An octopus is a soft-bodied, eight-armed cephalopod mollusk of the order Octopoda. True octopuses belong to the genus Octopus , a widespread group of shallow-water cephalopods. They possess a sac-like body with a poorly defined head, large complex eyes, and eight highly contractile arms lined with suckers for strong grip. Octopuses vary in size, with the smallest at about 5 cm long, while the largest, the giant Pacific octopus, can reach 5.4 meters in length with an arm span of almost 9 meters. They move by crawling with their arms and can swim swiftly backward by ejecting water from their siphon. When threatened, they release a melanin-based ink to create a screen for escape, and some species even produce a substance that paralyzes attackers. The common octopus ( O. vulgaris ) is considered the most intelligent invertebrate. They are known for their ability to rapidly change skin colors and patterns to blend in with their surroundings using specialized pigment-bearing cells called chromatophores.

Which group of animals does the octopus belong to?

The octopus belongs to the class Cephalopoda within the phylum Mollusca . Cephalopods are a small, highly advanced group of exclusively marine animals. Octopuses are part of the order Octopoda , characterized by eight arms with suckers. They lack an internal shell, and have mobile, contractile arms.

What are mollusks, and how is the octopus related to them?

Mollusks are a diverse group of soft-bodied invertebrate animals, usually enclosed in a calcium carbonate shell. Mollusks include an immense variety of forms, such as Gastropoda (limpets, snails, and slugs) and Bivalvia (clams, mussels, scallops, oysters, shipworms, and cockles). Octopuses are members of the class Cephalopoda , which is a subgroup within the mollusks. Cephalopods are characterized by having arms and tentacles, with eight or ten in most species. The ancestors of cephalopods appear to have the closest affinity with the ancestors of the class Gastropoda.

What is the scientific order of octopuses within the mollusk group?

Octopuses are cephalopod mollusks belonging to the order Octopoda . Octopoda is part of the subclass Coleoidea , which includes octopuses, squids, belemnites, and cuttlefish. Octopuses have existed from the Cretaceous period to the present day. The order Octopoda is further divided into three suborders: * Palaeoctopoda * Cirrata * Incirrata

What physical features help octopuses adapt to their environment?

Octopuses have several physical features that help them adapt to their environment. They possess eight contractile arms, each bearing two rows of fleshy suckers with great holding power, which they use for crawling, capturing prey, and manipulating objects. When alarmed, octopuses can swiftly shoot backward by ejecting a jet of water from their siphon. Additionally, they can eject an inky substance to create a screen, helping them escape from predators. Octopuses also have color pigment cells (chromatophores) and reflecting cells (iridocytes) in their skin that allow them to change colors and blend in with their surroundings for camouflage, attack, rest, or defense.

How do octopuses differ from other mollusks like snails or clams?

Octopuses, while mollusks like snails and clams, exhibit significant differences. Octopuses possess a saccular body with a slightly demarcated head, complex eyes, and eight contractile arms featuring powerful suckers. Unlike many mollusks, octopuses lack an external shell. They move by crawling or ejecting water from a siphon. Cephalopods like octopuses have attained the highest degree of development of the invertebrate nervous system among mollusks. They also display complex behaviors such as learning, tool use, and camouflage through color changes. In contrast to clams and snails, octopuses are active predators, using their intelligence and unique physical adaptations to hunt.

octopus

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A blue-ringed octopus with bright blue rings on its beige body crawls over a rocky, coral-strewn seabed.

An octopus with outstretched tentacles swims underwater near rising air bubbles, illuminated by sunlight from above.

A brown cow in a grassy field scratches its face with a long-handled brush, with trees and a house in the background.

