The octopus is the animal with three hearts and what is often described as nine brains. The “three hearts” part is straightforward anatomy: one systemic heart pumps blood through the body, and two smaller branchial hearts push blood through the gills. The “nine brains” claim requires more nuance, because an octopus has one central brain and eight large nerve clusters, one at the base of each arm, that operate with remarkable independence. Whether you call those clusters “brains” depends on your definition, but the effect is real: an octopus arm can taste, react to stimuli, and execute complex movements even after being severed from the central brain.
Why Three Hearts Instead of One
Octopuses and other cephalopods rely on hemocyanin, a copper-based protein, to carry oxygen through their blood. Hemocyanin is less efficient at binding oxygen than the hemoglobin found in vertebrate blood. It is also what gives octopus blood its distinctive blue color. Because of this less efficient oxygen carrier, the octopus circulatory system needs extra pumping power. The two branchial hearts sit at the base of the gills and push deoxygenated blood through gill tissue so it can pick up oxygen. The oxygenated blood then flows to the larger systemic heart, which sends it to the rest of the body.
This arrangement matters more than it might sound. Research on octopus hemocyanin shows that oxygen binding is heavily influenced by pH, temperature, and the ionic makeup of the blood. In the giant Pacific octopus, for instance, the effect of pH on hemocyanin’s oxygen affinity is dramatic, far larger than the comparable effect in most vertebrate blood proteins.1PubMed. Oxygen equilibria of Octopus dofleini hemocyanin The three-heart system compensates for hemocyanin’s limitations by keeping blood pressure high and oxygen delivery constant across all tissues, including the metabolically demanding brain.
One well-known quirk: the systemic heart stops beating when an octopus swims by jet propulsion. This is why octopuses generally prefer crawling along the seafloor. Jet swimming is fast but exhausting, because the body is temporarily running on whatever oxygen is already in the bloodstream. Crawling lets all three hearts work together smoothly.
How Temperature Reshapes the System
The three-heart system faces different challenges depending on water temperature. In cold-water species like the Antarctic octopus Pareledone charcoti, hemocyanin releases oxygen poorly at near-freezing temperatures. These octopuses compensate partly because cold water holds more dissolved oxygen, so the blood itself carries extra oxygen even when hemocyanin underperforms. At warmer temperatures, the same hemocyanin releases bound oxygen much more efficiently.2PubMed Central. Blue blood on ice: modulated blood oxygen transport facilitates cold compensation and eurythermy in an Antarctic octopod Warm-water octopuses face the opposite problem: their hemocyanin binds oxygen more tightly in cold conditions, making it harder to release where tissues need it.
Temperature also affects the nervous system in unexpected ways. Octopuses can edit their own RNA in response to temperature changes, altering the proteins that their cells produce without changing the underlying genetic code. In one species, roughly a third of the RNA editing sites examined showed significantly higher editing levels in cold water compared to warm water. Some of these edits directly affect proteins critical to nerve function, including one that changes how fast molecular motors transport cargo along nerve fibers and another that alters calcium binding in the machinery of synaptic transmission.3PubMed Central. Temperature-dependent RNA editing in octopus extensively recodes the neural proteome This is a remarkable form of molecular flexibility: the octopus fine-tunes how its nervous system functions as conditions change, without waiting for genetic mutations to accumulate over generations.
The Central Brain and Eight Arm Ganglia
An octopus has around 500 million neurons, roughly comparable to a dog. But the distribution of those neurons is strikingly different from any vertebrate. About two-thirds of them are located in the arms, not in the central brain. Each arm has a dense cluster of nerve cells at its base that acts as a local control center, processing sensory information and coordinating movement without needing instructions from the brain. Nerve fibers do run between the brain and the arms, carrying signals from higher-level brain regions directly to the arm nerve cords.4Philosophical Transactions of the Royal Society of London. B, Biological Sciences. Central pathways of the nerves of the arms and mantle of Octopus But the arms also have substantial autonomy.
This has been demonstrated in experiments where the nerve connections between an arm and the central brain are severed. The arm can still withdraw from a painful stimulus, explore objects, and perform basic grasping motions. In one study of pain responses, researchers found that even with the central nervous system removed entirely, injecting an irritant into an arm produced prolonged neural activity lasting over 30 minutes in the arm’s local nerve network, and that activity stopped only when a local anesthetic was applied to the injection site.5PubMed Central. Behavioral and neurophysiological evidence suggests affective pain experience in octopus The arm itself was generating and sustaining a sensory signal about the location of the painful stimulus, information that would normally travel to the brain for processing.
