An octopus has three hearts. Two branchial hearts push blood through the gills, and one larger systemic heart sends the oxygenated blood on to the rest of the body. The strange part, described by the Marine Biological Association, is that the systemic heart stops beating whenever the animal swims.
What does each heart do?
Think of it as a two stage pump.
The two branchial hearts sit at the base of the gills. Their only job is to force blood through the gill tissue, where it picks up oxygen from the water. Gills are dense and offer a lot of resistance, so this stage needs real pressure behind it.
The third heart, the systemic one, is larger. It takes that freshly oxygenated blood and drives it around the body: arms, brain, skin, everything.
So the octopus splits a job that our single heart does in one organ across three, with a dedicated pump for each side of the circuit.
Why does one heart stop when it swims?
An octopus swims by jet propulsion. It fills its mantle, the muscular sac behind the head, with water and then squeezes hard, firing the water out through a funnel and shooting itself in the opposite direction.
That squeeze is the problem. The systemic heart sits inside the mantle, and so do the veins feeding it. When the mantle contracts hard enough to launch the animal, the pressure inside rises far enough to pinch those veins shut.
No blood can reach the systemic heart. A pump with nothing arriving has nothing to move, so it stops. Every jet is a brief, self inflicted pause in the animal’s own circulation.
Why does an octopus need three hearts at all?
This is the part that ties it together, and it comes down to the blood.
Our blood carries oxygen using haemoglobin, which is built around iron. Octopus blood uses haemocyanin, built around copper, which is why it runs blue rather than red.
Haemocyanin has real advantages in cold, low oxygen water. What it is not is efficient. It carries less oxygen per unit of blood than haemoglobin does. To keep a large, active, famously intelligent animal supplied, the octopus has to move a greater volume of blood at higher pressure.
Three pumps are the answer to a fuel delivery problem. The blue blood explains the extra hearts, and the extra hearts explain the swimming.
So why do octopuses crawl?
Because swimming costs them their own circulation.
Jetting is fast but exhausting and it only carries the animal a few metres before it has to stop and recover. Crawling across the seabed on eight arms leaves the mantle relaxed, the veins open and the systemic heart beating normally.
Watch an octopus for long enough and you see the compromise in action. It walks almost everywhere and saves the jet for emergencies, because the escape mechanism briefly switches off the thing keeping it alive.
The short version
Two hearts for the gills, one for the body, blue copper based blood that carries oxygen poorly, and a swimming stroke that squeezes the main heart to a standstill. The three hearts are not a quirk. They are the fix for a circulatory system running on the wrong metal.
Source: Marine Biological Association.
