A soft-bodied mollusc with three hearts, blue blood and most of its neurons in its arms. It solves puzzles, changes colour in a blink while being colour-blind, and dies soon after it breeds, usually within a year or two of hatching.

How its arms thinkSee the numbers
ClassCephalopoda
Lives inEvery ocean
IUCN statusMostly Data Deficient
DietCrabs, clams, fish
OCTOPUSES AT A GLANCE

Octopuses (order Octopoda) are cephalopod molluscs, related to squid, cuttlefish and the nautilus, with about 300 species from tide pools to the deep sea. They have eight arms lined with suckers that touch and taste, three hearts and copper-based blue blood. Most live only one to two years, breed once and then die. They are among the most capable learners of any invertebrate.

Where a figure differs by species, this page uses the common octopus unless stated. Graded and sourced below.
KEY DISCOVERIES
1963Arms do the workJ. Z. Young’s anatomy shows most octopus neurons lie outside the brain, in the arms.
2009Tool carriersVeined octopuses carry coconut shells to use later as shelters (Finn et al.).
2014Longest broodA deep-sea octopus guards her eggs for 53 months, the longest known (Robison et al.).
2015Genome readThe California two-spot octopus genome reveals expanded families of neural genes (Albertin et al.).
2017RNA editingOctopuses edit their RNA far more than most animals, trading genetic evolution for flexibility (Liscovitch-Brauer et al.).
2022Sentience in lawUK law recognises cephalopods as sentient after a government-commissioned review (Birch et al. 2021).
ONLY IN THE OCTOPUS

What makes the octopus unlike any other animal?

Short answer: Its genetics, senses and life cycle all break the usual rules. These are the traits no other animal combines.

01It rewrites its own RNAOctopuses edit the messages copied from their genes at tens of thousands of sites, mostly in nerve cells. The cost: their DNA itself evolves unusually slowly.Liscovitch-Brauer et al. 2017
02Skin that senses lightThe skin of the California two-spot octopus contains a light-sensing protein, so it may detect light without its eyes.Ramirez & Oakley 2015
03Suckers that tasteEach sucker carries chemical receptors, so an arm can taste what it touches before the brain is involved.van Giesen et al. 2020
04A mother who never leavesA deep-sea octopus guarded one clutch for 53 months without being seen to feed, the longest brood of any known animal.Robison et al. 2014
05Programmed to dieAfter breeding, a gland behind the eyes changes its chemistry and triggers decline. Remove it and females stop brooding and live longer.Wang & Ragsdale 2018
06Two kinds of sleepOctopuses have a quiet sleep and a brief "active" sleep with rapid colour and skin changes, which some compare to dreaming.Medeiros et al. 2021
FACT SHEET

How many hearts and neurons does an octopus have?

Short answer: Three hearts and about 500 million neurons, two-thirds of them in the arms.

MeasuredEstimatedQuoted, unverified
NeuronsMeasured~500 millionIn the arms~2/3In the brain~1/3About as many as a dog. Young 1963; Hochner 2012
HeartsMeasured3Gill heartGill heartMain heartTwo pump blood through the gills, one around the body
SpeciesMeasured~300One dot per 10 species, from 2.5 cm to several metres across.Order Octopoda, WoRMS
Lifespan, common octopusMeasured1 to 2 yearsCommon1 to 2 yrsGiant Pacific3 to 5 yrsMost species breed once, then die
Longest egg guardingMeasured53 monthsGraneledone53 moCommon octopus~1 to 2 moWatched by remote vehicle at 1,397 m. Robison et al. 2014
Largest speciesEstimated~15 kg typicalGiant Pacific, typical~15 kgReported record~71 kgEnteroctopus dofleini. Large records are poorly documented
Colour visionMeasuredNoneOne type of light receptor in the eye, yet it matches backgrounds closely. How is still debated.Marshall & Messenger 1996
Nine brainsQuoted, unverifiedn/aOne central brain plus a nerve cord in each arm. The arm cords are not separate brains, though they do a lot of processing.Popular claim
Smarter than a dogQuoted, unverifiedn/aOctopus and dog intelligence evolved for different lives. No test fairly ranks them on one scale.Popular claim
ARMS AND BRAIN

Can an octopus arm think for itself?

Short answer: In part. Each arm has its own nerve cord that controls grasping and reaching locally; the brain sets goals rather than steering every sucker.