So calling each arm cluster a “brain” is a stretch, but only just. Each one processes local sensory data, initiates motor programs, and can operate independently. The central brain seems to set high-level goals like “reach toward that crab” while leaving the details of execution to the arms themselves. Researchers studying octopus arm motor control describe the system as engaging every division of the nervous system, from the nerve cords in the arms all the way up to the central brain, in a coordinated hierarchy.
Tasting the World Through Touch
The suckers lining each arm are not just for gripping. They are studded with specialized sensory cells that allow the octopus to taste whatever they touch. Scientists examining octopus suckers discovered two distinct types of receptor cells: chemosensory cells that detect chemicals emitted by objects, and mechanosensory cells that determine whether an object is moving. If something is moving, the octopus knows it might be food rather than a rock.6PubMed Central. Neural Control of Dynamic 3-Dimensional Skin Papillae for Cuttlefish Camouflage This “taste by touch” system is part of why the arm ganglia need so many neurons: each sucker is feeding the local nerve cluster a stream of chemical and tactile information that gets processed on the spot.
The skin itself is another sensory marvel. Cuttlefish and octopuses can raise and lower bumps on their skin called papillae, controlled by a complex system of muscles. These papillae can be held in a raised position for extended periods without continuous nerve input, thanks to a latch-like mechanism involving sustained muscle tension. Combined with rapid color and texture changes driven by chromatophore cells, the papillae let an octopus mimic the fine texture of coral, algae, or rock in seconds. The camouflage is not just visual; it is three-dimensional.
Problem-Solving and Individual Personality
An animal with this much neural hardware might be expected to be clever, and octopuses deliver. Classic experiments demonstrated decades ago that Octopus vulgaris can open transparent glass jars sealed with plastic plugs to reach a crab inside. The animals improved over repeated trials, needing fewer failed attempts and less time as they practiced, consistent with learning by either stimulus-response association or trial and error.7PubMed. Problem solving ability of Octopus vulgaris Lamarck (Mollusca, Cephalopoda)
More recent work has shown that individual octopuses differ in both personality and problem-solving ability. In experiments with common octopuses, researchers measured personality traits like neophilia (eagerness to investigate new objects) and individual learning rates, then tested how quickly each animal could open a puzzle box containing a crab. More curious octopuses approached the box faster and were more likely to solve it. Octopuses with larger brains made first contact with the box sooner than those with smaller brains. And animals that scored higher on individual learning measures solved the puzzle in less time, confirming that “smart” and “not as smart” is a real spectrum across individual octopuses.8PubMed Central. Octopus vulgaris Exhibits Interindividual Differences in Behavioural and Problem-Solving Performance
Octopuses also show behavioral flexibility in reversal learning tasks, where the “correct” answer is suddenly switched. When given appropriate feedback signals, octopuses learned a spatial discrimination task within a few sessions and then successfully adapted through several reversals, improving progressively each time.9PubMed Central. Reversal of a Spatial Discrimination Task in the Common Octopus (Octopus vulgaris) This kind of flexibility, learning a rule and then learning that the rule has changed, is a hallmark of cognitive sophistication that many vertebrates struggle with.
Sleep States That Mirror Vertebrate Patterns
Octopuses sleep, and they appear to dream, or at least go through something that looks a lot like dreaming. Researchers studying sleeping octopuses identified two distinct states that cycle back and forth. One is a quiet sleep state where the animal is still and pale. The other, called “active sleep,” involves dynamic changes in skin color and texture, rapid eye movements, sucker contractions, and muscular twitches, with episodes lasting a median of about 41 seconds.10PubMed Central. Cyclic alternation of quiet and active sleep states in the octopus The parallels to vertebrate REM sleep are striking: the rapid eye movements, the muscle twitches, the short cycling between states. Whether the octopus is actually experiencing anything like a dream is unknown, but the neural architecture for alternating sleep states evidently evolved independently in cephalopods and vertebrates.
