Octopuses Have Three Hearts and Blue Blood

The octopus's unique physiology is a remarkable adaptation to life in the ocean. The demanding task of pushing blood through the dense network of capillaries in the gills requires its own dedicated power source. That's why two of an octopus's hearts, known as branchial hearts, work exclusively to pump blood through the animal's gills to absorb oxygen from the water (Review). The third, more powerful systemic heart then circulates that oxygen-rich blood to the rest of the body. Interestingly, this systemic heart actually stops beating when the octopus swims, which helps explain why these creatures often prefer to crawl along the seafloor—it's simply less exhausting.

The blood being moved by this trio of hearts is just as unusual. Instead of the iron-based hemoglobin that makes our blood red, octopuses and other cephalopods use a copper-based protein called hemocyanin. This molecule is far more efficient at transporting oxygen in cold, low-oxygen conditions, giving the octopus a crucial survival advantage in the deep ocean environments it often inhabits. When deoxygenated, the blood is colorless, but once it binds with oxygen, the copper within the hemocyanin gives the fluid its distinct blue hue.