Insects & Invertebrates comparison

Octopus vs Squid: Key Differences Explained

The short answer

Count the appendages. An octopus has eight arms and nothing else; a squid has eight arms plus two longer feeding tentacles that shoot out to seize prey. A squid also has a stiffening internal structure called a pen, fins on the mantle, and a streamlined body built for open water. An octopus has no internal skeleton at all, which lets it squeeze through remarkably small gaps on the seafloor.

The main difference

A squid has ten appendages — eight arms and two extensible feeding tentacles — plus an internal stiffening pen and mantle fins, while an octopus has eight arms and no skeletal support of any kind.

Both are cephalopods with a similar general outline, and the differences between them are structural and easy to see once you know where to look.

At a glance

Comparison of Octopus and Squid across 6 dimensions, with an interpretation of each difference.
What differs	Octopus	Squid	What it means
Arms and tentacles	Eight arms of similar length; no feeding tentacles	Eight arms plus two longer extensible tentacles used to strike at prey	The two extra appendages are the fastest cue and are visible on any reasonably clear view.
Internal support	None, apart from a hard beak and small vestigial elements in some species	A chitinous pen running the length of the mantle, giving the body rigidity	This determines what each animal can do with its body: the octopus deforms, the squid holds its shape.
Body shape	Rounded, sac-like mantle with no fins in most species	Elongated, tapering mantle with fins along the sides or at the rear	Squid shape is built for sustained swimming; octopus shape is built for squeezing and manipulating.Deep-sea cirrate octopuses do have fins, which is a genuine exception to the simple rule.
Suckers	Soft, fleshy suckers without hardened rings	Suckers reinforced with toothed chitinous rings, and hooks in some species	Sucker armament reflects how each animal grips: sustained manipulation against rapid seizure of moving prey.
Typical habitat	Mostly bottom-dwelling, using dens, crevices and reef structure	Mostly open water, from coastal shallows to the deep ocean	Habitat matches body plan almost exactly, and it is often the easiest thing to observe.
Social behaviour	Largely solitary, with a small number of documented exceptions	Many species form shoals, sometimes very large ones	Shoaling suits open water where there is nowhere to hide; solitary living suits a seafloor animal with a den.

Arms and tentacles: OctopusEight arms of similar length; no feeding tentacles
SquidEight arms plus two longer extensible tentacles used to strike at prey
The two extra appendages are the fastest cue and are visible on any reasonably clear view.
Internal support: OctopusNone, apart from a hard beak and small vestigial elements in some species
SquidA chitinous pen running the length of the mantle, giving the body rigidity
This determines what each animal can do with its body: the octopus deforms, the squid holds its shape.
Body shape: OctopusRounded, sac-like mantle with no fins in most species
SquidElongated, tapering mantle with fins along the sides or at the rear
Squid shape is built for sustained swimming; octopus shape is built for squeezing and manipulating.Deep-sea cirrate octopuses do have fins, which is a genuine exception to the simple rule.
Suckers: OctopusSoft, fleshy suckers without hardened rings
SquidSuckers reinforced with toothed chitinous rings, and hooks in some species
Sucker armament reflects how each animal grips: sustained manipulation against rapid seizure of moving prey.
Typical habitat: OctopusMostly bottom-dwelling, using dens, crevices and reef structure
SquidMostly open water, from coastal shallows to the deep ocean
Habitat matches body plan almost exactly, and it is often the easiest thing to observe.
Social behaviour: OctopusLargely solitary, with a small number of documented exceptions
SquidMany species form shoals, sometimes very large ones
Shoaling suits open water where there is nowhere to hide; solitary living suits a seafloor animal with a den.

How they are related

Both are cephalopod molluscs, in the same class as cuttlefish and nautiluses, and both belong to the coleoids — the group that lost the external shell. Octopuses form the order Octopoda; squid are covered by the decapodiforms, animals with ten appendages, alongside cuttlefish. The shared ancestry is genuine and reasonably close, so this is a comparison of relatives that specialised in different directions rather than of look-alikes.

Telling them apart

Count appendages first: eight means octopus, ten means squid or cuttlefish. Then look at the body. A squid's mantle is elongated and tapered with fins along it, holding its shape as the animal swims. An octopus mantle is a rounded, boneless sac that changes shape constantly and shows no fins in the familiar shallow-water species. Sucker detail confirms it, being fleshy in octopuses and ringed with chitin in squid.

Eight arms, soft rounded body, on or near the seafloor: octopus. Eight arms plus two longer tentacles, torpedo-shaped body with fins, in open water: squid. If it has ten appendages but a broad flattened body and a thick internal cuttlebone, it is a cuttlefish — the third member of the group and the one most often mistaken for a squid.

Habitat and distribution

Octopuses are chiefly benthic, living on reefs, rocky bottoms and soft sediment, and typically occupying a den from which they forage. Squid are chiefly pelagic, occupying open water from the surface to the deep sea, and some undertake substantial daily vertical migrations. This split explains most of the anatomical differences: an animal that lives in crevices benefits from having no rigid parts, while one that swims continuously benefits from a stiffened body.

Diet and ecological role

Both are active predators. Octopuses take crabs, bivalves, snails and fish, using arms and suckers to explore, extract and manipulate prey, and can drill or prise open shelled animals. Squid strike at fish, crustaceans and other squid with their two extensible tentacles, seizing prey and drawing it back to the arms and beak. Both have a hard parrot-like beak and a radula, and many have venomous saliva used to subdue prey.

Behaviour

Octopuses are notable for problem-solving, den maintenance and manipulative skill with their arms, and most species are solitary and short-lived, typically reproducing once. Many squid are shoaling animals with rapid, coordinated swimming and elaborate skin signalling used between individuals. Both change colour and texture rapidly using chromatophores and other skin structures, for camouflage and for signalling.

