Fish & Marine Animals comparison

Octopus vs Nautilus: Key Differences Explained

The short answer

Both are cephalopods, but they sit on opposite branches of the class. A nautilus keeps the ancestral external shell, a coiled gas-filled buoyancy device, and gathers food with dozens of small suckerless tentacles while seeing through a pinhole eye that has no lens. An octopus has no external shell at all, eight suckered arms, a camera-type eye, an ink sac, and skin that changes colour and texture within a second. Nautiluses are slow and long-lived Indo-Pacific animals; most octopuses live a year or two.

The main difference

A nautilus carries an external chambered shell and dozens of suckerless tentacles, while an octopus has no shell, eight suckered arms and colour-changing skin.

The nautilus is routinely introduced as an octopus that kept its shell, which gets the relationship backwards: it is the surviving branch of an older shelled lineage, and almost everything about the two bodies follows from whether the shell was retained or abandoned.

At a glance

Comparison of Octopus and Nautilus across 6 dimensions, with an interpretation of each difference.
What differs	Octopus	Nautilus	What it means
Shell	None; the body is entirely soft, with no external or internal shell remaining	External coiled shell of aragonite divided into gas-filled chambers, linked by a tissue cord called the siphuncle	The shell is the fastest way to separate them, and it sets everything downstream: buoyancy, depth limit and swimming style.Female argonauts, often called paper nautiluses, secrete a thin egg case with two arms; it is not a chambered shell and argonauts are octopods.
Appendages	Eight arms, each lined with muscular suckers that grip and taste what they touch	Roughly sixty to ninety slender tentacles called cirri, with no suckers, sticky rather than gripping	Suckered arms allow fine manipulation and prey handling; the nautilus rakes items toward its beak instead.
Eyes	Camera-type eye with a lens and iris, resolving detail well	Open pinhole eye with no lens or cornea, flooded with seawater	Nautiluses navigate mainly by smell, while octopuses are strongly visual animals.Cephalopod eyes lack the multiple colour receptors most vertebrates use, and how octopuses perceive colour is still argued over.
Skin and display	Chromatophores, reflective iridophores and raisable papillae allow near-instant colour and texture change	Fixed pattern of orange-brown stripes on a pale shell; no chromatophores	Octopus concealment is a behaviour performed second by second; nautilus concealment is a fixed countershaded pattern it cannot alter.
Range and depth	All oceans, from tide pools and reefs to abyssal plains, depending on species	Tropical Indo-Pacific only, mostly on steep fore-reef slopes around 100 to 500 metres, rising shallower at night	The shell fails under pressure past a few hundred metres, so the nautilus is fenced in by its own buoyancy device.Quoted depth bands differ between nautilus species and between localities.
Life cycle	Usually one to two years, with a single terminal reproductive episode and many small eggs	Estimated fifteen to twenty years, maturing slowly and laying a few very large eggs at a time	Slow replacement means nautilus populations recover from collection far more slowly than octopus populations do.Nautilus ages come from growth studies and captive animals rather than long-term marked wild individuals.

Shell: OctopusNone; the body is entirely soft, with no external or internal shell remaining
NautilusExternal coiled shell of aragonite divided into gas-filled chambers, linked by a tissue cord called the siphuncle
The shell is the fastest way to separate them, and it sets everything downstream: buoyancy, depth limit and swimming style.Female argonauts, often called paper nautiluses, secrete a thin egg case with two arms; it is not a chambered shell and argonauts are octopods.
Appendages: OctopusEight arms, each lined with muscular suckers that grip and taste what they touch
NautilusRoughly sixty to ninety slender tentacles called cirri, with no suckers, sticky rather than gripping
Suckered arms allow fine manipulation and prey handling; the nautilus rakes items toward its beak instead.
Eyes: OctopusCamera-type eye with a lens and iris, resolving detail well
NautilusOpen pinhole eye with no lens or cornea, flooded with seawater
Nautiluses navigate mainly by smell, while octopuses are strongly visual animals.Cephalopod eyes lack the multiple colour receptors most vertebrates use, and how octopuses perceive colour is still argued over.
Skin and display: OctopusChromatophores, reflective iridophores and raisable papillae allow near-instant colour and texture change
NautilusFixed pattern of orange-brown stripes on a pale shell; no chromatophores
Octopus concealment is a behaviour performed second by second; nautilus concealment is a fixed countershaded pattern it cannot alter.
Range and depth: OctopusAll oceans, from tide pools and reefs to abyssal plains, depending on species
NautilusTropical Indo-Pacific only, mostly on steep fore-reef slopes around 100 to 500 metres, rising shallower at night
The shell fails under pressure past a few hundred metres, so the nautilus is fenced in by its own buoyancy device.Quoted depth bands differ between nautilus species and between localities.
Life cycle: OctopusUsually one to two years, with a single terminal reproductive episode and many small eggs
NautilusEstimated fifteen to twenty years, maturing slowly and laying a few very large eggs at a time
Slow replacement means nautilus populations recover from collection far more slowly than octopus populations do.Nautilus ages come from growth studies and captive animals rather than long-term marked wild individuals.

