The Octopus Has Three Hearts

The octopus is one of the ocean's strangest engineers. Two of its hearts pump blood through the gills, while a third circulates it to the rest of the body. Its blood is blue rather than red because it uses copper-based haemocyanin to carry oxygen, which works far better than iron in cold, low-oxygen water.

Curiously, the main systemic heart stops beating whenever the octopus swims, which is why these animals often prefer to crawl — swimming quite literally tires them out. Combined with their problem-solving intelligence and camouflage skills, octopuses are among the most remarkable invertebrates alive.