Quick Facts

Plural:: octopuses or octopi

Related Topics:: Do octopuses have ink?; How many hearts does an octopus have?; finned octopod; Incirrata; Cirrata

On the Web:: World Animal Rescue Network - Octopus: Facts, Threats and Conservation (Oct. 16, 2026)

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octopus

mollusk order

Also known as: Octopoda, octopi, octopuses

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Oct. 22, 2026 •History

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Octopus-inspired patch grips skin and reveals pulse through shifting colors • Oct. 20, 2026, 10:00 AM ET (Phys.org)

Top Questions

What is an octopus?
An octopus is a soft-bodied, eight-armed cephalopod mollusk of the order Octopoda. True octopuses belong to the genus Octopus, a widespread group of shallow-water cephalopods. They possess a sac-like body with a poorly defined head, large complex eyes, and eight highly contractile arms lined with suckers for strong grip.

Octopuses vary in size, with the smallest at about 5 cm long, while the largest, the giant Pacific octopus, can reach 5.4 meters in length with an arm span of almost 9 meters. They move by crawling with their arms and can swim swiftly backward by ejecting water from their siphon. When threatened, they release a melanin-based ink to create a screen for escape, and some species even produce a substance that paralyzes attackers.

The common octopus (O. vulgaris) is considered the most intelligent invertebrate. They are known for their ability to rapidly change skin colors and patterns to blend in with their surroundings using specialized pigment-bearing cells called chromatophores.
2 Britannica Sources
1. cephalopod: Reproduction and life cycles
2. octopus: Examples of species and behaviours
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Which group of animals does the octopus belong to?
The octopus belongs to the class Cephalopoda within the phylum Mollusca. Cephalopods are a small, highly advanced group of exclusively marine animals. Octopuses are part of the order Octopoda, characterized by eight arms with suckers. They lack an internal shell, and have mobile, contractile arms.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are mollusks, and how is the octopus related to them?
Mollusks are a diverse group of soft-bodied invertebrate animals, usually enclosed in a calcium carbonate shell. Mollusks include an immense variety of forms, such as Gastropoda (limpets, snails, and slugs) and Bivalvia (clams, mussels, scallops, oysters, shipworms, and cockles).

Octopuses are members of the class Cephalopoda, which is a subgroup within the mollusks. Cephalopods are characterized by having arms and tentacles, with eight or ten in most species. The ancestors of cephalopods appear to have the closest affinity with the ancestors of the class Gastropoda.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is the scientific order of octopuses within the mollusk group?
Octopuses are cephalopod mollusks belonging to the order Octopoda. Octopoda is part of the subclass Coleoidea, which includes octopuses, squids, belemnites, and cuttlefish. Octopuses have existed from the Cretaceous period to the present day.

The order Octopoda is further divided into three suborders:
* Palaeoctopoda
* Cirrata
* Incirrata
2 Britannica Sources
1. cephalopod: Annotated classification
2. octopus
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What physical features help octopuses adapt to their environment?
Octopuses have several physical features that help them adapt to their environment. They possess eight contractile arms, each bearing two rows of fleshy suckers with great holding power, which they use for crawling, capturing prey, and manipulating objects. When alarmed, octopuses can swiftly shoot backward by ejecting a jet of water from their siphon.

Additionally, they can eject an inky substance to create a screen, helping them escape from predators. Octopuses also have color pigment cells (chromatophores) and reflecting cells (iridocytes) in their skin that allow them to change colors and blend in with their surroundings for camouflage, attack, rest, or defense.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do octopuses differ from other mollusks like snails or clams?
Octopuses, while mollusks like snails and clams, exhibit significant differences. Octopuses possess a saccular body with a slightly demarcated head, complex eyes, and eight contractile arms featuring powerful suckers. Unlike many mollusks, octopuses lack an external shell. They move by crawling or ejecting water from a siphon.

Cephalopods like octopuses have attained the highest degree of development of the invertebrate nervous system among mollusks. They also display complex behaviors such as learning, tool use, and camouflage through color changes. In contrast to clams and snails, octopuses are active predators, using their intelligence and unique physical adaptations to hunt.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

octopus, in general, any eight-armed cephalopod (octopod) mollusk of the order Octopoda. The true octopuses are members of the genus Octopus, a large group of widely distributed shallow-water cephalopods. (See cephalopod.)

day octopusDay octopus (*Octopus cyanea*).

