An octopus is a soft-bodied sea animal with 8 arms, no bones, and a round head. Octopuses live in every ocean on Earth, from warm reefs to cold deep water. There are about 300 different species, which means 300 kinds. Scientists put octopuses in a group called cephalopods, a word that means “head-foot,” because their arms attach right under their head.
Why octopuses are amazing
Octopuses do things no other animal does. They have 3 hearts instead of 1. Their blood is blue, not red, because it carries oxygen using copper instead of iron. They can change the color of their skin in less than a second. And about two-thirds of their brain cells are inside their 8 arms, so each arm can taste, touch, and decide things on its own.
An octopus can also squeeze its whole body through any opening that is bigger than its hard beak. The beak is the main hard part of an octopus, so if the beak fits, the rest of the soft body can usually fit too.
Key facts about octopuses
Eight arms. Each arm has 2 rows of suckers. The suckers can grab, smell, and taste food. A giant Pacific octopus can have around 2,240 suckers in total.
Three hearts. Two small hearts (called branchial hearts) pump blood through the gills to pick up oxygen. One bigger heart (the systemic heart) sends the blood out to the rest of the body.
Blue blood. Octopus blood uses a copper-based protein called hemocyanin to carry oxygen. Copper turns the blood blue, the way iron turns our blood red.
Color-changing skin. Octopuses have tiny color bags in their skin called chromatophores. Muscles around each color bag stretch it open or pull it shut. This lets an octopus turn red, brown, yellow, or white in a fraction of a second.
Color-blind. Most octopuses cannot see color with their eyes. Scientists are still studying how octopuses match the colors around them so well even though they are color-blind.
Smart. Octopuses can solve puzzles, open jars, and remember people. They sometimes squirt water at zookeepers they do not like.
Big and small. The giant Pacific octopus is the biggest, with arms that stretch about 14 feet (4.3 m) across and a typical weight of about 33 lb (15 kg). The star-sucker pygmy octopus is the smallest at less than 1 inch (2.5 cm) long and less than 1 gram in weight.
Short lives. Most octopus species live 1 to 2 years. The giant Pacific octopus lives 3 to 5 years.
Tool users. The coconut octopus carries empty coconut shells across the seafloor and hides inside them. This is one of the only times an invertebrate (an animal without a backbone) has been seen using a tool.
Ink cloud. When an octopus is scared, it shoots out a cloud of black ink to confuse a predator while it swims away. The ink is mostly made of melanin, the same dark pigment found in your hair and skin.
Common myths about octopuses
Myth: An octopus is a fish. An octopus is not a fish. Fish have backbones and gills covered by hard plates. An octopus has no backbone at all and belongs to the group of soft-bodied animals called mollusks, the same big group as snails and clams.
Myth: Octopuses have tentacles. Octopuses have arms, not tentacles. Squid and cuttlefish have 8 arms and 2 longer tentacles, but octopuses only have arms. Tentacles have suckers only at the tip; arms have suckers all the way down.
Myth: All octopus arms come back the same way as a starfish leg. An octopus can grow back an arm if it loses one, but it takes weeks or months. The new arm starts small and grows out from the stump.
Myth: Octopuses are scary monsters. Octopuses almost always run, hide, or use ink to escape. They are very shy and prefer to stay hidden in dens between rocks.
Frequently asked questions
How many arms does an octopus have?
Eight. The name “octopus” comes from Greek words meaning “eight feet.”
Why is octopus blood blue?
Octopus blood carries oxygen using copper instead of iron. Copper makes the blood blue, the way iron makes our blood red.
How many hearts does an octopus have?
Three. Two pump blood to the gills, and one pumps blood to the rest of the body.
Are octopuses smart?
Yes. Octopuses can open jars, escape from tanks, recognize people, and use tools like coconut shells. They are among the smartest animals without a backbone.
Can octopuses see color?
Most octopuses cannot see color with their eyes. Scientists are still studying how they camouflage so well.
What is the biggest octopus?
The giant Pacific octopus. It usually weighs about 33 lb (15 kg) with an arm span around 14 feet (4.3 m). The largest one ever recorded weighed about 600 lb (270 kg).
What did Inky the octopus do?
Inky was an octopus at the National Aquarium of New Zealand. In 2016, Inky climbed out of his tank at night, slid across the floor, and slipped down a 165-foot (50 m) drainpipe back to the sea.
An octopus is a soft-bodied marine invertebrate with 8 arms, no internal skeleton, and a hard beak made of chitin (the same material in insect shells). Octopuses belong to the class Cephalopoda, the group that also includes squid, cuttlefish, and the chambered nautilus. About 300 octopus species have been described, ranging from the 1-inch (2.5 cm) star-sucker pygmy octopus to the 14-foot (4.3 m) giant Pacific octopus.
