Given eight seemingly interchangeable arms, an octopus still plays favorites: in a 2006 experiment, every animal tested reached into a maze with the same preferred arm again and again, and individuals favor one eye as well. Unlike human handedness, though, the species has no side — roughly half the animals lean left and half lean right, so the preference is personal rather than built into the octopus.
Octopuses use copper-based hemocyanin instead of iron-based hemoglobin to carry oxygen. Oxygenated hemocyanin is blue and remains effective in cold, oxygen-poor water, although it carries less oxygen per unit of blood than hemoglobin.
The parrot-like beak is the only hard part of an octopus; everything else is muscle and skin that can be poured through a gap. The Alaska Department of Fish and Game estimates that a 23-kilogram (50-pound) giant Pacific octopus can pass through a hole about 5 centimeters (2 inches) across. One researcher watched an octopus of that size cross a research vessel's deck, find the scupper drain and be gone inside a minute.
In 2016 staff at the National Aquarium of New Zealand in Napier found Inky's tank empty and a trail of water across the floor. He had exploited a lid left ajar after maintenance, crossed roughly 2.5 meters (8 feet) of floor and squeezed into a drainpipe about 50 meters (160 feet) long that emptied into Hawke's Bay. He was never seen again. His tankmate, Blotchy, stayed put.
During more than 500 diver hours off Indonesia, researchers watched veined octopuses dig up coconut-shell halves, stack them and carry them as far as 20 meters (66 feet) before assembling them into shelters. Because the shells provide no protection while being transported and are saved for later use, the behavior was classified as tool use.
Octopus marginatus and Abdopus aculeatus have been filmed walking on two alternating rear arms while wrapping the other six around the body. One resembles a drifting coconut and the other a clump of algae. The behavior was the first documented bipedal locomotion in an animal without a rigid skeleton.
The mimic octopus, first seen off Sulawesi in 1998 and named Thaumoctopus mimicus, flattens, contorts and recolors itself into imitations of flatfish, lionfish and banded sea snakes. Reports credit it with impersonating a dozen or more marine animals, nearly all of them venomous or unpalatable.
Female common blanket octopuses reach about 2 meters (6.5 feet) long, while males top out near 2.4 centimeters (1 inch). The weight ratio is at least 10,000-to-1 and may reach 40,000-to-1, among the most extreme size gaps known in any large animal. Males are so rarely encountered that the first live one was not photographed until 2002, off the Great Barrier Reef.
Male and juvenile blanket octopuses tear stinging tentacles from Portuguese men-of-war and hold them in their arms, apparently as weapons or deterrents. Skin chemistry is thought to keep the stinging cells from firing against them.
The argonaut is a small octopus that lives in open water, and the female builds herself a thin white shell using two of her arms. She swims to the surface, traps a gulp of air inside the shell, then dives — and deep down the squeezed air balances her weight exactly, so she floats in midwater without swimming.
Male argonauts detach a specialized mating arm inside the female, where it can continue moving on its own. In 1829, Georges Cuvier mistook these arms for parasitic worms and named them Hectocotylus octopodis. The term hectocotylus is still used for a cephalopod's specialized mating arm.
From 2007 to 2011, researchers from the Monterey Bay Aquarium Research Institute returned 18 times to the same site in Monterey Canyon and found the same scarred Graneledone boreopacifica female guarding about 160 eggs. She was never observed feeding and grew progressively thinner. Her 53-month brood remains the longest recorded for any animal.
At the base of Davidson Seamount off California, about 3,200 meters (10,500 feet) down, roughly 6,000 nesting pearl octopuses were counted in a 2.5-hectare patch, and researchers estimate the full site holds more than 20,000 nests. Named the Octopus Garden, it is the largest octopus aggregation known anywhere. The animals migrate there to mate, nest and die.
At the Octopus Garden, a nursery near the base of Davidson Seamount off California, surrounding seawater is about 1.6 °C (35 °F), while water seeping from cracks reaches nearly 11 °C (51 °F). Pearl octopus (Muusoctopus robustus) mothers cement their eggs to the rock and brood in the warmer crevices, where higher temperatures raise metabolic rates and shorten incubation from an expected five or more years to roughly 1.8 years.
In April 2019, cameras lowered into the Java Trench during the Five Deeps Expedition recorded a dumbo octopus foraging at 6,957 meters (22,825 feet), in water of about 1.5 °C (35 °F). A second animal turned up at 5,760 meters (18,898 feet). The previous reliable record, a black-and-white photograph, had stood at 5,145 meters for half a century.
