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).[3]
Various authors claim that the genus consists of from four to seven extant species; this remains the subject of debate.[4][5] Nautilus are typically found in shallow waters in tropical seas, mainly within the Indo-Pacific.[4] 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.[6]
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.[7] 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 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.[7][8] In 1951, he determined that the genus could only describe living species of Nautilus, despite many fossil species having already been assigned to it.[7]
In the years following this conclusion, fossil species were sometimes assigned to the genus, however, many authors insisted that these be excluded from the genus.[4] A 2021 review of the evidence concluded that the recognition of exclusively fossil genera was largely arbitrary, and instead recognized numerous fossil species within the genus Nautilus.[6]
Nomen Dubium: likely a giant form of N. pompilius[4][10]
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[5], and three more were described in 2023. Nautilus scrobiculatus, now Allonautilus scrobiculatus, has been assigned to a new genus,[2] and several species listed above have been identified as synonyms of this species, namely N. umbiculatus, N. perforatus, and N. texturatus.[4] 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.[11] For example, although N. pompilius was well studied in the 19th century,[12] it was not until 1996 that soft tissues of Allonautilus scrobiculatus had been dissected.[5]
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.[5] Furthermore, some biologists claim that N. stenomphalus and N. belauensis are members of N. pompilius based on both genetic and morphological data.[5] 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.[5]
Nautilus attracted to bait in American Samoa. Captured with Baited Remote Underwater Video Station (BRUVS).
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.[13] 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.[14]
A 2017 study determined that there were likely five Nautilus species, however these did not exactly correlate to the described species of the genus.[10] 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.[10] 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.[10] 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.[9]
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.[13] The divergence between the genus Nautilus, and its sister taxon Allonautilus likely occurred around New Guinea, and the Great Barrier Reef,[13] during the Mesozoic.[2] 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.[13]
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.[15] It has been suggested that this organ contains chemoreceptors, in order to detect food or sample the surrounding water.[15] 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.[15]
Habitat and distribution
Species within the genus Nautilus are localised to the Indo-Pacific, specifically the tropical seas within this area,[7] however the full extent of their geographic distribution has yet to be recorded.[4] 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.[4]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.[4] All Nautilus species are likely endangered, based on information from NautilusPompilius overfishing in the Philippines, which resulted in an 80% decline in the population from 1980 to 2010.[16]
A Nautilus shell from Broome, Australia. The majority of recovered shells, like this example, are found as drift shells.
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.[4] 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,[4] however the buoyancy of shells after death was found to be dependent on several factors, such as the rate of decay.[17] 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.[4] 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.[4]
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.[18][11] 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.[11] This behaviour may have ensured their survival during the Cretaceous-Paleogene extinction, when shallow areas of ocean became inhospitable.[11]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.[18][4]
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.[4] 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,[18] 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.[4]Nautilus are able to ascend at speeds of approximately 2 metres per minute and descend at speeds of 3 metres per minute.[4]
Predation
Several species have been observed to prey on Nautilus. Octopuses 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.[4] 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.[4]
Nautilus in aquaria
Live Nautilus pompilius at the Aquarium Finisterrae, Spain.
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.[4] In captivity, Nautilus are generally fed a diet of whole shrimp, fish, crab, and lobster moults.[4][19] 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.[4] Two Nautilus eggs were hatched at Waikiki Aquarium, however, these individuals both died months later.[19]
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.[4]
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.[4]
Nautilus eggs are laid in capsules, usually 3–4cm long,[19] which gradually harden when exposed to seawater.[4] 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. 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.[4]
A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
Ammonoids are extinct spiral shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living coleoids than they are to shelled nautiloids. The earliest ammonoids appeared during the Devonian, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event. They are often called ammonites, which is most frequently used for members of the order Ammonitida, which represented the only living group of ammonoids from the Jurassic onwards.
The nautilus is an ancient pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
Spirula spirula is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus Spirula, the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known as the ram's horn squid or the little post horn squid. Because the live animal has a light-emitting organ, it is also sometimes known as the tail-light squid.
The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species. All told, between 22 and 34 families and 165 to 184 genera have been recognised, making this the largest order of the subclass Nautiloidea.
The chambered nautilus, also called the pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it is not a golden spiral. The shell exhibits countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above.
Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species. They flourished during the early Paleozoic era, when they constituted the main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms (orthocones). Only a handful of rare coiled species, the nautiluses, survive to the present day.
Baculites is an extinct genus of heteromorph ammonite cephalopods with almost straight shells. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the K-Pg mass extinction event, was named by Lamarck in 1799.
