The “Bali Nautilus”, sometimes also called the “Indonesian Nautilus”, is one of only two nautilid species in the genus Allonautilus characterised by shells with large umbilicus and a thick, fuzzy periostracum. A very understudied species apparently restricted to Indonesian waters, in the past most known specimens have originated from around Bali; although recently it has surfaced from a range of localities between Madura Island and in the Banda Sea off the Maluku Islands (most recent specimens have originated around here).

Unlike its sister species Allonautilus scrobiculatus (Lightfoot, 1786) from Papua New Guinea and Solomon Islands which has been sighted and caught alive by scientists (although only a handful of times), there is no formal record of live A. perforatus and the animal has not been available for study. Though sometimes dubbed the ‘rarest living nautilus’, a considerable number of its shells have actually appeared on the shell trade in the past years. At least the majority of these are, however, supposedly drift shells, and no definitively live-taken material has been reported. As such, nothing is known about its ecology — although it is most likely a predatory and scavenging carnivorous cephalopod inhabiting moderately deep waters around -100~600 m like other living nautilids. Until recently it was not known if it possesses a thick periostracum like A. scrobiculatus, but the surfacing of fresh-looking, supposedly trawled, materials with nearly complete periostracum shows that it indeed does. These specimens may have even been live-taken, raising hopes that the animal may become available for study in the near future.

Its most distinct known morphological feature is the series of strong undulating radial ribs on both flanks of its shell, such strong radial sculpture is unknown from any other living nautilids, though many extinct nautiloids possessed similar ornamentation. This feature distinguishes it from A. scrobiculatus, which lacks these radial ribs and instead possess numerous zigzagging spiral lirae. These cross with fine radial growth lines to form a finely reticulate sculpture that is missing in A. perforatus. Despite these morphological differences, many remain doubtful if A. perforatus is truly a separate species from A. scrobiculatus. In the literature some have treated it as subspecies of A. scrobiculatus, and some merely as a local form. Resolving the status of A. perforatus conclusively requires obtaining live animals to compare the anatomical and genetic differences with A. scrobiculatus in the future. The genus Allonautilus is considered to differ from Nautilus in having a wide umbilicus, quadrate whorl cross-sections, and fuzzy periostracum, but also some anatomical features such as larger and more randomised papillae on the hood and smaller gills. These are currently only based on A. scrobiculatus and it would be of interest to see if similar anatomical features are shared by A. perforatus.

The name Allonautilus perforatus (Conrad, 1847) should not be confused with Nautilus perforatus Willey, 1896, which is a junior synonym of Nautilus pompilius Linnaeus, 1758 based on a freak specimen with a narrow open umbilicus. Since 2017 all nautilids, including A. perforatus, are listed on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), thus curbing the international trade of nautiluses globally. – from Chong Chen’s post