Nautilus is a marine cephalopod genus in the mollusk family Nautilidae. Species in this genus differ significantly, morphologically, from the two nautilus species in the adjacent 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).
Various authors claim that the genus consists of between four and seven extant species; this remains the subject of debate. Nautiloids are typically found in shallow ocean waters in tropical seas, mainly within the Indo-Pacific, from the Coral Triangle and Strait of Malacca to the open water South Pacific islands. The genus Nautilus includes several species represented in the fossil record; however, these have also been contentious in their placement, and some are only provisionally accepted.
Classification The classification of species within Nautilus has been contentious for decades, and the genus has been reconfigured and redefined several times throughout its history. Nautilus is the type genus of the family Nautilidae, originally defined as any coiled-shell species with simple sutures, or walls, between shell compartments. Any shells with complex sutures were assigned to the genus Ammonites. This definition persisted for nearly 200 years, from the time of the genera's 1758 inception by Carl Linnaeus to 1949, when paleobiologist Arthur K. Miller provided a detailed description of the shell of N. pompilius, becoming the type species of the genus. In 1951, he determined that only extant species of Nautilus should be placed in the genus, despite many fossil species having already been assigned to it.
In the years following this conclusion, fossil species were still sometimes assigned to the genus; however, many other authors insisted that these be excluded. In 2021, a review of the evidence concluded that the recognition of exclusively-fossil genera was largely arbitrary, instead recognising numerous extinct species within the genus Nautilus.
Species Species Authority Range Image†Nautilus altifrons(Chapman, 1915)Australia†Nautilus balcombensis(Chapman, 1915)AustraliaNautilus belauensisSaunders, 1981Palau†Nautilus butonensisMartin, 1933Indonesia†Nautilus campbelliMeek, 1862Western North America†Nautilus cookanusWhitfield, 1892North America†Nautilus geelongensis(Foord, 1891)Australia†Nautilus javanusMartin, 1879IndonesiaNautilus macromphalusG.B. Sowerby II, 1849New CaledoniaNautilus pompiliusLinnaeus, 1758Indonesia†Nautilus praepompiliusShimansky, 1957Nautilus samoaensisGregory J. Barord, David J. Combosch, Gonzalo Giribet, Neil Landman, Sarah Lemer, Job Veloso et Peter D. Ward, « Three new species of Nautilus Linnaeus, 1758 (Mollusca, Cephalopoda) from the Coral Sea and South Pacific », ZooKeys, vol. 1143, 2023, p. 51-69.Barord et al., 2023SamoaNautilus stenomphalusG.B. Sowerby II, 1849Great Barrier Reef†Nautilus taiwanusHuang, 2002TaiwanNautilus vanuatuensisBarord et al., 2023VanuatuNautilus vitiensisBarord et al., 2023Fiji
Other described species: Species StatusNautilus ambiguusNomen dubiumNautilus alumnusNomen dubiumNautilus scrobiculatusReclassified as Allonautilus scrobiculatusNautilus umbiliculatusNomen dubium, synonym of A. scrobiculatusNautilus perforatusReclassified as Allonautilus perforatusNautilus texturatusNomen dubium, synonym of A. scrobiculatusNautilus repertusNomen dubium, likely a giant form of N. pompilius
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, and three more were described in 2023. 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 specimens. The majority of described species have been determined on the drift shells of individuals alone, leading to inaccuracies when defining species divisions. For example, although N. pompilius was well studied in the 19th century, it was not until 1996 that soft tissues of Allonautilus scrobiculatus 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, using 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. macromphalus 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.
A 2017 study determined that there were likely five Nautilus species, however these did not exactly correlate to the described species of the genus. While 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, while the other from Fiji and American Samoa. While this study recorded five species, its results suggested that N. belauensis and N. repertus were synonyms with N. pompilius. This study resulted in the description of three new species in 2023, recognizing the species from Fiji and American Samoa as two taxa rather than one.
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, 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 several 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.