Octopuses are strange animals. They change colour in a blink, squeeze through gaps barely wider than their eyes, and their arms seem to have minds of their own. But one fact sounds almost made up: an octopus has three hearts, and its main heart stops beating when it swims by jet propulsion.
Plenty of websites mangle the details of this one, so I went back to the original research. Two things stood out. First, it’s the main heart that stops, not the two smaller hearts by the gills. Second, the reason it stops is far stranger than a simple energy-saving trick.
Why does an octopus have three hearts?
Each heart has its own job.
Two of them are branchial hearts, one at the base of each gill. They pump oxygen-poor blood through the gills. The third is the systemic heart. Once blood has picked up fresh oxygen in the gills, the systemic heart pumps it out to the rest of the body: the arms, the brain and every other organ.
It all works as one connected loop. Blood travels from the veins to the branchial hearts, through the gills, into the systemic heart, and out through the arteries, as ScienceBlog describes.
Why bother with three? Pushing blood through the fine vessels of a gill takes pressure. The gill hearts supply that extra push, leaving the systemic heart free for the high-pressure job of supplying the whole body.
We actually do something similar. The right side of a human heart pumps blood to the lungs, and the left side pumps it to the body. The octopus just keeps those jobs in separate hearts.
Which heart stops when an octopus swims?
The systemic heart. The main one.
This was shown in a 1987 study by M. J. Wells and colleagues in the Journal of Experimental Biology. They recorded blood pressure and flow in freely moving common octopuses (Octopus vulgaris) at rest and during exercise. When the octopuses walked around, their blood flow roughly doubled to meet the demand. But when they jetted, something very different happened. As the study’s abstract puts it, jet propulsion was accompanied by cardiac arrest.
How can an octopus stop its main heart and keep swimming?
The answer lies in how the jet works.
An octopus swims by jet propulsion using its mantle, the muscular body that surrounds most of its organs. It draws water into the mantle cavity, then squeezes hard, forcing the water out through its siphon. The jet shoots the octopus the other way.
That squeeze has a side effect. It sends the pressure inside the mantle soaring, and the 1987 study found that blood in the veins simply can’t flow back to the hearts against that pressure. In earlier work, Wells had also shown that the systemic heart only contracts when it’s filled. Put the two together and the chain looks like this:
- The mantle contracts hard.
- Water blasts out through the siphon.
- Pressure inside the body spikes.
- Blood stops returning through the veins.
- The systemic heart, with nothing to fill it, stops.
So the octopus isn’t deliberately switching off a heart to save energy. The jet itself cuts off the heart’s supply.
One detail often gets lost online. This happens during jet propulsion, not every time an octopus moves through the water. “Its heart stops whenever it swims” overstates it. The systemic heart stops during jetting, because of what the jet does to the pressure inside the mantle.
What about an octopus’s blue blood?
Yes, that’s real too, and it makes the whole system even more remarkable.
Our blood is red because it carries oxygen using haemoglobin, an iron-based protein packed inside red blood cells. Octopus blood, often called haemolymph, uses haemocyanin instead. It’s built around copper, it turns blue when it picks up oxygen, and it floats freely in the blood rather than sitting inside cells.
Haemocyanin has its limits, especially in the cold. A 2015 study led by University of Tasmania researchers compared an Antarctic octopus with a south-east Australian species and a Mediterranean one. It found that haemocyanin struggles to release oxygen to tissues at near-freezing temperatures, and that this limits how much cold octopuses can tolerate. The Antarctic species copes by carrying at least 40% more haemocyanin than the others, according to the study.
So the blood is already working hard to deliver oxygen. Jetting can stop circulation altogether. That combination has consequences.
Why crawling beats jet propulsion
Here’s where it all comes together. The 1987 study found that the oxygen debt an octopus can sustain is tiny, so jet-propelled movement is limited to a few metres at most.
That’s why jetting is the octopus’s emergency move: a burst to escape a predator or snatch prey, not a way to get around. Crawling along the sea floor with its arms avoids the huge pressure spikes, keeps the circulation running, and lets blood flow rise to meet the effort. It’s slower, but it’s the octopus’s sustainable everyday option.
Its whole lifestyle is shaped by the limits of its heart and blood.
Three hearts, one weak spot
So an octopus really does have three hearts: two to push blood through the gills, and one to send it around the body. Jet propulsion stops that main heart, because the pressure in the mantle blocks blood from returning to fill it. And all of it runs on blue, copper-based blood. What sounds like an impossible design turns out to be a well-matched system with one clear weak spot, and the octopus lives its life around it.
The ocean is full of animals stranger than the myths about them. The daddy long legs claim I looked into is a land-based example, and my post on why killer whales are actually dolphins is another favourite.
Sources
- Wells et al: Blood flow and pressure changes in exercising octopuses (Octopus vulgaris), Journal of Experimental Biology 131 (1987), University of Southampton repository
- ScienceBlog: During jet-propelled swimming, an octopus’s main systemic heart can briefly stop
- Oellermann et al: Blue blood on ice, modulated blood oxygen transport in an Antarctic octopod, Frontiers in Zoology (2015), University of Tasmania repository