An octopus is a mollusc, a relative of snails and clams, that has kept almost nothing of the shell and gained an unusually soft, flexible body. Its body is organized less like a single command center and more like a team of semi-independent parts. Almost every system, from its blood to its eyes, solves familiar problems in unfamiliar ways.
Why an Octopus Seems to Think With Its Arms
An octopus has about 500 million neurons, roughly what a dog has. But about two-thirds of them sit in its eight arms rather than its head.1Biomimetic Research. The Unique Structure of an Octopus Explained
That is where the popular claim of “nine brains” comes from: one central brain plus one per arm. The truth is a little different. The arms have large clusters of nerve cells, but they aren’t separate brains. The central brain still sets the goals, like where to go, while the arms handle local details such as gripping and feeling.
The arm nervous system works with considerable independence from the brain.2PubMed Central. Where Is It Like to Be an Octopus? An arm can start moving when something touches it, even without a command from the brain.3PubMed. Octopus as a comparative model for understanding the neural control of limb movement and limb-based behaviors One arm can probe a crevice while the others hold the animal in place. Coordinated actions like crawling across the seafloor still need the arms connected to the brain, though.4PubMed. Octopus as a comparative model for understanding the neural control of limb movement and limb-based behaviors
How a Boneless Arm Moves
An octopus arm is almost entirely muscle, with no rigid skeleton.5PubMed. Dynamic model of the octopus arm. I. Biomechanics of the octopus reaching movement. It supports itself with muscle and fluid pressure.6PubMed. Dynamic model of the octopus arm. I. Biomechanics of the octopus reaching movement. Because the arm keeps a constant volume, muscles running across it and along it can work against each other.7PubMed. Dynamic model of the octopus arm. I. Biomechanics of the octopus reaching movement. Squeezing the arm thinner makes it longer, the same way a tongue or an elephant trunk works. Ring-shaped muscles narrow the arm so it extends forward, while lengthwise muscles shorten one side so the arm bends that way.
Freedom like this comes with a control problem. A boneless arm can bend at any point, not just at joints, which gives it a nearly endless number of possible positions to manage. Spreading neurons through the arms helps carry that load. Octopuses also often stiffen part of an arm to create a temporary joint when reaching.
With no bones, an octopus can squeeze through openings barely larger than its beak, which lets it hide in narrow cracks and escape from tanks.
Suckers That Taste What They Touch
Octopus suckers hold a sensory system that combines touch and taste.8PubMed. A Sucker for Taste The inner lining of the suckers has a family of sensors tuned to molecules that don’t dissolve well in water.9Harvard Gazette. Octopus’ suction cups hold its taste and touch sensors So the octopus can tell what it’s touching, and whether it’s prey, just by feeling it.10Harvard Gazette. Octopus’ suction cups hold its taste and touch sensors Underwater, smells that dissolve spread and blur together, while molecules that stay stuck to surfaces reveal exactly what an object is. Signals from the suckers can be processed in the arm itself, so an arm can decide to grab food before the brain weighs in.
The suckers are also excellent grippers. Each one forms a seal and pulls on a surface, and hundreds of them together let an octopus hold rocks, shells and prey. They can manipulate objects finely too, a bit like fingers.
Skin That Changes Color and Shape
Under a clear outer skin lies a layer of thousands of pigment sacs, colored yellow, red or brown.11ASME 2011 Summer Bioengineering Conference, Parts A and B. Mechanics of the Cephalopod Chromatophore Layer: Structural Characterization of Cephalopod Chromatophores Below them sit reflective cells that bounce back light from blues and greens to near-infrared.12ASME 2011 Summer Bioengineering Conference, Parts A and B. Mechanics of the Cephalopod Chromatophore Layer: Structural Characterization of Cephalopod Chromatophores Mixing pigment with reflected light lets the skin show changing color, shimmer, brightness and patterns.13ASME 2011 Summer Bioengineering Conference, Parts A and B. Mechanics of the Cephalopod Chromatophore Layer: Structural Characterization of Cephalopod Chromatophores Cephalopods are also very sensitive to polarized light, and reflected polarized light can pass through the pigment layer, giving them a hidden signaling channel.14PubMed Central. Mechanisms and behavioural functions of structural coloration in cephalopods
Each color sac is ringed by tiny muscles. When the muscles pull the sac wide, the color shows; when they relax, the sac shrinks to a dot. Because nerves control these muscles directly, the changes happen in a fraction of a second.
