View source for Nautilus

{{Taxobox
| name = Nautilus
| image = Nautilus pompilius.jpg
| image_width = 280px
| image_caption = Chambered nautilus: ''Nautilus pompilius''
| regnum = [[Animal]]ia
| phylum = [[Mollusca]]
| classis = [[Cephalopod]]a
| subclassis = [[Nautiloid]]ea
| ordo = Nautilida
| subordo_authority = [[Louis Agassiz|Agassiz]], 1847 
| familia = '''Nautilidae'''
| familia_authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1825
| subdivision_ranks = [[Genus|Genera]]
| subdivision = 
''Allonautilus''<br>
''Nautilus''}}
[[File:Nautilus diagram-en.svg|thumb|right|280px|Anatomy of a nautilus]]

'''Nautilus'''<ref>from [[Greek language|Greek]] ''nautilos'', 'sailor'</ref> is the common name for [[cephalopod]]s of the [[family (biology)|family]] '''Nautilidae'''. They are the only living members of the subclass [[Nautiloidea]].<ref>Sweeney M.J. 2002. [http://tolweb.org/accessory/Nautilidae_Taxa?acc_id=2324 Taxa associated with the Family Nautilidae Blainville, 1825.] {{Webarchive|url=https://web.archive.org/web/20070930204032/http://tolweb.org/accessory/Nautilidae_Taxa?acc_id=2324 |date=2007-09-30 }} Tree of Life web project.</ref> 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 [[biologist]]s see them as '[[living fossil]]s', 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&nbsp;cm in diameter. However, most other nautiluses never exceed 20&nbsp;cm. The Bellybutton nautilus, ''Nautilus macromphalus'', is the smallest species, usually measuring only 16&nbsp;cm.

The [[radula]] is wide and has exactly nine teeth. There are two pairs of [[gill]]s.

==The shell==
[[Image:Nautilus species shells.png|thumb|right|200px|Nautilus shells: ''N. macromphalus'' (left), ''A. scrobiculatus'' (centre), ''N. pompilius'' (right)]]

Nautiluses are the only cephalopods who have an external [[Animal shell|shell]]. This is an ancestral or [[basal]], feature. The animal can withdraw completely into its shell. It can the close the opening with a leathery hood formed from two specially folded [[tentacle]]s. The shell is [[coil]]ed and [[calcareous]], with [[mother-of-pearl]] lining. It is [[pressure]] resistant, but will implode at a depth of about 800 m. The nautilus shell is made of two layers: the outer layer is a dull white, while the inner layer is a striking white with [[iridescence]]. The innermost portion of the shell is pearlescent, blue-gray. A '''osmena pearl''' is not a [[pearl]], but a [[jewellery]] product got from this part of the shell.

The shell is internally divided into chambers. All the walls between chambers ('septa') are pierced in the middle by a duct, the [[siphuncle]]. Through this the animal can change the air pressure in the chambers. As the nautilus matures its body moves forward, sealing each chamber in turn with a new ''septum'' (wall). The last fully open chamber, the largest one, is used as the living chamber. The number of chambers increases from around four at the moment of [[hatching]] to thirty or more in adults.

The shell colour keeps the animal [[camouflage|hidden]] in the water. When seen from the top, the shell is darker in color and marked with irregular stripes, which makes it blend into the darkness of the water below. The underside is almost completely white, making the animal indistinguishable from brighter waters near the ocean surface. This mode of camouflage is called ''countershading''.

The nautilus shell is one of the finest natural examples of a [[logarithmic spiral]].

<gallery>
Image:NautilusTop.jpg|A nautilus shell when viewed from above
Image:NautilusBottom.jpg|The same shell viewed from underneath
Image:NautilusCutawayLogarithmicSpiral.jpg|Hemishell showing the chambers in a [[logarithmic spiral]]
</gallery>

==Natural history==
===Buoyancy and movement===
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 (biology)|egg]] laying. The shell of the nautilus cannot withstand depths greater than approximately 800 m.

[[Image:Nautilus tentacles.jpg|thumb|right|175px|Nautilus tentacles]]
[[Image:NC Armoires.gif|thumb|The nautilus shell features prominently in the official emblem of [[New Caledonia]].]]

