MargenAn octopus does not have one heart, but three. And its blood is not red: it is blue.
Two of those hearts pump blood towards the gills, where it takes on oxygen; the third drives it towards the rest of the body. Curiously, when an octopus swims, the main heart stops beating, and that is why many prefer to move by crawling: swimming exhausts them.
The blue colour comes from haemocyanin, a copper-based molecule that carries oxygen in their blood, instead of the iron-based haemoglobin that gives ours its red.
Source: Natural History Museum (London)