A nautilus shell does several jobs at once. It is a float, a set of sealed gas-filled chambers that lets the animal hover without effort. It is armour, a coiled fortress the body can withdraw into. It is camouflage, striped above and pale below. It sets the deepest point a nautilus can go, because below about 800 m it implodes. And it is a diary, recording the animal’s growth, its depth and the moment it became an adult.

That last role is less familiar but just as important to science. Because nautiluses are the only living cephalopods with an external chambered shell, their shells are the key to interpreting the ammonites and nautiloids that dominated ancient seas. This page explains each job the shell does, how the shell is built and grown, what it has revealed about nautilus lives, and the long-running myth that it follows the golden ratio. The pumping mechanism itself is explained in more detail on the page about how a nautilus controls its buoyancy.

How is a nautilus shell built?

The shell is a coiled tube made of aragonite, a form of calcium carbonate. It has a matte outer layer that carries the colour pattern and a thick inner layer of iridescent nacre, or mother-of-pearl. Inside, curved walls called septa divide the coil into a series of chambers. The animal’s body occupies only the last and largest of them, the body chamber, which opens to the sea. All the older chambers behind it form the phragmocone, the buoyancy apparatus.

A thin strand of living tissue, the siphuncle, runs back from the body through a hole near the centre of every septum, connecting all the chambers. As the animal grows it moves forward in the shell, secretes a new septum behind itself and seals off another chamber. Each chamber starts filled with liquid, which the siphuncle then removes, leaving gas at low pressure. The nautilus genome, published in Nature Ecology & Evolution in 2021 by Zhang and colleagues, showed that the shell is built with a conserved molluscan toolkit of biomineralisation genes, combined with repetitive, low-complexity protein domains found only in the nautilus lineage.

Job one: a float

Nautiluses live on steep reef slopes and hunt and scavenge just above the bottom, rising and sinking along the slope. Muscle and shell are denser than seawater, so without help the animal would sink. The gas-filled chambers offset that weight, making the nautilus close to neutrally buoyant, so it can hover and drift while spending little energy. When the animal grows, adds weight or needs to adjust, the siphuncle shifts the balance of liquid and gas in the newest chambers. Because this works by moving salts and letting water follow by osmosis, it is a gradual adjustment rather than an instant one, which suits an animal that rises and sinks slowly along the reef.

The design has a hard limit. A 2016 study in PeerJ by Lemanis and colleagues noted that the gas inside cephalopod shell chambers is at only about one atmosphere. Outside, water pressure rises by roughly one atmosphere every 10 m, so at 1,000 m it is about 100 times sea-level pressure. A nautilus shell must resist that difference without deforming, or the buoyancy system fails.

Job two: a depth gauge and a barrier

Nautilus shells implode at depths greater than about 800 m. Tracking studies of Nautilus pompilius in Australia have shown that the animals range between an upper limit set by temperature, around 25 °C, and a lower limit near the shell’s implosion depth. In the Lemanis study, computer models built from CT scans showed that under water pressure the greatest stress falls on the last septum, while the rest of the shell is only lightly compressed. Compared with the small internal shell of the deep-sea cephalopod Spirula, the nautilus shell carried the higher stress.

That limit has shaped the evolution of nautiluses. In a 2023 paper in ZooKeys, Barord and colleagues described three new species, Nautilus samoaensis from American Samoa, N. vitiensis from Fiji and N. vanuatuensis from Vanuatu. Each lives on island groups separated from other nautilus populations by at least 200 km of water deeper than 800 m. Because a nautilus cannot cross water deeper than its shell can withstand, deep ocean acts as a wall between populations, and isolated populations have drifted apart into separate species. The shell’s strength, in other words, helps explain why there are several nautilus species scattered across the Indo-Pacific rather than one.

Within that limit, nautiluses use a wide band of depth. A 2026 study in Scientific Reports by Ward, Barord and colleagues used shell-mounted acoustic transmitters on seven species of Nautilus and Allonautilus, together with oxygen isotopes from their shells. Mature animals of all tracked species lived significantly shallower, at about 200 m, than immature animals from the same populations, which lived at about 350–400 m. Regular daily up-and-down migration was found in only one species of each genus. The same study compared shell chemistry from 18 extinct nautiloid species and found that most grew their shells in warmer water than living nautiluses, with the exception of Aturia, an ammonite-like genus of the Eocene to Miocene, though the authors cautioned that later chemical alteration of the fossils cannot be ruled out.

Horizontal travel is also constrained by the shell. Nautiluses normally move along the slope, following the bottom contours within a metre or so of the seabed, and can cover long distances that way: around Palau, Saunders and Spinosa recorded movements of as much as 150 km in 332 days. In 2015, acoustic data from one tagged N. pompilius, if correct, provided the first evidence of a living nautilus moving several kilometres between islands at mid-water depths rather than along the bottom. Even a mid-water crossing has to stay above the shell’s crushing depth, however deep the water below.

Job three: armour

When threatened, a nautilus withdraws its body into the body chamber and closes the opening with its hood, a thick, leathery pad on top of the head. The coiled form is strong, and the nacre lining, built of thin mineral plates bound by organic layers, makes the wall tougher than solid mineral would be. Octopuses, fish and other predators still attack nautiluses, and shells often show repaired breaks along the rim where the animal survived an attack and laid down new shell over the damage.

Armour has a cost. Every gram of shell must be supported by the buoyancy chambers, and a shell strong enough to resist both bites and deep-water pressure is heavier than one built for either alone. The nautilus shell is a compromise between those demands.

