Octopus, UnfoldedThe Science of a Soft-Bodied Mind

An octopus has no bones and no shell, so it can squeeze through any gap bigger than its beak. The beak is the only large hard part of the body, a parrot-like bite at the center of the ring of arms, and everything else can flatten and stretch to follow it through. Aquarium keepers work on a simple rule: if the beak fits, the octopus fits. A fully grown giant Pacific octopus can pass through a hole about the size of a lemon.⁴ Watch this fact10 s

Boneless does not mean soft all the way through. Besides the beak there is a case of cartilage around the brain, and many octopuses keep two small internal rods where head muscles anchor, all that is left of a shell their ancestors carried. The beak itself, that one large hard part, is made of chitin, not bone. In squid beaks, the best-studied kind, the chitin is toughened with proteins and stiffens gradually from a soft base to a very hard tip, with no minerals at all. The radula behind the beak, a tongue-like ribbon, carries tiny teeth, but nothing else in the body is as big and rigid.⁵

A mollusk that lost its shell

Octopuses are mollusks, related to snails and clams, and they belong to the cephalopods along with squid, cuttlefish and the nautilus. The first cephalopods appeared more than 500 million years ago, evolving from a small animal with a shell shaped like a witch's hat, and by 450 million years ago some of the fiercest predators in the ocean were shelled cephalopods. Octopuses came much later. Soft bodies seldom fossilize, so the octopus's own history is patchy; a fossil from Lebanon about 90 million years old is one of the oldest octopuses known.⁶

Of the living cephalopods, only the nautilus still lives inside an outside shell. The rest have shells that are greatly reduced and overgrown by the body. Cuttlefish carry a thick internal cuttlebone that they use to control buoyancy, and squid a thin horny pen. In octopuses the shell survives at most as small cartilage-like rods, or as the fin supports of the finned deep-sea kinds; in the giant Pacific octopus it is two small plates where head muscles anchor.⁷

Scientists know of about 300 species of octopus, and more are still being found and named. Most belong to the familiar finless kind: muscular, mostly bottom-living, usually carrying an ink sac. The others are finned deep-sea octopods, soft and partly gelatinous, with a single row of suckers flanked by finger-like cirri, and they are rarely found shallower than 600 meters except at high latitudes, toward the poles.⁸

Among the finned kinds are the dumbo octopuses, named for their large ear-like fins, which they use with their webbed arms to swim. In 2020 two researchers reported one filmed by a camera lander 6,957 meters (22,825 feet) down in the Java Trench, the deepest octopus yet seen. The sighting extended the known depth range of cephalopods by 1,812 meters and was the first sighting beyond doubt of any cephalopod in the hadal zone, deeper than 6,000 meters.¹⁰

Between these extremes, octopuses live in every ocean, from warm reefs to polar seas and from tide pools to the deep sea; the FAO names the Black Sea as the one exception for cephalopods. None lives in fresh water. Like nearly all cephalopods, octopuses tolerate low salt poorly, although Abdopus and some other intertidal octopuses, which live on the shore between high and low tide, may cope with rock pools that turn saltier in the sun or fresher after heavy rain.¹¹

Head, mantle and arms

The octopus body is easy to draw and easy to misread. The two large eyes sit on the head, and the rounded bulb behind them, which looks like an oversized head, is mostly the mantle, a muscular sac that holds the gut, the hearts and the other organs. The eight arms meet the head in a ring around the mouth, with the beak at the center, and in a typical octopus a web of skin joins the arms at their bases. All eight are arms. Zoologists keep the word tentacle for the extra, longer pair that squid and cuttlefish carry beside their eight arms, and an octopus has none.¹³

Muscle doing the work of bone

Each arm is a muscular hydrostat, a limb of densely packed muscle that holds itself up without a skeleton, as a tongue or an elephant's trunk does. Muscle is mostly water and barely changes volume, so when one set of fibers makes the arm thinner, the arm has to get longer. By working muscles that run across the arm, along it and at a slant around it against one another, an octopus can bend, shorten, lengthen or twist an arm at any point along it.¹⁴

