Octopuses (and other cephalopods like squid and cuttlefish) have blue blood due to the presence of a copper-rich protein called hemocyanin, which serves as their oxygen-carrying pigment instead of the iron-containing hemoglobin found in red blood cells of most vertebrates, including humans.

Hemoglobin gives blood its red color when it binds with oxygen; in contrast, hemocyanin turns blue when it binds with oxygen. Hemocyanin is dissolved directly in the plasma rather than being contained within specialized cells like red blood cells. When not bound to oxygen, hemocyanin appears colorless or slightly greenish, but when oxygenated, it produces a distinctive blue color that tints the octopus’s otherwise clear blood.

The reason for this different respiratory pigment may be related to the environments in which cephalopods typically live—cold, deep ocean waters where the pressure is high and oxygen levels can be lower. Hemocyanin is less efficient at binding oxygen than hemoglobin under normal atmospheric conditions, but it functions better under the high-pressure conditions typical of deeper depths in seawater. Additionally, hemocyanin has a higher affinity for oxygen at low temperatures compared to hemoglobin, making it advantageous for these cold-blooded marine animals.