Nautilus

Alternate meanings: The Nautilus, USS Nautilus, French Nautile[?]

Nautilus shell

The nautilus is a marine creature of the cephalopoda class. It is the sole member of the subclass Nautiloidea. They are found only in the western Pacific, inhabiting waters around coral reefs.

The nautilus is similar in general form to other cephalopods, with a prominent head and 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.

There are seven modern species: Allonautilus perforatus, A. scrobiculatus, Nautilus belauensis, N. macromphalus, N. pompilius pompilius, N. pompilius suluensis, N. stenomphalus. Common names are the Chambered nautilus, Emperor nautilus, and Bellybutton nautilus.

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.