Along each arm run the suckers, and an octopus's suckers can taste what they touch. Sensory cells in the suckers detect chemicals as well as touch, so an arm probing a crevice can tell what it is holding without the octopus seeing it. In 2020 researchers working with the California two-spot octopus identified a family of chemotactile receptors, sensors that taste by touch and are found only in cephalopods. They respond to poorly soluble chemicals, ones that do not dissolve easily in water, on the surfaces of prey and other animals.¹⁵ Watch this fact10 s

How many suckers there are depends on the species and on the animal's sex, so no single number fits octopuses in general. An adult female giant Pacific octopus has about 2,240 on her eight arms, and a male has about 100 fewer.¹⁶

An arm lost to a predator can be replaced, and without lasting harm. In the common octopus, where regrowth has been studied most, the wound starts closing within hours. A bud of unspecialized cells then forms at the stump and grows out, rebuilding muscle, nerves and suckers into a working arm over the following months.¹⁷

An octopus breathes with two feathery gills that hang inside the mantle cavity. The mantle squeezes and relaxes in a steady rhythm, pumping seawater through the cavity, and oxygen passes from the water into the blood as it flows through the gills. The used water leaves through the siphon, or funnel, a muscular tube the octopus can turn in different directions.¹⁸

An octopus has three hearts. Two branchial hearts, one at the base of each gill, push blood through the gills, where it picks up oxygen. The third, the systemic heart, then pumps that oxygen-rich blood around the rest of the body. Octopus blood is blue when it is carrying oxygen, because it carries oxygen on hemocyanin, a copper-based protein, where our blood uses iron-based hemoglobin.¹⁹ Watch this fact10 s

Hemocyanin differs from hemoglobin in another way. It is dissolved in the plasma, the liquid part of the blood, instead of being packed into blood cells. The copper is what gives oxygen-rich octopus blood its blue color, and blood that has lost its oxygen turns almost clear.²⁰

A closed loop

Octopuses, like squid and cuttlefish, keep their blood inside vessels, which is unusual among mollusks. A snail or a clam has an open circulation: the heart pumps blood into a body cavity, where it washes loosely around the organs. A closed system of arteries, capillaries and veins lets blood be pushed under pressure to where it is needed, which suits an active predator, and in the octopus the three hearts drive that loop.²¹

The jet

Squeezed hard, the same plumbing becomes an engine. Water drawn into the muscular mantle is forced out through the narrow siphon, pushing the octopus the other way. The octopus can point the siphon to steer, and it usually travels with its rounded mantle in front, so it can look as if it swims backward.²²

The heart that stops

Jetting has a cost, and the hearts pay it. Researchers who measured blood flow and pressure in freely moving common octopuses found that the systemic heart stops beating during vigorous jet swimming. A hard jet raises the pressure inside the mantle above anything the veins can push against, so blood stops flowing back to the hearts, and the systemic heart, which beats only when it fills, stalls.²³

Octopuses can run up only a small oxygen debt, and the same researchers concluded that jet-propelled travel is impossible for them over more than a few meters. This is one reason octopuses crawl far more than they swim. Everyday movement is crawling on the arms, and an octopus can save the jet for a quick escape.²⁴

The common octopus has about 500 million neurons, or nerve cells, roughly as many as a dog. A dog's neurons are mostly in its brain. In the octopus roughly two-thirds of them lie outside the brain, mostly in the nerve cords that run down the arms, and those cords handle much of the sensing and movement locally.²⁵

The central brain is large all the same. For its body size, the octopus has the largest brain of any invertebrate, and its brain-to-body ratio is higher than that of many vertebrates, though not mammals. The ratio is only a rough guide to intelligence, but it shows how much an animal invests in its brain.²⁶

It is an oddly shaped brain. Its nerve lobes are fused into a doughnut around the esophagus, the tube that carries food from the mouth to the stomach, so every meal passes through the middle of the brain. A sharp piece of swallowed prey can injure it. Vision takes up most of the space: the optic lobes, one behind each eye, make up most of the central brain. The octopus retina contains almost nothing but light-sensitive cells, which send their signals straight to the optic lobes, so much of the image processing that our retinas do happens inside the octopus's brain instead.²⁷

