Octopuses Have Three Hearts and Blue Blood
An octopus walks into a cardiologist's office. The cardiologist passes out. Three hearts is, generously, a lot of hearts.
Two of them — the branchial hearts — sit at the base of the gills, pushing blood through them so it can pick up oxygen. The third one, the systemic heart, then pumps that oxygenated blood out to the rest of the body. Most of the time. When the octopus swims, the systemic heart actually stops beating, which is why octopuses prefer crawling along the seafloor like a thoughtful, eight-legged commuter.
The blood is blue, by the way. Instead of iron-based haemoglobin like ours, octopuses use a copper-based protein called hemocyanin to ferry oxygen. It works better in cold, low-oxygen water — and it looks, frankly, fantastic.
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