Division of labor
3heartstwo for the gills, one for the body
Two gill hearts pump the blood through the two gills, where it picks up oxygen. The third, the systemic heart, sends it from there through the rest of the body.
Swimming? Rather not.
When an octopus swims, its systemic heart stops beating. It loses energy quickly. That's why it mostly crawls across the seafloor – and saves fast jet propulsion for when it's in a hurry.
And the blood is blue
In octopuses, oxygen isn't carried by iron-based hemoglobin as in us, but by hemocyanin. It contains copper and turns the blood bluish. In cold, low-oxygen water it even carries oxygen better than hemoglobin.