Arms that handle a lot on their own

In the common octopus, some arm movements are run by the arm's own nerves. In one experiment, an arm cut off from the brain still made a normal-looking reaching movement when it was stimulated, which shows that the program for a reach is built into the arm's nerve circuitry. The brain can send a simple go signal and leave the details to the arm. The arms are not independent, though. Octopuses can learn to steer a single arm to food by sight, and a 2020 study showed arms passing sensory information back to the brain, which does the learning.²⁸

Eyes built another way

Octopuses have camera-type eyes, with a lens that focuses an image onto a retina, as ours does. The two designs arose independently, since the last common ancestor of octopuses and vertebrates had no such eye, and they differ in one telling detail. In the octopus eye the light-sensitive cells face the incoming light and send their fibers straight out of the back of the eye to the brain. No nerve bundle has to pass through the retina, so there is no blind spot, the gap in our own retina where that crossing point lies.²⁹

In the common octopus the pupil narrows to a horizontal slit in bright light and opens into a circle in darkness, and it reacts quickly when the light changes. The slit lines up with a horizontal band of densely packed light-sensitive cells across the retina. Balance organs called statocysts help keep it that way. Each is a fluid-filled chamber lined with sensory hairs, in which a small dense mass shifts as the animal tilts and bends different hairs. Their signals turn the eyes to keep them level as the body moves, and octopuses with damaged statocysts are worse at telling shapes apart.³⁰

Octopuses can also see the polarization of light, the direction in which light waves vibrate. In their light-sensitive cells, the parts that catch light are lined up in two directions at right angles, so the eye can compare horizontally and vertically polarized light. Trained octopuses pick out patterns made of nothing but differences in polarization. The sense may help them spot transparent objects and see contrast in murky water.³¹

An octopus can change the color and pattern of its skin, and it can change the texture too, raising bumps, ridges and spikes to look like the rock, coral or seaweed around it. Most octopuses use the skill in two ways. Going pale, dark, blotchy or striped to match the surroundings hides an octopus from predators and from the animals it hunts, while the same changes can show mood or intent, warn off a rival or startle an attacker.³² Watch this fact10 s

The color comes from chromatophores, tiny elastic sacs of red, yellow, brown or black pigment packed into the skin. Each is a small organ rather than a single cell: a pigment sac ringed by radial muscles, with each muscle wired to nerves from the brain. When the muscles contract they stretch the sac wide and its pigment shows. When they relax, the elastic sac shrinks back to a dot.³³

Beneath the chromatophores lie two more kinds of cell, which work by reflecting light. Iridophores return it as shimmering blues and greens. Leucophores scatter white light and act as a bright backdrop for everything above them. Because every chromatophore is wired to the brain, an octopus can switch patterns almost at once, and the fastest changes take about a tenth of a second.³⁴

Texture is made with muscle too. Each papilla is a patch of skin with muscle fibers running in rings and in lines radiating outward. Tightening them pushes the skin up into a bump or a point, and relaxing them lets it lie flat again, a little like goosebumps. The day octopus, which hunts in daylight, can turn its skin into long lumpy ridges that mimic nearby corals, rocks or algae.³⁵

Put the layers together and one patch of skin has several controls. Chromatophores add or hide pigment at the surface, iridophores and leucophores beneath them shape the light that comes back, and papillae change the surface itself, so the same patch can go from smooth and pale to dark and spiky. Paired with a color change, a raised texture lets an octopus look like part of the seabed instead of an animal sitting on it.³⁶

Patterns and signals

Scientists sort octopus camouflage into three main kinds of body pattern. A uniform pattern is one even color, and a mottle is small light and dark blotches; both work by matching the background, such as plain sand or speckled gravel. A disruptive pattern uses bold contrasting patches that break up the body's outline, so a predator does not see the octopus as one animal. Researchers use the same three groups for cuttlefish.³⁷

