Look a little closer
Octopuses have not one but three hearts. Two of the hearts pump blood to the gills, while the third circulates blood throughout the rest of the body.
Even more surprising is that octopus blood isn't red — it's blue. While human blood relies on iron-based hemoglobin to carry oxygen, octopus blood uses a copper-based protein called hemocyanin instead.
Hemocyanin turns blue when bound to oxygen, and this allows octopuses to transport oxygen efficiently even in the cold, low-oxygen environment of the deep sea. However, its oxygen-carrying efficiency drops as temperatures fall, which makes octopuses especially sensitive to low-oxygen conditions.
Another fascinating fact is that about two-thirds of an octopus's neurons are located in its arms, allowing each arm to move and react with a surprising degree of independence.