Why octopuses are unusual
An octopus body looks nothing like a vertebrate body, and the differences run deep. The octopus has 3 hearts, blood that runs blue with copper instead of red with iron, and a nervous system in which most of the neurons sit outside the central brain.
Roughly 500 million neurons make up the octopus nervous system. About two-thirds of those neurons, around 330 million, are spread through the 8 arms in clusters called ganglia. Only about 40 to 45 million neurons are in the central brain itself. Each arm can taste with its suckers, decide which way to bend, and grab food without checking with the brain first.
Octopus vision is also unusual. The eye is a “camera-type” eye much like ours, with an iris, a circular lens, and a retina, but most octopus species are color-blind. Their retina has only 1 type of light-sensitive pigment. Even so, octopuses are excellent at matching the colors of their background, and scientists are still working out exactly how.
Key facts about octopuses
Eight arms with suckers. Each arm has 2 rows of suction cups. The suckers can grip, taste, and smell. A giant Pacific octopus has roughly 2,240 suckers in total, and the largest of them can hold about 35 lb (16 kg) each.
Three hearts. Two branchial hearts pump blood through the gills to load it with oxygen. One systemic heart circulates the oxygen-rich blood through the rest of the body. The systemic heart pauses while the octopus swims, which is one reason octopuses prefer to crawl.
Blue blood. Octopus blood uses hemocyanin, a protein that carries oxygen with copper rather than iron. Hemocyanin works better than hemoglobin in cold, low-oxygen water but is less efficient overall, which limits how fast and far an octopus can swim.
Color change in milliseconds. Octopus skin contains 3 layers of color cells: chromatophores (red, brown, yellow, black pigment sacs that open and close like little balloons), iridophores (mirror-like cells that reflect blues and greens), and leucophores (white scattering cells). The giant Pacific octopus can change its body color in about a tenth of a second.
Skin that senses light. A 2015 study at the University of California, Santa Barbara showed that octopus skin contains the same light-sensitive protein, rhodopsin, that the eye uses. The skin can detect changes in brightness even with no signal from the brain. This may help octopuses match background light when they cannot see their own body.
Camouflage and mimicry. The mimic octopus (Thaumoctopus mimicus), discovered in 1998 off Sulawesi, Indonesia, can imitate the shape and movement of lionfish, sea snakes, and flatfish.
Tool use. The coconut octopus (Amphioctopus marginatus) collects empty coconut and clamshell halves and carries them across the seafloor to assemble a portable shelter. The behavior, documented in Current Biology in 2009, is the first clear example of tool use in an invertebrate.
Smallest and largest. The star-sucker pygmy octopus (Octopus wolfi) is under 1 inch (2.5 cm) long and weighs less than 1 gram. The giant Pacific octopus (Enteroctopus dofleini) typically weighs about 33 lb (15 kg) with a 14-foot (4.3 m) arm span. The largest specimen on record was about 600 lb (270 kg) with a span around 32 feet (9.8 m).
All octopuses are venomous. Every octopus species injects venom through its beak to subdue prey. Most species are harmless to humans. The 4 species of blue-ringed octopus (Hapalochlaena), each about 5 to 8 inches (12 to 20 cm) long, carry tetrodotoxin, the same paralyzing toxin found in pufferfish.
Short lifespan. Most species live 1 to 2 years. The giant Pacific octopus lives 3 to 5 years. Octopuses are semelparous, meaning they reproduce only once and die soon after.
Famous escape.Inky, a New Zealand octopus at the National Aquarium of New Zealand in Napier, escaped his tank in 2016. He climbed out, crossed the floor, and slid down a 165-foot (50 m) drainpipe to the sea.
Common myths about octopuses
Myth: Octopuses have tentacles. Octopuses have arms, not tentacles. The difference matters in marine biology. Tentacles, like the 2 long feeding limbs of a squid, have suckers only at the tip. Arms have suckers along the entire length. An octopus has 8 arms and 0 tentacles.
Myth: Octopuses are mainly aggressive predators. Most octopuses are shy and prefer to hide. When threatened, they typically squirt ink, jet away, or hide in a den rather than attack. The few species dangerous to humans, like the blue-ringed octopus, only bite when handled or stepped on.
Myth: An octopus uses its ink to attack. The ink is a defense, not a weapon. It forms a cloud that hides the octopus and may also dull a predator’s sense of smell while the octopus jets to safety.
Myth: All octopus arms grow back perfectly. Lost arms can regenerate, but the new arm grows over weeks or months and may not always reach exactly the same length as before.
