An octopus tastes with its arms and runs much of its thinking through nine brains

An octopus does not think the way a person does, with a single command center calling all the shots. Its nervous system is spread out, with a central brain wrapped around its throat and a large cluster of nerve cells running down each of its eight arms. Biologists sometimes describe the arrangement as nine brains, one in the head and one in every limb, and it allows an octopus to solve problems in a way no vertebrate can. Its arms can explore, grip and even taste on their own, sampling the world by touch while the central brain attends to something else entirely.

That decentralized design is one reason octopuses seem so alien and so uncannily clever. It also makes them a favorite subject for scientists trying to understand how intelligence can arise from a body plan radically different from the human one, an eight-limbed invertebrate that last shared a common ancestor with humans more than half a billion years ago.

One brain around the throat, eight more in the arms

The octopus’s central brain sits between its eyes and is shaped like a doughnut, threaded around the animal’s esophagus. But the majority of its nerve cells are not there. Of the roughly 500 million neurons an octopus carries, about two-thirds are distributed through the arms rather than concentrated in the head, according to the Smithsonian’s overview of cephalopods and their biology. Each arm contains its own bundle of nerve tissue, a ganglion, that can process information and direct movement locally.

The total puts an octopus in surprising company. Its half a billion neurons are far more than most invertebrates possess and in the rough neighborhood of a small mammal, but they are wired in a fundamentally different architecture, dispersed across the body instead of centralized in a skull.

Arms that act with a mind of their own

Because so much of the nervous system lives in the arms, they enjoy a striking degree of independence. An octopus arm can carry out coordinated actions, curling to grasp an object or feeling its way into a crevice, with relatively little moment-to-moment instruction from the central brain. Experiments have shown that the neural circuitry within an arm can manage many of the details of movement on its own, freeing the animal’s central brain from micromanaging eight flexible, suckered limbs at once.

This semi-autonomy solves a real engineering problem. A rigid skeleton simplifies control, but an octopus arm can bend at any point and in any direction, giving it a nearly infinite range of positions. Delegating control to the arms themselves is how an octopus keeps such a boneless, hyper-flexible body from becoming unmanageable.

Suckers that taste whatever they touch

The arms do more than move; they perceive. The rows of suckers lining each arm are studded with sensory receptors that let the octopus taste by touching. Rather than needing to bring food to a mouth to sample it, the animal can identify what it is handling the instant its suckers make contact, distinguishing prey from pebbles inside a dark hole it cannot see into.

Research has identified specialized receptors in octopus suckers that detect chemicals directly from surfaces, a sense sometimes described as chemotactile, or taste-by-touch. Studies from laboratories investigating cephalopod biology have traced how these receptors respond to molecules that do not dissolve well in water, exactly the kind of chemical cue that would cling to potential prey. The result is an animal that can, in effect, reach into a crack and taste what is hiding there.

Nine brains, three hearts and blue blood

The distributed nervous system is only one of the octopus’s departures from the familiar animal body plan. It also runs on three hearts and blue blood. Two pump blood through the gills to pick up oxygen, and a third drives the oxygenated blood to the rest of the body, a system that shuts its main heart down when the animal swims, which is part of why octopuses often prefer to crawl.

Their blood is blue rather than red, because octopuses carry oxygen using a copper-based molecule called hemocyanin instead of the iron-based hemoglobin in human blood. Hemocyanin is more efficient at moving oxygen in the cold, low-oxygen conditions of the deep sea, another adaptation suited to the environments many octopuses inhabit.

What a distributed nervous system is good for

The payoff of all this is behavior that repeatedly startles researchers. Octopuses open jars, navigate mazes, use coconut shells and other objects as portable shelters, and recognize individual handlers. They can squeeze a large body through a gap barely wider than their beak, the only hard part of their anatomy, and change color and texture in a fraction of a second to vanish against a reef.

Whether an octopus experiences the world the way a centralized brain does remains an open scientific question, precisely because so much of its processing happens out in the limbs. What is clear is that intelligence does not require the vertebrate blueprint. In the octopus, evolution arrived at a sophisticated mind by a completely separate route, distributing the work across a body built for flexibility, and producing an animal that thinks, senses and tastes with its arms.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview