OctopusA Field Guide

Arms made of muscle

Scientists call an octopus arm a muscular hydrostat, a limb of packed muscle that holds itself up without a skeleton. Muscle is mostly water, so its volume hardly changes. When some fibers make an arm thinner, the arm gets longer. An octopus can bend, shorten, stretch or twist an arm anywhere along its length.

A giant's suckers

An adult female giant Pacific octopus has about 2,240 suckers, in two rows along each arm. A male has about 100 fewer. Other species have other numbers.

Spare parts

An octopus can regrow a lost arm. A bud of new cells forms at the stump, and over the following months it rebuilds muscle, nerves and suckers into a working arm.

Inside the mantle

The mantle is a muscular bag that holds the gut, the hearts and the other organs. Two feathery gills hang in the hollow underneath. As the mantle squeezes and relaxes, it pumps seawater over the gills, and oxygen passes into the blood. The water leaves through the siphon, a muscular tube the octopus can aim.

Beak and radula

At the center of the ring of arms is a parrot-like beak made of chitin, a tough material that is not bone. It is the only large hard part of the body. Behind it is the radula, a ribbon like a tongue covered in rows of tiny teeth.

An octopus has no bones and no shell, so its soft body can flatten and stretch to follow its hard beak. Aquarium keepers go by a simple rule: if the beak fits, the octopus fits. An octopus can squeeze through any gap bigger than its beak. A fully grown giant Pacific octopus can get through a hole about the size of a lemon. Watch this fact10 s

Blue blood

Our blood carries oxygen using iron. Octopus hemocyanin uses copper. The copper makes oxygen-rich blood look blue. Blood that has lost its oxygen turns almost clear.

Blood in tubes

Snails and clams have an open circulatory system, where blood washes loosely around the organs. Octopuses, squid and cuttlefish have a closed one, with the blood kept inside tubes. Blood in tubes can be pushed hard to where it is needed, which suits an active hunter.

The color puzzle

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 very well. Scientists are still testing explanations, such as reflective skin cells that take on the color of the light around them. A 2016 proposal suggested that an octopus's off-center pupil could pick up color from the way colors blur.

Slit pupils

In bright light, a common octopus's pupil narrows to a flat slit. In the dark it opens into a circle. Balance organs called statocysts turn the eyes to keep the slit level as the body tilts.

Suckers that taste

An octopus can taste with its suckers. Sensory cells in the suckers detect chemicals as well as touch, so an arm can tell what it is holding without the octopus seeing it. Watch this fact10 s

Neurons in the arms

The common octopus has about 500 million neurons, the cells that carry nerve signals. That is about as many as a dog. A dog keeps most of its neurons in its brain, but roughly two-thirds of a common octopus's neurons are in its arms. The arms handle much of the sensing and moving right where it happens.

One brain

An octopus has one central brain, shaped like a donut wrapped around the esophagus, the food tube. Swallowed food passes through the middle of the brain. Two optic lobes, one behind each eye, handle vision, and together they make up most of the central brain.

For its body size, the octopus has the largest brain of any invertebrate, an animal without a backbone.

How the colors work

Octopus skin is packed with chromatophores, tiny elastic sacs of red, yellow, brown or black pigment, each ringed by small muscles. When the muscles pull, the sac stretches wide and its color shows. When they relax, it shrinks back to a dot.

The muscles around each color sac are wired to nerves from the brain, so an octopus can switch patterns almost at once. The fastest changes take about a tenth of a second, quicker than a blink.

Just beneath the chromatophores lie iridophores, which reflect shimmering blues and greens. Leucophores lie deepest and scatter white light as a bright backdrop.

Three ways to hide

Scientists sort octopus camouflage into three main kinds of body pattern. Uniform is one even color. Mottle is small light and dark blotches. Both match the background, such as plain sand or speckled gravel. Disruptive uses bold patches that break up the body's outline, so a predator does not see it as one animal.