Octopuses can also raise skin bumps to mimic rough surfaces like rock or seaweed.15PubMed. Comparative morphology of changeable skin papillae in octopus and cuttlefish One ring of muscles lifts each bump and another set shapes it.16PubMed. Comparative morphology of changeable skin papillae in octopus and cuttlefish A third set of muscles then pulls the bump back flat.17PubMed. Comparative morphology of changeable skin papillae in octopus and cuttlefish In cuttlefish, close relatives, bumps can rise or fall in under a second and can be held for over an hour without effort, thanks to a catch-like muscle lock.18PubMed Central. Neural Control of Dynamic 3-Dimensional Skin Papillae for Cuttlefish Camouflage An octopus can match color, pattern and texture all at once, vanishing against coral or flashing warning colors.
Blue Blood and Three Hearts
Octopus blood uses haemocyanin, a copper-based protein, to carry oxygen, and that is what makes it blue. Human blood relies on iron-based hemoglobin packed inside red cells. Haemocyanin instead floats freely in the blood, and the copper gives it a blue tint when it carries oxygen.
This protein works less well as water gets colder.19Helmholtz-Zentrum für Polar-und Meeresforschung (Alfred-Wegener-Institut). Blue Blood on Ice: Cephalopod haemocyanin function and evolution in a latitudinal cline. So polar octopuses carry more of it and have a version that grabs oxygen differently to stay supplied near 0°C.20Helmholtz-Zentrum für Polar-und Meeresforschung (Alfred-Wegener-Institut). Blue Blood on Ice: Cephalopod haemocyanin function and evolution in a latitudinal cline.
Octopuses have three hearts: two push blood through the gills and one sends it around the body. The arrangement suits their blood. Haemocyanin blood carries less oxygen than ours, so the extra gill hearts boost flow through the gills. The main heart can then push the oxygen-rich blood out to the body. That main heart pauses while the octopus swims, which is one reason octopuses prefer crawling.
Eyes and Balance Organs
Octopus eyes look much like human eyes, with a lens and a retina.21PubMed Central. The Eye of the Common Octopus (Octopus vulgaris) But their light-sensing cells point toward the light, while ours point away.22PubMed Central. The Eye of the Common Octopus (Octopus vulgaris) As a result, octopus eyes have no blind spot where nerves exit, unlike ours.
Two balance organs near the brain tell the octopus its position and movement.23PubMed Central. A model biological neural network: the cephalopod vestibular system They contain hair cells that respond most strongly to movement from one direction, much like those in the human inner ear.24PubMed. Directional sensitivity of hair cell afferents in the Octopus statocyst The organs keep the octopus oriented during fast jets, and they also help steady its eyes as the body moves.
Jet Propulsion
The octopus draws water into its muscular mantle, the same cavity that bathes its gills.25Science Friday. Jet-setting Cephalopods It then squeezes the water out through a tube called the siphon, pushing its body the opposite way.26Science Friday. Jet-setting Cephalopods Aiming the siphon and changing the force lets it steer and change speed.27Science Friday. Jet-setting Cephalopods
The trade-off is cost. An octopus breathes efficiently, pulling 35 to 50% of the oxygen from water passing its gills.28Canadian Journal of Zoology. Oxygen extraction and jet propulsion in cephalopods That leaves little room to extract more when it needs extra energy.29Canadian Journal of Zoology. Oxygen extraction and jet propulsion in cephalopods So jetting is expensive, and octopuses use much higher jet pressures than squid when they swim.30Canadian Journal of Zoology. Oxygen extraction and jet propulsion in cephalopods Most octopuses crawl most of the time; jetting is mainly for quick escapes.
