Octopus Has Three Hearts

Instead of the iron-based hemoglobin that makes our blood red, an octopus’s circulatory system is built around a copper-based protein called hemocyanin, which turns its blood a distinct blue. While effective in cold, low-oxygen environments, hemocyanin is less efficient at transporting oxygen than our own blood. To compensate for this less potent blood, the octopus evolved a remarkably powerful and specialized circulatory system to keep its active body and large brain supplied with the oxygen it needs to survive.

This system is powered by a trio of hearts. Two smaller branchial hearts are dedicated solely to pumping blood through the octopus's large gills, ensuring maximum oxygen absorption from the water (Review). Once oxygenated, the blue blood flows to a larger, more muscular systemic heart, which circulates it throughout the rest of the body. Curiously, this powerful systemic heart stops beating when the animal swims, a physiological quirk that makes swimming incredibly taxing. This explains why octopuses often prefer to use their intelligent arms to crawl along the ocean floor, saving their energy-intensive jet propulsion for short bursts of speed to hunt or escape danger.