Where the neurons are
Arms ~2/3Optic lobesCentral brain
Approximate split, about 500 million neurons in total. Young 1963.
Local reflexesA severed arm still reaches and grasps for a while when touched.
Taste by touchReceptors in the suckers let arms sample food without looking.
No fixed shapeNo skeleton: only the beak is hard, so it can squeeze through gaps the width of its beak.
Self-avoidanceSuckers avoid gripping the octopus’s own skin, thanks to a chemical in it.
SKIN

How does an octopus change colour if it is colour-blind?

Short answer: Muscles open and close thousands of pigment sacs in milliseconds, under nerve control. How it matches colours it cannot see is still unsolved.

ChromatophoresElastic sacs of yellow, red and brown pigment, each ringed by muscles.
IridophoresStacked plates that reflect blues and greens.
LeucophoresWhite cells that scatter light, matching the ambient colour.
PapillaeMuscles raise the skin into bumps and spikes, changing texture.
Fast controlNerves fire the sacs directly, so patterns change in under a second.
Ideas for the colour puzzle include light-sensing skin and the odd U-shaped pupil splitting light by wavelength. Neither is proven.
INTELLIGENCE

How intelligent is an octopus?

Short answer: Very capable for an invertebrate: it learns by trial and error, remembers solutions, uses objects as tools and recognises individual people.

Opens jarsPulls the plug from a glass jar to seize the crab inside, with fewer failed attacks over trials.Fiorito et al. 1990
Carries its shelterVeined octopuses stack coconut half-shells, carry them and reassemble them as a den.Finn et al. 2009
Knows facesGiant Pacific octopuses treated a keeper who fed them differently from one who poked them, even when both dressed alike.Anderson et al. 2010
Watches and learnsCommon octopuses learned a choice by watching another octopus, though this classic result has been hard to repeat.Fiorito & Scotto 1992
Shared densAt "Octopolis" and a second site, "Octlantis", gloomy octopuses live close together in shell-bed dens.Scheel et al. 2017
EscapesAquarium escapes are well documented, though many famous stories are anecdote rather than study.Anecdotal
HEARTS AND BLOOD

Why does an octopus have blue blood and three hearts?

Short answer: Its blood carries oxygen with copper, not iron, which turns blue. Copper is less efficient, so two extra hearts push blood through the gills.

HaemocyaninCopper-based, dissolved in the blood. Works well in cold, low-oxygen water.
Haemoglobin (ours)Iron-based, packed in red cells. Carries more oxygen per drop.
Main heart stopsThe body heart pauses while swimming, one reason octopuses prefer to crawl.
Jet propulsionWater forced out of the siphon drives fast escapes.
Ink cloudA dark cloud of melanin confuses predators and can dull their sense of smell.
A beak and a tongueA parrot-like beak and a radula of tiny teeth drill shells.
SPECIES

Which octopus is which?

Short answer: From a thumb-sized blue-ringed octopus to the Giant Pacific, each has its own speciality.

Common octopusOctopus vulgarisMediterranean and eastern Atlantic shallows; the best-studied species.
Giant Pacific octopusEnteroctopus dofleiniThe largest; lives 3 to 5 years in the cold North Pacific.
Blue-ringed octopusesHapalochlaenaSmall and deadly; tetrodotoxin in saliva can kill a person.
Mimic octopusThaumoctopus mimicusImitates lionfish, sea snakes and flatfish.
Coconut octopusAmphioctopus marginatusCarries shells as portable shelters.
Dumbo octopusesGrimpoteuthisDeep-sea octopuses that swim with ear-like fins.
EVOLUTION

What is an octopus related to?

Short answer: It is a mollusc, like snails and clams, that lost its shell. Its closest living relative is the vampire squid; squid and cuttlefish are a separate branch.

Where octopuses sit among cephalopodsRelationships only, not to time scale
NautilusShelled, the oldest lineageSquidDecapodiformes, 10 limbsCuttlefishDecapodiformes, internal cuttleboneVampire squidNot a squid; the octopus’s closest relativeDumbo octopusesCirrata, finned deep-sea octopusesCommon octopusIncirrata, finless octopusesColeoideaOctopodiformesOctopoda
Soft bodies fossilise poorly. The oldest known octopus-line fossil, Syllipsimopodi, is about 328 million years old (Whalen & Landman 2022).
LIFE CYCLE

Why do octopuses die after breeding?

Short answer: They are built to breed once. After mating, males decline within weeks; females stop eating while guarding eggs and die soon after they hatch.

1HatchTiny larvae drift in plankton for weeks.
2SettleYoung sink to the seabed and find a den.
3Grow fastCan double weight in weeks; most of life is growth.
4MateA male passes sperm with a modified arm, the hectocotylus.
5BroodThe female cleans and aerates her eggs without feeding.
6DeclineOptic gland signals trigger decline; she dies as eggs hatch.
OCTOPUSES AND PEOPLE

Should octopuses be farmed?

