The validity of Nautilus macromphalus as the species occurring around New Caledonia was established quite early (Bonacum et al. 2011), and has recently been confirmed (Combosch et al. 2017).

Molecular studies of N. pompilius sensu lato suggest that this name covers a suite of cryptic species. Genetic studies have identified divergence between material from the following locations (1) Fiji, (2) Vanuatu, (3) American Samoa, (4) Coral Sea Islands (including Osprey Reef, Shark Reef and Bougainville Reef), (5) Papua New Guinea and the Great Barrier Reef (including Port Moresby, Admiralty Islands, Mantis Reef, Wishbone Reef, Carter Reef) and (6) Indonesia, NW Australia, Philippines and Palau (Sinclair et al. 2007, 2011; Bonacum et al. 2011; Williams et al. 2012, 2015; summarised in Nikolaeva et al. 2015; Vandepas et al. 2016).

Indonesia, Western Australia, Philippines and Palau
An application was made to the International Commission on Zoological Nomenclature (ICZN) to set aside all previous type designations for Nautilus pompilius and to designate a neotype from Ambon Indonesia (Nikolaeva et al. 2015). The commission ruled in favour of this proposal in (ICZN 2018). The species has now been redefined (Saunders et al. 2017) but may yet undergo further revisions. With Ambon, Indonesia as the type locality for N. pompilius, we assume the distribution of N. pompilius pompilius to be Indonesia/NW Australia/Andaman Islands. Approximate Bayesian Computation analysis showed that the apparent lack of genetic divergence between Western Australia and Philippine material is because of limited genetic drift in these isolated populations since their separation rather than gene exchange (Williams et al. 2015). This supports a Philippine clade as separate from an Indonesia/NW Australia clade. Habe and Okutani (1988) described a subspecies N. pompilius suluensis from near Palawan Island, Philippines. Thus we treat Philippine nautilus under the name N. pompilius suluensis. Using genomic data, Combosch et al. (2017) found little genetic differentiation between samples from the Philippines, and samples from Palau, Indonesia and NW Australia. However, the latter three locations were represented by only five specimens in total, so the data should be treated with caution. We treat Palau nautilus under the widely applied name N. belauensis. We treat material from SW Australia as N. repertus, thus using the name under which specimens from this region were originally described. While all these species/subspecies may not be valid, the names are currently available and allow us to differentiate widely spaced populations that likely need separate management.

Fiji, Vanuatu, American Samoa
Combosch et al. (2017) consider that populations from Vanuatu, Fiji, and American Samoa could represent a single species, or could represent two species, with specimens from American Samoa and Fiji comprising one species, and those from Vanuatu comprising a second. In the absence of a suitable name, we treat specimens from these three countries as Nautilus pompilius West Pacific population, but herein acknowledge that a new name is required and that this grouping may indeed comprise two or three species.

Coral Sea Islands, Papua New Guinea and the Great Barrier Reef
Combosch et al.(2017) consider populations from the Coral Sea Islands, Papua New Guinea and the Northern Great Barrier Reef to potentially comprise a single species, although, like Williams et al. (2015), they did find the Coral Sea Island population to be genetically different to specimens from the Northern Great Barrier Reef and Papua New Guinea. There are species names available from this region (e.g., N. stenomphalus, N. moretoni), but use of these names would require a full taxonomic revision since they would not be being used in their original context. Thus we treat specimens from the Coral Sea Islands as Nautilus pompilius Coral Sea subpopulation, and specimens from Papua New Guinea and the Northern Great Barrier Reef as Nautilus pompilius PNG/GBR subpopulation, despite being fully aware that these populations do not form part of the species Nautilus pompilius as currently defined (Saunders et al. 2017). Although no genetic data are available for specimens from the Solomon Islands, they may form part of the PNG/GBR subpopulation due to their geographic closeness and lack of deep-water barriers.

Nautilus belauensis is assessed as Near Threatened (nearly meets criterion B). Its extent of occurrence (EOO) is around 6,000 km². Within this range, the area of occupancy (AOO) is likely more restricted due to the species' distribution being constrained by the area of habitat available on this small oceanic atoll. However, at present it is not possible to accurately estimate AOO (the AOO provided in this assessment is very much an overestimate and is not used for this assessment). Although fishing is restricted, and the coral reefs of Palau are in relatively good condition compared to other Pacific Reefs, any impacts of ecotourism or future climate change could have a devastating effect on this species because it is so geographically constrained. There is evidence that Nautilus species are very vulnerable to fishing pressure. For example, at least three fisheries of N. pompilius suluensis are known to have crashed in the Philippines and the majority of N. p. suluensis fisheries investigated in Palawan, Philippines, are showing serious population declines of more than 80%. Slow growth, low fecundity and the uneven sex-ratio of Nautilus species make them already susceptible to overfishing even when levels of exploitation are relatively low. Hence Nautilus belauensis is potentially threatened by fishing activity, individuals can be found for sale on the internet, but the extent of the current fishing pressure on the population is not documented. There is anecdotal evidence that the National Marine Sanctuary covering 80% of Palau's waters, and excluding foreign tuna vessels from its water, is putting local fishing pressure on inshore reefs, as local fishermen try to supply the gap left by the tuna fishery exclusion. Using a precautionary attitude, and keeping in mind additional threats from climate change and declining reef quality, we infer a continuing decline in population size and recommend further studies into the population size estimates and a clearer understanding of the geographic extent of populations to support our inference in future reassessments.

This species is known from the Western Caroline Islands, Palau (Ward 2008) occurring in depths from 66 to 504 m (Saunders et al. 1987). Coral reef in Palau is estimated to extend approximately 525 km² (Yukihira et al. 2007). The range map is based on the limited data currently available. The estimated extent of occurrence is approximately 6,329 km², based on the generalized range map supplied.

