Octopuses have three hearts and blue blood.

Human blood is red because of iron, but octopus blood uses copper to move oxygen around. This copper-based protein is called hemocyanin. It works much better than our blood when the water is very cold and doesn't have much oxygen. When the copper connects with oxygen, it turns a bright blue color.The three hearts have very specific jobs. Two small hearts, called branchial hearts, sit right next to the gills. They push blood through the gills to pick up oxygen. Then, a larger heart in the middle, called the systemic heart, pumps that oxygen-rich blood to the brain and muscles.There is a catch to having three hearts. When an octopus swims fast, the big heart actually stops beating. This is because the muscles they use to swim put too much pressure on that heart. This makes swimming very tiring for them. To save energy, most octopuses prefer to crawl along the ocean floor instead of swimming.A study in 2013 by the Alfred Wegener Institute showed how amazing this blood is. It helps octopuses live in water that is almost freezing and also in water as warm as 30 degrees Celsius. Even though they have less energy than fish, their blue blood makes them masters of the deep, dark ocean.