Chambered Nautilus (Nautilus pompilius): Facts and Shell

What does a chambered nautilus look like?

The most obvious feature is the smooth, coiled shell. It is creamy white with bold reddish-brown stripes across the top and sides, while the underside is almost plain white. This pattern is a form of camouflage called countershading: seen from above, the striped shell blends with the dark water below; seen from below, the pale underside blends with the brighter water near the surface.

Only the head and tentacles stick out of the shell opening. A thick, leathery hood, formed from fused tentacles, covers the top of the head. When a nautilus is disturbed, it pulls its whole body into the shell and closes the opening with this hood, like a door.

Around the mouth are many thin tentacles, called cirri, which can be pulled back into protective sheaths. A chambered nautilus has roughly 60 to 90 of them, and females usually have more than males. Unlike the arms of octopuses and squid, they carry no suckers. Instead, ridges on the tentacles release a sticky secretion that lets them grip food and rocks firmly. Hidden among them is a hard, parrot-like beak.

Below the head is a muscular funnel called the hyponome. By squirting water out of it, the nautilus moves by jet propulsion, usually backward with the shell leading. Like an octopus, it has blue blood based on copper rather than iron, as explained in why octopus blood is blue.

How big is a chambered nautilus?

The chambered nautilus is the largest living nautilus species. Most adults have a shell 13–23 cm (5–9 in) across and weigh around 1 kg (2 lb), but size varies a lot between populations.

At maturity the shells of males are slightly larger than those of females. The giant form from Indonesia and northern Australia was once described as a separate species, Nautilus repertus, but most researchers treat it as the same species.

How does the chambered shell work?

Inside, the shell is divided by curved walls, called septa, into a row of chambers that get bigger toward the opening. The animal lives only in the last and largest chamber. As it grows, it moves its body forward and builds a new wall behind it, sealing off another empty chamber. A hatchling has only a few chambers, while an adult has about 30 or more.

A thin tube of living tissue called the siphuncle runs through the middle of every wall, linking all the chambers. A newly sealed chamber is full of liquid. The siphuncle pumps salts out of this liquid into the blood, the water follows by osmosis, and gas fills the space left behind. By slowly removing or letting in liquid, the nautilus keeps itself almost weightless in the water.

This system is slow. It keeps the animal balanced as it grows, or after part of the shell breaks off, but it is not used for quick dives. To move up and down the reef, the nautilus swims with its funnel. The coiled shell is strong, yet it has a limit: it is thought to implode at roughly 800 m (2,600 ft), which sets the deepest point a nautilus can live.

The inside of the shell is lined with nacre, or mother-of-pearl, which gives the species its other name, pearly nautilus. A shell cut in half shows a near-perfect logarithmic spiral of chambers. Whether that spiral follows the famous golden ratio is checked in is the nautilus shell a golden spiral?

Senses: pinhole eyes and a keen sense of smell

The eyes of a chambered nautilus are unusual. They have no lens: each eye is an open cup with a small hole that lets seawater in, working like a pinhole camera. This gives only a dim, blurry image, far poorer than the sharp vision of octopuses, squid and cuttlefish.

Smell matters much more. Below each eye is a small organ called a rhinophore that detects chemicals in the water, and some of the tentacles sense odors too. In experiments, nautiluses followed scent trails to bait, and they also feel their way along the reef with their tentacles.

The nautilus brain is simpler than that of other cephalopods and lacks the specialized learning centers of octopuses. Even so, a 2008 study showed that chambered nautiluses could learn to link a flash of blue light with food. They showed signs of remembering it for up to about 12 hours, but had forgotten it by the next day.

Where do chambered nautiluses live?

The chambered nautilus lives in the tropical Indo-Pacific. It is found from the Andaman Sea, the Gulf of Thailand and Vietnam east through Malaysia, Indonesia and the Philippines to Papua New Guinea, the Solomon Islands and northern Australia, including the Great Barrier Reef and Coral Sea reefs. Nautilus populations in Fiji, Samoa and Vanuatu that were once included in this species were described as three separate species in 2023.

