Octopuses Have Three Hearts and Blue Blood

Octopuses possess a circulatory system that seems straight out of science fiction. They have three hearts: two pump blood to the gills, while the third pumps it to the rest of the body. Even more remarkably, their blood is blue instead of red because it uses copper-based hemocyanin to carry oxygen, rather than the iron-based hemoglobin found in humans and most vertebrates. This copper-based system is actually more efficient in cold, low-oxygen environments like the deep ocean where many octopuses live. When an octopus swims, the heart that delivers blood to the body actually stops beating, which is why these creatures prefer crawling along the seafloor to swimming—it's less exhausting. This unique physiology, combined with their nine brains (one central brain plus a mini-brain in each arm), makes octopuses one of nature's most alien-like creatures, despite living right here on Earth.