Octopus blood is blue because it uses hemocyanin, a copper-based protein, to carry oxygen.

Unlike humans, whose blood is red due to iron-based hemoglobin, octopuses use hemocyanin to transport oxygen. Hemocyanin contains copper, which turns blue when it binds to oxygen. This blue blood works better in cold, low-oxygen ocean environments but isn’t as efficient as hemoglobin at carrying oxygen, especially in warmer water. Because of this, octopuses need more hearts to pump blood quickly enough to meet their oxygen needs.