Octopuses Have Three Hearts and Blue Blood
Two branchial hearts pump blood through the gills, while a third systemic heart circulates it around the body. Their blood contains hemocyanin, a copper-based protein that appears blue when oxygenated. This adaptation suits cold, low-oxygen seawater environments.
💡 Fun detail:
The systemic heart actually stops beating when an octopus swims, which is why they prefer crawling or jet propulsion.
Source: National Geographic