The population size of this species is not known. The sex ratio of nautiluses generally is known to be highly biased in favour of males with typically only 25% of any given nautilid population being female (Saunders et al. 1987). The recorded sex ratio of individuals in Palau reflected this (Saunders and Spinosa 1978). In 1977, 375 nautiluses were trapped in Palau, and, of these, 72.3% were male (Saunders et al. 2017). However, there is some evidence that females avoid each other (Basil 2014) which might limit their numbers in traps, so the sex ratio may be artificial. In Palau, in 1977, 80.5% of trapped animals were mature (Saunders et al. 2017). It seems that fairly small-sized subpopulations are associated with each area. For example, in a mark and recapture experiment in Palau in 1982, 15% of almost 1,000 animals released during a 3-month field season were recaptured. Animals were recaptured as often as five times and many traps comprised 30% or more recaptures. However, it is also possible that animals are aggregating at bated traps. Individuals of Nautilus species have been tracked using radio telemetry (Dunstan et al. 2011c) and mark and recapture (e.g., Saunders et al. 1987) studies. These have found that animal move up to 3 km per day, with mean movement rates of about 1 km per day. Longer distant movements have also been recorded with animals recaptured up to 150 km from their release site. This suggests that subpopulations extend over a large area even though the population size might be small. Overall, the current population trend is unknown, but could very feasibly be declining given that there is evidence that Nautilus species are very vulnerable to fishing pressure, even when fishing intensity is relatively low. Where fishing pressure is higher, declines can be more catastrophic: for example, at least three fisheries of N. pompilius suluensis are known to have crashed in the Philippines and the majority of N. pompilius suluensis fisheries investigated in Palawan, Philippines are showing serious population declines of more than 80%. While anecdotal evidence suggests that Nautilus belauensis has recovered somewhat from previous heavy fishing (Aguiar 2000), levels of fishing, fishing impacts, and recovery are not properly documented.

Nautiluses live on the steep sides of coral reefs and adjacent substrates (Jereb and Roper 2005, Saunders and Ward 2010). They are probably best characterised as mobile bottom-dwelling fore-reef scavengers and opportunistic scavengers (Jereb and Roper 2005, Saunders and Ward 2010, Dunstan et al. 2011a); they will also take small crustaceans such as crabs. They themselves are preyed upon by sharks and other fishes and, in some regions, by octopuses that drill into the shell (Saunders et al. 1991). Attacks from triggerfish and groupers have been observed on numerous occasions (Saunders et al. 2011). Nautiluses lay a maximum of ten eggs per batch, and experiments in aquaria have indicated that eggs take up to a year to hatch (Barord and Basil 2014). Juveniles hatch at 25–30 mm diameter growing to 6 or 7 cm diameter within a year. Saunders suggested that nautiluses live to more than 20 years old. Dunstan et al. (2011b) suggest, as have others previously, that these characteristics make nautiluses particularly vulnerable to over-exploitation. The coral reefs of Palau are generally in good condition. Palau has not experienced any major bleaching events since the 1997–1998 El Niño event and disease prevalence is low. The 2010 La Niña was less severe and mortality from bleaching was less (Colin 2018). Although temperatures of the 2016 El Niño event were similar to those of the 1997–1998 El Niño, only sensitive coral species bleached in Palau, and these recovered as temperatures decreased (Colin 2018). Run-off and siltation have been noted as a problem although efforts to mitigate this through, for example, paving of dirt roads, are underway (Marino et al. 2008).

Slow growth, low fecundity and the uneven sex-ratio of Nautilus species make them susceptible to overfishing even where levels of exploitation are relatively low. Hence Nautilus belauensis is potentially threatened by fishing activity, and shell collecting, but the extent of the current fishing pressure on the population is not documented. Nevertheless, specimens of Nautilus belauensis can be found for sale on the internet. Ecotourism may also be a threat, for example, this species has been captured by dive operators to allow tourists to be photographed with live animals (Aguiar 2000). In addition, these trapped nautiluses may then be released into shallow waters where they may suffer increased predation, particularly as trigger fish become attracted to common release sites (Carlson and Awai 2015, cited in Miller 2017). There is anecdotal evidence that the National Marine Sanctuary covering 80% of Palau's waters, and excluding foreign tuna vessels from its water, is putting local fishing pressure on inshore reefs, as local fisherman try to supply the gap left by tuna imports.

Climate change, which may impact the coral reef habitat, is a potential future threat. There are also impacts on due to a decline in reef quality, for example, run-off and siltation had been noted as a problem although efforts to mitigate this through, for example, paving of dirt roads, were underway (Marino et al. 2008).

Anecdotal evidence suggests that this species has recovered somewhat from previous heavy fishing (Aguiar 2000), however levels of fishing, fishing impacts, and recovery are not properly documented. This species has been captured by dive operators to allow tourists to be photographed with live animals (Aguiar 2000). Marino et al. (2008) identified eco-tourism as an economically important activity with over 80% of Palau’s visitors coming to dive among the coral reefs.

As of 2007, Palau had designated 31 Marine Protected Areas, covering more than 40% of Palau's nearshore marine area with various fishing restrictions in place. In 2015, Palau designated 80% of its waters National Marine Sanctuary, the marine protected area becoming fully operational in 2020. The family Nautilidae was listed under CITES Appendix II in 2017 and includes all Nautilus and Allonautilus species. Monitoring of population trends, harvest trends and trade trends is required. Education is required to ensure ecotourism operates in a way to minimise impacts on the species.