Why Octopuses Have Three Hearts

A simple explanation of octopus anatomy, blue blood, and why these sea creatures are so bizarrely intelligent.

ScrollFacts  ·  Marine Biology  ·  Animal Science  ·  7–9 min read

Three hearts. Blue blood. Nine brains. A doughnut-shaped brain wrapped around its own throat. The octopus is one of the most anatomically bizarre creatures on Earth — and yet every single strange feature it has exists for a brilliant reason. This is the story of how evolution built one of the ocean's most intelligent animals from the ground up, one weird design choice at a time.

The three-heart question is the perfect place to start. It sounds excessive — surely one heart is enough? But once you understand the blood flowing through those hearts, the whole system clicks into place. And once you understand the brains controlling those arms, you start to wonder whether "bizarre" is really the right word at all.

01 — Why Octopuses Have Three Hearts

Most animals get by with a single heart. So why does an octopus need three? The answer lies in the unique demands of their circulatory system — and specifically in the blood those hearts are pumping.

An octopus has two types of heart, each doing a very different job. Two of the hearts — called branchial hearts, or gill hearts — sit at the base of each gill. Their sole job is to push deoxygenated blood through the gills, where it picks up oxygen from the water. Think of them as the intake valves of the system. Without them, the blood never gets oxygenated in the first place.

The third heart — the systemic heart — then takes that freshly oxygenated blood and pumps it out to the rest of the body: all eight arms, the internal organs, and the brain. This is the main circulation pump, the one keeping the whole animal alive and moving.

Why does this system need three hearts rather than one? Because octopus blood is far less efficient at carrying oxygen than human blood — and so it needs extra pumping power to compensate. Which brings us to the blue blood question.

02 — Why Octopus Blood Is Blue

Human blood is red because it uses an iron-based protein called haemoglobin to carry oxygen through the body. When iron binds to oxygen, it produces the familiar red colour. Octopus blood works entirely differently.

Instead of haemoglobin, octopuses use a copper-based protein called haemocyanin. When copper binds to oxygen, it turns blue. When it releases oxygen and becomes deoxygenated, it turns completely colourless. So octopus blood is not blue all the time — it is blue when oxygenated, and transparent when it is not.

So why use haemocyanin at all if it is less efficient? Because in the cold, low-oxygen depths of the ocean, haemocyanin actually outperforms haemoglobin. The copper-based protein is far better at binding and transporting oxygen in freezing water with little dissolved oxygen — exactly the conditions in which octopuses live. It is a perfect adaptation to an extreme environment.

There is a catch, however. Haemocyanin makes octopuses extremely sensitive to changes in water acidity. If the ocean's pH drops too low, their blood cannot circulate enough oxygen to keep them alive. As ocean acidification increases due to climate change, this is a growing concern for octopus populations worldwide.

03 — The Heart That Stops When They Swim

Here is one of the strangest facts about octopus anatomy: every time an octopus swims, its main heart stops beating.

When an octopus needs to move quickly — to escape a predator, for example — it uses jet propulsion, sucking water into its mantle cavity and forcing it out through a siphon. The muscular contractions required for this jet-propelled movement physically compress the systemic heart, preventing it from pumping. The main circulation pump simply shuts down mid-swim.

"This is why octopuses strongly prefer crawling to swimming. Swimming is genuinely exhausting for them — not because they are unfit, but because their main heart stops working every time they do it."

Once an octopus stops swimming and settles back onto the seafloor, the systemic heart restarts and circulation resumes. But the animal still needs recovery time. It is a remarkable design limitation in an otherwise extraordinarily capable creature — and it explains why you will almost always see an octopus crawling gracefully along the ocean floor rather than swimming through open water.

04 — Nine Brains: The Arms That Think for Themselves

Three hearts is impressive. But the octopus brain situation is genuinely extraordinary. An octopus does not have one brain — it has nine.

There is a central brain, which is doughnut-shaped and wrapped around the oesophagus — meaning every piece of food an octopus eats passes directly through the middle of its brain on the way to its stomach. And then there are eight additional mini-brains, one located at the base of each arm.

