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Introduction
Nautilus is one of two genera within the family Nautilidae. The current consensus is that the genus consists of four valid species, although this remains the subject of debate. Nautilus are typically found in shallow waters in tropical seas, mainly within the Indo-Pacific . The genus Nautilus has previously included several species represented in the fossil record, however these have since been reclassified, and the genus now only includes extant species.

Classification
The classification of nautilus species has been a source of contention for decades, and the genus has been redefined at several points throughout its history. Nautilus is they type genus of the family Nautilidae, and was originally defined as any coiled shell species with simple sutures, or walls between compartments. Any shells with complex sutures were assigned to the genus Ammonites. This definition of the genus persisted from its inception in 1758 by Carl Linnaeus up to the mid-1900s, where paleo-biologist Arthur Miller provided a detailed description of the shell of Nautilus Pompilius, which became the type species of the genus. In 1951, he determined that the genus could only describe living species of Nautilus, despite many fossil species having already been assigned to it.

In the years following this conclusion, two newly discovered fossil species were still assigned to the genus however, namely Nautilus Ucrainicus, and Nautilus Praepompilius, recovered from Ukraine, and the Ust-Urt Plateau respectively. These species have since been removed from the genus however.

As of 2010, 11 species have been described, some of these featuring several variants, or subspecies. The details of their classification are listed below.

Controversy over species
There has been much debate over the vailidity of species within the genus, and several identified species have since been reclassified, or determined as taxonomic synonyms or nomen dubium (a doubtful classification). As of 2015, only four Nautilus species have been recognised, specifically N. Pompilius, N. Macromphalus, N. Stenomphalus,  and N. Belauensis ''. Nautilus Scrobiculatus, now Allonautilus Scrobiculatus has been assigned to a new genus, and several species listed above have been identified as synonyms of this species, namely N. Umbiculatus, N. Perforatus, and N. Texturatus ''. Much of the confusion regarding the classification of species is due to the rarity of live species. The majority of described species have been determined on the drift shells of individuals alone, leading to inaccuracies when defining species divisions. In fact, it was not until 1996, that soft tissues of any Nautilus species had been dissected.

Genetic Studies
Several genetic studies have also been conducted on select species of Nautilus, from 1995 onwards, most of which focus on a single gene, COI, which ultimately lead to the decision to remove N.Scrobiculatus from the genus. Furthermore, some biologists claim that N.Stenomphalus and N. Belauensis are members of N. Pompilius based on both genetic and morphological data. One study, sampling nautiluses in 2012, demonstrated that the features of Nautilus Pompilius and Nautilus Stenomphalus exist along a spectrum, with a range of individuals displaying a combination of characteristics, further invalidating them as separate species.

Additionally, mitochondrial DNA studies, utilising two gene regions, also have led to the notion that many of the morphological differences between different nautilus populations are simply localised variations within the single nautilus species. This same 2011 study however, suggested that N. Macronphalus was a species synonymous with A.Scrobiculatus, leading to further debate over classification. These findings were also reinforced by the initial DNA studies conducted on the genus, which only revealed two phylogenetic species.

More recently, a 2017 study determined that there were likely 5 nautilus species, however these did not exactly correlate to the described species of the genus. Whilst the status of N. Macromphalus, N. Stenomphalus, and N. Pompilius were validated by the genetic study, two undescribed, but genetically distinct, species were discovered in the South Pacific. One of these cryptic species was recorded from Vanuatu, whilst the other from Fiji and American Samoa. Whilst this study recorded five species, its results suggested that N. Belauensis and N. Repertus were synonyms with N. Pompilius.

Evolution
In addition to defining species, genetic studies have also provided evidence for the evolution of the genus over time. Mitochondrial DNA studies have indicated that the genus is currently undergoing evolutionary radiation in the indo-pacific. The divergence between the genus Nautilus, and its sister taxon Allonautiluslikely occurred around New Guinea, and the Great Barrier Reef, during the mesozoic. From there, populations of Nautilus split diverged further, involving migrations east to Vanuatu, Fiji, and American Samoa, as well as west, to the Phillipines, Palau, Indonesia, and Western Australia.

Habitat and Distribution
Species within the genus Nautilus are localised to the Indo-pacific, specifically the tropical seas within this area, however the full extent of their geographic distribution has yet to be recorded . The movements of Nautilus species are greatly restricted by water depth. Nautilus are unable to easily move across areas deeper than 800m, and most of their activity occurs at a depth of 100-300m deep . Nautilus can occasionally be found closer to the surface than 100m, however the minimum depth they can reach is determined by factors such as water temperature and season . All Nautilus species are likely endangered, based on information from Nautilus Pompilius overfishing in the Phillipines, which resulted in an 80% decline in the population from 1980-2010.