Myth: Octopuses live in groups. Most species are solitary and only meet other octopuses to mate. The larger Pacific striped octopus is a notable exception. Researchers have observed up to 40 of them living in close company off the coasts of Nicaragua and Panama.
Frequently asked questions
How many hearts does an octopus have?
Three. Two branchial hearts feed oxygen-poor blood to the gills, and a single systemic heart pushes oxygen-rich blood out to the body.
Why is octopus blood blue?
The oxygen-carrying protein in octopus blood is hemocyanin, which uses copper. Copper makes the blood blue when oxygen is attached. Vertebrate blood uses hemoglobin and iron, which is why our blood is red.
How many neurons does an octopus have?
Around 500 million. Roughly two-thirds of them sit in the 8 arms in nerve clusters called ganglia, with only 40 to 45 million in the central brain.
How can octopuses match colors if they are color-blind?
Octopus eyes have only 1 type of color-sensitive pigment, so the eye itself cannot distinguish red from green the way a human can. Possible explanations include light-sensitive proteins in the skin, the unusual shape of the octopus pupil, and the way the lens spreads different colors to slightly different focal points. The full mechanism is still being studied.
Are octopuses dangerous?
Most octopus species pose no real threat to humans. The blue-ringed octopus is the main exception. Its bite carries enough tetrodotoxin to paralyze a person, and there is no antivenom. Bites usually happen only when the animal is picked up.
How long do octopuses live?
Most species live 1 to 2 years. The giant Pacific octopus lives 3 to 5 years. After mating and laying eggs, female octopuses brood the eggs without eating and die soon after the eggs hatch.
What is the smartest thing an octopus has done?
Documented behaviors include opening child-proof bottles, fitting through small openings, using coconut shells as shelters, mimicking other animals, and learning to recognize individual humans. Inky’s escape from the National Aquarium of New Zealand in 2016 is the most famous case.
An octopus is a soft-bodied cephalopod mollusk in the order Octopoda, distinguished from other cephalopods by 8 arms, no internal shell, no tentacles, and a dorsal mantle that houses the visceral organs. Roughly 300 living species have been described, ranging from the star-sucker pygmy octopus (Octopus wolfi) at under 1 inch (2.5 cm) and less than 1 gram to the giant Pacific octopus (Enteroctopus dofleini), which typically weighs about 33 lb (15 kg) with a 14-foot (4.3 m) arm span and reached a record 600 lb (270 kg) and 32 feet (9.8 m) of arm span. Octopuses occupy every ocean on Earth, from intertidal pools to depths of 6,600 feet (2,000 m).
Why octopuses resist easy summary
Three biological features of octopuses defy intuition built around vertebrate animals.
The first is the cardiovascular system. An octopus has 3 hearts. Two branchial hearts push deoxygenated hemolymph through the gills, and a single systemic heart pumps the oxygenated hemolymph through the body. The hemolymph carries oxygen using hemocyanin, a copper-based protein dissolved freely in plasma rather than packaged inside cells. Hemocyanin is blue when oxygenated and colorless when not. It performs better than vertebrate hemoglobin in cold, low-oxygen seawater but is less efficient overall, which constrains sustained swimming. The systemic heart in fact stops beating during fast swimming, one reason octopuses prefer crawling on their arms.
The second is distributed cognition. The octopus nervous system contains roughly 500 million neurons, similar to a dog’s brain in raw count, but the layout is unlike any vertebrate. About two-thirds of these neurons (around 330 million) sit in the 8 arms, organized into a chain of axial nerve cords and ganglia that run the length of each arm. Only 40 to 45 million neurons reside in the central brain, which sits between the eyes and surrounds the esophagus. Each arm can taste with its suckers, generate goal-directed movement, and coordinate with neighboring arms with limited oversight from the central brain. The octopus arm is best understood as a muscular hydrostat, a structure (like an elephant trunk or a human tongue) whose constant volume lets sets of muscles act against each other without a skeleton.
The third is the disconnect between color vision and color change. The octopus eye is a camera-type eye with an iris, lens, vitreous body, and retina that converges on vertebrate eyes by independent evolution. Most octopus retinas, however, contain only 1 opsin, so the animal is effectively color-blind by retinal measurement. Yet the skin produces extremely detailed color matches to the substrate. The skin contains a 3-layer chromatic system: chromatophores (pigment-loaded sacs of red, brown, yellow, and black under direct neural control by radial muscles), iridophores (stacks of reflectin platelets that interfere-tune blues and greens through plate spacing), and leucophores (broadband white scatterers that supply a bright background). A 2015 paper by Ramirez and Oakley showed that Octopus bimaculoides skin contains rhodopsin and other phototransduction components, and that isolated skin patches expand chromatophores in response to light without input from eye or brain. The current best hypothesis is that the skin samples light independently and helps tune the body pattern even when the animal cannot see itself.
