A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae, suborder Nautilina, order Nautilida, and subclass Nautiloidea.
It comprises nine living 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 of the Nautilidae. All are protected under CITES Appendix II. Depending on species, adult shell diameter is between .
The Nautilidae, both extant and extinct, are characterized by involute or more or less convoluted shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass Nautiloidea, and are often considered "living fossils".
thumb|The first and oldest fossil of [[chambered nautilus displayed at Philippine National Museum.]]
Anatomy
upright=1.8|thumb|Diagram of the anatomical structure of a female N. pompilius including most of its internal organs
Tentacles
The arm crown of modern nautilids (genera Nautilus and Allonautilus) is very distinct in comparison to coleoids. Unlike the ten-armed Decapodiformes or the eight-armed Octopodiformes, nautilus may possess any number of tentacles (cirri) from 50 to over 90 tentacles depending on the sex and individual. These tentacles are classified into three distinct categories: ocular, digital, and labial (buccal). There are two sets of ocular tentacles: one set in front of the eye (pre-ocular) and one set behind the eye (post-ocular). The digital and labial tentacles are arrayed circularly around the mouth, with the digital tentacles forming the outermost ring and the labial tentacles in between the digital tentacles and the mouth. There are 19 pairs of digital tentacles that, together with the ocular tentacles, make up the 42 appendages that are visible when observing the animal (not counting the modified tentacles that form the hood). The labial tentacles are generally not visible, being smaller than the digital tentacles, and more variable both in number and in shape. Males modify four of their labial tentacles into the spadix, which delivers spermatophores into the female during copulation. The sheaths of the digital tentacles are fused at their base into a single mass referred to as the cephalic sheath. The digital cirri can be fully withdrawn into the sheath and are highly flexible, capable of extending just over double their fully retracted length and show a high degree of allowable bendability and torsion. Release is triggered through contraction of the tentacle musculature rather than the secretion of a chemical solvent, similar to the adhesion/release system in Euprymna, though it is unclear whether these adhesives are homologous. The ocular tentacles show no adhesive capability but operate as sensory organs. Both the ocular tentacles and the eight lateral digital tentacles show chemoreceptive abilities; the preocular tentacles detect distant odor and the lateral digital tentacles detect nearby odor.
Digestive system
The radula is wide and distinctively has nine teeth.
The mouth consists of a parrot-like beak made up of two interlocking jaws capable of ripping the animal's food — mostly crustaceans — from the rocks to which they are attached. Males can be superficially differentiated from females by examining the arrangement of tentacles around the buccal cone: males have a spadix organ (shaped like a spike or shovel) located on the left side of the cone making the cone look irregular, whereas the buccal cone of the female is bilaterally symmetrical. Oxygenated blood arrives at the heart through four ventricles and flows out to the animal's organs through distinct aortas but returns through veins which are too small and varied to be specifically described. The one exception to this is the vena cava, a single large vein running along the underside of the crop into which nearly all other vessels containing deoxygenated blood empty. All blood passes through one of the four sets of filtering organs (composed of one pericardial appendage and two renal appendages) upon leaving the vena cava and before arriving at the gills for re-oxygenation. Blood waste is emptied through a series of corresponding pores into the pallial cavity.
Nervous system
The central component of the nautilus nervous system is the oesophageal nerve ring which is a collection of ganglia, commissures, and connectives that together form a ring around the animal's oesophagus. From this ring extend all of the nerves forward to the mouth, tentacles, and funnel; laterally to the eyes and rhinophores; and posteriorly to the remaining organs.
The nerve ring does not constitute what is typically considered a cephalopod "brain": the upper portion of the nerve ring lacks differentiated lobes, and most of the nervous tissue appears to focus on finding and consuming food (i.e., it lacks a "higher learning" center). Nautili also tend to have rather short memory spans, and the nerve ring is not protected by any form of brain case.
Shell
thumb|Nautilus half-shell showing the camerae in a [[logarithmic spiral|alt=]] thumb|Section cut of a nautilus shell
Nautili are the sole living cephalopods whose bony body structure is externalized as a planispiral 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, nacreous and pressure-resistant, thought to be imploding at a depth of about . and a striking white iridescent inner layer. The innermost portion of the shell is a pearlescent blue-gray. The osmeña pearl, contrarily to its name, is not a pearl, but a jewellery 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 4 at the moment of hatching to 30 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 called 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.
Size
N. pompilius is the largest species in the genus. One form from Indonesia and northern Australia, once called N. repertus, may reach in diameter. However, most nautilus species never exceed . Nautilus macromphalus is the smallest species, usually measuring only . A dwarf population from the Sulu Sea (Nautilus pompilius suluensis) is even smaller, with a mean shell diameter of . It is thought that this is related to the use of asymmetrical contractile cycles and may be an adaptation to mitigate metabolic demands and protect against hypoxia when foraging at depth. While water is inside the chamber, the siphuncle extracts salt from it and diffuses it into the blood.
The animal adjusts its buoyancy only in long term density changes by osmosis, either removing liquid from its chambers or allowing water from the blood in the siphuncle to slowly refill the chambers. This is done in response to sudden changes in buoyancy that can occur with predatory attacks of fish, which can break off parts of the shell. This limits nautiluses in that they cannot operate under the extreme hydrostatic pressures found at depths greater than approximately , and in fact implode at about that depth, causing instant death. The maximum depth at which they can regulate buoyancy by osmotic removal of chamber liquid is not known.
The "ear" of the nautilus consists of structures called otocysts located immediately behind the pedal ganglia near the nerve ring. They are oval structures densely packed with elliptical calcium carbonate crystals.
Brain and intelligence
Nautiluses are much closer to the first cephalopods that appeared about 500 million years ago than the early modern cephalopods that appeared maybe 100 million years later (ammonoids and coleoids). They have a seemingly simple brain, not the large complex brains of octopus, cuttlefish and squid, and had long been assumed to lack intelligence. But the cephalopod nervous system is quite different from that of other animals, and recent experiments have shown not only memory, but a changing response to the same event over time.
Reproduction and lifespan
Nautiluses reproduce by laying eggs. Gravid females attach the fertilized eggs, either singly or in small batches, to rocks in warmer waters (21–25 Celsius), whereupon the eggs take eight to twelve months to develop until the 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 cirri modified into an organ, called the "spadix", which transfers sperm into the female's mantle during mating. At sexual maturity, the male shell becomes slightly larger than the female's. Males have been found to greatly outnumber females in practically all published studies, accounting for 60 to 94% of all recorded individuals at different sites. However, nautiluses typically do not reach sexual maturity until they are about 15 years old, limiting their reproductive lifespan to often less than five years.
Nautiluses have several reproductive organs whose functions are not yet entirely known. In nautilus males, this is the Van der Hoeven's organ; and in nautilus females, these are the Organ of Valenciennes and Owen's laminated organ.