How they are related

Both belong to the class Cephalopoda, but they split near its base. Nautiluses sit in Nautiloidea, the surviving remnant of a long-lived lineage of externally shelled cephalopods with a rich fossil record. Octopuses sit in Coleoidea alongside squid and cuttlefish, where the shell was first internalised and then, in octopods, lost altogether. Neither common name covers a single species: about six living nautilus species across two genera are usually recognised, while octopus covers roughly three hundred described species worldwide.

A note on naming: Octopus covers roughly three hundred species in the order Octopoda, and nautilus covers about six living species in two genera. Both names describe groups, so every figure on this page is a range across members rather than a fixed value.

Telling them apart

Nothing else is needed once the shell is visible. A nautilus swims inside a coiled cream-and-orange striped shell, with a leathery hood and a fringe of thin tentacles protruding from the opening. An octopus is a bag-shaped, boneless animal with eight visible arms lined with suckers, two prominent eyes with slit or rectangular pupils, and a muscular funnel below the head. Octopus skin also shifts colour and texture continuously, whereas the striped pattern on a nautilus shell never changes.

Shell on the outside, coiled and striped, with a mass of thin tentacles at the opening: nautilus. No shell, eight suckered arms, skin that shifts colour while you watch: octopus. On a deep Indo-Pacific reef slope a nautilus is genuinely possible. Anywhere else in the world, a small floating shelled cephalopod is far more likely to be an argonaut egg case, which belongs to an octopod despite the paper nautilus name.

Habitat and distribution

Nautiluses are restricted to the tropical Indo-Pacific, from northern Australia and the Philippines through to Fiji and Palau, where they hug steep fore-reef slopes at depths usually quoted between about 100 and 500 metres and move into shallower water at night. Octopuses occur in every ocean and at almost every depth, from rock pools and seagrass beds to hydrothermal vents and abyssal sediment, with each species tied to a particular depth band and substrate rather than the group as a whole.

Diet and ecological role

An octopus is an active predator that prises open bivalves, drills shells, and takes crabs and other crustaceans, subduing them with saliva before the beak breaks them apart. Nautiluses feed far more opportunistically, following scent trails to carrion, shed crustacean moults and slow-moving items on the bottom, then sweeping them toward the beak with sticky cirri. The contrast reflects an energy budget: the octopus spends heavily and grows fast, while the nautilus runs on a very low metabolic rate.

Behaviour

Octopuses are strikingly exploratory in the wild and in captivity, opening containers, carrying coconut halves and shells as portable shelter, and learning routes back to a den. Nautiluses show nothing comparable. They drift at neutral buoyancy, jet slowly, and organise their behaviour around chemical searching rather than manipulation. Laboratory work on nautilus memory has found short-term and longer-term retention of odour associations, but the behavioural range remains narrow beside that of any coleoid cephalopod.

Senses and adaptations

The nautilus eye is an open pinhole flooded with seawater, with no lens or cornea, producing a dim low-resolution image; olfaction dominates instead, and paired rhinophores track scent plumes over long distances. Buoyancy is managed through the siphuncle, which moves salts and water between shell chambers. The octopus invests in the opposite direction: a lens eye, sensitivity to polarised light, and chemotactile suckers that taste surfaces directly, with a large share of its nervous system distributed into the arms.

More on this topic: Animal senses and adaptations.

Lifespan and life stages

Most octopus species live roughly one to two years, growing quickly and dying after a single reproductive episode, though cold deep-sea species live longer and one brooding female in the northeast Pacific was observed guarding eggs for more than four years. Nautilus estimates run very differently: slow growth, maturity often placed somewhere around ten to fifteen years, and total lifespans commonly estimated at fifteen to twenty. Both sets of figures carry wide uncertainty and depend on species and temperature.

People and these animals

Octopuses are fished commercially across the world, displayed in public aquaria, and are now covered by animal welfare provisions in several jurisdictions on the strength of evidence for their cognitive complexity. Nautiluses are collected mainly for the ornamental shell trade, and Nautilus pompilius was added to CITES Appendix II in 2016 so that trade could be regulated. Because nautiluses mature slowly and lay few eggs, sustained collection is far harder for a local population to absorb.