The pale deep-sea octopod nicknamed Casper was filmed near Hawaii in 2016 at more than 4,000 meters (13,100 feet) — the deepest record was 4,290 meters (14,100 feet) — a record depth for its group. Related octopods were then found attaching their eggs to the stalks of dead sponges, and those sponges grow only on manganese nodules — potato-shaped metal lumps that take millions of years to form and are prime targets for seabed mining. Remove the nodules and the sponges go, and with them the nursery.
The male seven-arm octopus (Haliphron atlanticus) hides its eighth arm, a specialized mating arm, in a pouch beneath the right eye, so only seven are visible. This deep-sea species feeds on jellyfish, eating the bell and sometimes carrying the trailing tentacles.
The giant Pacific octopus (Enteroctopus dofleini) can span about 4.3 meters (14 feet) from arm to arm. The heaviest individual reliably weighed alive was 71 kilograms (156 pounds), while most adults are far smaller.
The star-sucker pygmy octopus, Octopus wolfi, lives in the shallow coastal waters of the western Pacific Ocean, measures under 2.5 centimeters (1 inch) in total length and weighs less than a gram (0.04 ounce) — smaller and lighter than a small coin, and small enough to sit on a fingernail. It is named for the ring of tiny papillae fringing the suckers near its arm tips, which let it taste whatever it touches.
A 2009 genetic survey found venom proteins across octopuses, cuttlefish and some squid, tracing them to a single ancient venomous ancestor. Most species use theirs quietly, paralyzing a clam until its shell relaxes open. Only the blue-ringed octopuses of the genus Hapalochlaena carry a toxin dangerous to humans.
Octopuses are carnivores, and for nearly every species studied the staple is crustaceans: crabs above all, with shrimp and lobsters behind them. Mollusks come second — clams, snails and scallops, pulled apart, drilled through or relaxed with paralyzing saliva. Larger species take fish as well. And the giant Pacific octopus is recorded eating smaller octopuses, its own species included.
When an octopus cannot pull a shell open, a muscular salivary papilla studded with minute teeth — separate from the radula, its primary toothed ribbon — can drill through it. Secretions from the posterior salivary glands then help relax the muscle holding the shell closed.
Blue-ringed octopus saliva contains tetrodotoxin, a potent neurotoxin with no known antivenom. Symbiotic bacteria in the posterior salivary glands produce the toxin, and females also coat their eggs with it.
The common octopus runs on roughly 500 million neurons, by far the largest nervous system of any invertebrate — a honeybee has about a million. Two-thirds of them sit in the arms rather than the central brain, each arm carrying a nerve cord that handles touch, taste and movement locally. The brain sets the mission; the arms work out the details.
An octopus has one brain, but a highly distributed nervous system. The central brain in its head does the learning, the remembering and the deciding, while each of the eight arms carries its own large cluster of neurons that can sense touch and taste and control much of the arm's movement without waiting for orders. Those clusters are nicknamed mini-brains, but nothing in an arm learns or remembers.
The central brain is not a lump above the mouth. It is a donut of fused lobes encircling the esophagus, split into a supraesophageal mass handling learning and memory and a subesophageal mass handling movement. Food passes through the middle of the brain on its way to the gut. Whatever an octopus swallows, it swallows through its own head.
Harvard researchers identified a family of chemotactile receptors found only in cephalopods and expressed in the suction cups themselves. They detect molecules that dissolve poorly in water — exactly the compounds that would cling to a crab's surface rather than drift away. So an octopus probing a dark crevice is not groping blindly. Each sucker is reporting flavor.
A giant Pacific octopus has roughly 200 to 250 suckers on each arm. Monterey Bay Aquarium reports up to 2,240 suckers in a female, about 100 more than in a male. Each sucker can grip independently, and those near the arm tips are the smallest and most sensitive.
Hundreds of suckers per arm will stick to almost anything they meet, which raises an obvious problem for an animal with eight boneless arms. Israeli researchers found that amputated octopus arms grabbed petri dishes readily but would not grab octopus skin, and gripped dishes smeared with skin extract about ten times more weakly. The signal works in the arm itself, with no help from the brain. The octopus solved knots with chemistry.
Detached arms continue responding to stimuli and activating their suckers long after they lose contact with the brain — in laboratory observations, for more than an hour. In one experiment an octopus handled its own amputated arm freely but held it only at the skinless cut end and would not eat it. The self-recognition signal apparently survives the separation.
Researchers at UC Santa Barbara shone light on isolated patches of skin from the California two-spot octopus and watched the color-changing chromatophores expand, then relax in darkness. The tissue responded fastest to blue light near 480 nanometers, the same wavelength the eye's pigment favors, and the skin turned out to express rhodopsin. They named the effect light-activated chromatophore expansion. The skin does not form images — but it knows when the lights change.
