Insects & Invertebrates comparison

Squid vs Cuttlefish: Key Differences Explained

The short answer

Both have eight arms and two feeding tentacles, 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 cuttlebone used to control buoyancy. Cuttlefish are broader and flatter with a fin skirting the whole mantle and a distinctive W-shaped pupil, and they hover near the bottom rather than cruising open water.

The main difference

A cuttlefish has a thick chambered cuttlebone controlling buoyancy and a fin running the whole mantle; a squid has a thin pen, fins at the rear, and a body built for open-water swimming.

Once someone learns that eight arms means octopus and ten means squid, the cuttlefish breaks the rule — and it needs its own comparison to resolve.

At a glance

Comparison of Squid and Cuttlefish across 6 dimensions, with an interpretation of each difference.
What differs	Squid	Cuttlefish	What it means
Internal structure	A thin flexible chitinous pen giving the mantle rigidity	A thick porous chambered cuttlebone used to regulate buoyancy	The cuttlebone is a genuine buoyancy organ, not just a stiffener.
Fins	Fins at the rear of the mantle, often triangular	A continuous narrow fin skirting the whole length of the mantle	The full-length fin allows precise hovering and fine positional control.
Body shape	Elongated and tapering, built for sustained forward swimming	Broader and flattened, built for manoeuvring near the bottom	Shape follows habitat: open water against structured seafloor.
Pupil shape	Round	Distinctive W-shaped	The W-shaped pupil is close to diagnostic on a clear view of the eye.
Habitat	Open water from coastal shallows to the deep ocean	Coastal shallows and shelf waters, close to the bottom	Cuttlefish are absent from some ocean regions entirely, including American coasts.
Camouflage	Rapid colour change, used heavily for signalling in shoaling species	Exceptional — colour, pattern and three-dimensional skin texture combined	Cuttlefish camouflage against a structured seafloor is among the best documented in any animal.

Internal structure: SquidA thin flexible chitinous pen giving the mantle rigidity
CuttlefishA thick porous chambered cuttlebone used to regulate buoyancy
The cuttlebone is a genuine buoyancy organ, not just a stiffener.
Fins: SquidFins at the rear of the mantle, often triangular
CuttlefishA continuous narrow fin skirting the whole length of the mantle
The full-length fin allows precise hovering and fine positional control.
Body shape: SquidElongated and tapering, built for sustained forward swimming
CuttlefishBroader and flattened, built for manoeuvring near the bottom
Shape follows habitat: open water against structured seafloor.
Pupil shape: SquidRound
CuttlefishDistinctive W-shaped
The W-shaped pupil is close to diagnostic on a clear view of the eye.
Habitat: SquidOpen water from coastal shallows to the deep ocean
CuttlefishCoastal shallows and shelf waters, close to the bottom
Cuttlefish are absent from some ocean regions entirely, including American coasts.
Camouflage: SquidRapid colour change, used heavily for signalling in shoaling species
CuttlefishExceptional — colour, pattern and three-dimensional skin texture combined
Cuttlefish camouflage against a structured seafloor is among the best documented in any animal.

How they are related

Both are decapodiform cephalopods with ten appendages, and both are coleoids that lost the external shell. Cuttlefish form the order Sepiida and squid are covered by several orders. They are close relatives, and the cuttlebone is a remnant of the ancestral shell retained in a modified, functional form.

Telling them apart

Body shape and fins do the work. A cuttlefish is broad and flattened with a narrow fin running the entire length of the mantle, hovering rather than cruising. A squid is elongated and tapered with fins concentrated at the rear. If the eye is visible, the cuttlefish's W-shaped pupil is close to conclusive.

Broad, flattened, hovering near the bottom, fin running the whole mantle, W-shaped pupil: cuttlefish. Elongated, tapered, cruising in open water, fins at the rear: squid. Both have ten appendages, so the count that separates them from octopuses will not separate them from each other.

Habitat and distribution

Cuttlefish are coastal and shelf animals associated with the seafloor, seagrass and reef, and are absent from some regions including the Americas. Squid occupy open water worldwide from the surface to the deep sea, and several species undertake substantial daily vertical migrations between depths.