Key octopus facts
Classification. Order Octopoda, subclass Coleoidea, class Cephalopoda, phylum Mollusca. The name “cephalopod” combines the Greek roots for head and foot. Sister groups within Coleoidea include the squids (Decapodiformes) and the cuttlefishes.
Species count. Around 300 described species, with new ones added every few years, especially from deep-sea expeditions.
Hearts and blood. Three hearts, hemocyanin-based blood. The 2 branchial hearts feed oxygen to the gills; the 1 systemic heart drives the body circuit. Hemocyanin appears blue when bound to oxygen.
Neurons. Approximately 500 million in total, with about two-thirds distributed through the 8 arms in axial nerve cords and ganglia. The central brain has 40 to 45 million neurons divided across roughly 40 lobes.
Beak. The octopus mouth is a parrot-like beak made of chitin, the same nitrogenous polysaccharide found in insect exoskeletons. The beak is the main hard part of the body. An octopus can squeeze through openings larger than its beak because most of the body is soft and deformable.
Suckers. Each arm has 2 rows of suckers under independent muscular control. A giant Pacific octopus has approximately 2,240 suckers in total. The largest GPO suckers reach 2.5 inches (6.4 cm) across and can hold about 35 lb (16 kg) of force each.
Camouflage system. Three skin layers (chromatophores, iridophores, leucophores) generate the body pattern. Chromatophore expansion is direct and fast: the giant Pacific octopus can change body color in about 1 tenth of a second.
Color vision. Most octopuses have a single retinal opsin and are color-blind by retinal physiology. Working hypotheses for matched coloration include extraocular skin photoreception (Ramirez and Oakley, 2015), chromatic blur from the rectangular pupil (Stubbs and Stubbs, 2016), and behavioral cues from the substrate.
Venom. All octopuses are venomous. The 4 species of blue-ringed octopus (Hapalochlaena) carry tetrodotoxin in their salivary glands, the same sodium-channel-blocking toxin found in pufferfish. There is no antivenom; survival depends on assisted ventilation until the toxin clears.
Ink. Defensive ink is produced by an ink sac in the rectum and discharged through the funnel. The pigment is a melanin generated by tyrosinase, suspended in mucus. The cloud disrupts visual pursuit and may dull a predator’s chemoreception.
Tool use. The coconut octopus (Amphioctopus marginatus) was documented in Current Biology in 2009 carrying nested halves of coconut shells across the seafloor and reassembling them into a portable shelter. The behavior is the first uncontested case of tool use in an invertebrate.
Mimicry. The mimic octopus (Thaumoctopus mimicus) was discovered in 1998 off Sulawesi, Indonesia, and formally described as a new species by Norman and Hochberg in 2005. Field observations report context-appropriate impersonation of lionfish, banded sea snakes, and flatfish.
Lifespan and reproduction. Most octopuses are semelparous: they reproduce once and die. Most species live 1 to 2 years; the giant Pacific octopus lives 3 to 5 years. After egg-laying, the optic gland (between the eyes) drives a programmed senescence, including loss of appetite, tissue degeneration, and death by starvation while the female broods the clutch (Wang and Ragsdale, 2018).
Sociality. Octopuses are usually solitary outside mating. The larger Pacific striped octopus, described by Caldwell and colleagues in PLOS ONE in 2015, is the main exception, with groups of up to 40 observed in the wild and pair-bonded mating in captivity.
Famous escape. Inky, a Pinnoctopus cordiformis, escaped the National Aquarium of New Zealand in Napier in 2016. Tracks across the floor showed he climbed out of a slightly open tank lid and traversed a 165-foot (50 m) drainpipe to the sea.
Welfare and farming. The OCTOPUS Act of 2024, introduced in the U.S. Senate, would prohibit commercial octopus aquaculture and the import of farmed octopus, carrying civil penalties up to $100,000 per violation. Washington state and California have passed similar bans at the state level.
Common octopus myths
Myth: Octopuses have tentacles. They have arms. The terminology is not arbitrary. In cephalopod anatomy, “tentacles” refers specifically to the 2 elongated feeding limbs of squid and cuttlefish, which are smooth along their length and bear suckers only at the distal club. Octopus arms are sucker-lined for their entire length. Octopuses have 8 arms and 0 tentacles.
