The chambered nautilus, Nautilus pompilius, also called the pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it is not a golden spiral. The shell exhibits countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above.

The range of the chambered nautilus encompasses much of the south Pacific; It has been found near reefs and on the seafloor off of the coasts of Australia, Japan, and Micronesia.

The eyes of the chambered nautilus, like those of all Nautilus species, are more primitive than those of most other cephalopods; the eye has no lens and thus is comparable to a pinhole camera. The species has about 90 cirri that do not have suckers differing significantly from the limbs of coleoids. Chambered nautiluses, again like all members of the genus, have a pair of rhinophores located near each eye which detect chemicals, and use olfaction and chemotaxis to find their food.
The oldest fossils of the species are known from Early Pleistocene sediments deposited off the coast of Luzon in the Philippines.

The first and oldest fossil of Chambered Nautilus displayed at the Philippine National Museum. Although once thought to be a living fossil, the chambered nautilus is now considered taxonomically very different from ancient ammonites, and the recent fossil record surrounding the species shows more genetic diversity among nautiluses now than has been found since the extinction of the dinosaurs. Indeed, the taxon of the chambered nautilus, Nautilus pompilius is actually a grouping of tens of different species of nautilus under one name.

All nautilus species are threatened due to overfishing for their shell, which primarily is used for jewelry and other ornamental artifacts. In 2016, they were moved to CITES Appendix II, which restricts international trade, and later the chambered nautilus was recognized as a threatened species under the Endangered Species Act.

As a carnivore, it feeds on both underwater carrion and detritus, as well as living shellfish and crab. Mainly scavengers, chambered nautiluses have been described as eating “anything that smells”. This food is stored in a stomach-like organ known as a crop, which can store food for a great deal of time without it denaturing.

The shell of the chambered nautilus fulfills the function of buoyancy, which allows the nautilus to dive or ascend at will, by controlling the density and volume of the liquid within its shell chambers. This was found during research done in New Caledonia on nautiluses whose shell chamber fluid densities were tested at various depths, weeks apart. Generally speaking, chambered nautiluses inhabit a depth around 1000 feet, although further tests demonstrated that they can, and do, dive deeper. However, there are hazards associated with extreme depth for the nautilus: the shells of chambered nautiluses slowly fill with water at such depths, and they are only capable of withstanding depths up to 2000 feet before imploding due to pressure.

The chambered nautilus inhabits different segments of the shell as it grows, continuously growing new, larger “cells” into which it moves its internal organs as it grows in maturity. All of the smaller chambers, once uninhabited, are used in the method described above to regulate depth.