Which sea creature has three hearts?

An octopus runs on three hearts and blue blood, and the two facts are connected. Its oxygen carrier is haemocyanin, which uses copper rather than the iron in our haemoglobin, and that turns the blood bluish. Haemocyanin is less efficient at carrying oxygen, but it works better in cold, low-oxygen water, which is where many octopuses live. To compensate for the weaker carrier the animal needs higher blood pressure through the gills, so two branchial hearts pump blood through them while a third systemic heart drives the oxygenated blood around the rest of the body. There is a curious consequence: the systemic heart stops beating when the octopus swims by jet propulsion, which tires it quickly and is one reason these animals prefer to crawl along the seabed rather than swim. The circulatory arrangement is closed, unusual among molluscs, which supports their active, predatory way of life.

Most of an octopus's neurons are in its arms rather than its brain, so each arm can explore and grip with a good deal of independence.