This paper introduces Xanthomelon amurndamilumila sp. nov., a newly-discovered land snail species inhabiting the North East Isles, offshore from Groote Eylandt (= Ayangkidarrba), north-western Gulf of Carpentaria, in the Northern Territory, Australia. Specimens of this species were first collected during the 2021 Bush Blitz expedition to Groote Eylandt, a large offshore archipelago previously unexplored for land snails. The taxonomic status of the new species was established through a comprehensive analysis of comparative morphology and mitochondrial genetics: X. amurndamilumila forms a maximally supported clade closely related to X. arnhemense and is characterised by a unique combination of morphological characteristics, including smaller shell size, distinctive sculpture of collabral ridges and specific features of its reproductive anatomy. The genetic divergence and phylogenetic relationships suggest historical isolation. While the discovery of X. amurndamilumila sp. nov. enriches our understanding of land snail diversity in the Northern Territory, its conservation status is of concern on North East Island because of habitat degradation caused by feral deer.

distribution, Gastropoda, Helicoidea, Pulmonata, taxonomy

The genus Xanthomelon E. von Martens, 1860 comprises several species of Australian camaenid land snail, typically characterised by their large, globose shells of mostly uniform yellowish-brown to ochre colour. The distribution of this genus encompasses the Australian Monsoon Tropics as well as eastern Queensland. Of the 11 currently accepted species of Xanthomelon, five occur in the ‘Top End’ of the Northern Territory, according to the latest revision of the genus (Köhler and Burghardt 2016). These species exhibit varying degrees of morphological distinctive characteristics, including some that are highly similar with one another and have distributional ranges that vary from narrow to wide. It is a testament to our rather poor knowledge of land snails in the ‘Top End’ generally that two of these five species have been described only a few years ago. Xanthomelon darwinense was only described in the 21^st century (Köhler and Burghardt 2016), even though it occurs in the Territories’ capital, Darwin and is readily distinguished from most other congeners by its much smaller size. This species has a narrow distribution in and around Darwin and is currently known only from two separate locations.

In contrast, X. arnhemense Köhler & Burghardt, 2016 has a wide distribution, encompassing most of Arnhem Land. However, this species had also remained unrecognised for at least a century due to its highly cryptic nature, even though specimens are well represented in museum collections. However, shells of X. arnhemense are indistinguishable from those of its close ally, X. interpositum Iredale, 1938. Indeed, these two species can be distinguished only through comparative reproductive anatomy or genetics. The previously-poor documentation of Xanthomelon is but one example of how the scarcity of modern systematic studies impairs the documentation of the Territories’ land snail fauna to this day. The recent discovery of another undescribed species of land snail in the Northern Territory, Parglogenia cobourgensis Köhler & Shea, 2022, which had been overlooked for centuries due to its externally cryptic shell, is another example illustrating our slow progress towards a more complete documentation of the land snail fauna of the Northern Territory (Köhler and Shea 2022).

The presumably still incomplete knowledge of Australian land snails also hampers our ability to manage and conserve the fauna that has already been described. Indeed, Willan et al. (2009) showed that four species of Torresitrachia Iredale, 1939 described in 2009, which are endemic to the Daly Basin Bioregion and have very limited distribution ranges, are under threat of extinction due to the impact of changed fire regimes and the transformation of native savannah by introduced grasses.

To remedy the lack of documentation of biotic patterns throughout the Northern Territory, more systematic studies are urgently needed that employ contemporary methods and analyse evidence from multiple sources of information to reliably identify and delimit species. Oftentimes, new species may be discovered when understudied museum samples are examined, especially by using novel scientific techniques. Parglogenia cobourgensis and Xanthomelon arnhemense are two such species that were discovered in museum collections. Secondly, new discoveries may be made in hitherto under-surveyed areas.

In the present study, we examine newly-collected samples of a presumed new species that were collected on small islets offshore from Groote Eylandt, which were never surveyed for land snails previously. The specimens examined here were first collected in 2021 during the Bush Blitz expedition to Groote Eylandt that aimed to close a pre-existing survey gap for multiple groups of organisms through a targeted survey (Willan and Bourke 2022).

Preliminarily identified in the field by one of us (RCW) as a putative new species of Xanthomelon, we here employ comparative morphology and mitochondrial genetics to resolve its taxonomic status. To ensure an accurate taxonomic assessment, we compare all new data collected on this putative new species with the current knowledge of the Xanthomelon species in the ‘Top End’ as summarised in the latest systematic revision by Köhler and Burghardt (2016).

The present study is based on samples collected on North East Island (= Amburrkba) and Hawk Island (= Ayangkijirumanja) in the Gulf of Carpentaria during the Bush Blitz on Groote Eylandt conducted in 2021. Additional specimens were collected in 2022 on Lane Island (= Milyekaluwakba) and also on Hawk Island (Fig. 1).

Figure 1. 

Map of the North East Isles off Groote Eylandt, Arnhem Land, Northern Territory. Specimens of Xanthomelon amurndamilumila sp. nov. have been found on North East Island as well as Hawk and Lane Islands. Small circles indicate sampling sites.

Most of the specimens have been deposited in the malacological collection of the Museum and Art Gallery of the Northern Territory in Darwin (NTM). Supplementary specimens have been deposited in the malacological collection of the Australian Museum in Sydney (AM).

Material was collected by hand while conducting visual searches of habitats on North East Island, the largest of the North East Isles. Living specimens were fixed and preserved in 95% ethanol. Shells were photographed and reproductive anatomy was studied using a binocular microscope with drawing mirror. Height of shell (H) and diameter (D) of fully mature shells as recognised by a finished apertural lip were measured with callipers precise to 0.1 mm and shell whorls were counted as described by Köhler (2011).

