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For other uses, see Nautilus (disambiguation).
Nautilus (from Greek ναυτίλος, 'sailor') is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of suborder Nautilina. It comprises six species in two genera, the type of which is genus Nautilus. Though it more specifically refers to 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 "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). However, this octopus is not closely related to the Nautiloidea.
[edit] Anatomy 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. These are the only remnants of the ancestral metamerism to be visible in extant cephalopods.[1]:56 Nautilus pompilius is the largest species in the genus. One form from western Australia may reach 26.8 centimetres (10.6 in) in diameter. However, most other nautilus species never exceed 20 centimetres (7.9 in). Nautilus macromphalus is the smallest species, usually measuring only 16 centimetres (6.3 in). [edit] ShellNautiluses are the sole living cephalopods whose bony body structure is externalized as a shell. The animal can withdraw completely into its shell and close the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled, aragonitic,[2] nacreous and pressure resistant, imploding at a depth of about 800 metres (2,600 ft). The nautilus shell is composed of 2 layers: a matte white outer layer, and a striking white iridescent inner layer. 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. Internally, the shell divides into camerae (chambers), the chambered section being called the phragmocone. The divisions are defined by septa, each of which is pierced in the middle by a duct, the siphuncle. As the nautilus matures it creates new, larger camerae, and moves its growing body into the larger space, sealing the vacated chamber with a new septum. The camerae increase in number 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 above, the shell is darker in color and marked with irregular stripes, which helps it blend into the dark water below. The underside is almost completely white, making the animal indistinguishable from brighter waters near the surface. This mode of camouflage is named countershading. The nautilus shell presents one of the finest natural examples of a logarithmic spiral, although it is not a golden spiral. The use of nautilus shells in art and literature is covered at nautilus shell.
[edit] TentaclesNautilus tentacles differ from those of other cephalopods. Lacking pads, the tentacles stick to prey by virtue of their ridged surface.[3] Nautiloids have a powerful grip. Attempts to take an object already seized by a Nautilus, may tear tentacles away from the creature, which remain firmly attached to the surface of the object.[3] Two pairs of tentacles are separate from the other 90-ish, the pre-ocular and post-ocular, situated before and behind the eye. These are more evidently grooved, with more pronounced ridges. They are extensively ciliated and serve an olfactory purpose.[3] [edit] Physiology[edit] Buoyancy and movement  • File format: Ogg multiplexed audio/video file, Theora/Vorbis • File size: 1.29 MB • Duration: 5.0s  In order to swim, the nautilus draws water into and out of the living chamber with its hyponome, which uses jet propulsion. While water is inside the chamber, the siphuncle extracts salt from it and diffuses it into the blood. The animal adjusts its buoyancy by osmotically pumping gas and fluid into or out of the camerae along the siphuncles. This limits them; they cannot operate under the extreme hydrostatic pressures found at depths greater than approximately 800 metres (2,600 ft). In the wild, nautiluses usually inhabit depths of about 300 metres (980 ft), rising to around 100 metres (330 ft) at night to feed, mate and to lay eggs. [edit] Senses.jpg) Unlike many other cephalopods, they do not have good vision; their eye structure is highly developed but lacks a solid lens. They have a simple "pinhole" eye open to the environment. Instead of vision, the animal is thought to use olfaction as the primary sense for foraging, locating or identifying potential mates. [edit] Reproduction and lifespanNautiluses 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 millimetres (1.2 in) juveniles hatch. Females spawn once per year and regenerate their gonads, making nautiluses the only cephalopods to present iteroparity or polycyclic spawning. 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. The lifespan of nautiluses is about 20 years, which is exceptionally long for a cephalopod. [edit] Ecology[edit] DietNautiluses are predators that feed mainly on shrimp, small fish and crustaceans, which are captured by the tentacles. Due to the limited energy they expend in swimming, nautiloids only need to eat once a month. [edit] Range and habitatNautiluses 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. [edit] EvolutionFossil records indicate that nautiluses have not evolved much during the last 500 million years. 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 metres (8 ft 2 in) 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. Nautiloids were much more extensive and varied 200 million years ago. [edit] TaxonomyThe genus Nautilus contains six extant species and several extinct species.
[edit] Dubious or uncertain taxaThe following taxa associated with the family Nautilidae are of uncertain taxonomic status:[4]
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