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Octopuses Have Blue Blood and Three Hearts
Octopuses have blue blood due to copper-based hemocyanin instead of iron-based hemoglobin, and they have three hearts to pump it efficiently.
About this fact
Unlike humans who have red blood due to iron-based hemoglobin, octopuses have blue blood because they use copper-based hemocyanin to carry oxygen. This blue blood is more efficient in cold, low-oxygen environments. To pump this unusual blood, octopuses have three hearts: two pump blood to the gills, while the third pumps blood to the rest of the body. Interestingly, the main heart stops beating when they swim, which is why octopuses prefer crawling - swimming exhausts them quickly.