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.

Having survived relatively unchanged for millions of years, nautiluses represent the only living members of the subclass Nautiloidea, and are often considered to be "living fossils."

The name "Nautilus" originally referred to the Argonauta, otherwise known as paper nautiluses, because they allegedly use their two expanded arms as sails (cf. Aristotle Historia Animalium 622b).

Description

The nautilus is similar in general form to other cephalopods, with a prominent head and tentacles. Nautiluses typically have more tentacles than other cephalopods, up to ninety. These tentacles are arranged into two circles and, unlike the tentacles of other cephalopods, they have no suckers, are undifferentiated and retractable. The radula is wide and distinctively has nine teeth. There are two pairs of gills.

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.

The shell

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.

The nautilus shell presents one of the finest natural examples of a logarithmic spiral. (It is sometimes incorrectly claimed to be a golden spiral as well.)

Buoyancy and movement

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.

Diet and sensory system

Nautiluses are predators and feed mainly on shrimp, small fish and crustaceans, which are captured by the tentacles. Due to the limited energy expended in 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. They have a simple "pinhole" lens through which water can pass. Instead of vision, the animal is thought to use olfaction as the primary sense for foraging, locating or identifying potential mates.

Reproduction and lifespan

Nautiluses are sexually dimorphic, in that males have four tentacles modified into an organ, called the "spadix," which transfers sperm into the female's mantle during mating. 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 mm juveniles hatch. Females spawn once per year and regenerate their gonads, making nautiluses the only cephalopods to present iteroparity or polycyclic spawning. The lifespan of nautiluses is about 20 years, which is exceptionally long for a cephalopod.

Classification

The genus Nautilus contains six extant species and several extinct species.

• Genus Allonautilus
• Allonautilus perforatus
• Allonautilus scrobiculatus
• Genus Nautilus
• Nautilus belauensis
• †Nautilus clarkanus
• †Nautilus cookanum
• Nautilus macromphalus
• Nautilus pompilius (type)
• Nautilus pompilius pompilius
• Nautilus pompilius suluensis
• †Nautilus praepompilius
• Nautilus stenomphalus

Dubious or uncertain taxa

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)]

Distribution

Nautiluses are only found in the Indo-Pacific, from 30° N to 30° S latitude and 90° to 185° W longitude. They inhabit the deep slopes of coral reefs.

Natural history

Fossil records indicate that nautiluses have not evolved much during the last 500 million years, and nautiloids were much more extensive and varied 200 million years ago. Many were initially straight-shelled, as in the extinct genus Lituites. They developed in the Cambrian period and became a significant sea predator in the Ordovician period. Certain 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. Extinct relatives of the nautilus include ammonites, such as the baculites and goniatites.

See also

• Cephalopod size, for maximum shell diameters
• The Chambered Nautilus, a poem of Oliver Wendell Holmes
• Coelacanth, another "living fossil"
• History of animals by Conrad Gesner, first book with fossil illustrations.

References

• Ward, P.D. 1988. In Search of Nautilus. Simon and Schuster.

External links

• Nautilidae discussion forum at TONMO.com
• Waikïkï Aquarium: Marine Life Profile: Chambered Nautilus
• A molecular and karyological approach to the taxonomy of Nautilus