Chambered nautilus (Nautilus)
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Nautilus (from Greek ναυτίλος, 'sailor') is the common name of any marine creatures of the cephalopod family Nautilidae, the sole family of the suborder Nautilina. It comprises six species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to the species Nautilus pompilius, the name chambered nautilus is also used for any species of the Nautilidae.
Nautilus pompilius is the largest species in the genus. One form from western Australia may reach 26.8 cm in diameter. However, most other nautilus species never exceed 20 cm. Nautilus macromphalus is the smallest species, usually measuring only 16 cm.
Nautiluses are the sole cephalopods whose bony structure of the body is externalized as a shell. The animal can withdraw completely into its shell, closing the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled, calcareous, nacreous and pressure resistant (imploding at a depth of about 800 m). The nautilus shell is composed of 2 layers: the outer layer is a matte white, while the inner layer is a striking white with iridescence. The innermost portion of the shell is a pearlescent blue-gray. The osmena pearl, contrarily to its name, is not a pearl, but a jewelry product derived from this part of the shell.
The shell is internally divided into chambers, the chambered section being called the phragmocone. The phragmocone is divided into camerae by septa, all of which are pierced in the middle by a duct, the siphuncle. As the nautilus matures its body moves forward, sealing the camerae behind it with a new septum. The last fully open chamber, also the largest one, is used as the living chamber. The number of camerae increases from around four at the moment of hatching to thirty or more in adults.
The shell coloration also keeps the animal cryptic 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. On the contrary, the underside is almost completely white, making the animal indistinguishable from brighter waters near the ocean surface. This mode of camouflage is named countershading.
In order to swim, the nautilus draws water into and out of the living chamber with the hyponome, which 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 osmotical pumping of gas and fluid into or from the camerae along the siphuncles. The control of buoyancy in this manner limits the nautilus; they cannot operate under extreme hydrostatic pressures.
In the wild, nautiluses usually inhabit 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.
The following taxa associated with the family Nautilidae are of uncertain taxonomic status:
|Binomial name and author citation||Current systematic status||Type locality||Type repository|
|Nautilus alumnus Iredale, 1944||Species dubium [fide Saunders (1987:49)]||Queensland, Australia||Not designated [fide Saunders (1987:49)]|
|Nautilus ambiguus Sowerby, 1848||Species dubium [fide Saunders (1987:48)]||Not designated||Unresolved|
|Nautilus beccarii Linne, 1758||Non-cephalopod; Foraminifera [fide Frizzell and Keen (1949:106)]|
|Nautilus calcar Linne, 1758||?Non-cephalopod; Foraminifera Lenticulina||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus crispus Linne, 1758||Undetermined||Mediterranean Sea||Unresolved; Linnean Society of London?|
|Nautilus crista Linne, 1758||Non-cephalopod; Turbo [fide Dodge (1953:14)]|
|Nautilus fascia Linne, 1758||Undetermined||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus granum Linne, 1758||Undetermined||Mediterranean Sea||Unresolved; Linnean Society of London?|
|Nautilus lacustris Lightfoot, 1786||Non-cephalopod; Helix [fide Dillwyn (1817:339)]|
|Nautilus legumen Linne, 1758||Undetermined||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus micrombilicatus Joubin, 1888||Nomen nudum|
|Nautilus obliquus Linne, 1758||Undetermined||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus pompilius marginalis Willey, 1896||Species dubium [fide Saunders (1987:50)]||New Guinea||Unresolved|
|Nautilus pompilius moretoni Willey, 1896||Species dubium [fide Saunders (1987:49)]||New Guinea||Unresolved|
|Nautilus pompilius perforatus Willey, 1896||Species dubium [fide Saunders (1987:49)]||New Guinea||Unresolved|
|Nautilus radicula Linne, 1758||?Non-cephalopod; Foraminifera Nodosaria||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus raphanistrum Linne, 1758||Undetermined||Mediterranean Sea||Unresolved; Linnean Society of London?|
|Nautilus raphanus Linne, 1758||Undetermined||Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus semi-lituus Linne, 1758||Undetermined||Liburni, Adriatic Sea||Unresolved; Linnean Society of London?|
|Nautilus sipunculus Linne, 1758||Undetermined||"freto Siculo"||Unresolved; Linnean Society of London?|
|Nautilus texturatus Gould, 1857||Nomen nudum|
|Octopodia nautilus Schneider, 1784||Rejected specific name [fide Opinion 233, ICZN (1954:278)]|
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