An octopus has three hearts. Two of these hearts are responsible for pumping blood to the gills, where oxygen is absorbed. The third heart pumps the oxygenated blood to the rest of the body. Despite having three hearts, the octopus has a unique circulatory system because it has blue blood, which contains hemocyanin instead of hemoglobin for transporting oxygen. When an octopus swims, the heart that delivers blood to the body stops beating, which is why they prefer to crawl rather than swim; swimming is more tiring for them due to this physiological trait.