Octopuses are already among the strangest animals in the ocean. They can change colour, squeeze through remarkably small spaces and control much of their movement with their arms. But one fact about them sounds almost impossible: an octopus has three hearts, and its main heart stops beating when it uses jet propulsion to swim.
It sounds like one of those internet facts that has been exaggerated over time, and plenty of websites get the details wrong. So I went back to the original research. The key detail is that it is the systemic heart, the main heart that pumps blood around the body, that stops during jet propulsion, not the two hearts attached to the gills. And the reason it stops turns out to be far stranger than a simple energy-saving trick.
Why does an octopus have three hearts?
The three hearts work together as part of the octopus’s closed circulatory system, but they do different jobs. Two are branchial hearts, one attached to each gill, and they pump oxygen-poor blood through the gills. The third is the systemic heart. Once blood has passed through the gills and picked up oxygen, the systemic heart pumps it around the rest of the body, including the arms and other organs.
As ScienceBlog describes it, the blood travels in one connected loop: from the veins to the branchial hearts, through the gills, into the systemic heart, then out through the arteries. The arrangement makes sense for an animal that breathes through gills. Pushing blood through the fine vessels of a gill takes pressure, so the gill hearts provide extra pumping power for that part of the circuit, leaving the systemic heart to handle the high-pressure job of supplying the whole body. Humans have a similar division of labour inside one organ: the right side of our heart pumps blood to the lungs and the left side to the body. The octopus has simply split those jobs into separate hearts.
Which heart stops when an octopus swims?
The systemic heart stops during jet propulsion. This was shown in a landmark 1987 study by M. J. Wells and colleagues, published in the Journal of Experimental Biology. The researchers measured blood pressure and blood flow in freely moving common octopuses (Octopus vulgaris) at rest and during exercise, and found that jet propulsion was accompanied by cardiac arrest. Just as importantly, their measurements of the pressure inside the animal’s body explained why.
How can an octopus stop its main heart and keep swimming?
An octopus swims by jet propulsion using its mantle, the muscular body structure surrounding most of its internal organs. It draws water into the mantle cavity, then contracts the mantle hard to force the water out through its siphon, and the jet of water pushes it in the opposite direction. The problem is that the same squeeze that produces the jet also sends the pressure inside the mantle soaring.
The 1987 study found that this rise in pressure makes it difficult for blood in the veins to flow back to the hearts. That matters because, as Wells had shown in earlier work described by ScienceBlog, the systemic heart only contracts when it is filled. So the chain of events runs like this: the mantle contracts, water is forced out through the siphon, pressure inside the body rises, blood stops returning through the veins, and the systemic heart, with nothing to fill it, stops beating. In other words, the octopus is not deliberately switching off a heart to save energy. The jet itself cuts off the heart’s supply.
It is worth being precise about one thing that often gets lost online. The famous finding relates specifically to jet propulsion, not to every movement an octopus makes in the water. Saying an octopus’s heart stops “whenever it swims” is a slight oversimplification. The more accurate version is that the systemic heart stops during jet-propelled swimming, because the pressure inside the mantle interferes with blood returning to it.
What about an octopus’s blue blood?
The three-heart system gets even more interesting when you look at what the hearts are pumping. Human blood is red because it carries oxygen using haemoglobin, a protein built around iron and packed inside red blood cells. Octopus blood (which researchers often call haemolymph) uses a protein called haemocyanin instead. It is built around copper, turns blue when it picks up oxygen, and is dissolved directly in the blood rather than held inside cells.
Haemocyanin comes with real limitations, especially in the cold. A 2015 study led by researchers at the University of Tasmania, which compared an Antarctic octopus with a south-east Australian species and a Mediterranean one, found that haemocyanin is poor at releasing oxygen to tissues at near-freezing temperatures, and that it limits how much cold octopuses can tolerate. The Antarctic species copes by having at least 40% more haemocyanin in its blood than the others, as Sci.News reported. A blood supply that is already working hard to deliver oxygen makes it all the more striking that jetting can interrupt circulation altogether.
Why crawling is easier than jet propulsion
Put those facts together and the octopus’s behaviour starts to make sense. The 1987 study found that jet propulsion leaves the animal with an oxygen debt it can only sustain for a short time, so jetting suits short dashes rather than long journeys. Wells later went as far as calling jet propulsion in cephalopods “a locomotory loser”, and later researchers note that jetting almost always leads to oxygen debt. Crawling along the sea floor with its arms avoids those huge pressure spikes and lets the circulation keep running normally.
So jet propulsion is fast but expensive, a high-performance escape system for fleeing a predator or making a sudden move rather than an economical way to cruise. Crawling is slower, but it is the octopus’s sustainable everyday option. Its whole lifestyle is shaped by the limits of its heart and blood. It is another case of animal biology being stranger than the myths about it, like the daddy long legs claim I looked into, and the ocean is full of them; my post on why killer whales are actually dolphins is another favourite.
The short version
An octopus really does have three hearts. Two branchial hearts pump oxygen-poor blood through the gills, and the larger systemic heart pumps the freshly oxygenated blood around the body. When an octopus uses jet propulsion, the systemic heart temporarily stops, because the pressure generated inside the mantle stops blood returning to fill it. The octopus can still manage short bursts of jetting, but it pays a price in oxygen, which is why it usually crawls. And the blue blood is real too: octopuses carry oxygen with copper-based haemocyanin rather than our iron-based haemoglobin. What sounds like an impossible design is a remarkably well-matched system, with every strange feature tied to how an octopus breathes, moves and lives.
Sources
- Wells et al: Blood flow and pressure changes in exercising octopuses (Octopus vulgaris), Journal of Experimental Biology 131 (1987)
- 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)
- Sci.News: How Antarctic octopus Pareledone charcoti survives extreme temperatures