The nautilus is a marine mollusc of the cephalopod family nautilidae, 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. Having survived relatively unchanged for millennia, nautiluses represent the only living members of the subclass nautiloidea, and are often considered “living fossils.”
Nautilus pompilius is the largest species in the genus. One form from northwestern Australia may reach 26.8 centimetres (10.6 in) in diameter. However, most nautilus species never exceed 20 centimetres (7.9 in). Nautilus macromphalus is the smallest species, usually measuring only 16 centimetres (6.3 in). A dwarf population from the Sulu Sea (Nautilus pompilius suluensis) is even smaller, with a mean shell diameter of 115.6 mm.
Each nautilus tentacle is composed of a long, soft, flexible cirrus and is retractable into a corresponding hardened sheath. Nautiluses typically have more tentacles than other cephalopods— up to ninety. The radula is wide and distinctively has nine teeth. The mouth consists of a parrot-like beak made up of two interlocking jaws capable of ripping the animal’s food— mostly crustaceans— from the rocks to which they are attached. The animal can withdraw completely into its shell and close the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled and pressure resistant, imploding at a depth of about 800 metres (2,600 ft). To swim, the nautilus draws water into and out of the living chamber with its hyponome, which uses jet propulsion. Unlike many other cephalopods, they do not have good vision; their eye structure is highly developed but lacks a solid lens. They have a simple “pinhole” eye open to the environment. Nautiluses reproduce by laying eggs. Gravid females attach the fertilized eggs to rocks in shallow waters, whereupon the eggs take eight to twelve months to develop until the 30 millimetres (1.2 in) juveniles hatch. Females spawn once per year and their lifespan may exceed 20 years, which is exceptionally lengthy for a cephalopod. However, nautiluses typically reach sexual maturity when they are about 15 years old.