Short answer: It is contested. Wild catches have grown for decades, a first commercial octopus farm has been proposed in Spain, and scientists and several governments have raised welfare concerns.

Endangered register →
Fishing pressureHIGHOver 350,000 tonnes are caught a year; some local stocks are overfished.
Farming plansDEBATEDSolitary, curious animals may suffer in crowded tanks; no welfare standards exist.
Data gapsWATCHMost species are listed as Data Deficient by the IUCN.
Sentience recognisedUK law covers cephalopods under the Animal Welfare (Sentience) Act 2022.
Research rulesEU law has protected cephalopods used in research since 2010.
Short lives, fast recoveryShort generations mean stocks can rebound quickly when fishing eases.
Catch figure is an approximate recent annual total from FAO reporting.
MYTH OR FACT

Six octopus claims, checked

Short answer: The famous oddities, blue blood and death after breeding, are real. The "nine brains" line is a simplification.

2Myths2Half true2Facts
01HALF TRUE“Octopuses have nine brains.”WHERE IT CAME FROMA neat way to describe a central brain plus eight arm nerve cords.
WHAT THE EVIDENCE SAYSThe arms do a lot of local processing, but their nerve cords are not separate brains and stay linked to the central one.
WHAT IS TRUE INSTEADOne brain, with an unusually independent nervous system.Sources: 04
02MYTH“Octopuses have eight tentacles.”WHERE IT CAME FROMEveryday usage.
WHAT THE EVIDENCE SAYSBiologists call octopus limbs arms: suckers along their full length. Tentacles, as in squid, have suckers only at the tip.
WHAT IS TRUE INSTEADEight arms, no tentacles.Sources: standard biology texts
03FACT“Octopus blood is blue.”WHERE IT CAME FROMObserved whenever blood is exposed to air.
WHAT THE EVIDENCE SAYSCopper-based haemocyanin turns blue when it carries oxygen.
WHAT IS TRUE INSTEADBlue when oxygenated, nearly clear without oxygen.Sources: standard biology texts
04MYTH“Giant octopuses attack ships.”WHERE IT CAME FROMSailors’ tales and the kraken legend.
WHAT THE EVIDENCE SAYSThe largest octopus weighs tens of kilograms. The kraken story is more likely based on giant squid.
WHAT IS TRUE INSTEADNo octopus is big enough to threaten a boat.Sources: standard biology texts
05HALF TRUE“Octopuses can live out of water.”WHERE IT CAME FROMFootage of octopuses crossing rocks at low tide.
WHAT THE EVIDENCE SAYSSome, like Abdopus aculeatus, crawl between pools for short spells, but they breathe with gills and soon suffocate in air.
WHAT IS TRUE INSTEADBrief trips on land, not a life there.Sources: standard biology texts
06FACT“Octopuses die after breeding.”WHERE IT CAME FROMField and lab observations over a century.
WHAT THE EVIDENCE SAYSNearly all species breed once. Removing the optic glands in lab experiments stopped the decline.
WHAT IS TRUE INSTEADA built-in end, not old age.Sources: 06
OPEN QUESTIONS

What science still doesn’t know

Short answer: Some of the most basic questions. Much of what we know comes from a few species kept in labs.

?How does it match colours?It has one light receptor type, yet blends into colourful backgrounds. Skin light-sensing and its pupil shape are leading ideas.
?What is it like to be one?How a brain spread through the arms produces one experience is an open question in philosophy and neuroscience.
?Do they dream?Active sleep looks like replaying waking patterns, but this is interpretation, not proof.
?How many species?About 300 are named; many deep-sea species are seen only on video.
?Can they be farmed humanely?No one has shown it can be done; welfare needs are poorly understood.
?Why so short-lived?Why evolution favoured a single breeding and rapid death is debated.
FAQ

Frequently asked questions

How many hearts does an octopus have?+

Three. Two branchial hearts push blood through the gills and one systemic heart pumps it around the body.

How long do octopuses live?+

Most species live one to two years. The Giant Pacific octopus lives three to five years.

Are octopuses dangerous?+

Almost all are harmless to people. Blue-ringed octopuses carry tetrodotoxin and have caused deaths.

What is the plural of octopus?+

Octopuses is standard English. Octopi is common but based on a mistaken Latin ending; octopodes follows the Greek.

Do octopuses feel pain?+

Evidence suggests they do: they guard and tend injuries and avoid places where they were hurt. This led the UK to protect them by law.

