How many hearts does an octopus have?

An octopus runs on three hearts because it runs on the wrong kind of blood. Its oxygen carrier is haemocyanin, which uses copper rather than the iron in our haemoglobin, and copper-based blood is a poorer carrier. To compensate, the animal pushes that blood through the gills under dedicated pressure: two branchial hearts serve one gill each, and a third systemic heart sends the freshly oxygenated blood on to the body. The arrangement has an odd side effect. The systemic heart stops beating while the octopus swims, which is exhausting enough that most species prefer to crawl along the seabed instead of jetting through open water. Haemocyanin also explains the colour: octopus blood is blue rather than red, and works better than haemoglobin in the cold, low-oxygen water where many species live.

About two thirds of an octopus's neurons sit in its arms rather than its brain.