Some octopuses can send a dark band of color rippling out from the head across the body and arms before it fades, a display called the passing cloud. It has been filmed in several species, including the common octopus, the day octopus and Octopus laqueus. Scientists are still finding out what it is for. One idea is that it makes a still octopus look as if it is moving, tricking prey into leaving its hiding place.³⁸

Color also carries messages between octopuses. The giant Pacific octopus can change color in ways that show its mood and intentions. Filming wild gloomy octopuses at a crowded site in Jervis Bay, Australia, researchers found that a dark octopus was acting aggressively and that one which turned pale usually backed off. When an approaching dark octopus met an equally dark one, the encounter was more likely to turn into grappling. The octopuses there also stood tall on high ground with their web spread. It was the first study to show octopuses using signals systematically in contests.³⁹

The color-blind puzzle

All of this is harder to explain because most octopuses are probably color-blind. Their eyes have only one kind of light-sensitive pigment, and in tests they cannot learn to tell objects apart by color alone, yet they match colored backgrounds well. Scientists are still testing explanations. Reflective skin cells might take on the color of the surrounding light, or light-sensitive proteins in the skin might play a part. In 2016 researchers proposed that the octopus's off-center pupil could recover color from the way different colors blur. The one-pigment finding is solid; whether octopuses get color information some other way is still open.⁴⁰

The idea of light-sensing skin has some evidence behind it, in the California two-spot octopus. In pieces of skin separated from the body, light made the chromatophores expand, fastest under blue light, and sensory cells in the skin carry opsin, the same light-detecting protein that the eyes use. Sensing light is a long way from seeing, though, and there is no evidence that the skin forms images. Scientists do not yet know how, or how much, the living animal uses this sense.⁴¹

An octopus has no shell to retreat into and no bones, so most octopuses keep their soft bodies safe in tight spaces: a den in a crack in the rocks or a hole in the sand.⁴² Camouflage works only until a predator spots the octopus. When a hiding octopus is found, many species switch from blending in to a sudden bold display: the octopus turns dark, darkens its eyes and spreads its arms to look bigger. Scientists call this a deimatic, or startle, display, and in cephalopods such alarm patterns use strong light-and-dark contrast, dark outlines or eye-like spots. It may make a predator hesitate long enough for the octopus to jet away.⁴³

Most octopuses can squirt a cloud of dark ink that hides them from an attacker while they get away. The ink is made in an ink sac inside the body and is mostly dark melanin pigment mixed with mucus. It leaves through the siphon, usually as the octopus jets off, and an octopus often pales, inks and jets all at once, so the attacker is left facing a dark cloud while a hard-to-see octopus shoots away.⁴⁴ Watch this fact10 s

The ink sac is a pouch off the end of the gut. A gland inside it makes the ink, and the sac empties near the anus, where the ink mixes with mucus from a gland in the funnel on its way out. Most octopuses have one, but some deep-sea octopuses have no ink sac at all, among them the finned deep-sea octopods.⁴⁵

Ink can also be a decoy. Some octopuses, like many squid, can squirt ink thickened with mucus that holds together as a dark blob about their own size, which a predator may attack instead. Scientists call such a blob a pseudomorph, meaning 'false body', and how much mucus goes in changes the result, from a spreading cloud to a blob that hangs in the water. The evidence from octopuses is slim. In one small test, green turtle hatchlings chasing Bock's pygmy octopus snapped at the ink blobs, then gave up the chase while the octopus escaped.⁴⁶

Disguises

A few octopuses escape by passing for something else. The mimic octopus of Indonesia can pose as other animals, including a flatfish, a lionfish and a banded sea snake, by changing its shape, its color and the way it moves. It forages over open sand in daylight instead of hiding. Scientists filming it in Sulawesi and Bali saw it swim flat like a banded sole, trail its banded arms like the spines of a lionfish, and hide six arms in a hole while waving two banded arms like a sea snake. Several of the animals it copies are venomous or toxic, so looking like them may keep fish predators away.⁴⁷