How smart is an octopus?+

It can learn, solve puzzles and use tools. Comparisons with dogs or children are not meaningful because the minds evolved separately.

COMPARE AND EXPLORE

Keep exploring cephalopods

Short answer: Squid and cuttlefish solved the same problems differently.

ARTICLECuttlefish: colour mastersOpen →ARTICLEColossal squidOpen →PROFILEMantis shrimp profileOpen →TOOLAnimal QuizOpen →
HOW WE KNOW

Sources and how we check

This page is written by Strange Animals and checked against primary research. Every figure carries a grade and a numbered source.

Cite this pageStrange Animals. (2026, November 13). Octopus (Octopoda): arms, brain, skin, intelligence and life cycle. https://strangeanimals.info/marine-life/octopuses/octopus
  1. 01Young, J. Z. The number and sizes of nerve cells in Octopus. Proceedings of the Zoological Society of London 1401963
  2. 02Liscovitch-Brauer, N. et al. Trade-off between transcriptome plasticity and genome evolution in cephalopods. Cell 169: 191-2022017
  3. 03Albertin, C. B. et al. The octopus genome and the evolution of cephalopod neural and morphological novelties. Nature 524: 220-2242015
  4. 04Hochner, B. An embodied view of octopus neurobiology. Current Biology 22: R887-R8922012
  5. 05Robison, B. et al. Deep-sea octopus (Graneledone boreopacifica) conducts the longest-known egg-brooding period of any animal. PLoS ONE 9: e1034372014
  6. 06Wang, Z. Y. and Ragsdale, C. W. Multiple optic gland signaling pathways implicated in octopus maternal behaviors and death. Journal of Experimental Biology 2212018
  7. 07Finn, J. K., Tregenza, T. and Norman, M. D. Defensive tool use in a coconut-carrying octopus. Current Biology 19: R1069-R10702009
  8. 08van Giesen, L. et al. Molecular basis of chemotactile sensation in octopus. Cell 183: 594-6042020
  9. 09Ramirez, M. D. and Oakley, T. H. Eye-independent, light-activated chromatophore expansion in Octopus bimaculoides. Journal of Experimental Biology 2182015
  10. 10Medeiros, S. L. de S. et al. Cyclic alternation of quiet and active sleep states in the octopus. iScience 24: 1022232021
  11. 11Anderson, R. C. et al. Octopuses (Enteroctopus dofleini) recognize individual humans. Journal of Applied Animal Welfare Science 132010
  12. 12Fiorito, G. and Scotto, P. Observational learning in Octopus vulgaris. Science 256: 545-5471992
  13. 13Scheel, D. et al. A second site occupied by Octopus tetricus at high densities. Marine and Freshwater Behaviour and Physiology 502017
  14. 14Birch, J. et al. Review of the evidence of sentience in cephalopod molluscs and decapod crustaceans. LSE Consulting2021
  15. 15Marshall, N. J. and Messenger, J. B. Colour-blind camouflage. Nature 382: 408-4091996
  16. 16Whalen, C. D. and Landman, N. H. Fossil coleoid cephalopod from the Mississippian Bear Gulch Lagerstätte. Nature Communications 13: 11072022
WATCH

Three films worth your time

ClipBBC · Spy in the OceanThe Octopus with Nine BrainsA coconut octopus builds a shelter from shells, with a robot camera watching.Pairs with: Intelligence →
ClipBBCThe octopus that can walk on dry landAbdopus leaves the water to cross between rock pools at low tide.Pairs with: Myths →
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CephalopodsIn depth · Dec 7, 2025Blue-Ringed Octopus: A Tetrodotoxin Bite
CephalopodsIn depth · Dec 6, 2025Coconut Octopus: Tool Use in an Invertebrate
CephalopodsIn depth · Nov 30, 2025How the Mimic Octopus Impersonates Other Species
CephalopodsIn depth · Nov 28, 2025Octopus Intelligence: From Problem Solving to Escape
Deep SeaIn depth · Nov 17, 2025Dumbo Octopus Life in the Deep Sea
OctopusesIn depth · Oct 21, 2025Octopuses: Two-Thirds of Neurons in Arms
OctopusesIn depth · Aug 14, 2025Giant Pacific Octopus: 2,240 Suckers
OctopusesIn depth · Jun 18, 2025Mimic Octopus: First Seen Off Sulawesi
OctopusesIn depth · Apr 20, 2025Blue-Ringed Octopus: Painless, Lethal Bite
OctopusesIn depth · Feb 19, 2025Common Octopus: Arms That Think for Themselves
Deep SeaIn depth · Feb 16, 2025Dumbo Octopus: Found at 6,957 m Deep