A new species of Diancta of the staircase snail family Diplommatinidae is described from Mt. Savusavu, Vanua Levu Island, Fiji. Due to its left coiling shell and a constriction before the last whorl, it is placed in the genus Diancta. Micro-CT imaging reveals two apertural teeth and an inner lamella that is situated at the zone of constriction. The shell abruptly changes coiling direction by 45 degrees before the last whorl. Up to now, this coiling modus had not yet been documented for any species of Diplommatinidae from the Fiji Islands.

Diancta, Diplommatinidae, Fiji, new species, Oceania, terrestrial malacology

Micromolluscs are defined being smaller than 5 mm and can be found in all parts of the world. They belong to different gastropod groups, are diverse in their habitat needs, appearance and, due to their small size, certainly underexplored. Still, many new species are found in all parts of the world. Usually, little is known about their ecology, distribution patterns and morphological variability. Because of their limited dispersal capabilities and microhabitat needs, microsnails demonstrate a high endemism rate. Terrestrial island snails especially show a high endemism rate of about 75% (Proios et al. 2021). Although the Diplommatinidae are distributed worldwide, there is one group among the terrestrial island micromolluscs that is particularly well represented in the Indo-Pacific region from Southeast Asia to the south-west Pacific and Australia (Stanisic et al. 2010). It is also one of the most locally and regionally diverse land snail families (Webster et al. 2012), showing high endemism. For example, from the Papuan and Wallacean region, 127 species are known (Greķe 2021), on Borneo nearly 170 diplommatinid taxa are found (Liew and Schilthuizen 2016), and on the Fiji islands 42 species had been documented so far (Neubert and Bouchet 2015).

Diplommatinids are known for having a zone of constriction close to the aperture (Egorov and Greķe 2003), internal teeth and lamellar structures (Thiele 1929), while some genera are characterized by a change in the coiling direction of the shell axis (Webster et al. 2012). The shell of the new species presented here is remarkable because it changes its coiling direction upwards to the apex and again back to its original coiling axis. This coiling mode was not yet known for any other species of Diplommatinidae from Fiji Islands.

This study is based on a dry sample collected by Otto Degener in 1941 on the island Vanua Levu of Fiji, previously housed in the MCZ collection (Cambridge, Massachusetts, USA). It aims to provide further information on the land snail richness of the Fiji Islands, particularly that of Vanua Levu, by describing this new diplommatinid species and providing the first diplommatinid record from western Vanua Levu. Dating from the Late Eocene, Vanua Levu is the second largest island within the Fiji Archipelago, which consists of more than 332 volcanic islands (Neall and Trewick 2008). As is known for other Fijian islands, some areas are covered by isolated limestone blocks presenting ideal ecological niches for Diplommatinidae. Despite its large area of 5807 square kilometres, only two localities of Vanua Levu are known for diplommatids, Waivunia village and Netewa Peninsula, from which nine species have been identified so far (Neubert and Bouchet 2015; Barker and Narosamalua 2017).

So far, the Fiji Islands are home to the diplommatinid genera Diancta E. von Martens, 1864, Moussonia O. Semper, 1865 and Palaina O. Semper, 1865 (Neubert and Bouchet 2015). We tentatively assign the new species to the genus Diancta based on the zone of constriction as described in Martens (1864) and in Kobelt’s (1902) emendation, “somewhat irregularly coiled”. As is the case with many Pacific islands, the Fiji Archipelago remains malacologically underexplored (Greke 2017). Phylogenetic data are underrepresented in the available data, and none of the Fijian Diplommatinidae has so far been molecularly assessed. Subsequently, it is not clear whether the unusual shape of the shell is simply a species-specific trait or whether it belongs to another genus.

Internal structures, such as the lamellae or plicae, were examined using X-ray microtomographic (micro-CT) imaging. Unfortunately, the shell broke during removal from the sample holder. Some dry remains of the animal itself could be found inside the shell. This mummified tissue could potentially be used for DNA extraction and sequencing.

The description of this new species is based on a single dry shell from the type locality. There has been no living individual of this species collected or documented to date. Before scanning, the shell was manually cleaned of dried mud and moss with a fine brush and distilled water.

