Hemocyanin is the blood pigment that makes horseshoe crab blood blue.
Hemocyanin transports oxygen using copper atoms. When oxygenated, the copper-containing molecule appears blue, unlike hemoglobin, the iron-containing pigment that makes human blood red.
Horseshoe crabs have an open circulatory system and use hemolymph rather than blood in the vertebrate sense. Their immune cells can react rapidly to bacterial endotoxins, a property that has made horseshoe crab blood important in testing injectable medical products.
The blue color is not unique to horseshoe crabs; several arthropods and molluscs use hemocyanin. It is also different from the blue appearance sometimes seen in animals whose skin or shell reflects light. Horseshoe crabs are ancient marine arthropods, more closely related to arachnids than to true crabs.