that detects minute levels of bacteria, forming a protective shield within the body akin to a "primitive antibiotic."
Learn more about the Nature film "Crash: A Tail of Two Species."