Genomic DNA was extracted from small pieces of foot muscle using a QIAGEN DNA extraction kit for animal tissue following the standard procedure of the manual. Fragments of two mitochondrial genes, 16S rRNA (16S) and cytochrome c oxidase subunit 1 (COI), were amplified by PCR using the primer pairs 16Scs1 (5′-AAACATACCTTTTGCATAATGG-3′) (Chiba 1999) and 16Sbd1 (5′-CTGAACTCAGATCATGTAGG-3′) (Sutcharit et al. 2007) and L1490 (5′-GGTCAACAAATCATAAAGATATTGG-3′) and H2198 (5′-TAAACTTCAGGGTGACCAAAAAATCA-3′) (Folmer et al. 1994), respectively. Reactions were performed with an annealing step of 60 s at 55 °C for 16S and at 50 °C for COI with elongation times of 90 and 60 s, respectively. PCR fragments were purified with ExoSAP (Affymetrix) and both strands were cycle sequenced by use of the PCR primers. Chromatograms were merged into one sequence contig using Sequencher (Gene Codes Corporation. Ann Abor) and misreads were manually corrected where necessary. New sequences have been deposited in GenBank under the accession numbers OR610390-OR610392 and OR612298-OR612302. For the phylogenetic analyses, we built a sequence dataset by incorporating all available GenBank sequences of Xanthomelon (16S and COI). The 16S sequences were aligned using the online version of MAFFT (version 7.4) (Katoh et al. 2002) available at http://mafft.cbrc.jp/alignment/server/ by employing the iterative refinement method E-INS-i. The final sequence alignments of 16S and COI were concatenated into one partitioned dataset. Two partitions were designated, one for each gene fragment. Phylogenetic relationships were estimated by employing a Maximum Likelihood-based method of tree reconstruction (ML) using IQ-TREE v. 2.3 (Nguyen et al. 2015). We used the integrated ModelFinder (Kalyaanamoorthy et al. 2017) to identify the best-fit models of sequence evolution for each sequence partition. We employed Ultrafast Bootstrap Approximation (Minh et al. 2013) to estimate the statistical branch support of the best Maximum Likelihood tree.

Abbreviations used: 16S = 16S rRNA; COI = cytochrome c oxidase subunit 1; H = shell height; D = shell diameter; SD = standard deviation.

The mitochondrial phylogeny revealed that the putative new Xanthomelon species on the North East Isles is most closely related to X. arnhemense, which occurs on Groote Eylandt, the large land mass closest to the North East Isles (Fig. 1). In fact, both species are sister species forming a single clade in the mitochondrial phylogeny of Xanthomelon (Fig. 2). The genetic divergence between both species (p-distances of on average 8.8% in COI) is consistent with the amount of interspecific genetic divergence found in this genus (between 14.5% and 19.1%; Köhler and Burghardt (2016)), although perhaps at the lower end of the range. This genetic distance supports the conclusion that there has been a considerable duration of evolutionary isolation of the populations on the North East Isles and that there is no evidence for contemporary genetic exchange between them and X. arnhemense on Groote Eylandt.

The largely identical reproductive anatomy is testimony to the close phylogenetic relationship between both species. We hypothesise that the putative new species likely diverged from X. arnhemense because of peripatric speciation after an historical event of long-distance dispersal from mainland Groote Eylandt, but that there has been little if any genetic exchange between the two of them subsequently. That both species now form a sister pair of clades in the mtDNA phylogeny shows that there has been a sufficiently long period of isolation to allow any pre-existing ancestral polymorphisms to sort out.

However, comparative morphology provides the strongest arguments for the recognition of the Xanthomelon species presently living on the North East Isles as an independent species. The North East Isles species is characterised by several unique features, which distinguish it from any other species of Xanthomelon, including X. arnhemense (refer to taxonomic description for details).

The close relationship of both species is also evident from their closely similar reproductive anatomy. However, the significant differences in shell size and sculpture in combination with substantial amounts of mitochondrial variation between both taxa are indicative of the existence of largely distinct gene pools in both forms, hence indicative of the distinct species status of the North East Isles species.

We conclude that molecular and morphological data support the conclusion that the Xanthomelon snails from the North East Isles indeed represent a distinct species or an evolutionarily distinct entity, that has a distinct gene pool (as expressed in a consistently distinct morphology) and with a high probability of lacking genetic admixture with X. arnhemense (as expressed in substantial mitochondrial differentiation).

Fieldwork on the North East Isles was conducted as part of the Bush Blitz Groote Eylandt 2021 expedition. Firstly, we thank the Anindilyakwa Traditional Owners of the North East Isles – Freda Wurramarrba and her four sons, Fabian, Ishmael, Hamilton and Veron – for allowing us on-to their land to collect molluscs. We also thank the Anindilyakwa Land and Sea Rangers, specifically, Katie Oxenham and Dan Keynes, for co-ordinating access to the North East Isles during the Bush Blitz survey. Lastly, we are indebted to Kirsten Eden (Indigenous Protected Areas Operations Co-ordinator, Groote Eylandt Language Centre) for organising the collaboration with Traditional Owners and other Anindilyakwa elders that resulted in the selection of the species’ name. Vince Kessner’s opinion that this taxon constitutes a new species of Xanthomelon is greatly appreciated.

All the specimens were collected under CDU Animal Ethic Approval 20013 – Fauna Surveys and Monitoring in the Anindilyakwa Protected Area. Thanks go to two reviewers, John Stanisic and Bernhard Hausdorf, for their constructive comments, which helped to improve the quality of this article.