Passing clouds

Some octopuses can send a dark band rippling from the head over the body and arms. This passing cloud has been filmed in the common octopus, the day octopus and other species. Scientists are still finding out what it is for. It might make a still octopus look as if it is moving, tricking prey into leaving a hiding spot.

Color as a signal

At one crowded site in Australia, researchers filmed gloomy octopuses meeting. Going dark signaled aggression, and an octopus that turned pale usually backed off.

Look big, look scary

When a predator spots a hiding octopus, many species switch to a sudden bold startle display. They turn dark, darken the eyes and spread their arms to look bigger. This may make an attacker hesitate long enough for the octopus to jet away.

The mimic octopus

Instead of hiding, the mimic octopus of Indonesia forages over open sand in daylight. It can pose as other animals by changing its shape, its color and the way it moves.

How a jet works

To jet, an octopus fills its mantle with water and squeezes hard. The water rushes out of the narrow siphon and pushes the octopus the opposite way, usually mantle first. It can steer by turning the siphon.

Where the ink comes from

Most octopuses have an ink sac, a pouch off the end of the gut that makes and stores the ink. Some deep-sea octopuses have none.

Walking in disguise

Two species of tropical octopus can walk backward on just two arms. The veined octopus wraps its other six arms around itself and looks like a tiptoeing coconut. The algae octopus looks like drifting seaweed. The two back arms roll along the sand like tank treads, so the octopus can leave danger behind and stay hidden.

Getting out of the tank

Octopuses in aquariums and labs are well known for climbing out of their tanks. A survey of 38 scientists and aquarium keepers found that some species try to escape far more often than others. Escaping can be dangerous for an octopus, so the study also gave keepers tips to prevent it.

Getting into a shell

A clam shuts tight, but an octopus can drill its way in. Saliva softens the shell, and the octopus scrapes a tiny hole with its radula and a small drilling organ called the salivary papilla. Saliva sent through the hole weakens the animal inside, so the octopus can get at its meal.

Hunting with fish

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

Home is a den

Most bottom-living octopuses live in a den, such as a crack in the rocks, a hole dug in the sand or an empty shell. A tight space keeps a soft body safe, and an octopus often carries its catch home to eat. The common octopus shelters in its den by day and hunts mostly at night.

Home improvements

Some octopuses fix up their dens. Young common octopuses in Bermuda cleared rocks out of their dens and pulled objects across the opening like a door.

Neighbors

Most octopuses live alone and hunt alone, so scientists were surprised to find places where many live close together.

Off eastern Australia, gloomy octopuses gather at two known sites nicknamed Octopolis and Octlantis, with up to about 15 at the second. Each site sits on a bed of old scallop shells, probably built up from the octopuses' own meals. There they signal, fight and even push one another out of dens. The scientists stress that these are clusters of separate homes, not cities.

Tests and puzzles

In lab experiments, octopuses can learn by trial and error. In Naples, common octopuses learned to tell apart shapes that meant a reward, and they worked through simple mazes.

In another lab test, common octopuses had to pull an L-shaped food box through a tight hole in a clear wall. All seven learned the trick of pulling, letting go and pushing at the right moments. When the puzzle changed, each slowed down briefly, then coped.

Play?

Eight giant Pacific octopuses were each given a floating pill bottle. Two of them kept aiming water jets to send it across the tank, and the current brought it back. Scientists call this play-like, because they cannot ask an octopus whether it is having fun.

Carrying a coconut

Some veined octopuses carry their shelter with them. The veined octopus of Indonesia digs up halves of coconut shell, 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. The payoff only comes later, when it stops somewhere open and needs a place to hide. Watch this fact10 s

Two kinds of sleep

Octopuses studied in labs switch between two kinds of sleep. In quiet sleep, the octopus is pale and still. About every half hour to an hour, it slips into roughly a minute of active sleep. Its skin flickers through colors and textures, and its eyes and arms twitch. In one species, brain recordings during active sleep look like those of an awake octopus. Scientists are still finding out what active sleep is for.