Myth: Octopus arms are independent because the brain is too small to control them. The central brain is small relative to body mass, but the better description is that control is distributed, not absent. The arms execute local reflexes and coordinate among themselves through inter-arm connectives, and the brain still issues high-level goals. Recent work suggests tighter brain-arm coupling than the original “8 mini-brains” framing implied.
Myth: Octopuses live for decades. Most species live 1 to 2 years. The giant Pacific octopus is one of the longest-lived at 3 to 5 years. The deep-sea brooding octopus Graneledone boreopacifica is the known outlier, with one female observed brooding eggs for 53 months at 4,600 feet (1,400 m) off Monterey, California, but the brooding period is not the same as a long total lifespan.
Myth: Octopus ink is highly toxic. The ink is a visual and chemical screen, not a poison. It contains melanin, mucus, dopamine, and amino acids; it can irritate gills and dull olfaction in predators but does not function as a venom.
Myth: Octopuses are alien. The wider context is that cephalopod evolution split from the vertebrate lineage roughly 600 million years ago, but octopuses are unmistakably terrestrial Earth animals. They share the basic mollusk body plan with snails and clams. The deep difference is in nervous-system organization, not provenance.
Frequently asked questions
How many hearts does an octopus have, and why?
Three. The 2 branchial hearts pump deoxygenated hemolymph through the gills; the 1 systemic heart distributes oxygenated hemolymph to the body. The arrangement compensates for the relatively low oxygen-carrying capacity of hemocyanin-based blood.
Why is octopus blood blue?
The oxygen carrier is hemocyanin, a copper-based protein, rather than the iron-based hemoglobin found in vertebrates. Oxygen binding to copper produces a blue color; deoxygenated hemocyanin is colorless.
How smart is an octopus?
Octopus cognition is well documented: maze learning, observational learning, tool use (coconut shells), object manipulation (childproof bottles), and individual recognition of human handlers. Octopus intelligence drove the United Kingdom’s 2021 update to its Animal Welfare (Sentience) Act to formally recognize cephalopods as sentient.
Can octopuses see color?
Most species have a single retinal opsin and cannot distinguish wavelengths through the eye alone. Skin opsins provide a separate light-sensing pathway (Ramirez and Oakley, 2015). The combined system supports the spectacular camouflage match without classical color vision.
Why do octopuses die after reproducing?
The optic gland between the eyes triggers a hormonal cascade after mating that suppresses feeding and digestion. In females, the cascade also drives egg brooding and death by starvation around the time of hatching. Wang and Ragsdale (2018) identified at least 4 distinct steroid and feeding-peptide pathways involved.
What is the largest octopus?
The giant Pacific octopus (Enteroctopus dofleini). Typical adult mass is 33 lb (15 kg) with a 14-foot (4.3 m) arm span. The largest specimen on record reached approximately 600 lb (270 kg) and a 32-foot (9.8 m) span.
What is the most dangerous octopus?
The 4 species of blue-ringed octopus (Hapalochlaena) carry tetrodotoxin in their salivary glands. A bite can paralyze respiratory muscles within minutes. There is no antivenom; treatment is supportive ventilation until the toxin clears. Confirmed human fatalities are rare (about 11 documented).
Are octopuses really being farmed?
There is currently no commercial octopus farm operating in the United States. A Spanish company has proposed an industrial farm for the common octopus, drawing scientific opposition on welfare grounds and prompting the bipartisan OCTOPUS Act of 2024 in the U.S. Senate.
An octopus is a soft-bodied cephalopod mollusk in the order Octopoda, characterized by a derived body plan of 8 sucker-lined arms, a closed cardiovascular system with 3 hearts and copper-based hemolymph, a centralized but heavily decentralized nervous system of approximately 500 million neurons, and an integumentary chromatic system of chromatophores, iridophores, and leucophores capable of subsecond pattern reconfiguration. The order contains roughly 300 described extant species across 2 main suborders, Cirrina (finned, mostly deep-sea) and Incirrina (the familiar benthic and pelagic octopuses without lateral fins), with a fossil record extending into the Late Cretaceous.
Why octopus biology defies vertebrate intuition
The cephalopod lineage diverged from the gastropod and bivalve molluscs roughly 540 million years ago and from the deuterostome ancestors of vertebrates around 600 million years ago. The result, in the modern octopus, is convergent evolution that produced superficially familiar features (a camera eye, complex cognition, behavioral flexibility) on top of a non-vertebrate substrate. Several specific differences are worth marking precisely.
