Octopus

An octopus is a soft-bodied, eight-limbed mollusc belonging to the order Octopoda, within the class Cephalopoda that also includes squids, cuttlefish and nautiloids. Around 300 species are known, displaying remarkable diversity in form and behaviour. Octopuses possess bilateral symmetry, two highly developed eyes and a central beaked mouth situated at the point from which the eight flexible arms radiate. They are renowned for their intelligence, adaptability and complex behavioural repertoire, making them among the most sophisticated of all invertebrates.
Octopuses inhabit a wide range of marine environments, from coral reefs and coastal shallows to pelagic waters and the deep sea. Some species live in intertidal habitats while others occupy abyssal depths. They grow rapidly, mature early and typically have short lifespans. Reproduction is semelparous: males usually die shortly after mating, and females die after guarding and tending their eggs until hatching.

Etymology and Pluralisation

The English plural used in this article is octopuses, which is standard, widely accepted and historically attested. The word derives from Neo-Latin octopus, itself from the Ancient Greek compound meaning “eight-footed”. Historically, several plural forms have been used:

• octopuses – the regular and preferred English plural
• octopi – a common but etymologically incorrect hypercorrection formed as if octopus were a Latin second-declension noun
• octopodes – the Ancient Greek plural; linguistically accurate but rare and often considered pedantic

Modern usage guides, including Fowler’s Modern English Usage and the New Oxford American Dictionary, recommend octopuses. Major dictionaries record the other variants but note their limited or nonstandard character.

Anatomy and Physiology

Size Variation

Octopus size varies widely across species. The giant Pacific octopus (Enteroctopus dofleini) is often cited as the largest, with adults usually weighing several tens of kilograms and possessing arm spans exceeding several metres. The largest scientifically documented specimen had a mass of around 71 kg. Unverified reports suggest even greater sizes. At the other end of the scale, Octopus wolfi is among the smallest, measuring around a centimetre and weighing less than a gram.

External Characteristics

The octopus body plan comprises a muscular, bulbous mantle, housing most internal organs, and eight flexible, prehensile arms. These arms evolved from the molluscan foot and are connected near their bases by a web of skin. Each arm contains powerful longitudinal, circular and transverse muscles functioning as muscular hydrostats, allowing extraordinary flexibility, extension and torsion.
The suckers lining the inner surfaces of the arms are muscular structures composed of a disc-like infundibulum and a cuplike acetabulum. Through precise muscular control, they create strong adhesion, enabling manipulation, locomotion and anchorage. Each arm contains axial nerves and large numbers of sensory receptors; notably, the arms can respond to light independently, enabling localised control even when the head is obscured.
The skin consists of an epidermis with mucous and sensory cells and a collagen-rich dermis embedded with specialised chromatophores and related structures that allow rapid colour change. As most of the body is soft, octopuses can squeeze through incredibly small openings—gaps scarcely larger than their beaks.

Head and Sensory Organs

The head contains the brain and the chitinous beak, positioned centrally beneath the arm crown. The eyes, housed in cartilaginous capsules, are large and anatomically similar to vertebrate eyes, featuring a lens, iris, pupil and retina. The pupil can expand or contract and retinal pigments help screen intense light.
Some species, especially among the cirrate (Cirrina) suborder, deviate from the typical form. These have gelatinous bodies, prominent fins above the eyes, a shell remnant and arms with fleshy papillae or cirri beneath the suckers.

Locomotion and Circulatory System

Octopuses primarily move by crawling, using the arms for coordination, though they can swim by jet propulsion, expelling water forcefully from the siphon. When swimming, the systemic heart temporarily stops, causing rapid fatigue and reinforcing their preference for crawling.
Their closed circulatory system includes three hearts: a systemic heart that pumps blood around the body, and two branchial hearts that pump blood through the gills. Octopus blood contains the copper-based oxygen-transport protein haemocyanin, which is dissolved in the plasma and gives the blood a bluish tint. Haemocyanin is particularly effective in cold, low-oxygen environments.

Behaviour and Life Cycle

Octopuses exhibit a remarkable array of behaviours. They are capable of problem-solving, tool use, exploration and learning. They display camouflage through chromatophores, structural colour cells and muscular skin movements. Defence mechanisms include ink release, rapid jetting, startling deimatic displays and deceptive behaviours.
Mating involves the male using a specialised arm, the hectocotylus, to transfer spermatophores into the female’s mantle cavity. After laying and attaching strings of eggs to the walls of a den, the female guards and cleans them until hatching, during which she ceases feeding and eventually dies. Males also die soon after mating due to programmed senescence.

Diet, Habitat and Predation

Octopuses feed on crustaceans, molluscs, fish and other marine animals. Their habitats range from rocky crevices and coral reefs to sandy bottoms and open waters. Defence strategies combine crypsis, mimicry, rapid movement and hiding. All octopuses possess venom used to subdue prey, though only the blue-ringed octopuses produce toxins dangerous to humans.

Cultural Significance

Octopuses occupy a prominent role in global culture and mythology. They appear as sea monsters in Norse legends (the kraken), in Ainu tradition (Akkorokamui) and possibly influence Greek representations such as the Gorgon. The creature features famously in literature, including Victor Hugo’s Toilers of the Sea, and is a recurrent motif in Japanese shunga art.
Culinarily, octopuses are eaten in many parts of the world, especially throughout Mediterranean and Asian cuisines where they are regarded as delicacies.