Nautilus

For other uses, see Nautilus (disambiguation).

*Nautilus*

**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

 File:Nautilus species shells.png
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

 File:Nautilus.ogg
 File:Nautilus oceanworld thailand.png
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. 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. They have a simple "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.

### 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:

## 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

 File:NC Armoires.gif
* Cephalopod size, for maximum shell diameters
* The Chambered Nautilus, a poem of Oliver Wendell Holmes
* Coelacanth, another "living fossil"

## References

1. ↑ Sweeney, M.J. 2002. Taxa Associated with the Family Nautilidae Blainville, 1825. Tree of Life web project.

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

## External links

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Wikimedia Commons has media related to: ***Nautilus***
* CephBase: Nautilidae
* Nautilidae discussion forum at TONMO.com
* Waikïkï Aquarium: Marine Life Profile: Chambered Nautilus
* A molecular and karyological approach to the taxonomy of *Nautilus*

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