Horseshoe Crab

Horseshoe crab are marine arthropods of the family Limulidae that live primarily in and around shallow ocean water on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, a decline in the number of individuals has occurred as a consequence of coastal habitats in Japan and overharvesting along the coast of North America. Horseshoe crab are considered living fossils. Unlike vertebrates, horseshoe crab do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play the role of white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins. Harvesting horseshoe crab blood involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation.

http://www.pbs.org/wnet/nature/episodes/crash-a-tale-of-two-species/the-benefits-of-blue-blood/595/

http://izismile.com/2011/08/16/amazing_horseshoe_crabs_with_blue_blood_20_pics.html