page_title: Nautilus
section_title: Buoyancy and movement
hierarchical_section_title: Nautilus / Natural history / Buoyancy and movement
caption_attribution_description: Nautilus, photographed at Siam Ocean World aquarium, Bangkok
context_page_description: Nautilus is the common name for cephalopods of the family Nautilidae. They are the only living members of the subclass Nautiloidea. In appearance, they have not changed much in millions of years. The term chambered nautilus is also used for any species of the Nautilidae.
  Many biologists see them as 'living fossils', because they are the only living descendents of the group which gave rise to the ammonites in the Palaeozoic era.
  Today, there are six living species in two genera. The species Nautilus pompilius, from Western Australia is the largest and best known. It get up to 27 cm in diameter. However, most other nautiluses never exceed 20 cm. The Bellybutton nautilus, Nautilus macromphalus, is the smallest species, usually measuring only 16 cm.
  The radula is wide and has exactly nine teeth. There are two pairs of gills.
context_section_description: In order to swim, the nautilus draws water into and out of the living chamber and makes use of jet propulsion. When water is inside the chamber, the siphuncle extracts salt from it and diffuses it into the blood. When water is pumped out, the animal adjusts its buoyancy with the gas contained in the chamber. Buoyancy can be controlled by the  pumping of gas and fluid into or from the chambers using the siphuncle. The control of buoyancy like this limits the nautilus; they cannot operate under extreme pressure.
  In the wild, nautilus usually lives in depths of about 300 m, rising to around 100 m at night for feeding, mating and egg laying. The shell of the nautilus cannot withstand depths greater than approximately 800 m.
caption: Nautilus tentacles
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