Copper instead of iron
Our blood is red because it carries oxygen with a molecule rich in iron, hemoglobin. The octopus picked a different metal. Its blood relies on hemocyanin, a molecule built around copper. And oxygenated copper does not turn red: it turns blue. Open an octopus and a bluish liquid runs out.
This is no whim of colour. Hemocyanin performs worse than hemoglobin under our own conditions, warm and well oxygenated. But in cold, deep, oxygen-poor water it pulls ahead: it grabs and releases oxygen more efficiently exactly where it grows scarce. For an animal that lives in the sea, sometimes far from the surface, it is precisely the right tool.
Why three, not one
A flaw remains. Hemocyanin blood flows less easily: it isn’t packed into cells like ours, it drifts freely, and it pushes with less force. A single heart would never keep up. So the octopus has three, laid out along a very clear logic.
Two of them, the branchial hearts, sit at the base of each gill. Their only job: push the blood through the gills to recharge it with oxygen. The third, the systemic heart, takes over and sends that freshly oxygenated blood out to the rest of the body. A two-step pumping chain, to make up for blood that drags its feet.
The heart that stops when it swims
The strangest part lies elsewhere. When the octopus jets through open water, its systemic heart stops beating. Swimming therefore costs it dearly: each burst of propulsion robs it, for an instant, of its main pump, and wears it out. That is why an octopus almost always prefers to crawl along the bottom, arm after arm, rather than dart off swimming. It spares a temperamental heart.
Three hearts, blue metal in the veins, a pump that switches off under strain: the octopus is not a botched animal. It is one shaped for a world we never visit, the world of cold, salty water. Down under the sea, red blood was never the obvious choice.