These arm brains are not just relay stations. Each one is capable of independent thought and action. An octopus arm can taste, touch, smell, and move entirely on its own, without waiting for instructions from the central brain. About two-thirds of all the octopus's neurons — roughly 500 million in total, comparable to a dog — are located in the arms rather than the head.

This distributed intelligence system means an octopus can multitask in a way that is almost impossible for vertebrates. While its central brain focuses on hunting or navigating, its arms are simultaneously tasting the seafloor, probing coral crevices, prying open a clam, and testing the water — all acting semi-independently at the same time.

05 — Just How Intelligent Are Octopuses?

The short answer is: remarkably, surprisingly, almost uncomfortably intelligent for an animal so evolutionarily distant from us.

Octopuses have demonstrated in laboratory settings that they can navigate mazes, solve multi-step puzzles to reach food, open screw-top jars, recognise individual human faces, and use tools — including carrying coconut shells across the ocean floor to use as portable shelters later. That last behaviour is considered one of the clearest examples of tool use ever recorded in an invertebrate.

Perhaps the most striking anecdote comes from an aquarium where fish kept disappearing from a tank overnight. Staff set up a camera and discovered the culprit: an octopus from a nearby tank was climbing out of its own enclosure during the night, crossing the floor, climbing into the fish tank, eating the fish, then returning to its own tank and closing the lid behind it. The behaviour had been happening for weeks.

"Octopuses have also been observed playing — repeatedly releasing objects into a circulating current and catching them again, with no food reward. Play behaviour is considered a strong indicator of higher cognitive function."

What makes octopus intelligence so philosophically interesting is how independently it evolved. The last common ancestor shared by octopuses and humans was a simple, flat worm-like creature that lived over 750 million years ago. Complex intelligence evolved in octopuses and in vertebrates completely separately — two entirely different evolutionary paths arriving at a surprisingly similar destination.

06 — Other Bizarre Octopus Facts

The three hearts and nine brains barely scratch the surface. Here are a few more reasons the octopus is one of nature's most extraordinary designs.

They are colour blind, yet masters of camouflage. Octopuses can change their colour, pattern, brightness, and even skin texture in a fraction of a second — yet their eyes contain only one type of colour receptor, making them technically colour blind. Scientists believe their unusual dumbbell-shaped pupils may act like prisms, splitting light into separate wavelengths and allowing them to perceive colour indirectly. It is a mystery still being actively researched.

Their ink is a chemical weapon. Octopus ink is not simply a smokescreen. It contains melanin — the same dark pigment in human skin and hair — along with a compound called tyrosinase, which irritates and temporarily disables a predator's senses of smell and taste. The ink cloud buys the octopus time while its attacker is distracted and disoriented.

They are one of the oldest animal lineages on Earth. The oldest known fossil of an octopus ancestor dates back roughly 330 million years — long before the dinosaurs, and in the same ancient timeframe as sharks. By the time dinosaurs appeared on land, octopuses had already been refined by hundreds of millions of years of evolution.

All octopuses are venomous. Every species of octopus produces venom, injected through their beak to paralyse prey. Most are harmless to humans — but the blue-ringed octopus of Australia is a notable exception. Small enough to sit on a human palm, it carries enough venom to kill a person within minutes, with no known antidote.

Why the Octopus Matters

The octopus is not just a collection of strange facts. It is one of the most compelling arguments in nature that intelligence, complexity, and sophisticated behaviour can emerge from a completely different evolutionary starting point than our own.

Every unusual feature — the three hearts, the blue blood, the distributed brains — is an elegant solution to a real problem. The cold, dark, low-oxygen ocean floor is one of the most demanding environments on Earth. The octopus did not survive there by accident. It built itself into something perfectly suited to that world, piece by remarkable piece, over hundreds of millions of years.

The late cephalopod researcher Dr Martin Wells once said that the octopus is essentially an alien. With three hearts, blue blood, and arms that think for themselves, it is hard to argue otherwise. And yet here it is, sharing the same planet, shaped by the same evolutionary forces, arriving at intelligence by a completely different road.

That might be the strangest fact of all.