Two tropical species can walk on two arms while holding the other six in a disguise. The veined octopus wraps six arms tightly around its body so that it looks like a coconut tiptoeing along, and the algae octopus holds its arms like a clump of drifting seaweed. In both, the two back arms roll along the sand like tank treads. Each arm stays on the sand for more than half of each stride, so scientists count it as true walking, and the octopus moves away from danger while staying hidden.⁴⁸

Out of the tank

Octopuses kept in aquariums and labs are well known for climbing out of their tanks, and some species try to escape far more often than others. A survey of 38 scientists and aquarists who had kept 25 octopus species found that the tendency depends on the species. Many aquariums have stories of octopuses that can leave their tanks at night, sometimes to visit a neighboring tank for food. Escaping is dangerous for an octopus, so the study also gave keepers ways to keep octopuses safely in their tanks.⁴⁹

In 2016 New Zealand's National Aquarium, in Napier, reported that its octopus Inky had escaped from his tank at night. A fisherman had brought Inky in after finding him in a crayfish pot. The lid of the tank had been left slightly ajar, and staff believe he climbed out, crossed the floor and squeezed into a drainpipe that runs to Hawke's Bay, a route they worked out from the wet trail he left. Nobody saw it happen, and the aquarium also floated a second route, through a pipe at the top of the tank, so the path is the staff's reconstruction.⁵⁰

Most octopuses are hunters of other animals, especially crabs, clams and other shellfish, and fish, although a few kinds feed on plankton instead. Crabs are a favorite of many; the giant Pacific octopus also eats clams, scallops, abalone, fish and even other octopuses. An octopus often carries its catch back to its den to eat.⁵¹

Some octopuses hunt by pouncing. They spread their arms and the web of skin between them over a rock or a patch of seabed like a net, then feel underneath for trapped prey. A hunting day octopus may pounce every meter or two, pausing a few seconds each time before moving on, and aquarium staff describe a giant Pacific octopus billowing out like a parachute to envelop a crab.⁵²

In the Red Sea, day octopuses sometimes hunt in teams with fish such as the blue goatfish, which scout crevices and show where prey is hiding while the octopus decides whether and when the group moves on. Now and then the octopus punches a fish with a quick jab of an arm, and the fish is pushed to the edge of the group or leaves. Only a few hunts have been filmed, so scientists are still finding out how common this is and why octopuses punch.⁵³

Shellfish have to be opened. Octopuses can pull shells apart or bite them open, but prey that is hard to open is drilled. Saliva softens the shell, and the octopus scrapes a small hole with its radula, a tongue-like ribbon covered in rows of tiny teeth, and a small drilling organ called the salivary papilla. Worn teeth at the front of the radula are replaced by new rows growing from behind. Saliva sent through the hole carries venom that paralyzes the prey and starts to digest it, so the meat comes away from the shell more easily. In 2009 Fry and colleagues concluded that all octopuses are venomous, and that the venom proteins share a single early origin across octopuses, cuttlefish and some squid. Only the blue-ringed octopuses are known to be dangerous to people.⁵⁴

Blue-ringed octopuses carry tetrodotoxin, a venom that can kill a person, and bites have killed people in Australia. The bite may not be felt, but within minutes the lips and tongue go numb and breathing becomes hard, so helpers call for aid and keep the person breathing. The octopuses themselves are shy. They use the venom on crabs and small fish, hide under ledges, show their bright blue rings only when disturbed, and bite only when picked up or handled.⁵⁵

Most octopuses live in a den, such as a crack in the rocks, a hole dug in the sand or an empty shell, and tight spaces keep a soft body safe from predators. On sandy seabeds, common octopuses dig dens and may line them with shells, and young ones can hide between the halves of an empty clam shell. Some octopuses fix up their dens. Young common octopuses in Bermuda were seen clearing rocks out of their dens and bringing objects to block the entrance like a door, and gloomy octopuses in Australia sometimes gather collections of objects around the rims of their den holes.⁵⁶

Some veined octopuses carry a shelter with them. The veined octopus of Indonesia digs up coconut-shell halves, carries them across the seabed and later fits them together as a shelter to hide inside. Carrying the shells slows the octopus down, because it has to walk on stiff arms with the shells held underneath, so it gains nothing until it stops somewhere open and needs a hiding place. That delayed payoff is why the biologists who described it in 2009 called it tool use. Coconut carrying is known only from the veined octopus.⁵⁸ Watch this fact10 s

Puzzles

The word mind is used carefully in this chapter. What researchers can measure is behavior: what an animal learns, how quickly, and whether it copes when the task changes. Several of the studies here are small, some with only a handful of animals, and where the text gives their numbers you can weigh them.