Cardiovascular and respiratory physiology. The octopus has 3 hearts. Two branchial hearts perfuse the ctenidia (gills) under pressure that is lower than the systemic heart but adequate to drive countercurrent oxygen uptake; the systemic heart then propels oxygenated hemolymph to the rest of the body. Oxygen is carried by hemocyanin, a copper-binding metalloprotein dissolved freely in plasma at concentrations around 60 to 110 g/L in Octopus vulgaris. Hemocyanin’s oxygen affinity is strongly pH-dependent (Bohr effect), and in most octopuses the molecule is most efficient in cold, well-oxygenated water. The systemic heart is known to arrest during sustained jet swimming, partly explaining the energetic preference for benthic crawling and the limited stamina of pelagic escape jets.
Distributed nervous system. The 500-million-neuron count is roughly equal to a domestic dog and roughly an order of magnitude larger than the next-most-neurally-endowed invertebrate (cuttlefish). Of these, only 40 to 45 million neurons are in the central brain itself, organized into about 40 lobes wrapped around the esophagus. The remainder reside in 8 axial nerve cords running the length of each arm and in the large optic lobes flanking the brain. Each axial nerve cord contains tens of millions of neurons organized into segmental ganglia, each governing local sucker control and inter-segmental coordination. The arms are muscular hydrostats: incompressible, near-isovolumetric structures whose longitudinal, transverse, and oblique muscle layers act against each other in the absence of a skeleton. Consequence: an arm can elongate, shorten, bend, or stiffen at any point, and many of these moves are organized at the local axial-cord level rather than by descending command from the brain.
Sensory paradox: color-blind, color-matching. Most octopus retinas express a single rhodopsin-class opsin, with peak sensitivity in the blue-green (around 475 to 500 nm in O. vulgaris). By the standard retinal definition, the animal is color-blind. The skin nonetheless reproduces local color and texture with high fidelity. Three plausible mechanisms have been proposed and are not mutually exclusive. First, extraocular photoreception: Ramirez and Oakley (2015) demonstrated rhodopsin and downstream phototransduction components in Octopus bimaculoides skin and showed eye-independent, light-activated chromatophore expansion (LACE) in isolated tissue. Second, chromatic blur: Stubbs and Stubbs (2016) argued that the octopus’s annular pupil and high refractive-index lens spread different wavelengths to slightly different focal planes, so accommodation by the lens converts wavelength into a depth cue. Third, behavioral and proprioceptive sampling of the substrate via the suckers, whose chemoreceptors and mechanoreceptors are extremely dense.
Reproduction and programmed senescence. Octopuses are semelparous. Most species live 1 to 2 years; the giant Pacific octopus reaches 3 to 5 years. After mating, females undergo a hormonally triggered senescence: appetite suppression, retreat to a den, brooding of the egg clutch with continuous oxygenation by funnel jetting, and death around the time of hatching. The driver is the optic gland, a small endocrine organ between the eyes that is anatomically homologous to the vertebrate pituitary. Wodinsky’s classic 1977 paper in Science showed that surgical removal of the optic gland in brooding females arrests the senescence program and the female resumes feeding and may live longer. Wang and Ragsdale (2018) traced the program to a 4-pathway hormonal cascade: a sterol synthesis surge, depletion of cholesterol and accumulation of 7-dehydrocholesterol, upregulation of insulin-like and feeding peptide pathways, and changes in catecholamine signaling. The molecular logic is closer to a developmental program than to a fatigue or starvation model.
Key octopus facts
Phylogeny. Order Octopoda is divided into the suborders Cirrina (cirrate, finned, mostly bathyal-abyssal: Cirroteuthis, Grimpoteuthis the “Dumbo” octopus) and Incirrina (incirrate, no fins, benthic or pelagic: Octopus, Enteroctopus, Hapalochlaena, Argonauta, Vampyroteuthis historically allied here but now placed in its own order Vampyromorphida).
Species inventory. Approximately 300 species described, with active discovery in the deep sea. Among the most studied are Octopus vulgaris (Mediterranean and tropical Atlantic), Octopus bimaculoides (California two-spot, the genome-sequenced model), Enteroctopus dofleini (giant Pacific), Hapalochlaena (4 species of blue-ringed octopus), Thaumoctopus mimicus (mimic, observed 1998 and formally described 2005), and Amphioctopus marginatus (coconut, tool-use 2009).
Cardiovascular. Three hearts, hemocyanin-based hemolymph (copper, blue when oxygenated), systemic heart arrest during jet swimming.
Oxygen transport. Hemocyanin operates as a multi-subunit didecamer dissolved in plasma; oxygen-binding capacity is roughly one-quarter that of vertebrate hemoglobin per unit volume. In Antarctic octopods such as Pareledone charcoti (Oellermann et al., 2015), cold-water performance is sustained not by improved oxygen affinity (the species in fact has lower affinity and the lowest measured Bohr coefficient among octopods studied) but by elevated hemocyanin concentrations and a pH-shifted oxygen binding range that supports tissue offloading at near-zero temperatures.