Diet and ecological role

Both are active predators taking crustaceans, fish and other cephalopods, striking with the two extensible tentacles and drawing prey to the beak. Cuttlefish hunt over structured bottom habitat, often stalking prey with a mesmerising rippling display of moving colour bands. Squid pursue prey in open water, often cooperatively in shoaling species.

Behaviour

Cuttlefish are largely solitary and territorial, with elaborate visual signalling during courtship and rivalry, including the well-documented behaviour of males displaying courtship patterns on one side of the body and female-like patterns on the other to deceive rivals. Many squid are shoaling animals coordinating movement and using skin signals within the group.

Senses and adaptations

The cuttlebone is a chambered structure whose gas-to-liquid ratio the animal adjusts to control buoyancy, letting it hover with minimal effort — an ability squid lack. Both are believed to be colourblind in the conventional sense despite their extraordinary colour displays, and cuttlefish may extract colour information from chromatic blur produced by their unusual pupil shape.

More on this topic: Animal senses and adaptations.

People and these animals

Both are commercially fished, squid in very large quantities. Cuttlebone has long been used as a calcium supplement for caged birds and, historically, as a polishing agent, and the ink of both has been used as a pigment — sepia takes its name from the cuttlefish genus. Both are widely studied in neuroscience and camouflage research.

What they have in common

Both are decapodiform cephalopods with eight arms and two feeding tentacles.
Both change colour and skin texture rapidly using chromatophores under nervous control.
Both are active predators that strike with extensible tentacles and have a hard beak.
Both are short-lived, typically maturing and dying within one to two years.

Key differences

Cuttlefish have a thick chambered cuttlebone; squid have a thin flexible pen.
A cuttlefish fin skirts the whole mantle, while squid fins sit at the rear.
Cuttlefish are broad and flattened; squid are elongated and tapered.
Cuttlefish have W-shaped pupils, whereas squid pupils are round.
Cuttlefish hover near the seafloor, while squid cruise open water.

Where people go wrong

Using appendage count to separate them, when both have ten.
Assuming cuttlefish occur worldwide, when they are absent from the Americas.
Treating the cuttlebone as a simple stiffener rather than a buoyancy organ.

Frequently asked questions

What is the main difference between a squid and a cuttlefish?

The internal structure. A squid has a thin flexible pen that stiffens the mantle, while a cuttlefish has a thick chambered cuttlebone it uses to control buoyancy by adjusting the ratio of gas to liquid inside. That difference shapes everything else: cuttlefish hover near the bottom with minimal effort, whereas squid must keep swimming to hold position in open water.

Why do cuttlefish have W-shaped pupils?

The unusual shape is thought to help balance light entering the eye from above and below, which matters for an animal that hovers in bright shallow water. One leading hypothesis is that it also spreads colours differently across the retina, and that the cuttlefish extracts colour information from that chromatic blur — a possible resolution to the puzzle of how a colourblind animal produces such precise colour matching.

Is a cuttlefish a kind of squid?

No, though they are close relatives. Both are decapodiform cephalopods with ten appendages, but cuttlefish form their own order, Sepiida. The everyday habit of calling anything with ten appendages a squid obscures a real distinction — the cuttlebone, the full-length fin and the bottom-associated lifestyle are all genuine differences at order level.

Where can cuttlefish be found?

In coastal and shelf waters of Europe, Africa, Asia and Australia, generally close to the seafloor over sand, seagrass and reef. They are notably absent from the Americas, which surprises people who assume cephalopod groups are globally distributed. Squid, by contrast, occur worldwide in open water from the surface to the deep ocean.

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 — 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
ReferenceBritannica — Cuttlefish — Editor-reviewed encyclopedia entry
Wildlife referenceIUCN Red List of Threatened Species — Authoritative source for current conservation status
ReferenceSmithsonian Ocean — National Museum of Natural History — Smithsonian Institution educational ocean-science resource

Last updated: 2026-08-04