Why do octopuses have three hearts and blue blood?
An octopus slides through the kelp forest, arms rippling, skin flashing from brown to red to speckled white. Inside that soft, boneless body, three hearts are beating. Three! And if you could peek at a drop of octopus blood under a microscope, you'd see it's not red like yours — it's blue. Why would evolution build such a strange circulation system?
Start with the blood. Your blood is red because it's full of hemoglobin, an iron-based molecule that grabs oxygen in your lungs and ferries it to your muscles and brain. Iron rusts red, so hemoglobin makes your blood red. But octopuses use a different molecule: hemocyanin, built around copper instead of iron. Copper oxidizes blue-green — think of the Statue of Liberty. So octopus blood, loaded with hemocyanin, runs pale blue.