Nervous system. ~500 million neurons total. ~40 to 45 million in the central brain across roughly 40 lobes. ~330 million distributed through 8 axial nerve cords. Optic lobes are the largest single brain structure.
Arm mechanics. Muscular hydrostat with longitudinal, transverse, and helical oblique muscle layers. Suckers under independent neural control. Chemoreception is taste-by-touch via dedicated chemotactile receptors (van Giesen et al., Cell, 2020 identified the receptor family).
Skin chromatic system. Three layers, distally to proximally. Chromatophores: pigment-loaded sacs (red, brown, yellow, black) under direct neural control by radial muscles attached to a central elastic sacculus. Iridophores: stacks of reflectin-protein platelets that produce structural color through thin-film interference, tunable by hydration and protein conformation. Leucophores: broadband white scatterers built from leucosomes that diffusely reflect ambient light.
Pupil and lens. Horizontal slit pupil under bright conditions, dilating to round in the dark. The lens has a graded refractive index, similar to vertebrate fish lenses, and corrects for spherical but not chromatic aberration; chromatic blur may carry color information into the otherwise monochromatic retina.
Beak and venom. Beak made of chitin with a mineralized tip (jaw mineralization gradient described by Miserez et al., Science, 2008). All octopuses are venomous; venom is delivered through the beak via salivary glands. The 4 Hapalochlaena species sequester tetrodotoxin through symbiotic bacteria, the same sodium-channel blocker found in pufferfish and rough-skinned newts.
Ink. Produced in an ink sac off the rectum, ejected through the funnel. Chemistry: melanin (synthesized via tyrosinase from tyrosine), suspended in mucus, plus tyrosinase itself, dopamine, and free amino acids. Defensive function is both visual (smoke-screen and pseudomorph decoy) and chemical (predator olfactory disruption).
Locomotion. Two principal modes. Crawling using arm tips and suckers, the most metabolically efficient option. Jet propulsion by mantle contraction and funnel ejection, fast but expensive; the systemic heart arrests during sustained jets, capping range and stamina. Argonauts (Argonauta) and pelagic species use jet primarily.
Reproduction. Internal fertilization via a modified arm called the hectocotylus, which transfers spermatophores into the female’s mantle cavity. Argonaut males have a free-swimming hectocotylus that detaches during mating. Females lay 50,000 to 200,000 eggs in Octopus vulgaris; clutch size scales widely with species. The deep-sea Graneledone boreopacifica female brooded for 53 months at 4,600 feet (1,400 m) off Monterey, California (Robison et al., PLOS ONE, 2014), the longest known brood in the animal kingdom.
Lifespan and senescence. 1- to 2-year species lifespan typical; Enteroctopus dofleini reaches 3 to 5 years. Death follows a programmed senescence driven by the optic gland (Wodinsky, 1977; Wang and Ragsdale, 2018). Senescent giant Pacific octopuses show peripheral neural degeneration and epithelial loss in the final weeks (Anderson et al., 2022).
Sociality. Most species are solitary outside mating. The larger Pacific striped octopus (Caldwell et al., 2015) shows pair bonding, beak-to-beak food sharing, and tolerated cohabitation, with field observations of up to 40 individuals together. Whether this represents true sociality or aggregation around resources is unsettled.
Cognition and behavior. Documented capabilities: reversal learning, observational learning, individual recognition of conspecifics and humans, defensive tool use (coconut-shell shelters, Finn et al., 2009), context-dependent mimicry (mimic octopus), object exploration, and play behavior with neutral objects.
Welfare and law. The United Kingdom Animal Welfare (Sentience) Act 2022 formally recognized cephalopods and decapod crustaceans as sentient. The U.S. OCTOPUS Act of 2024 (S.4810), introduced by Senators Whitehouse and Murkowski, would prohibit commercial octopus aquaculture and the import of farmed octopus, with civil fines up to $100,000 per violation.
Common octopus myths
Myth: Octopuses have tentacles. Octopus appendages are arms in cephalopod terminology. Tentacles are the 2 elongated, sucker-club-bearing limbs of squid and cuttlefish. The distinction is anatomical, not stylistic. Octopuses have 8 arms and 0 tentacles.
Myth: Octopus arms operate fully autonomously. The “8 mini-brains” framing oversimplifies. Each axial nerve cord supports rich local computation and reflex control, but inter-arm coordination, goal-directed reaching, and the high-level body pattern are organized at brain level. The arms can execute sucker-by-sucker chemotactile sampling without the brain, but a coordinated reach toward a target involves descending input.