Octopuses in labs and aquariums can learn to open jars to reach food inside. In a 1990 study in Naples, common octopuses learned to pull the plug out of a glass jar holding a live crab and got faster with practice. In captivity octopuses have also learned to open screw-top jars, and aquariums describe their giant Pacific octopuses learning to open jars and solve puzzles. Other lab work, much of it with common octopuses at the zoological station in Naples, showed that octopuses can learn by trial and error to tell shapes apart for a reward and to find their way through simple mazes.⁵⁹

One study set seven common octopuses a harder task: pulling an L-shaped food container through a tight hole in a clear wall. The container only comes through if the octopus pulls, releases and pushes at the right moments. All seven learned it, and each coped when the container was turned a new way, put behind an opaque barrier or offered at random angles, though they slowed down briefly at each change.⁶⁰

Play, people and neighbors

Octopuses in experiments have shown play-like behavior with objects that are not food. Eight giant Pacific octopuses were each given only a floating pill bottle. After exploring it, two of them repeatedly aimed water jets to send it to the far end of the tank, where the circulating water carried it back to them. In another study, nine of fourteen common octopuses given Lego objects showed play-like handling after a few days of exploring them. Scientists say play-like because they cannot ask an octopus whether it is having fun.⁶¹

In one study at the Seattle Aquarium, eight giant Pacific octopuses were visited over two weeks by two people wearing the same aquarium T-shirt. One fed them; the other touched them with a bristly stick. By the end the octopuses reacted differently to each person: reaching toward or away, where they pointed the siphon, and whether they showed a dark bar across the eye. The researchers did not work out which cues the octopuses used, and the study does not show that octopuses recognize faces.⁶²

Most octopuses live alone and hunt alone, and when two meet they may fight, so the few places where many live close together surprised scientists. Off eastern Australia, gloomy octopuses gather at two known sites that scientists nicknamed Octopolis and Octlantis; at Octlantis, up to about 15 have been counted living close together. Each site sits on a bed of discarded scallop shells, probably built up from the octopuses' own meals, and the shells make good dens where hiding places are otherwise scarce. There the octopuses signal to each other, mate, fight and even evict one another from dens. The scientists stress that these are clusters of separate homes, not planned cities.⁶³

At one crowded site in Jervis Bay, gloomy octopuses sometimes gather silt, shells or seaweed in their arms and fling it through the water with a blast from the siphon. Researchers counted 102 throws in their 2015 videos, and 17 of them hit another octopus. The team found evidence that some throws may have been aimed, but scientists are still finding out whether they were. It is the first report of throwing by any octopus.⁶⁴

Sleep and the genes

Octopuses studied in the lab, Octopus insularis in Brazil and Octopus laqueus in Japan, alternate quiet sleep with short bouts of active sleep. In quiet sleep the octopus is pale and still. Every half hour to an hour it switches, for about a minute, into active sleep, when its skin flickers through colors and textures and its eyes and arms twitch, although it is still hard to wake. In Octopus laqueus, brain recordings during active sleep look like waking brain activity, and the skin patterns resemble ones used while awake. What active sleep is for, scientists are still finding out.⁶⁵

Octopuses, squid and cuttlefish also edit the RNA copies of their genes far more than most animals do, especially in the nervous system. The editing swaps single letters in a gene's message after it is copied from DNA, so one gene can yield slightly different proteins. When researchers cooled the water of California two-spot octopuses, editing changed at more than 13,000 sites in their nervous systems, altering proteins used in nerve signaling. Scientists are still finding out how much this flexibility matters to the living animal.⁶⁶

