Nautilus

This article is about the mollusc species. See Nautilus (disambiguation) for more terms.

The nautilus is a marine creature of the Cephalopoda class. It is the sole member of the subclass Nautiloidea, order Nautilida. Nautilus also happens to be one of the two genera in the Nautilidae family. Allonautilus is the other genus and is very similar to Nautilus. They are found only in the western Pacific, inhabiting waters around coral reefs. The species has survived relatively unchanged for millions of years, much like the coelacanth.

The nautilus is similar in general form to other cephalopods, with a prominent head and undifferentiated tentacles, the nautilus have up to ninety tentacles, although without suckers. Unlike the others of the class the bony structure of the body is externalised as a shell, providing protection and buoyancy.

The shell is calcareous and internally divided in chambers, phragmocone, that are divided by septa and are all pierced by a tube, the siphuncle. The last fully open chamber is the living chamber. As the nautilus matures its body moves forward, sealing the shell behind it. Adults can have thirty or more chambers to their shell. The buoyancy is generally neutral, but it can be controlled by gas and fluid being pumped into or from the chambers by an osmotic process along the siphuncle. The control of buoyancy in this manner limits the nautilus; they cannot operate under high hydrostatic pressures and few venture below 300 m. Around 750 m is the maximum depth of any species.

Like other cephalopods they swim by jet action, using their hyponome and by pistoning water by head movements into and out of the living chamber. They are predators and feed mainly on shrimp and other small sea-life.

Unlike other cephalopods they do not have good vision. Their eye structure is highly developed but lacks a lens - there is simply a hole through which water can pass.

The nautilis are sexually dimorphic and reproduce by eggs. Attached to rocks in shallower waters they take twelve months to develop before hatching out at around 30 mm long. The largest adults are no more than 300 mm in diameter.

Classification

• CLASS CEPHALOPODA
  • Subclass Nautiloidea: Nautiluses
    • Order Nautilida
      • Family Nautilidae
        • Allonautilus perforatus
        • Crusty Nautilus, Allonautilus scrobiculatus
        • Palau Nautilus, Nautilus belauensis
        • Bellybutton Nautilus, Nautilus macromphalus
        • Nautilus pompilius
          • Emperor Nautilus, Nautilus pompilius pompilius
          • Chambered Nautilus, Nautilus pompilius suluensis
        • White-patch Nautilus, Nautilus stenomphalus
    • Order Bacritina: extinct ancestors of ammonites and coleoids
  • Subclass Coleoidea: Squids, octopuses, cuttlefish

Fossil records indicate that the class was much more extensive and varied in the past. They developed in the Cambrian period and were a significant sea predator in the Ordovician period, certain species reaching over 2.5 metres in size. The other subclass Coleoidea diverged over 400 million years ago and the nautilis is relatively unchanged since that time.

The Chambered Nautilus in literature and art

Chambered Nautilus is the common name for Nautilus pompilius. The shell, when cut away as in the photograph above reveals a lining of lustrous mother-of-pearl, and displays a nearly perfect equiangular spiral with pleasing proportions that bear a mathematical relationship to the golden section. Small natural history collections were common in mid-1800s Victorian homes, and chambered nautilus shells were popular decorations.

The Chambered Nautilus is the title and subject of a poem by Oliver Wendell Holmes, in which he admires the "ship of pearl" and the "silent toil/That spread his lustrous coil/Still, as the spiral grew/He left the past year's dwelling for the new." He concludes with the peroration:

Build thee more stately mansions, O my soul,

As the swift seasons roll!

Leave thy low-vaulted past!

Let each new temple, nobler than the last,

Shut thee from heaven with a dome more vast,

Till thou at length art free,

Leaving thine outgrown shell by life's unresting sea!

A painting by Andrew Wyeth, entitled "Chambered Nautilus," shows a woman in a canopied bed; the composition and proportions of the bed and the window behind it mirror those of a chambered nautilus lying on a nearby table.