Myth: All cephalopods are color-blind. Most are. The interesting outliers include Octopus aegina (which appears to express more than 1 retinal opsin) and the firefly squid Watasenia scintillans, which has 3 retinal opsins and is one of the few demonstrably color-vision cephalopods. The general statement is “octopuses are color-blind”; the more accurate version is “most octopuses, including all the well-studied species, are color-blind.”
Myth: Octopuses live for many years. Most species live 1 to 2 years. The 5-year giant Pacific octopus is the upper end of the typical octopus lifespan. The 4.5-year Graneledone brood off Monterey is a brooding record, not a typical lifespan; the female died around the time of hatching as in shorter-lived species.
Myth: Octopuses are color-blind and so chromatic camouflage is just luck. The match is neither retinal nor random. Skin photoreception (LACE) provides eye-free brightness sampling, the lens delivers chromatic depth-of-focus information, and the suckers sample the substrate’s chemistry and texture directly. The body pattern is a multi-modal fusion, and the absence of classical retinal color vision is one input rather than the whole story.
Myth: Octopus venom is just for prey. All species are venomous, and most use venom to subdue crustacean prey through paralysis, then proteolytic digestion delivered via the beak. The blue-ringed octopus is unusual for both the potency (tetrodotoxin) and the role of bacterial symbionts in toxin synthesis.
Frequently asked questions
How does hemocyanin compare with hemoglobin in oxygen transport?
Hemocyanin uses 2 copper atoms per oxygen-binding site (Type 3 copper protein) versus the iron-porphyrin of hemoglobin. Per unit volume, hemocyanin carries roughly one-quarter the oxygen of vertebrate hemoglobin, but its dissolved-in-plasma form (rather than packed in cells) gives lower viscosity at low temperatures, and the strong Bohr response in some species supports oxygen unloading at active tissues. Cold, oxygen-rich water favors hemocyanin; warm, hypoxic water exposes its limits, contributing to the heat sensitivity of many octopuses.
What is the cellular basis for chromatophore color change?
Each chromatophore is a small organ comprising a central pigment sac, an elastic sacculus, and 15 to 30 radial muscle fibers attached to the sacculus periphery. Activation of the radial muscles is under direct motoneuron control from the chromatophore lobes of the brain. Muscle contraction stretches the sacculus laterally, expanding the pigment sac and noticeably darkening the overlying skin. Relaxation lets the sacculus snap back. Activation latency is on the order of tens to hundreds of milliseconds, supporting subsecond body-pattern transitions.
How is the optic gland senescence program triggered?
Mating in females, and to a lesser extent maturation in males, releases an inhibitory tonic input from the central brain on the optic gland. The disinhibited gland upregulates a sterol synthesis pathway, leading to depletion of cholesterol and accumulation of 7-dehydrocholesterol and other intermediates. Insulin-like, feeding peptide, and catecholamine pathways shift simultaneously, and downstream effects include suppression of feeding, atrophy of digestive and salivary glands, retreat to a den, and continuous brooding. Surgical removal of the optic gland in brooding females (Wodinsky, 1977) reverses the appetite suppression and extends life.
How big is the largest octopus?Enteroctopus dofleini averages 33 lb (15 kg) and 14 feet (4.3 m) of arm span; the largest specimen on record was approximately 600 lb (270 kg) with a 32-foot (9.8 m) span. Older claims of larger octopuses, including the alleged “St. Augustine monster,” are not supported by histological analysis, which identified the 1896 carcass as decomposed whale collagen rather than cephalopod tissue (Pierce et al., 1995, Biological Bulletin).
Why is the deep-sea brooding record so long?
Cold abyssal water (~37 °F / 3 °C) slows embryonic development. Graneledone boreopacifica was observed by MBARI at 4,600 feet (1,400 m) off Monterey to brood a single clutch for 53 months, the longest known brood for any animal. The female maintained the clutch on a vertical rock face, oxygenating it with funnel jetting and refusing food throughout, before dying near hatching as in shorter-lived shelf-water species.
Are octopuses sentient by current scientific consensus?
The 2021 LSE-commissioned review for the United Kingdom government, drawing on 8 categories of evidence including learning, integrated information, and analgesic-modulated nociception, concluded there is “strong scientific evidence” of sentience in cephalopod molluscs. The conclusion was the basis for cephalopod inclusion in the Animal Welfare (Sentience) Act 2022 and underlies welfare arguments behind the U.S. OCTOPUS Act of 2024.