=== Diet and sensory system ===
Nautiluses are [[predator]]s and feed mainly on [[shrimp]], small [[fish]] and [[crustacean]]s, which are captured by the tentacles. However, due to the very little energy they devote to swimming, they need only eat once a month. Unlike other cephalopods, they do not have good vision; their eye structure is highly developed but lacks a solid [[Lens (anatomy)|lens]]. They have a simple "[[Pinhole camera|pinhole]]" lens through which water can pass. Instead of vision, the animal is thought to use [[olfaction]] as the primary sensory means during [[foraging]], locating or identifying potential mates.

=== Evolution ===
[[Fossil]] records indicate that nautiloids have not changed much (at least in their body form) for the last 500 million years.<ref>Kümmel,B. 1964. Nautiloidae-Nautilida. In Teichert and Moore (eds) ''Treatise on invertebrate paleontology''. Geological Society of America and Univ. of Kansas Press.</ref><ref>Saunders  W.B. & Landman N.H. (eds) 2010. ''Nautilus: the biology and paleobiology of a living fossil''. Springer, 25–32.</ref><ref>Clarkson E.N.K 1998. ''Invertebrate palaeontology''. Oxford: Blackwell Science, 233. {{ISBN|0-632-05238-4}}</ref> Nautiloids were much more common and varied 200 million years ago. Many of the early ones were straight-shelled, as in the extinct genus ''Lituites''. They developed in the [[Cambrian]] and became a significant sea [[predator]] in the [[Ordovician]] period. Some species reached over 2.5 meters in size. The other cephalopod subclass, [[Coleoidea]], diverged from the Nautilidae long ago and the nautilus has remained relatively unchanged since. [[Extinction|Extinct]] relatives of the nautilus include [[ammonite]]s, which were very important for many millions of years.  Other groups were the baculites and goniatites.

=== Reproduction and lifespan ===
Nautiluses are [[sexual dimorphism|sexually dimorphic]] and reproduce by laying [[egg (biology)|egg]]s. Attached to rocks in shallow waters, the eggs take twelve months to develop before hatching out at around 30&nbsp;mm long. Females spawn once per year and regenerate their [[gonad]]s, making nautiluses the only cephalopods to repeat reproduction. The lifespan of nautiluses is about 20 years, which is exceptionally long for a cephalopod.

=== Distribution ===
Nautiluses are only found in the tropical [[Indian Ocean|Indian]] and [[Pacific Ocean]]s, from 30° N to 30° S latitude and 90° to 185° W longitude. They live on the deep slopes of [[coral reef]]s.

== Related pages ==
*[[Ammonite]]

== References ==
{{reflist}}
*Ward, P.D. 1988. ''In search of Nautilus''. Simon and Schuster.

== Other websites ==
{{wikispecies|Nautilidae}}
*[http://www.arkive.org/nautilus/nautilus-pompilius/ ''Nautilus pompilius'' at ARKive] {{Webarchive|url=https://web.archive.org/web/20111112012942/http://www.arkive.org/nautilus/nautilus-pompilius/ |date=2011-11-12 }}
* [http://www.tonmo.com/forums/forumdisplay.php?f=49 Nautilidae discussion forum at TONMO.com] {{Webarchive|url=https://web.archive.org/web/20061111153002/http://www.tonmo.com/forums/forumdisplay.php?f=49 |date=2006-11-11 }}
* [http://waquarium.mic.hawaii.edu/MLP/root/html/MarineLife/Invertebrates/Molluscs/Nautilus.html Waikïkï Aquarium: Marine Life Profile: Chambered Nautilus] {{Webarchive|url=https://web.archive.org/web/20060409175523/http://waquarium.mic.hawaii.edu/MLP/root/html/MarineLife/Invertebrates/Molluscs/Nautilus.html |date=2006-04-09 }}
*[http://www.cephbase.utmb.edu/refdb/pdf/8036.pdf A molecular and karyological approach to the taxonomy of ''Nautilus''] {{Webarchive|url=https://web.archive.org/web/20050426132101/http://www.cephbase.utmb.edu/refdb/pdf/8036.pdf |date=2005-04-26 }}

[[Category:Cephalopods]]
[[Category:Living fossils]]