Senses and adaptations

Both have large, image-forming eyes of a design that evolved independently of the vertebrate eye and lacks its blind spot. Both control colour and texture through chromatophores under direct nervous control, allowing changes within a fraction of a second, and both are believed to be colourblind in the conventional sense despite this. Octopuses have unusually distributed nervous systems, with a large share of neurons in the arms.

More on this topic: Animal senses and adaptations.

Lifespan and life stages

Most shallow-water species in both groups are short-lived, commonly maturing and dying within one to two years, and many reproduce a single time before death. Deep-sea species can live substantially longer. Lifespan figures vary with species, temperature and food supply, and captive figures do not transfer reliably to wild populations, so any single number should be treated as species-specific.

People and these animals

Squid support very large commercial fisheries worldwide and are a major food source, while octopuses are fished and, more recently and controversially, farmed. Both are widely studied in neuroscience and behaviour, and cephalopods are included in animal welfare legislation in a growing number of jurisdictions on the grounds of their neural complexity. Both are also significant prey for marine mammals, sharks and seabirds.

What they have in common

Both are coleoid cephalopod molluscs with a hard beak, ink defence and jet propulsion.
Both change colour and skin texture rapidly using chromatophores under direct nervous control.
Both are active predators with large, image-forming eyes and complex nervous systems.
Most shallow-water species in both groups are short-lived and reproduce once.

Key differences

An octopus has eight arms; a squid has eight arms plus two extensible feeding tentacles.
Squid have an internal chitinous pen; octopuses have no skeletal support beyond the beak.
Squid have fins on the mantle and a streamlined body; most octopuses have neither.
Squid suckers are reinforced with chitinous rings and sometimes hooks; octopus suckers are fleshy.
Octopuses are mostly solitary bottom-dwellers, while many squid shoal in open water.

Where people go wrong

Calling a squid's two feeding tentacles arms, which makes the appendage count appear the same.
Assuming any ten-appendage cephalopod is a squid, when cuttlefish also have ten.
Believing octopuses have no hard parts at all — the beak is rigid and sets the limit on what gaps they can pass.

Frequently asked questions

How many arms does a squid have?

Ten appendages, but they are not all the same. Eight are arms of roughly similar length lined with suckers along their length, and two are longer feeding tentacles with suckers concentrated in a club at the tip. Those two tentacles shoot out to seize prey and then retract. An octopus has eight arms and no tentacles at all, which is why appendage count is the quickest way to separate the two.

Can an octopus really fit through a small hole?

Yes, and it is a direct consequence of anatomy. An octopus has no internal or external skeleton, so the only rigid part of its body is the beak. Anything the beak can pass through, the rest of the animal can generally follow. Squid cannot do this: the chitinous pen running the length of the mantle holds the body in shape, which is useful for swimming but rules out squeezing through gaps.

Are octopuses and squid closely related?

Yes. Both are coleoid cephalopods, the group of molluscs that lost the external shell, which also includes cuttlefish. They share a hard beak, jet propulsion, ink defence, rapid colour change and a large complex nervous system. Their divergence is in lifestyle rather than deep ancestry — one specialised for life on the seafloor, the other for open water — and their anatomy follows from that split.

What is the difference between a squid and a cuttlefish?

Both have ten appendages, so appendage count does not separate them. The internal structure does: a squid has a thin flexible pen, while a cuttlefish has a thick, chambered, buoyancy-regulating cuttlebone. Cuttlefish are also broader and flatter with a fin skirting the whole mantle, and they tend to hover near the bottom rather than swim continuously in open water as many squid do.

Why do both change colour so quickly?

Both have chromatophores — pigment sacs in the skin surrounded by muscle fibres under direct nervous control — so a change takes a fraction of a second rather than the minutes required by hormonal colour change in other animals. Layers of reflective cells underneath add iridescence and brightness. The system serves camouflage against predators, signalling to other individuals, and startling prey or threats.

Read the full profiles

Sources and methodology

How this comparison is built: FaunaHub comparisons are assembled from the sourced animal profiles behind them rather than written as standalone claims. Figures such as size, mass, and lifespan are typical ranges that vary by subspecies, population, region, sex, age, and — where relevant — whether an animal lives in the wild or under human care, so they are presented as ranges rather than fixed values. Where a common name covers a group rather than a single species, the page says so instead of implying a species-to-species match. Conservation status changes over time and should be confirmed against the current IUCN Red List. Dimensions that could not be supported from the underlying profiles are omitted rather than filled in.

What this page does not do: it does not rank the two animals, predict the outcome of a confrontation, or present either one as stronger, smarter, or better. It is not veterinary, medical, first-aid, handling, capture, hunting, or pest-control guidance. For an animal in your care, or any situation involving injury, contact a qualified veterinarian, your local wildlife authority, or emergency services.

Sources and further reading

Authoritative references used for general educational context. External links open in a new tab. These sources do not endorse FaunaHub.

UniversityAnimal Diversity Web — Octopus vulgaris (common octopus) — University of Michigan species account
ReferenceBritannica — Octopus — order Octopoda — Editor-reviewed encyclopedia entry
Wildlife referenceIUCN Red List of Threatened Species — Authoritative source for current conservation status
UniversityAnimal Diversity Web — Cephalopoda (cephalopods, including squid) — University of Michigan class-level account
GovernmentNOAA Fisheries — Marine Life — U.S. government science agency for marine species and habitats
ReferenceEncyclopaedia Britannica — Animals reference — Editor-reviewed encyclopedia overview entries
ReferenceWoRMS — World Register of Marine Species — Authoritative register of marine species names
ReferenceSmithsonian Ocean — National Museum of Natural History — Smithsonian Institution educational ocean-science resource

Last updated: 2026-08-04