It lives on steep reef slopes and the sea floor around them, usually between about 100 and 500 m (330–1,640 ft) deep, with sightings to about 703 m (2,306 ft). Nautiluses need cool water and avoid temperatures above about 25 °C (77 °F), which is why they stay below the warm surface layer in the tropics.

Many nautiluses spend the day deeper on the slope and move up into shallower water at night to forage, a pattern called vertical migration, though tracking studies show it varies between reefs and individuals. They usually travel and feed alone. Because adults cannot cross very deep water or warm shallow seas, populations on different reefs are small and isolated from one another. Empty shells, on the other hand, float after death and can drift thousands of kilometers; they have washed up as far away as Japan, India and Africa.

What do chambered nautiluses eat?

Chambered nautiluses are carnivores and, above all, scavengers. Guided mainly by smell, they eat dead fish and other carcasses, crabs and hermit crabs, shrimp, and the shed shells of lobsters, which are rich in minerals. They also take slow-moving live prey when they can catch it.

The sticky tentacles grip the food and pass it to the beak, which tears it into pieces. Food is stored in a stretchy pouch called the crop before digestion, so a nautilus can eat a big meal when it finds one. With a slow metabolism, it can then go a long time before feeding again, an advantage in deep, food-poor water.

Reproduction, growth and lifespan

Chambered nautiluses grow slowly and live much longer than most cephalopods. The majority of octopuses and squid live only one or two years and die after breeding once, as described in how long octopuses live. A chambered nautilus takes about 10–15 years to mature and can live 15–20 years or more.

In males, four tentacles form an organ called the spadix, which passes a packet of sperm to the female. Unlike other living cephalopods, a female nautilus does not die after spawning and can breed year after year. She lays only a small number of large eggs, gluing them one at a time or in small batches to rocks or coral. There is no larval stage: the young develop inside the egg for about a year and hatch as miniature adults with a shell about 3 cm (1.2 in) across.

Wild nautilus eggs have almost never been found, so most of what is known about breeding comes from aquariums, where it is still rare. In most field studies males make up the majority of the animals caught, often 60–90 percent.

Predators and threats

Natural predators include octopuses, which can drill holes through the shell, and sharks and large bony fish such as triggerfish, which bite or crack it. Broken shell edges can be repaired, and the siphuncle adjusts the buoyancy to make up for the change in weight.

The greatest threat is people. Nautiluses are caught in baited traps lowered down reef slopes, mainly so their shells can be sold to tourists and collectors or cut up for jewelry and shell inlay. Because they mature late, lay few eggs and live in small, isolated populations, they recover very slowly. In heavily fished parts of the Philippines, nautilus numbers are estimated to have fallen by about 80 percent between 1980 and 2010, and the animals have become scarce in other fished areas of Indonesia and the Philippines.

Is the chambered nautilus endangered?

The IUCN Red List classifies the chambered nautilus as Vulnerable, with a decreasing population, based on an assessment carried out in 2023. The main reason is the ongoing decline expected from fishing for the shell trade.

In 2016, all nautilus species were added to Appendix II of CITES, the treaty that regulates international wildlife trade, so shells and shell products can only cross borders with permits. In 2018, the United States listed the chambered nautilus as threatened under the Endangered Species Act. Several countries, including Indonesia, protect it by law, but illegal fishing and trade continue.

Nautiluses and people

Nautilus shells have been prized for centuries. In Renaissance and Baroque Europe, goldsmiths mounted them as ornate nautilus cups for cabinets of curiosities, and in Samoa nautilus shell decorates a traditional ceremonial headdress. The animal inspired Oliver Wendell Holmes’s 1858 poem “The Chambered Nautilus” and shares its name with Captain Nemo’s submarine in Jules Verne’s novel and with USS Nautilus, the first nuclear-powered submarine.

Nautiluses are the last survivors of a group of shelled cephalopods that goes back about 500 million years, so they are often called living fossils. The real story is more complicated, as explained in is the nautilus a living fossil? For their closest living relatives, see the common octopus and octopus vs squid.