Description

Octopuses vary greatly in size: the smallest (O. arborescens) is about 5 cm (2 inches) long, while the largest species, the reddish pink giant Pacific octopus (Enteroctopus dofleini), may grow to 5.4 metres (18 feet) in length and have an arm span of almost 9 metres (30 feet). The typical octopus has a saccular body: the head is only slightly demarcated from the body and has large complex eyes and eight contractile arms. Each arm bears two rows of fleshy suckers that are capable of great holding power. The arms are joined at their bases by a web of tissue known as the skirt, at the centre of which lies the mouth. The latter organ has a pair of sharp horny beaks and a filelike organ, the radula, for drilling shells and rasping away flesh.

common octopus The common octopus (*Octopus vulgaris*) moving through water.

The octopus takes water into its mantle and expels the water after respiration through a short funnel or siphon. Most octopuses move by crawling along the bottom with their arms and suckers, though when alarmed they may shoot swiftly backward by ejecting a jet of water from the siphon. When endangered, they eject an inky substance—a type of melanin-based ink (produced by a gland in the hindgut) mixed with mucus (produced by the octopus’s funnel gland)—through a duct near the anus. The ink is used as a screen that helps to cover the octopus’s escape; the substance produced by some species paralyzes the sensory organs of the attacker.

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

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Examples of species and behaviours

The best-known octopus is the common octopus, O. vulgaris, a medium-sized animal that is widely distributed in tropical and temperate seas throughout the world. It lives in holes or crevices along the rocky bottom and is secretive and retiring by nature. It feeds mainly on crabs and other crustaceans. This species is thought to be the most intelligent of all invertebrate animals. O. vulgaris has highly developed pigment-bearing cells and can change its skin colours to an astonishing degree with great rapidity. (The giant Pacific octopus can reportedly change color in one-tenth of a second, quickly blending in with rocky and coral surroundings.) Each pigment-bearing cell (chromatophore) is individually innervated from the brain.

The veined octopus (Amphioctopus marginatus) is also known for its intelligence. In 2009 biologists reported having observed the animals excavating coconut half shells from the ocean floor and carrying them for use as portable shelters. Such behaviour is regarded by biologists as the first documented example of tool use by an invertebrate.

Several species of octopuses appear to rely heavily on certain pairs of arms. In 2025 scientists examined the behaviors of octopuses in the Caribbean Sea and near Spain. They reported that while each octopus they observed could raise, lower, extend, curl, and tuck each of its arms, anterior pairs of arms were used more often than posterior pairs to perform complex activities—such as moving and grasping objects, searching the substrate, walking along the bottom, standing, and attacking prey.

Reproduction

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Octopuses have separate sexes, and the male has a specially modified arm, called a hectocotylus, by means of which it inserts spermatophores, or packets of sperm, directly into the female’s mantle cavity. O. vulgaris mates during the winter, and the eggs, about 0.3 cm (1/8 inch) long, are laid under rocks or in holes, the total number of eggs amounting to more than 100,000. During the four to eight weeks required for the larvae to hatch, the female guards the eggs, cleaning them with her suckers and agitating them with water. Upon hatching, the tiny octopods, which closely resemble their parents, spend several weeks drifting in the plankton before taking refuge on the bottom.

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A Dumbo octopus (Grimpoteuthis species) with ear-like fins and curled arms swims in dark ocean water in the Gulf of Mexico. — Dumbo octopusA Dumbo octopus (*Grimpoteuthis* species) swimming.

Diet and predators

Octopuses feed mainly upon crabs and lobsters, although some are plankton feeders.

Octopuses are fed upon by a number of marine fishes, and they have long been considered a culinary delicacy by humans.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Meg Matthias.

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Examples of species and behaviours

Reproduction

Diet and predators