Nautilus (genus)
Nautilus is a genus of cephalopods in the family Nautilidae. Species in this genus differ significantly in terms of morphology from those placed in the sister taxon "Allonautilus". The oldest fossils of the genus are known from the Late Eocene Hoko River Formation, in Washington State and from Late-Eocene to Early Oligocene sediments in Kazakhstan. The oldest fossils of the modern species "Nautilus pompilius" are from Early Pleistocene sediments off the coast of Luzon in the Philippines. The commonly used term 'nautilus' usually refers to any of the surviving members of "Nautilidae", and more specifically to the  Nautilus pompilius species. The entire family of "Nautilidae", including all species in the genera "Nautilus" and "Allonautilus", is listed on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). 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 species within "Nautilus" has been contentious for decades; the genus has been redefined at several points throughout its history. "Nautilus" is the 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 1949, when the paleobiologist Arthur K. 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 which feature several variants, or subspecies. The details of their classification are listed below. Controversy over species. There has been much debate over the  validity 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, called COI. These studies 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 five "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 "Allonautilus" likely 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 Philippines, Palau, Indonesia, and western Australia. Sensory organs. "Nautilus" have unique sensory organs, which differ from  related genera in several ways. Unlike other cephalopods, the eyes of "Nautilus" species lack ocular muscles, and instead move via a 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. It has been suggested that this organ contains chemoreceptors, in order to detect food, or sample the surrounding water. Additionally, the tentacles of the "Nautilus" also perform several sensory functions. Their ocular and preocular tentacles feature cilia, and operate as mechanoreceptors , while their digital tentacles have been hypothesised to feature a range of receptor cells. 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 800 metres, and most of their activity occurs at a depth of 100-300 metres deep. "Nautilus" can occasionally be found closer to the surface than 100 metres, 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 Philippines, which resulted in an 80% decline in the population from 1980 to 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 genus "Nautilus". Shells have been found across a wide range of coastal areas, including Korea, Australia, Seychelles, Mauritius, the Philippines, 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 can float to the surface, where they can remain for a considerable time period, however the buoyancy of shells after death was found to be dependent on a number of factors, such as the rate of decay. 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 Kuroshio Current carries shells from the Philippines 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. Here, they  engage in scavenging activity, seeking out animal remains, and the moults of crustaceans. "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-Paleogene extinction, when shallow areas of ocean became inhospitable. "Nautilus" have been noted to exhibit an extensive range of depth, close to 500 metres, however they were demonstrated to be at risk of implosion, when exceeding their depth and pressure limits. Depending on the species, the shells of live  Nautilus will collapse at depths of 750 metres or deeper. The feeding behaviour of the genus 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 observed to feed on "Nautilus", by violently charging at individuals to break their shells. In response to attacks from predators, "Nautilus" withdraw into their shells. Nautilus in aquaria. It is possible to keep "Nautilus" in aquaria, however specific care is necessary in order to ensure their survival in captivity. The survival rate of "Nautilus" in captivity is relatively poor, primarily due to the  stress that individuals are subjected to during transportation. 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 around the world host specimens of the genus, however there have not yet been any successful attempts of breeding in captivity, despite viable eggs being produced at several locations. Two "Nautilus" eggs were hatched at Waikiki Aquarium, however these individuals both died months later. In addition to observing wild specimens, our  knowledge of "Nautilus" temperature thresholds is also supplemented by the study of captive individuals in aquaria. Captive "Nautilus" specimens 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, as 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, usually 3–4 cm long, 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 hypothesised that they are  able to chew their way out, using their beak. The genus exhibits a skewed sex ratio, biased towards male individuals. This phenomenon has been observed at several locations around the globe, with population samples consisting of up to 95% males. The reason for this is currently unknown.