Octopuses Have Three Hearts and Blue Blood

Most animals have one heart pumping red blood, but octopuses operate on a completely different system. Two of their three hearts pump blood to the gills, while the third pumps it to the rest of the body. The blood itself is blue because octopuses use a copper-based protein called hemocyanin to carry oxygen instead of the iron-based hemoglobin that makes human blood red. This blue blood adaptation serves a purpose. Hemocyanin works better in cold, low-oxygen ocean environments where octopuses hunt. It's more efficient at transporting oxygen in those conditions than hemoglobin would be. There's a quirk to this system, though. When an octopus swims, the heart that delivers blood to the body actually stops beating. This is why octopuses prefer crawling along the ocean floor to swimming, they tire quickly in the water because their circulation essentially pauses during movement. Researchers have observed that captive octopuses will swim only when absolutely necessary, usually to escape something or reach food they can't otherwise access.