The color changes during active sleep are especially vivid. An octopus that was resting in pale camouflage might suddenly flash through a rapid sequence of dark patterns, spots, and stripes before returning to quiet stillness. Some researchers have speculated that the octopus might be “replaying” waking experiences, cycling through the skin patterns it used during the day, though this remains speculative.
Evidence That Octopuses Feel Pain
The question of whether invertebrates experience pain, rather than simply reacting reflexively to harmful stimuli, has been debated for decades. The octopus is one of the strongest cases for invertebrate pain. Research has shown that when an irritant is injected into one arm, octopuses do not just flinch. They avoid the affected arm for extended periods, groom the injection site, and prefer locations in their environment associated with pain relief. The neural activity generated at the injury site travels to the central brain, which processes the information and drives these lasting behavioral changes.5PubMed Central. Behavioral and neurophysiological evidence suggests affective pain experience in octopus These are not the hallmarks of a simple reflex. They suggest an experience that has an emotional dimension, what pain researchers call “affective pain.” This evidence has had real-world consequences: the United Kingdom amended its Animal Welfare Act in 2022 to recognize cephalopods as sentient beings, meaning that octopuses, squid, and cuttlefish now have legal protections against cruelty.
A Brain That Evolved on a Completely Separate Path
Octopus intelligence did not evolve from the same ancestor as vertebrate intelligence. The last common ancestor of octopuses and humans lived over 500 million years ago, and it was almost certainly a simple worm-like creature with a minimal nervous system. Everything about the octopus brain, from the distributed arm ganglia to the complex central lobes, evolved independently. This makes octopuses one of the most important counterexamples in the study of how intelligence evolves. Various hypotheses have proposed that large brains require long lifespans, complex social structures, or extended parental care. Octopuses have none of these: most species live only one to two years, they are generally solitary, and maternal care typically ends with the female’s death shortly after the eggs hatch.11PubMed Central. Evolution of cephalopod nervous systems Yet they evolved brains with hundreds of millions of neurons and sophisticated behavioral repertoires.
This independent evolution means that any similarities between octopus cognition and vertebrate cognition, such as the alternating sleep states, the capacity for learning, and the evidence for pain experience, arose through convergent evolution. Two completely different lineages arrived at similar solutions to the same biological problems. Studying where those solutions converge and where they diverge is one of the reasons neuroscientists find cephalopods so valuable.
Robotics Inspired by Octopus Anatomy
The octopus body plan has become a popular template for soft robotics. The distributed control architecture, where a central coordinator sets goals and local controllers handle the details, maps neatly onto engineering problems for flexible robotic arms. One recent system, inspired by octopus arm control, models each section of a soft robotic arm as an independent agent that cooperates with its neighbors, learning to reach targets and navigate obstacles without needing a complete map of its surroundings.12arXiv. Octopus-inspired Distributed Control for Soft Robotic Arms: A Graph Neural Network–Based Attention Policy with Environmental Interaction
The suckers have also attracted engineering interest. Researchers have built soft robotic systems that mimic the hierarchical structure of octopus neuromuscular control, using simple suction cups as both sensors and actuators. By exploiting the energy and information contained in suction itself, these robots can achieve complex grasping and manipulation without rigid joints or conventional electronic sensors.13PubMed. Embodying soft robots with octopus-inspired hierarchical suction intelligence The appeal is obvious: octopus arms can squeeze through gaps, conform to irregular shapes, and manipulate objects with a dexterity that rigid robotic arms cannot match. If you could replicate even a fraction of that flexibility in a machine, applications in surgery, search and rescue, and deep-sea exploration become much more practical.
Other Animals With Multiple Hearts
The octopus gets the most attention for its three hearts, but it is not the only animal with more than one. Squid and cuttlefish, its closest relatives, share the same three-heart arrangement. Earthworms have five pairs of aortic arches that function like simple hearts, pumping blood through a closed circulatory system. Hagfish, sometimes called the most primitive living fish, have one main heart plus several accessory hearts distributed along their body that help push blood through their unusually low-pressure circulatory system. None of these animals, though, combine multiple hearts with anything close to the octopus’s neural complexity. The pairing of a high-performance circulatory system with a distributed, semi-autonomous nervous system is what makes the octopus genuinely unusual, not just in the “fun animal facts” sense but in a way that has reshaped how biologists think about the evolution of intelligence and body plan integration.