Phylogenetic and Morphological Evidence for Two New Species of Terrestrial Micro Snail Genus Georissa Blanford, 1864 (Neritimorpha: Hydrocenidae) from Central Thailand

This paper introduces two newly discovered endemic micro snails, Georissa chatra Buathanom & Pholyotha, sp. nov. and G. principalis Buathanom & Tongkerd, sp. nov., from limestone hills of Uthai Thani and Nakhon Sawan Provinces in central Thailand. These findings expand the number of Georissa species recorded in Thailand to seven. The taxonomic status and species boundaries of these new species were determined through a comprehensive analysis of comparative morphology and mitochondrial genetics. Georissa chatra sp. nov. is distinguished by a conical shell with tall and prominent carinas. Georissa principalis sp. nov. is characterized by an elongate conical or turreted shell with prominent cord-like spiral ridges, and an operculum with two layers. The cytochrome c oxidase subunit I (COI) phylogeny supports the monophyly of both lineages, but the relationships with other congeners remain unresolved. The analysis also revealed interspecific genetic distances among examined species ranging from 6.8% to 18.6%. Additionally, intraspecific genetic distances are surprisingly high, with 6.1% for G. chatra sp. nov. and 7.5% for G. principalis sp. nov. The observed high intraspecific genetic divergences are likely due to the significant endemism and fragmented distribution of populations within karst ecosystems. As a result, the discovery of these two new species significantly advances our understanding of micro snail diversity and represents a crucial first step in establishing a comprehensive DNA barcode reference library for micro snails in Thailand.