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🐙Biology9 min read

The Octopus

An octopus has nine brains, three hearts, blue blood, and the ability to change the colour and texture of its skin in under a millisecond. It is also almost certainly colourblind, and may see colour anyway, through its skin.

In plain English

Most animals with significant intelligence share a common ancestor. Mammals, birds, even fish: the neural structures underlying their cognition trace back to the same vertebrate lineage. Intelligence, in most cases, is a family trait.

The octopus is the exception. Its most recent common ancestor with vertebrates is a simple flatworm that lived over 500 million years ago. Everything the octopus has, its distributed nervous system, its problem-solving ability, its manipulation of colour and texture, evolved completely independently. From scratch. Along a totally different path.

This makes the octopus uniquely interesting. When we look at a dog or a crow or even a fish, we are looking at intelligence built on recognisable foundations. When we look at an octopus, we are looking at what intelligence looks like when it evolves in a completely different direction.

The answer is: very strange indeed.

Five things to file under "wait, what?"

  • It has nine brains. One central brain sits between its eyes. But each of its eight arms contains its own neural cluster, a ganglion capable of processing sensory information and directing movement independently. Around two-thirds of an octopus's 500 million neurons are in its arms, not its central brain. When you cut off an octopus arm, the arm continues to behave purposefully for up to an hour, recoiling from painful stimuli and even passing food toward where the mouth used to be.

  • It has three hearts and blue blood. Two branchial hearts pump blood through the gills. One systemic heart pumps it to the body. The blood is blue because it uses haemocyanin, a copper-based protein, to carry oxygen, rather than the iron-based haemoglobin that makes vertebrate blood red. Haemocyanin is less efficient at oxygen transport, which is why octopuses tire quickly in sustained movement. But it works better in cold, low-oxygen environments, useful for a deep-sea animal.

  • It can change colour and texture despite being almost certainly colourblind. Octopus skin contains millions of chromatophores, tiny, elastic pigment sacs controlled directly by muscles and neurons, as well as iridophores that create structural colour through light interference, and papillae that can change the physical texture of the skin within milliseconds. The octopus can mimic a rock, a piece of coral, a patch of sand, or another animal with speed and precision. The puzzle is that octopuses have a single type of photoreceptor in their eyes, the standard basis for colourblindness. The leading hypothesis is that they use the pupil shape (a distinctive horizontal W) to take in light from different angles simultaneously, effectively reading colour through chromatic aberration.

  • They edit their own RNA. Almost all animals alter their proteins through DNA mutations: slow, intergenerational change. Octopuses do something different: they chemically modify their RNA after transcription, effectively reprogramming individual proteins in real time without changing the underlying DNA. This allows rapid adaptation to temperature change. Octopuses in colder water reconfigure the same proteins that function in warmer water, without waiting for genetic evolution. This RNA editing is rare in nature and extensive in cephalopods. It may be one reason they can adapt so rapidly to changing environments.

  • They use tools. Octopuses have been filmed collecting coconut shell halves, carrying them across the sea floor, and then assembling them into portable shelters, behaviour that requires planning and delayed gratification. This is one of the clearest examples of tool use outside vertebrates. They also regularly unscrew jars from the inside to retrieve food, navigate mazes, recognise individual human faces, and, in captivity, have been known to squirt water at lights they find annoying until the lights are turned off.

The full story

A short, intelligent life

One of the most striking facts about octopus intelligence is the context in which it exists: a lifespan of one to two years. Most animals with comparable cognitive complexity (primates, corvids, elephants, cetaceans) are long-lived. The investment in intelligence seems to require time to pay off. The octopus confounds this.

An octopus reaches sexual maturity within months, mates once, and dies. The female octopus guards her eggs, thousands of them, for weeks or months without eating, then dies shortly after they hatch. She does not survive to teach her offspring anything. The young octopuses emerge fully alone, with no parental guidance, no cultural transmission, no accumulated learning from previous generations.

And yet they are demonstrably intelligent. They play. They have been shown to have individual personalities, consistent differences in how boldly or cautiously different individuals approach new situations, differences that persist across time and context. They learn from observation. They solve novel problems.

All of this intelligence is bootstrapped, from nothing, in under two years. This challenges how we think about the origins and nature of intelligence: it apparently requires neither longevity, nor social structure, nor intergenerational learning.

The distributed body

The octopus's nervous system raises deep questions about the nature of control and selfhood. With two-thirds of its neurons in its arms, the central brain does not micromanage the arms. Instead, it issues high-level commands, reach for that, move toward here, and the arms figure out the details. The arms are semi-autonomous agents.

This differs from the vertebrate model, where the brain is the unambiguous command centre and the limbs are executors of its instructions. The octopus is more like a distributed committee. The central brain sets goals; the arms negotiate the implementation.

The practical consequences of this architecture are visible. An octopus can explore eight different crevices simultaneously, with each arm operating its own decision-making process. When an arm encounters a potential food item, it can investigate and pass it toward the mouth without waiting for instructions from the central brain. The arm knows what to do.

After an arm is amputated, it continues to function purposefully for up to an hour. It will attempt to pass food to a mouth that no longer exists. It will withdraw from pain. It is, in a meaningful sense, still working.

The skin as sense organ

Recent research has revealed that octopus skin contains functional photoreceptors, light-sensing proteins of the same type found in the eyes. The skin may be able to detect light directly, independently of the visual system. This has led to speculation that octopuses may in some sense "see" with their skin. Not forming images, but detecting patterns of light and colour across the body surface.

This would help explain the otherwise paradoxical ability to match colour while being colourblind. It would also make the octopus's body one of the most complex sensory organs in nature, simultaneously the thing that detects the environment and the thing that responds to it, skin that is both eye and camouflage at once.

What it means to be an octopus

The philosopher Peter Godfrey-Smith, who studies octopus cognition, has written that encountering an octopus in the wild is "the closest we can come to meeting an intelligent alien." He does not mean this metaphorically. The octopus is not alien in the science-fiction sense. But it is an independent solution to the problem of being intelligent, a system that evolved complex cognition without any of the architectural preconditions we usually associate with it.

It lives fast, dies young, leaves no cultural legacy, and has no social structure to transmit accumulated knowledge. And yet it solves problems, has a personality, and almost certainly experiences something.

What that experience is like from the inside, what it is like to have nine semi-independent decision-making processes operating simultaneously in a body that sees with its skin, is a question we have no way of answering. It is worth sitting with.

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