An octopus has three hearts, and that is why it would rather walk than swim

An octopus has three hearts. It is the kind of fact that gets passed around on its own, which is a shame, because the interesting part is not the number. It is what the arrangement forces the animal to do, and the consequence is that a creature living in open water would generally rather crawl along the bottom than swim.

What the three hearts do

The work is divided. Two branchial hearts, one at the base of each gill, pump blood through the gills so it can pick up oxygen. A third and larger systemic heart then pushes that oxygenated blood out to the rest of the body.

Splitting the job this way is a reasonable engineering response to a problem, and to see the problem you have to look at what octopus blood is made of.

Blue blood, and why it is a compromise

Vertebrates carry oxygen on haemoglobin, which uses iron and turns red. Octopuses and other cephalopods use haemocyanin, which uses copper and turns their blood blue. Haemocyanin dissolves freely in the blood plasma rather than being packed into cells.

Haemocyanin has a genuine advantage: it is efficient at low temperatures and in low-oxygen water, which is exactly the environment much of the deep sea provides. But it carries considerably less oxygen per unit of blood than haemoglobin does. To deliver enough oxygen to an active animal, an octopus has to move a lot more blood, at higher pressure, than a fish of the same size.

Hence the extra pumps. The branchial hearts exist because getting blood through the fine vessels of the gills at sufficient rate is itself a demanding job, and the systemic heart alone could not both fill the gills and supply the body.

Why it would rather walk than swim

Here is the part that makes the fact worth knowing. When an octopus swims by jet propulsion, filling its mantle with water and forcing it out through the siphon, the contraction of the mantle raises the pressure inside the body cavity. That pressure works directly against the systemic heart, which sits inside it.

The result is that the systemic heart effectively stops, or slows dramatically, during vigorous jetting. The animal is swimming on whatever oxygen is already in circulation, and it accumulates an oxygen debt fast. Studies of swimming cephalopods have found that octopuses tire quickly and need a substantial recovery period afterwards.

Crawling has none of that cost. The arms move independently, largely under local nervous control, and the mantle stays relaxed so the heart keeps beating normally. For an animal built like this, walking across the seabed is not the slow option. It is the sustainable one, and jetting is reserved for escape.

The rest of the body follows the same logic

• Nine brains, loosely speaking. A central brain and a large nerve cluster in each arm. Roughly two thirds of an octopus’s neurons are in the arms, which can carry out movements without central instruction.
• No skeleton at all. The only hard part is the beak, which is why an octopus can pass through any gap the beak fits through.
• Skin that sees light. Octopus skin contains light-sensitive proteins of the kind normally found in eyes, which may contribute to how they match colour despite being, by the usual tests, colourblind.
• A short life. Most species live only one to two years, which sits oddly with how much intelligence the animal packs into that time.

Elsewhere in the same territory

For physics rather than biology, why the sky is blue.