All different perspectives of the shell were captured using a Leica MC190 HD digital camera connected to a Leica M205 C stereo microscope (Leica Microsystems GmbH, Wetzlar, Germany). The multifocal images were processed using the Leica proprietary software LAS X EDOF version 3.6.0.20104 (Leica Microsystems).

Micro-CT was conducted at the Anatomical Institute in Bern, Switzerland. The sample was mounted in a small custom-made cylindrical sample holder (3D-printed: https://git.io/Jc4De) and imaged on a Bruker SkyScan 1272 high-resolution microtomography machine (Control software version 1.4, Bruker microCT, Kontich, Belgium). The X-ray source was set to a tube voltage of 50.0 kV and a tube current of 200.0 µA, and the sample was imaged with an unfiltered x-ray spectrum. A set of 322 projection images of 1632 × 1092 pixels were taken at every 0.6° over a 180° recorded sample rotation. Every single projection was exposed for 339 ms. Three projections were averaged to reduce image noise. This resulted in a scan time of approximately 16 minutes. The projection images were then reconstructed into a 3D stack of images with NRecon (Version 2.0.0.5, Bruker microCT, Kontich Belgium). The whole process resulted in a dataset with an isometric voxel size of 7.5 µm. The 3D images and videos were visualized using the CTvox software Version 3.3.1 (Bruker microCT) and the Image J software version 1.53c 2020.

The raw data from the micro-CT scan as well as the reconstructions are-in the spirit of reproducible research-available online (Haberthür et al. 2021): https://doi.org/10.17605/OSF.IO/CSGKQ.

Measurements were made using the LAS X software measuring tool and are given in mm. Abbreviations used are: SH = shell height, SW = shell width, AH = aperture height, AW = aperture width, W = number of whorls after Kerney et al. (1983).

Diplommatinidae are mainly still assessed using shell characters. The original descriptions of the three genera from Fiji are quite short and have been emended via additional shell characters by subsequent authors such as Kobelt (1902) and Egorov (2013) The classification into genera, subgenera and species has already been regarded as difficult when focusing only on shell characters (Rundell 2008; Webster et al. 2012; Neubert and Bouchet 2015). For example, Köhler and Kessner (2020) found a high variation in shell ribbing in a single population of Diplommatina fluminis B. Rensch, 1931. Many species are only known from a single shell or a limited number of specimens, which hampers any serious conclusions about the variability of shell morphology. The risk that the specimen described herein is an aberrant form must be considered, due to the lack of comparative material from the type locality. However, the probability of finding a new species is quite high, considering that Diplommatinidae are very small in size, are local endemics and have only been documented in three localities from three regions of Vanua Levu. Clements et al. (2008) mentioned a mutant form, but also that the intraspecific variation among shell dimensions seems to be low. Therefore, we conclude that it is more probable to have a new species rather than an abnormal form, considering the clear differences in shell morphology compared to previously described species in Fiji.

For further sampling of fresh material, it is necessary to explore the northwestern part of the island in the Savu Sau region, as well as the Savu Savu mountain in the central-southern region, to find out exactly where the new species is found. The assignment to the genus Diancta is tentative. Here, the inclusion of the type species of the genus Diancta, Diplommatina constricta Martens, 1864, from the Moluccas in Indonesia, would be mandatory to confirm this generic assignment. Micro-CT is a highly useful and seldomly used method for revealing important diagnostic characters such as the inner dentition and the lamellae, especially in micromolluscs, which are difficult to handle. This method was malacologically pioneered and successfully used for assessing inner shell characters and variability in the genera Plecostoma and Opisthostoma (Liew et al. 2014a; Liew and Schilthuizen 2016), and far surpasses the need to break rare and valuable shells to expose internal structures (Budha et al. 2017).

We are deeply indebted to Adam J. Baldinger and Murat Recevik from the Museum of Comparative Zoology in Harvard for providing the holotype of the new species. We would like to thank Charles Huber, Bern, for his support when searching for more information on the locus typicus, as well as Merewalesi Vakarewa from the public library of Fiji, Angelika Principe from Zentralbibliothek Zürich, Matt Capper and Marita Manley from Talanoa Consulting, Suva, Fiji and Christine and Sam Tawake-Bachofner from Lawaki Beach House, Fiji for additional research and personal comments. Also, a great thank to Adrienne Jochum for improving the linguistic quality of our text. This work was supported by the NMBE.