Octopuses have nine brains and blue blood, and can taste with their arms

Few animals rearrange basic assumptions about biology as thoroughly as the octopus. It gets by with nine separate brains, pumps blue blood instead of red, and can taste an object simply by touching it with one of its eight arms. Marine biologists have spent decades mapping exactly how this unusual anatomy works, and the answers help explain why octopuses are considered among the most intelligent invertebrates on the planet, despite having no backbone and a lifespan of only a few years.

Nine Brains, One Central and Eight Local

An octopus has one central brain located between its eyes, but roughly two-thirds of its half-billion neurons are distributed into eight separate clusters, one running down each arm. Each of those arm-based neuron clusters functions almost like its own miniature brain, capable of directing movement and texture sensing without waiting for instructions to travel all the way to the central brain and back.

Researchers who study the species, including scientists affiliated with NOAA Fisheries, have found that a severed octopus arm can still respond to stimuli and even attempt to grasp food for a short time, a striking demonstration of how much independent processing power is packed into the limbs themselves.

Blood Built for Cold, Low-Oxygen Water

Instead of the iron-based hemoglobin that makes human blood red, octopuses rely on a copper-based protein called hemocyanin to carry oxygen through their bodies, which turns their blood pale blue. Hemocyanin is less efficient at binding oxygen in warm water than hemoglobin, but it performs better in cold, low-oxygen conditions, an adaptation suited to the deep, chilly seafloor habitats many octopus species favor over warmer shallows.

That same blood chemistry is one reason octopuses are especially sensitive to changes in water temperature and acidity, since warming oceans reduce how effectively their copper-based blood can pick up and release oxygen.

Three Hearts to Move That Blood

Circulating hemocyanin efficiently requires more pumping power than a single heart can provide, which is why an octopus has three: two smaller hearts push blood through each of its two gills, while a third, larger heart circulates freshly oxygenated blood to the rest of the body. That main heart briefly stops beating when the octopus swims by jetting water through its siphon, part of why octopuses generally prefer crawling over swimming when they can.

That heart-pausing pattern, combined with the energy cost of jetting, is why octopuses tend to save swimming for short bursts of escape from predators, relying on their arms for most day-to-day movement across the seafloor instead.

Arms That Taste What They Touch

Each of an octopus’s roughly 240 suckers per arm is lined with chemoreceptor cells that let the animal taste a surface the instant it makes contact, effectively combining touch and taste into a single action. That ability lets an octopus identify prey hidden in a crevice, or check whether an object is food at all, without ever bringing it near its mouth first.

Because so much of the nervous system operates locally within the arms, this tasting and gripping behavior can happen with minimal input from the central brain, letting each arm explore independently while the rest of the animal watches for predators or manages other tasks at the same time.

Camouflage, Puzzle-Solving, and Apparent Play

The combination of a capable central brain and eight semi-autonomous arms is part of why octopuses have been documented solving mazes, opening jars, and using coconut shells or discarded shells as portable shelters, behaviors described by researchers at London’s Natural History Museum as evidence of unusual problem-solving skill among invertebrates. Their skin also contains light-sensitive cells that work alongside their eyes to help them match colors and patterns during camouflage, even though octopuses themselves are colorblind.

In laboratory settings, octopuses have also shown the ability to distinguish between individual humans and recognize objects from previous encounters, with some documented cases of apparent play behavior toward objects that offer no food reward, signs researchers point to as evidence of flexible intelligence rather than simple instinct.

Why Engineers Study Octopus Neurology

The decentralized nervous system that lets an octopus arm act semi-independently has drawn interest from robotics researchers designing soft, flexible limbs that do not rely on a single central processor for every movement. Studying how an octopus coordinates nine brains without constant central oversight offers a working biological model for building machines that adapt and respond locally rather than routing every decision through one control unit.

Because arm neurons can process sensory information and trigger movement without waiting on the central brain, some robotics teams have built prototype grippers modeled on octopus arms that adjust their grip to oddly shaped or fragile objects, a capability standard rigid robotic arms still struggle to match consistently.

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

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