Octopuses Have Three Hearts and Blue Blood

When you cut open an octopus — which marine biologists obviously do more often than the rest of us — you find a body plan that looks like it was designed by someone who had heard of vertebrates but disagreed with most of the conclusions. Three hearts instead of one. Blue blood instead of red. A brain that is mostly not in the head. The closer you look, the stranger it gets.

The three hearts are not just a fun trivia fact. They are a clue to a much bigger story about how a soft-bodied animal solved the problem of moving oxygen around a body that has no skeleton to anchor it.

Why three hearts, not one

Two of the octopus's hearts are called branchial hearts. Each sits at the base of one of the two gills, and its only job is to push deoxygenated blood through the gill so it can pick up oxygen from the water. The third heart — the systemic heart — then pumps that freshly oxygenated blood out to the rest of the body.

This division of labor exists because the octopus's blood is harder to pump than ours. The respiratory pigment it uses, hemocyanin, is dissolved freely in the blood plasma rather than packed inside red blood cells (as our hemoglobin is). That makes octopus blood thick and inefficient at carrying oxygen — about a quarter as good at it as vertebrate blood, according to the work summarized in Frontiers in Zoology on hemocyanin biology. Two dedicated gill pumps help compensate by keeping pressure high right where the oxygen exchange happens.

The trade-off is real but the design works. Octopuses thrive in cold, low-oxygen environments where hemocyanin [actually](/post/ai-in-daily-life-invisible-co-pilot) outperforms hemoglobin, which is one reason this same blood chemistry is found across most mollusks and many arthropods.

Why the blood is blue

Hemoglobin uses iron to bind oxygen, and the iron-oxygen complex looks red — which is why our blood is red.

Hemocyanin uses copper instead. When copper binds oxygen, the complex looks pale blue. Deoxygenated octopus blood is almost colorless; oxygenated blood is a faint blue. (If you have ever seen vivid royal-blue octopus blood in a YouTube thumbnail, the saturation has been turned up.)

The copper-based system is older and slower than the iron-based one, but it has one big advantage: it handles cold water better. Octopuses living in the deep sea, where temperatures hover near freezing and oxygen is scarce, would struggle with our blood chemistry. Their blue blood is one reason they can live somewhere we cannot.

The heart that stops when they swim

This is the part people usually do not believe. When an octopus swims — jetting through the water by squirting a stream out of its siphon — its systemic heart stops beating.

The contraction of the body wall that produces the jet is so strong that it temporarily compresses the systemic heart and interrupts its rhythm. The branchial hearts keep going, but the main pump goes quiet. Researchers have measured this in lab conditions and it consistently shows up.

This is why octopuses, despite being capable of fast jet propulsion, mostly walk along the seafloor on their arms. Swimming is metabolically expensive for them, partly because of this heart problem and partly because hemocyanin cannot keep up with the oxygen demand of sustained activity. Crawling is their default; jetting is what they do when they need to escape.

It is a useful reminder that "the best way to move" is relative to the body doing the moving. For us, sprinting is more efficient than crawling. For an octopus, the opposite is true.

Nine brains, sort of

You will see this claim everywhere: octopuses have nine brains. The accurate version is more interesting.

An octopus has about 500 million neurons — comparable to a dog. But unlike a dog, only about a third of those neurons live in the central brain. The other two-thirds are distributed across the arms, with a large nerve cluster in each one. Those arm clusters can process information and coordinate movement locally, without checking in with the central brain.

This is not a metaphor. In experiments, octopus arms that are severed from the body keep reaching, grabbing, and reacting to food for a short time. The arm "knows" what to do because the neural machinery to do it is right there. Researcher Binyamin Hochner's lab in Jerusalem has studied this distributed architecture for decades and found that the central brain seems to issue goals — "move toward that crab" — while the arms work out the details on their own.

Peter Godfrey-Smith, the philosopher who wrote Other Minds, describes octopus cognition as the closest thing on Earth to encountering an alien intelligence. He is not exaggerating for effect. The cephalopod nervous system evolved independently of the vertebrate one over 500 million years ago. It is one of two places on this planet where complex intelligence appears to have shown up — and the two solutions look almost nothing alike.

The RNA editing trick

There is one more weird thing worth knowing about, partly because it was only discovered recently and partly because it changes what "biology" even means.

A 2017 paper in Cell showed that octopuses and other cephalopods edit their own RNA at a rate orders of magnitude higher than humans do. RNA editing means changing the message between gene and protein — effectively tweaking how proteins behave on the fly, in response to the environment. We can do this too, but only on a tiny scale. Cephalopods do it constantly, especially in nervous-system tissues.

What this might mean is still debated, but one hypothesis is that RNA editing gives the octopus brain a way to adapt without having to wait for DNA mutations and evolution. It is a faster, more local form of change. Whether that is actually why cephalopods are unusually flexible learners is open, but the discovery alone is the kind of thing that makes evolutionary biologists rewrite their slides.

Why this matters beyond trivia

You do not need to care about octopuses to find this useful. The lesson hiding inside the three hearts and the blue blood and the distributed brain is that the body plan we happen to have — central pump, iron blood, brain in the skull — is not the only one that works. It is one solution. Cephalopods are a different solution.

When you encounter biology that looks strange, the temptation is to think it must be inefficient or primitive — the same impulse that makes radioactive bananas seem alarming until you understand the chemistry. Usually it just means evolution found a different local optimum, given different constraints. The octopus is what happens when you optimize a body for cold, dim water, soft tissue, and a need to think with your arms.

If anything, the surprise is not that octopuses are weird. It is that we expected biology to converge — the same mistake as assuming honey preserves itself through mystery rather than stacked biological chemistry.