Job four: camouflage

Most nautilus shells are boldly marked with brown or orange-brown stripes on the upper surface that fade towards a pale underside. The usual interpretation is countershading: seen from above against the dark slope, the stripes break up the outline of the shell, while the pale underside is harder to see from below against the light coming down from the surface. The stripe pattern differs between species and populations, and Barord and colleagues used shell colour patterns, along with the soft anatomy of the hood and genetics, in defining the three new species.

Job five: a diary of the animal’s life

Because a nautilus adds septa one at a time and never sheds them, its shell keeps a record of its life. Nautiluses develop inside an egg capsule attached to the seafloor, a process that takes as long as a year in aquariums, and the first few chambers are already formed at hatching. Growth to sexual maturity takes about 12 to 20 years, much slower than other cephalopods, and unlike squid and octopuses, nautiluses do not die after breeding. Each septum takes time to form: a review of nautilus growth rates by Landman and Cochran estimated that early septa take weeks to secrete, while those formed near maturity take months.

The shell shows when the animal has stopped growing. At maturity the last few septa are spaced more closely together, a feature called septal approximation, and a black band forms at the edge of the shell opening. Shell and tissue growth then stop. Mature Nautilus belauensis from Palau, one of the two largest species, average 200–220 mm across, while the endemic nautilus of Osprey Reef in the Coral Sea, the smallest known, matures at about 120–130 mm.

The chemistry of each septum records conditions when it was made. In a 2022 study in PLOS ONE, Tajika, Landman, Cochran and colleagues analysed oxygen and carbon isotopes in the septa of two Nautilus macromphalus from New Caledonia, together with water samples from the area. They calculated that the animals lived at about 140 m before hatching and about 370 m after. Because tracking living nautiluses is so difficult, this kind of shell chemistry is a key tool, and it is the same method palaeontologists use on fossil ammonites and nautiloids.

What nautilus shells reveal about ammonites

Ammonites, extinct relatives of the nautilus, had chambered shells with septa folded into elaborate, frilly patterns. For centuries, the leading explanation was that complex folding strengthened the shell against pressure and allowed ammonites to live deeper. Finite-element models built from real shells, including the nautilus, have challenged that idea. A 2020 study by Robert Lemanis in Proceedings of the Royal Society B found that adding folds to the septum had little or no effect on the stress in the shell wall or septum, and that higher-order folds reduced stress in the wall but increased it in the septum. He concluded that the evolution of complex septa does not reflect a persistent trend towards deeper water. The 2016 PeerJ study had found that more strongly fluted septa actually raised stress under water pressure, though they reduced stress from point loads such as a predator’s bite along the suture lines.

The living nautilus also helps test ideas about how quickly chambers could be emptied. A 2016 study in PLOS ONE by Lemanis and colleagues used high-resolution CT scanning to compare chamber shapes in ammonites with those of Spirula and nautilids. Ammonites did not have persistently higher chamber surface areas, as had been thought, but at least one ammonite had a larger siphuncle surface relative to chamber volume than either living group, which in theory would allow faster emptying and faster growth.

Is the nautilus shell a golden spiral?

No. The nautilus shell is a logarithmic spiral: it grows by a constant proportion with each turn, so it keeps the same shape as it gets bigger. It is often claimed to follow the golden ratio of about 1.618. When the mathematician Clement Falbo measured nautilus shells in the collection of the California Academy of Sciences in 1999, he found that their spirals fit rectangles with sides in a ratio of about 1.33, with measurements ranging from 1.24 to 1.43. He wrote that it “seems highly unlikely that there exists any nautilus shell that is within 2 percent of the golden ratio.” John Sharp, measuring shells for a 2002 article, likewise concluded that the golden spiral and the nautilus spiral simply do not match.

The constant-proportion growth is the real point of interest. Because each new section of shell is a scaled-up copy of the last, the animal’s body chamber and buoyancy system keep the same proportions throughout life, which is exactly what a growing animal that depends on precise buoyancy needs.

Not to be confused with the paper nautilus

The “paper nautilus” sold in shell shops is a different animal entirely. Argonauts are open-ocean octopuses, and the delicate, white, ridged “shell” is an egg case secreted by the female from the tips of two greatly expanded arms. It is not a true cephalopod shell, has no gas-filled chambers or siphuncle, and is used as a brood chamber for the eggs. Females grow to about 10 cm and make egg cases up to about 30 cm across, while males rarely exceed 2 cm and make no shell at all. Argonauts do use their egg case for buoyancy, but in a different way: the female gulps a bubble of air at the surface and traps it inside, then controls its position. The resemblance between the argonaut’s egg case and the shells of ammonites is a case of similar forms arising independently, not of close relationship.

The comparison underlines what is unusual about the true nautilus shell. It is part of the animal’s body from the embryo onward, it grows throughout life, and it combines buoyancy control, protection and a lifelong record in a single structure, the last working example of a design that once filled the oceans.

Why shells are a conservation problem

The same features that make the shell useful to the nautilus make it valuable to people. Polished shells are sold as decoration and their nacre is cut for jewellery and inlay, and nautiluses can be caught in baited traps lowered down reef slopes. Because they take more than a decade to mature, populations on heavily fished reefs can be emptied quickly. All nautilus species are listed on Appendix II of CITES, which regulates international trade, and some countries protect them locally. Anyone offered a nautilus shell should assume it came from a wild animal killed for the trade. The page are nautilus endangered covers their status and the evidence for declines.