Beak, Venom and Gut
All octopuses studied make venomous saliva in glands behind the mouth.31PubMed. Combined Transcriptomic and Proteomic Analysis of the Posterior Salivary Gland from the Southern Blue-Ringed Octopus and the Southern Sand Octopus A muscular gland wall pushes the toxin through a duct into the mouth.32Egyptian Journal of Aquatic Biology and Fisheries. New Vision for Structure of Posterior Salivary Gland of Octopus vulgaris from the Red Sea, Egypt The venom is a mix of many proteins, with protein-cutting enzymes making up most of it in common and blue-ringed octopuses.33PubMed. Combined Transcriptomic and Proteomic Analysis of the Posterior Salivary Gland from the Southern Blue-Ringed Octopus and the Southern Sand Octopus That lets it start breaking down prey from the inside.34PubMed Central. Putative Antimicrobial Peptides of the Posterior Salivary Glands from the Cephalopod Octopus vulgaris Revealed by Exploring a Composite Protein Database
For people, most octopus bites are harmless or cause only local pain. The blue-ringed octopus is the dangerous exception, so it’s best not to handle wild octopuses, especially brightly ringed ones.
The beak, the only hard part of the body, looks like a parrot’s. It cracks shells and tears flesh after the venom has softened the prey.
Swallowed food passes into a storage pouch whose wider front half holds about three-quarters of the meal.35PubMed Central. Methodological considerations in studying digestive system physiology in octopus: limitations, lacunae and lessons learnt From there it moves to the stomach area and a coiled tube with folded walls that increases the surface for absorbing nutrients.36PubMed Central. Methodological considerations in studying digestive system physiology in octopus: limitations, lacunae and lessons learnt These chambers share a common space without strong valves, so food can shift back and forth as muscles squeeze.37PubMed Central. Methodological considerations in studying digestive system physiology in octopus: limitations, lacunae and lessons learnt
Regrowing Arms and the Mating Arm
After an arm is damaged, the skin edges first close around the wound.38PubMed Central. Imaging Arm Regeneration: Label-Free Multiphoton Microscopy to Dissect the Process in Octopus vulgaris A clot of blood cells then covers the exposed tissue and forms a mass of new cells.39PubMed Central. Imaging Arm Regeneration: Label-Free Multiphoton Microscopy to Dissect the Process in Octopus vulgaris Next, a small knob grows from the healed stump and becomes a new arm.40PubMed Central. Imaging Arm Regeneration: Label-Free Multiphoton Microscopy to Dissect the Process in Octopus vulgaris New nerve fibers grow into the forming arm tip among the old damaged fibers,41PubMed Central. Imaging Arm Regeneration: Label-Free Multiphoton Microscopy to Dissect the Process in Octopus vulgaris and cut nerves can start regrowing within three days.42PubMed Central. Nerve degeneration and regeneration in the cephalopod mollusc Octopus vulgaris: the case of the pallial nerve
This is useful because octopuses may lose arms to predators. A regrown arm restores grip, taste and movement, though regrown arms may start smaller before reaching full size.
In males, one arm, usually the third right arm, is a mating arm built to carry sperm packets to the female.43Marine Biology. Hold it close: male octopus hold their hectocotylus closer to their body The packets slide along a groove in the skin to the arm tip.44PubMed Central. A sensory system for mating in octopus The tip has small suckers that may help it sense its way,45PubMed Central. A sensory system for mating in octopus and males tend to hold this arm close to the body.46Marine Biology. Hold it close: male octopus hold their hectocotylus closer to their body
A Hidden Shell and Self-Editing Genes
Octopuses keep tiny rod-like remnants of their ancestral shell.47Journal of the Marine Biological Association of the United Kingdom. Stylet (vestigial shell) size in Octopus vulgaris (Cephalopoda) hatchlings used to determine stylet nucleus in adults These lay down a growth ring each day, which lets scientists estimate an octopus’s age.48Journal of the Marine Biological Association of the United Kingdom. Stylet (vestigial shell) size in Octopus vulgaris (Cephalopoda) hatchlings used to determine stylet nucleus in adults Snails and clams keep full shells for protection. Octopuses traded theirs for squeezing and hiding, and camouflage and venom help replace the shell’s protection.
Octopuses also edit many genetic messages in their nerves, at thousands of spots.49PubMed Central. The evolution and adaptation of A-to-I RNA editing A gene is like a recipe that gets copied into working notes. The octopus changes letters in the notes, not in the recipe, so the same gene can make slightly different proteins. When water temperature changes, they can alter over 13,000 parts of nerve proteins.50PubMed Central. Temperature-dependent RNA editing in octopus extensively recodes the neural proteome That means they can adjust nerve function to cold or warm water without changing their genes.