Many shells recovered from areas of the world have not yet been identified down to the level of species, however are still identifiable as members of the Nautilus genus. Shells have been found across a wide range of coastal areas, including Korea, Australia, Seychelles, Maritius, the Phillipines, Taiwan, Japan, Thailand, India, Sri Lanka, Kenya, and South Africa. This does not necessarily imply live populations of Nautilus at these sites however, as Nautilus drift shells are able to make their way across oceans via currents. Following the death of an individual, Nautilus shells float to the surface, where they can remain for a considerable time period. An experiment with a Nautilus shell in an aquarium resulted in the shell floating for over two years, and one recovered shell was revealed to have been afloat for a period of 11 years. Furthermore, shells have been demonstrated to drift considerable distances in this time, contributing to their extensive distribution across coastal areas. Several ocean currents have been identified to contribute to this process. The Koroshio current carries shells from the Phillipines to areas such as Japan, and the Equatorial current is responsible for many of the shells recovered from the Marshall Islands.

Behaviour
Nautilus have been observed to spend days in deeper areas around coral reefs, to avoid predation from turtles and carnivorous fish, and ascend to shallow areas of the reef during nights. Nautilus species usually travel, and feed alone. Nautilus return to deeper areas following daybreak, and also lay eggs in these locations, which take approximately one year to hatch. This behaviour may have ensured their survival during the cretaceous extinction, when shallow areas of ocean became inhospitable. Nautilus exhibit a sex ratio biased towards male individuals. This phenomenon has been observed at several locations, with population samples consisting of up to 95% males. The reason for this is currently unknown.

Nautilus have been noted to show a large vertical range of depth, close to 500m, and were demonstrated to be at risk of implosion, at greater pressures and depths. Depending on the species, the shells of live Nautilus will collapse at depths from 750 m and deeper. Feeding behaviour has been identified from observation of captive individuals, as well as the stomach contents of wild specimens. Nautilus are opportunistic scavengers, and feed on a variety of crustaceans, including their moults, and fish, however they have been observed to feed on chicken and bat bait. Initially, Nautilus were thought to actively hunt certain prey, however this activity has only been recorded in traps, where prey species are confined in close proximity to Nautilus. Nautilus locate these food sources by using their tentacles, which have chemosensory functions, as well as by sight. Nautilus participate in routine vertical migration, in which they ascend to shallow areas of reefs, between 100 and 150 metres deep, during the night to feed, and later descend to depths of 250-350 metres during the day, however these depths may vary depending on local geographic characteristics. Nautilus are able to ascend at speeds of approximately 2 metres per minute, and descend at speeds of 3 metres per minute.

Predation
Several species have been observed to prey on Nautilus. Octopi were listed as predators of the genus, following an incident where an octopus was shown to have partially consumed a Nautilus in a trap. Additionally many drift shells exhibit small holes which match the patterns produced by octopus boring into shell to feed. Teleosts, such as triggerfish, have also been shown to feed on Nautilus, by violently charging at individuals to break their shells. In response to attacks from predators,  Nautilus withdraw into their shells.

Sensory Organs
Nautilus have unique sensory organs, which differ from related genera in several ways. The eyes of Nautilus species lack ocular muscles, unlike other cephalopods, and instead move their eyes via the stalk, which contains both muscle and connective tissue. Additionally, Nautilus eyes lack any lens or cornea, and only have an aperture to allow for light.

Below their eyes, Nautilus also feature rhinophores, which are small sacs with cilia. This organ likely contains chemoreceptors, in order to detect food, or sample the surrounding water. The tentacles of the Nautilus perform several sensory functions as well. Their ocular and preocular tentacles feature cilia, and probably operate as mechanoreceptors, while their digital tentacles have been hypothesized to feature a range of receptor cells.

Nautilus in Aquaria
It is possible to keep Nautilus in aquariums, however great care is necessary in order to ensure their survival in captivity. The survival rate of Nautilus in captivity is not great, due to the stress that individuals are subjected to during transportation however. As many as 50-80% of Nautilus die during transportation, and this percentage can be higher, if individuals are exposed to high temperatures.

In captivity, Nautilus are generally fed a diet of whole shrimp, fish, crab, and lobster moults. Several Aquaria feature the genus, however there have not yet been any successful attempts of breeding in captivity, despite viable eggs being produced at several locations.

In addition to the geographic distribution of Nautilus, our knowledge of the Nautilus habitat also comes from captive specimens in aquaria. Keeping Nautilus in captivity, and holding cages, have demonstrated that prolonged exposure to temperatures over 25 degrees Celsius will eventually result in death after several days. However, individuals have been documented to experience temperatures higher than this, and survive, so long as they are not exposed to these temperatures for longer than 10 hours. Optimal temperatures for the genus tend to range from 9-21 degrees Celsius.

Reproduction
The majority of our knowledge regarding Nautilus reproduction comes from captive species in Aquaria. From these specimens, it appears that Nautilus do not have an elaborate courtship process. Males have been observed to attempt to mate with any object the same size and shape as another Nautilus. If a male is successful in finding a female however, the mating process follows, and afterwards, the male may continue to hold onto the female for a period ranging from minutes to hours.

Nautilus eggs are laid in capsules, which gradually harden when exposed to sea water. It is not yet known how exactly the juveniles break out of these capsules, yet it has been hypothesized that they are able to chew their way out, using their beak.