Distribution and Habitat Associations of the Genus Diplommatina (Gastropoda: Diplommatinidae) in Sarawak

Nurul Syafiqah, Mat Nasir (2025) Distribution and Habitat Associations of the Genus Diplommatina (Gastropoda: Diplommatinidae) in Sarawak. Masters thesis, UNIMAS.

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Abstract

The formation of the South China Sea separated Peninsular Malaysia and Borneo, influencing the evolutionary paths of many species, including land snails, which serve as excellent models for biogeographical studies. Land snails, particularly those of the genus Diplommatina, mainly found in limestone habitats due to the abundance of calcium carbonate, resulting in high levels of endemism and restricted distribution ranges. The habitat association of land snails is influenced by environmental factors such as soil pH, moisture, and calcium levels, with minute land snail species being particularly sensitive to changes to in habitat conditions. Despite the focus on limestone habitats, recent studies have shown that some Diplommatina species are also found in non-limestone areas, highlighting the need for more research on their distribution and ecological preferences in these regions. Hence, the main objectives of this study are to determine the distribution of the genus Diplommatina in Sarawak and the habitat association of selected species. The study was conducted across eight sites in Sarawak, Malaysia, including limestone areas such as Gua Rumbang, Gua Rambus, Gunung Silabur, and Niah National Park, as well as non-limestone areas like Gunung Gading National Park, Kiyau Campsite in Sapit, Mutiara Campsite in Baleh, and Lambir Hills National Park. Fieldwork for this study was conducted from October 2022 to January 2024 across six 20 × 20 m plots per site, where both living snails and empty shells were collected using direct searching method and soil floatation habitat assessment focused on substrate types (e.g., leaf litter, vegetation, and limestone), abiotic factors (e.g., leaf litter depth, soil pH, soil temperature, ambience temperature, humidity, luminosity) and biotic factors (e.g., canopy cover and diameter of tree at breast heigth). A total of 23 Diplommatina species, including both living snails and empty shells, were recorded for distribution mapping. 16 out of 23 species found alive were included in the habitat association analysis. The findings of this study suggest the distribution of Diplommatina species in Sarawak is primarily influenced by the presence of limestone outcrops, species chirality, and shell size, leading to region-specific endemism which allows the understanding of the current distribution of this genus. Additionally, most living Diplommatina individuals were found to be significantly abundant in leaf litter compared to limestone or vegetation habitats. Notable observations, such as the presence of D. baritensis on banana tree trunks in non-limestone areas, provide valuable insights into the occurrence of this genus beyond limestone outcrops and emphasise the need to include these areas in future surveys. Furthermore, the discovery of a new species, D. rumbangensis, highlights the potential for further taxonomic discoveries in the region and underscores the importance of conserving the areas. Soil pH was the only factor that significantly affected species, with D. onyx favoring lower pH levels and D. baritensis associated with near-neutral soils. The other tested biotic and abiotic factors did not show significant differences between species (p > 0.05). These findings underscore the importance of further exploring the ecological factors that contribute to the unique distribution patterns of this genus in Sarawak.

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