Unlike humans with red iron-based blood, octopuses have blue blood containing copper instead of iron.
Octopus blood contains hemocyanin, a copper-rich protein that turns blue when oxygenated. This adaptation helps them survive in cold, low-oxygen deep ocean environments where iron-based hemoglobin would be less efficient. Their unique circulatory system includes three hearts - two pump blood through the gills while the third circulates it to the rest of the body. Interestingly, when swimming, the systemic heart actually stops beating temporarily.