Most octopuses live only about one to two years. The giant Pacific octopus lives unusually long for an octopus, about three to five years, and like most octopuses it breeds once, near the end of its life. The FAO's world catalog of octopods gives examples from both ends: Diguet's pygmy octopus completes its whole life cycle in 125 to 240 days in the laboratory, while some polar species are estimated to live at least six years. The Two Oceans Aquarium gives the tiny star-sucker pygmy octopus a lifespan of roughly six months. How long most deep-sea octopods live is still poorly known.⁶⁷

In most octopuses the male passes packets of sperm to the female with a specially shaped arm, the hectocotylus, usually his third right arm. It has a groove along its length ending in a spoon-shaped tip. During mating he places the tip inside the female's mantle cavity and passes spermatophores, sealed packets of sperm, along the groove, and she can store the sperm and use it later to fertilize her eggs. In argonauts it is the third left arm, and the finned deep-sea octopods have no such arm.⁶⁸

Eggs

Octopuses hatch from eggs their mothers lay, one by one or in strings, usually fixed to rock or shell; some species carry them in their arms or web instead. The common octopus lays hers under rocks or in holes. In one open-ocean octopus, Ocythoe, the eggs hatch inside the mother's long oviducts, the tubes her eggs travel along, before the young leave her.⁶⁹

Clutch sizes vary enormously. A giant Pacific octopus hangs tens of thousands of eggs, up to about 100,000, from the roof of her den; the Monterey Bay Aquarium gives a usual range of 18,000 to 74,000, in hundreds of strands of about 250 eggs each. The common octopus lays even more, 100,000 to 500,000, while species with large eggs lay far fewer: Diguet's pygmy octopus lays 50 to 250.⁷⁰

Female argonauts, open-ocean octopuses also called paper nautiluses, build a thin white shell in which they carry their eggs. It is a brood case, secreted by webs on the tips of the female's first pair of arms, and she can repair or rebuild it. Male argonauts are tiny and never make one.⁷¹

Having laid her eggs, a mother octopus stays with them. She guards them, cleans them and blows fresh water over them, which brings oxygen and stops algae and bacteria from growing, and she usually stops eating while she does. A common octopus mother cleans her eggs with her suckers. A giant Pacific octopus mother fans hers with her arms or shoots streams of water over them for months without eating.⁷²

In most octopus species the mother breeds only once and dies around the time her eggs hatch. While she broods she does not eat, and she slowly weakens. It is the ordinary end of an octopus life, the last part of it spent caring for eggs. Males die too: like most octopuses, a male breeds once, near the end of his life, and a male giant Pacific octopus dies a few weeks after mating.⁷³

The optic gland

In the octopus mothers studied so far, the brooding, the fasting and the decline are driven by the optic gland, and scientists are still finding out exactly how. It is a small gland that works somewhat like the pituitary, the gland at the base of our own brain. In 1977 Jerome Wodinsky removed the optic glands from brooding Caribbean two-spot octopus mothers. They abandoned their eggs, began eating again and lived for months longer.⁷⁴

Later work has found no single self-destruct hormone. In the California two-spot octopus, Wang and Ragsdale reported in 2018 that the gland switches several signaling pathways up and down. These are the chains of chemical messages that tell the body's cells what to do, among them steroid, insulin, catecholamine and feeding-peptide pathways. A 2022 study found large shifts in the gland's cholesterol and steroid chemistry after reproduction. Some captive mothers injure themselves near the end. Scientists are still finding out how these changes fit together, and nothing shows that the mother chooses any of it.⁷⁵

The longest wait

In May 2007, researchers from the Monterey Bay Aquarium Research Institute led by Bruce Robison found a female Graneledone boreopacifica on a rock ledge about 1,400 meters deep in Monterey Canyon, California. They went back 18 times. She was never seen eating, and her skin grew pale and loose. By October 2011 she was gone, leaving about 160 empty egg cases: she had guarded a single clutch for about 53 months, the longest egg-brooding period known for any animal. The water there is about 3 °C, and cold slows the eggs' development; the young hatch large and well developed. The previous octopus record, 14 months, came from a laboratory study.⁷⁶