The Palau nautilus, Nautilus belauensis, is mainly found in Palau in the Western Carolines as its name suggests. It can be found on fore reef slopes in depths ranging from 95m to 504m but typically prefers to remain within 150-300m where water temperatures range between 16.6 and 9.4 °C. N. belauensis are highly mobile epibenthic scavenging herbalist and opportunistic predators that rely mostly on scent detection. They are active both diurnally and nocturnally within the preferred depth range although most shallow-water-incursions are usually nocturnal events that coincide with greatly diminished fish activities.
Nautilus stenomphalus, also known as the white-patch nautilus, is a species of nautilus native to the Great Barrier Reef. N. stenomphalus is very similar to N. pompilius and may in fact represent a subspecies. It is separated by the absence of a thickened callus and the presence of white patches in the umbilical and shoulder regions of the shell. The sheaths of this species have scalloped edges compared with the smooth sheaths of N. pompilius. N. stenomphalus also differs slightly in hood ornamentation. The shell is usually up to around 180 mm in diameter, although the largest specimen ever recorded measured 201 mm.
Allonautilus scrobiculatus, also known as the crusty nautilus or fuzzy nautilus, is a species of nautilus native to the waters around New Guinea, specifically New Britain and Milne Bay, and the Solomon Islands. A. scrobiculatus is recognizable by the large open umbilicus, which is around 20% of the shell diameter at its widest point. This species, along with the closely related A. perforatus, were originally placed in the genus Nautilus, but have recently been given their own genus on account of significant morphological differences. The most obvious are features of the shell, including crease and an encrusting layer (periostracum) that covers most of the shell. Gills and reproductive structures also differ significantly from members of the genus Nautilus. The shell is usually up to around 18 cm in diameter, although the largest specimen ever recorded measured 21.5 cm. The species was thought to have gone extinct after 1986, but was rediscovered in July 2015.
Allonautilus perforatus also known as the Bali chambered nautilus, is a species of nautilus native to the waters around Bali, Indonesia. It is known only from drifted shells and, as such, is the least studied of the six recognized nautilus species. Thus, not much is known about it outside of the shell.
Nautilus cookanus is an extinct species of nautilus. It lived during the Eocene epoch. N. cookanus placed within the genus Nautilus, together with extant species based on their shared shell characters. Fossils of the species from the Late Eocene Hoko River Formation are noted as one of the two oldest occurrences for the genus. Its name has frequently been misspelled as "cookanum".
Nautilus praepompilius is an extinct species of nautilus. It lived from the Late Paleocene through Oligocene epochs. The first fossil specimens discovered in the Late Eocene to Oligocene-aged Chegan Formation of Kazakhstan: an additional, older specimen was found in the Late or Latest Paleocene-aged Pebble Point Formation in Victoria, Australia. N. praepompilius has been grouped into a single genus together with extant species based on their shared shell characters. It is morphologically closest to N. pompilius, hence the name. The nepionic constriction shows that the hatching size was approximately 23 mm, close to that for N. pompilius. N. praepompilius, along with aff. N. cookanum fossils from the late Eocene Hoko River Formation in Washington state are the oldest occurrences of the genus.
Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid or modifying the tentacles into thin filaments. Octopodiformes is often considered the crown group of octopuses and vampire squids, including all descendants of their common ancestor. Some authors use the term Vampyropoda for the same general category, though others use "Vampyropoda" to refer to the total group. Another term is Octobranchia, referring to cephalopods without prominent tentacles.
All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles.
Cephalopod ink is a dark-coloured or luminous ink released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the Cirrina, are able to release ink to confuse predators.
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.
The Hoko River Formation is a Late Eocene marine sedimentary geologic formation. The formation is exposed in outcrops along the Strait of Juan de Fuca on the Olympic Peninsula in Washington state, USA. It is known for containing numerous fossils of crabs. It overlies the older Lyre Formation and underlies the younger Makah Formation.
Eutrephoceras is an extinct genus of nautilus from the Late Jurassic to the Miocene. They are characterized by a highly rounded involute shell with slightly sinuous suture patterns.
1 2 3 Barber, V. C.; Wright, D. E. (1969). "The fine structure of the sense organs of the cephalopod mollusc Nautilus". Zeitschrift für Zellforschung und Mikroskopische Anatomie. 102 (3): 293–312. doi:10.1007/bf00335442. ISSN0302-766X. PMID4903946. S2CID19565604.
↑ Dunstan, A.; Alanis, O.; Marshall, J. (November 2010). "Nautilus pompilius fishing and population decline in the Philippines: A comparison with an unexploited Australian Nautilus population". Fisheries Research. 106 (2): 239–247. doi:10.1016/j.fishres.2010.06.015. ISSN0165-7836.
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