Keeping them: what to understand first

Octopuses are sometimes kept by experienced marine aquarists and are demanding in ways that are easy to underestimate: naturally short lives, persistent escape behaviour, sensitivity to water chemistry, and a need for enrichment. Nautiluses are effectively a public-aquarium animal with cold deep-water requirements. Whether any cephalopod belongs in a given household depends on local law, welfare capacity and long-term responsibility, and those decisions belong with a qualified veterinarian and an experienced aquarium professional.

Conservation context

Nautilus trade regulation reflects concern about localised depletion around fished island groups rather than one uniform global picture, and status differs between species and countries. Octopus assessments also vary by species, with fishery pressure rising as global demand grows. Neither group should be given a single blanket label, so check the current IUCN Red List entry for the particular species rather than relying on a general statement about cephalopods.

Trade in Nautilus pompilius is regulated under CITES Appendix II, but status differs by species and region and assessments change. Check the current IUCN Red List entry for the species you are researching.

What they have in common

Both are cephalopods with a chitinous beak, a radula and a muscular funnel used for jet propulsion.
Both are marine-only animals with no freshwater representatives anywhere in the group.
Both are predators or scavengers that locate food partly by chemical cues in the water.
Both names cover several species rather than one animal, so figures are ranges across a group.

Key differences

The nautilus has an external chambered shell; the octopus has no shell at all.
Octopuses have eight suckered arms, while nautiluses have dozens of suckerless cirri.
The octopus eye has a lens; the nautilus eye is an open pinhole flooded with seawater.
Only the octopus can change colour and skin texture, and only the octopus has an ink sac.
Nautiluses are confined to the tropical Indo-Pacific, while octopuses occur in every ocean.
Octopus lives are usually one or two years, whereas nautilus lifespans are estimated in the decades.

Where people go wrong

Calling the argonaut a paper nautilus and assuming it is a small nautilus rather than an octopod.
Describing the nautilus as a primitive octopus, when the shell is the ancestral cephalopod feature the octopus lineage lost.
Assuming a nautilus can camouflage or ink like other cephalopods; it has neither chromatophores nor an ink sac.

Frequently asked questions

Is a nautilus just an octopus that kept its shell?

No, and the phrasing reverses the history. The external shell is the older cephalopod condition, and nautiluses are the surviving branch that retained it. Octopuses descend from a separate lineage in which the shell was internalised, as it still is in cuttlefish, and then lost entirely. A nautilus is not a modified octopus; the two are distant cousins within one class, separated by a very long stretch of cephalopod evolution.

Why can a nautilus not change colour like an octopus?

It has no chromatophores. Octopus skin carries pigment sacs that muscles expand and contract, backed by reflective cells and by papillae that alter surface texture, all under direct nervous control. A nautilus has none of that machinery, and its visible surface is a hard shell rather than living skin. Its concealment is instead a fixed countershaded pattern, dark stripes above and pale below, which works against a lit surface when seen from below.

How deep can a nautilus go before the shell gives way?

Published implosion depths sit in the region of several hundred metres beyond its normal range, which is why nautiluses are usually recorded between about 100 and 500 metres and move up the reef slope at night. The chambered shell is a pressure vessel, so it sets a hard ceiling on depth that a shell-less octopus does not face. Exact figures vary with species, shell thickness and the individual animal.

Is the paper nautilus a real nautilus?

No. Paper nautilus is a common name for argonauts, which are open-ocean octopods. The female secretes a thin, brittle egg case using two specialised arms, and that case is not a chambered buoyancy shell and is not homologous with the nautilus shell. It also is not permanently attached in the same way. The resemblance is a coincidence of shape, and the name has confused shell collectors for centuries.

Why do octopuses live only a year or two when nautiluses last decades?

The two run opposite life strategies. Octopuses grow fast on a high metabolic rate, reproduce once, and die soon after, with females typically ceasing to feed while brooding. Nautiluses grow slowly, mature late, and lay a few large eggs repeatedly over many years. Deep, cold-water octopuses live longer than shallow ones, so the contrast is a tendency rather than an absolute rule, and every figure here is an estimate with real spread.

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
ReferenceWoRMS — World Register of Marine Species — Authoritative register of marine species names
ReferenceBritannica — Nautilus — Editor-reviewed encyclopedia entry
ReferenceSmithsonian Ocean — National Museum of Natural History — Smithsonian Institution educational ocean-science resource
ReferenceEncyclopaedia Britannica — Animals reference — Editor-reviewed encyclopedia overview entries
UniversityAnimal Diversity Web — University of Michigan Museum of Zoology — Peer-edited reference accounts for animal species

Last updated: 2026-08-04