Nautilus
Nautilus is the common name of any marine creatures of the
cephalopod family Nautilidae, the sole family of the suborder Nautilina. It comprises 6 very similar species in 2 genera, the type of which is the genus
Nautilus. The name
chambered nautilus is also used for any species of the Nautilidae, though it more specifically refers to the species
Nautilus pompilius.
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."
Encyclopedia
Nautilus is the common name of any marine creatures of the
cephalopod family
Nautilidae, the sole family of the suborder
Nautilina. It comprises 6 very similar species in 2 genera, the type of which is the genus
Nautilus. The name
chambered nautilus is also used for any species of the Nautilidae, though it more specifically refers to the species
Nautilus pompilius.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 disk-bearing arms as sails .
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 268 mm in diameter. However, the majority of other nautiluses 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,
mother-of-pearl-lined and pressure resistant . 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 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 camera 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.
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 it
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.
The animal can also crawl on land or on the seabed.
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. 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 and reproduce by laying eggs. Attached to rocks in shallow waters, the eggs take twelve months to develop before hatching out at around 30 mm long. 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.
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 is an extinct genus [i] of the nautiloid [i]s, and is one of the most primitive known cephalopod [i] ...
. 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
External links