How many eggs?

A giant Pacific octopus lays up to about 100,000 eggs, hung in long strings from the roof of her den. According to the FAO, a common octopus lays even more, 100,000 to 500,000. Species with big eggs lay far fewer: Diguet's pygmy octopus lays only 50 to 250, the FAO says.

Drifting young

In many species, the hatchlings drift in the plankton for weeks or months before settling. These drifting young are called paralarvae. In other species, the young hatch as tiny copies of the adults and crawl away. No octopus parent cares for its young after they hatch.

The giant Pacific octopus, the largest octopus, hatches from an egg about the size of a grain of rice. The Monterey Bay Aquarium says its hatchling is about 6 to 7 mm long and already has about 14 tiny suckers on each arm.

A mother's last job

In most octopus species, the mother breeds only once. She spends the last part of her life caring for her eggs, and she usually stops eating. Slowly she grows weaker, and she dies around the time her young hatch. This is the ordinary end of an octopus life. Male giant Pacific octopuses die a few weeks after mating.

How long do octopuses live?

Most octopuses live only about one to two years. The giant Pacific octopus lives longer, about three to five, and some cold-water and deep-sea species live several years. For most deep-sea octopuses, scientists are still finding out.

• Diguet's pygmy octopus: 125 to 240 days in the lab, says the FAO
• Star-sucker pygmy octopus: roughly six months, says the Two Oceans Aquarium
• Most octopuses: about 1 to 2 years
• Giant Pacific octopus: about 3 to 5 years
• Some polar species: at least 6 years, the FAO estimates

Where did the shell go?

Cephalopods first appeared more than 500 million years ago as small animals with shells. Octopuses came much later. Their soft bodies rarely become fossils, but one from Lebanon, about 90 million years old, is among the oldest octopus fossils known, says National Geographic.

Along the way, octopuses lost the shell. The nautilus still has one on the outside. A cuttlefish keeps a cuttlebone inside, and a squid has a thin strip inside, called a pen. An octopus has, at most, tiny leftovers.

Finless and finned

Octopods, the whole octopus group, come in two main kinds. Finless octopuses include most species. They are muscular, mostly live on the bottom and usually have an ink sac. Finned octopods are soft and jelly-like, with fins and no ink sac, and they live only in the deep sea.

Where octopuses live

Octopuses live in every ocean, from warm tropical reefs to icy polar seas and from tide pools to the deep sea. No octopus lives in fresh water. Most bottom-living octopuses keep a den, such as a crack in the rocks.

The giant

The giant Pacific octopus is the largest known octopus species. Adults average about 20 to 30 kg (45 to 65 lb) and span about 2 to 4 m (7 to 13 ft) from arm tip to arm tip. Record animals are bigger, but these are typical sizes.

The tiny

The smallest known octopuses are a few centimeters long when fully grown and can weigh under a gram. Sources disagree about which species is smallest. The star-sucker pygmy octopus is one of the smallest: the FAO gives it at most about 4.5 cm long.

Species cards

More species cards

The Inky story

In 2016, New Zealand's National Aquarium reported that Inky, a common New Zealand octopus, had got out of his tank one night. The lid had been left slightly open. Staff believe he climbed out, crossed the floor and squeezed into a drainpipe that runs to the sea. They worked out the route from the wet trail he left. Nobody saw it happen, so the route is their best guess.

Laws for octopuses

In the United Kingdom, a 2022 law counts octopuses and all other cephalopods as sentient animals, able to have feelings. It does not ban fishing for octopuses or eating them.

In the European Union, a law from 2010 protects octopuses and other cephalopods used in research, as it protects animals with backbones.

In 2024, two US states, Washington and California, passed laws banning octopus farming. California's law also bans selling farmed octopus. Both states pointed to octopus intelligence and the ability to feel pain. These are state laws, so octopus farming is not banned across the whole country.