Adrift

In many octopus species the hatchlings drift in the plankton for weeks or months before settling on the seafloor, while in others the young hatch as miniature adults and crawl away. Small eggs tend to give drifting young, called paralarvae, and large eggs crawl-away young; the deep-sea Graneledone hatches with no drifting stage at all. Giant Pacific octopus hatchlings float near the surface for weeks to several months, depending on water temperature, before they settle. No octopus parent cares for the young after they hatch.⁷⁷

The giant Pacific octopus starts very small. It hatches from an egg about the size of a grain of rice, which the FAO catalog gives as about 6 to 8 millimeters long. The hatchling is just over a quarter of an inch long, weighs about 22 milligrams and already has about 14 suckers on each arm. By age two it may weigh around 9 kilograms (20 pounds).⁷⁸

Adults of the species are the largest known living octopuses, averaging roughly 20 to 30 kilograms (45 to 65 pounds) and spanning about 2 to 4 meters (7 to 13 feet) from arm tip to arm tip. Those are typical sizes. Record claims vary widely: the heaviest well-reported animals weighed 71 to 91 kilograms. The species lives in the cold North Pacific, from Japan to Alaska and Baja California, and the FAO names one other giant, the open-ocean seven-arm octopus, Haliphron atlanticus, with arm spans likely over 3 to 4 meters.⁷⁹

A common octopus is usually about 30 to 90 centimeters (1 to 3 feet) long, arms included, and weighs up to about 10 kilograms (22 pounds), though most weigh much less. The biggest reported sizes differ between sources, and averages are much smaller than the biggest animals. The smallest known octopuses are only a few centimeters long when fully grown and can weigh less than a gram. Sources disagree about which species is the very smallest; the star-sucker pygmy octopus of the western Pacific is one of the smallest.⁸⁰

In 2021 a review commissioned by the UK government and led by Jonathan Birch at the London School of Economics concluded that there is strong scientific evidence that cephalopods and decapod crustaceans are sentient. To be sentient is to have feelings, such as pain; the decapods are the ten-legged crustaceans, among them crabs, lobsters and shrimp. In November of that year both groups were added to the bill that became the Animal Welfare (Sentience) Act 2022, which defines an animal as any vertebrate other than humans, any cephalopod mollusk and any decapod crustacean. The Act sets up an Animal Sentience Committee to report on how government policy weighs animal welfare. It does not itself change fishing or restaurant practice.⁸¹

The European Union had already acted for animals in laboratories. Under Directive 2010/63/EU, adopted in September 2010 and applied from January 2013, octopuses and other live cephalopods used in research are protected by the same law that covers vertebrates. The reason given is scientific evidence that cephalopods can experience pain, suffering, distress and lasting harm. Bringing this class of about 700 species under the Directive led researchers to write the first guidelines for cephalopod care and welfare in research. A 2024 amendment adds rules for the care and humane killing of cephalopods, which apply from December 4, 2026.⁸²

Farming

In 2024 the states of Washington and California passed laws banning octopus farming. Washington's governor signed House Bill 1153 on March 13, and it took effect on June 6. California's AB 3162, approved on September 27, makes it unlawful to farm any species of octopus for human consumption and bars businesses from selling, possessing or transporting farmed octopus. Both laws drew on octopus intelligence and the capacity for pain: in Washington through testimony at the bill's hearings, and in California through the law's written findings, which also cite the 2021 LSE review. These are state laws; octopus farming is not banned across the United States.⁸³

In the net

People catch about 350,000 metric tons of octopus a year, a reported world catch that roughly doubled between 1980 and 2014. More than twenty described species are harvested in some 90 countries. The FAO estimates that more than 100 species are taken, although catch statistics name only four, and much of the catch goes unreported. Octopuses are caught by hand, by spear, on lines, in traps and pots, and in trawls, and the largest single-species fishery is for the common octopus off northwest Africa.⁸⁴