Chromatophores hold only yellow, red, brown and black pigments, so no combination of them can produce the blue-green sheen a living octopus sometimes shows. Two other cell types do that work: iridophores, stacks of protein plates that split light by interference, and leucophores, which scatter whatever light falls on them and so look red in red light and blue in blue. Part of octopus color is not paint at all. It is optics.
Every octopus studied carries a single type of visual pigment, which by the usual definition makes the animal color-blind. Yet octopuses produce camouflage that matches background colors closely enough to fool spectrophotometers. The paradox has been open since the pigment was first measured in 1958.
Vertebrate and cephalopod camera eyes evolved independently, but their retinas are wired differently. In vertebrates, nerve fibers lie in front of the photoreceptors and the optic nerve creates a blind spot. In octopuses, the photoreceptors face incoming light and the nerve fibers exit behind them, so there is no comparable blind spot.
Octopus photoreceptors are arranged in two channels that detect the orientation of polarized light. Behavioral tests show they can distinguish differences in polarization angle of only a few degrees. This can make transparent prey visible even when brightness and color provide little contrast.
Octopuses retune their nervous systems with the seasons: they can extensively edit their RNA, altering the proteins produced from the same genes. When California two-spot octopuses were acclimated to 13 °C (55 °F) instead of 22 °C (72 °F), editing changed at more than 20,000 sites, including sites in proteins involved in nerve signaling and intracellular transport.
The California two-spot octopus has a genome about 2.7 billion DNA base pairs long, slightly smaller than ours. Yet researchers identified about 33,600 protein-coding genes, compared with roughly 20,000 in humans. Surprisingly, this extra genetic complexity did not come from duplicating the entire genome; instead, certain gene families expanded and the genome was extensively rearranged.
The octopus genome contains 168 protocadherin genes, which help regulate how neurons develop and connect. That is about ten times the number found in other invertebrates and more than twice the number in mammals. Large protocadherin families had previously been considered mainly a vertebrate feature.
Voluntary arm shedding, or autotomy, has been documented in at least Abdopus aculeatus and Ameloctopus litoralis. The arm separates at a defined break point and continues writhing while the octopus escapes.
An octopus can rebuild a lost arm, including its muscles, suckers and nerve cord. The wound is first covered by a layer of epithelium, then new tissue is organized into the structures of a functional replacement arm rather than a simple scar.
In round-the-clock observations at 22 °C (72 °F), nocturnal octopuses averaged about 12 quiet-sleep bouts of 50 minutes each plus roughly 10 brief active-sleep bouts. Together, these amounted to about 10 hours of sleep per day. Warmer water shortened the interval between active bouts.
A resting octopus lies pale and still, then erupts for about a minute into twitching arms, darting eyes, quickened breathing and racing skin patterns before going quiet again. The phase has a raised arousal threshold, is regulated by prior sleep, and produces neural activity resembling wakefulness, making it broadly comparable to mammalian REM sleep.
Cameras trained on gloomy octopuses in Jervis Bay, Australia, caught 102 throws in 24 hours of footage from a group of about ten animals. Each throw involves gathering silt, shells or algae in the arms and repositioning the siphon under the arm web to blast the material several body lengths. Throws in social contexts were more forceful, more often silt than shells, and more often made by dark-colored individuals — and they frequently connected. Targeted throwing had previously been seen only in some social mammals.
In the Red Sea, day octopuses hunt alongside goatfish, groupers and other reef fish, and researchers recorded them delivering swift explosive jabs with one arm at particular fish partners. The team called the behavior active displacement. Sometimes it clears a competitor off prey; sometimes there is no prey to grab at all. The scouting fish get punched least, and the loafers get punched most.
Three-dimensional tracking of octopus-fish hunting groups showed goatfish scouting ahead and signaling discoveries, while the octopus largely determined whether the group moved toward them. Group composition also changed how much each species contributed and how successful the hunt was.
At the Seattle Aquarium, eight giant Pacific octopuses met two unfamiliar people dressed the same way for two weeks: one always fed them, the other always touched them with a bristly stick. By the end, the animals showed measurably different body patterns, arm movements and funnel orientation toward each person, and the larger ones changed their breathing rate. The uniform trick was the point. They were not reading the clothes.
Repeated behavioral tests have found consistent individual differences in octopus activity, reactivity and avoidance. Later work in another species found similar temperament differences early in life, showing that individuals of the same species can behave predictably differently.
Captive octopuses routinely learn to open screw-top containers for food. At the Seattle Aquarium, some also worked out childproof pill-bottle caps. In a baby-bottle test, one octopus bypassed the screw top entirely by tearing off the nipple.
