The nautilus is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any of the Nautilidae. All are protected under CITES Appendix II. Depending on species, adult shell diameter is between 4 and 10 inches.
Nautilidae, both extant and extinct, are characterized by involute or more or less convolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered "living fossils".
The word nautilus is derived from the Greek word ναυτίλος nautílos "sailor", it originally referred to a type of octopus of the genus Argonauta, also known as 'paper nautilus', which were thought to use two of their arms as sails.
Nautiluses are only found in the Indo-Pacific, from 30° N to 30° S latitude and 90° E to 175° E longitude. They inhabit the deep slopes of coral reefs.
Nautiluses usually inhabit depths of several hundred metres. It has long been believed that nautiluses rise at night to feed, mate, and lay eggs, but it appears that, in at least some populations, the vertical movement patterns of these animals are far more complex. The greatest depth at which a nautilus has been sighted is 703 m (2,306 ft) (N. pompilius). Implosion depth for nautilus shells is thought to be around 800 m (2,600 ft). Only in New Caledonia, the Loyalty Islands, and Vanuatu can nautiluses be observed in very shallow water, at depths of as little as 5 m (15 ft). This is due to the cooler surface waters found in these southern hemisphere habitats as compared to the many equatorial habitats of other nautilus populations – these usually being restricted to depths greater than 100 m (300 ft). Nautiluses generally avoid water temperatures above 25 °C (75 °F).