In trials with eight giant Pacific octopuses, two repeatedly jetted water at a floating pill bottle so a tank current carried it back, then jetted it away again. Researchers classified the repeated, non-feeding behavior as exploratory play. Later studies found similar play with other objects and in adults as well as juveniles.
Researcher Jennifer Mather observed a wild octopus leave its den, collect three rocks, set them aside, carry them back and stack them across the entrance before resting. The sequence has been discussed as possible planning, although that interpretation remains debated.
Pairs of larger Pacific striped octopuses mate face to face with their suckers aligned, share food and may occupy the same den for days. Females produce multiple clutches over months instead of a single terminal brood.
Octopuses drag prey home and discard the shells outside the entrance, building a heap called a midden. Divers use these piles of cracked crab and clam shells to locate dens that would otherwise be invisible. The habit is old enough that Aristotle noted it more than 2,000 years ago.
A female giant Pacific octopus hangs her eggs from the roof of her den in hundreds of strands of roughly 250 eggs each, commonly totaling 18,000 to 74,000 and occasionally reaching 100,000. For six or seven months she fans them, cleans them and blows water over them, and does not eat. As the eggs hatch, she blows the young free of the den with her last strength, then dies. Each hatchling emerges the size of a rice grain, weighing about 22 milligrams, and of the tens of thousands, a handful will reach adulthood.
A giant Pacific octopus can grow to tens of kilograms, but a hatchling weighs only about 22 milligrams and is roughly the size of a rice grain. The young drift near the surface as plankton for weeks or months before settling to the seafloor, and only about one in a hundred survives that stage.
Most octopus species live for roughly a year or less. The giant Pacific octopus is unusually long-lived at about three to five years, while the deep-sea Graneledone boreopacifica has been recorded brooding its eggs for 53 months.
The animal long called the common octopus, Octopus vulgaris, is now understood as a species complex rather than a single worldwide species. Genetic studies have identified at least six regional species that can be extremely difficult to distinguish by appearance alone.
The Greek roots behind Cephalopoda translate as head and foot, describing a body plan in which the two are fused. The eight arms are modifications of the same muscular foot that a snail crawls on, rearranged into a ring around the head. The mouth and beak sit underneath, at the center of that ring. An octopus is a mollusk that walks on a reinvented snail foot.
In cephalopod anatomy, arms carry suckers along their whole length while tentacles are longer, retractable and suckered mainly at the tip. Squid and cuttlefish have both — eight arms plus two tentacles. Octopuses have eight arms and nothing else. The word "tentacle" is almost universally applied to them and is almost universally wrong.
Octopuses is the standard English plural. Octopi is also widely used, but it applies a Latin-style ending to a word derived from Greek. Octopodes follows the Greek plural pattern. Major dictionaries record all three forms.
In History of Animals, Aristotle called the octopus a stupid creature because it would approach a person's lowered hand. Yet he also accurately described its specialized mating arm, a feature European naturalists later had to rediscover.
With no internal or external skeleton, an octopus is roughly 90% muscle. Its arms act as muscular hydrostats, stiffening, bending and extending by contracting muscles against one another rather than pulling on bones. This structure gives the animal both strength and extreme flexibility.
Cirrate octopuses, the deep-sea group nicknamed dumbo for the ear-like fins above their eyes, propel themselves by slowly beating those fins rather than by jetting water. A web of skin joins the arms into an umbrella. The deepest octopus ever filmed was one of them, foraging almost 7 kilometers down.
Common octopuses can lay about 100,000 to 500,000 small eggs that hatch into planktonic paralarvae, while giant Pacific octopuses lay tens of thousands. Some deep-sea octopods produce only about 30 large, yolk-rich eggs whose young hatch as miniature bottom-dwelling octopuses.
The siphon is a muscular funnel that forcefully expels water from the body. An octopus uses it for jet propulsion, to eject ink, and to aim water beneath the arm web strongly enough to throw silt, shells and other debris.
Octopuses are semelparous: females produce a single clutch, guard it without feeding, and die around the time it hatches. Males typically die not long after mating. The one clear exception found so far, the larger Pacific striped octopus, spawns repeatedly and keeps eating — which is why researchers now study its optic glands looking for what the rest of the order is missing.
A blue-ringed octopus is small and usually drab, but vivid iridescent blue rings appear when it feels threatened. The display serves as a warning; the animal generally tries to escape, and nearly every recorded bite has come after someone picked it up.
Two branchial hearts drive blood through the gills; a third systemic heart sends it out to the arms, organs and brain. When the animal jets, pressure inside the mantle stops blood returning, and the systemic heart — with nothing to pump — stops until the jetting ends. Fast swimming therefore starves an octopus of oxygen at the exact moment it needs it most. This makes sustained jetting physiologically costly compared with crawling.
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