Morphological Differentiation Among Three Mitochondrial Lineages of Hydrobioides nassa Theobald, 1865 (Gastropoda: Bithyniidae) from Thailand

The identification of species complexes in freshwater snails remains challenging due to limited diagnostic morphological characters and incomplete taxonomic knowledge in many taxa. Within the family Bithyniidae, species have traditionally been classified using shell morphology and genital anatomy to distinguish intraspecific variation from interspecific differences. However, extensive morphological plasticity has hindered reliable species delimitation, and the presence of cryptic diversity further complicates taxonomy. Recent DNA barcoding studies of <i>Hydrobioides</i> have provided evidence of such cryptic diversity, highlighting the need for taxonomic reassessment within the genus. In the present study, we examined morphological variation in <i>Hydrobioides nassa</i> from Thailand in conjunction with mitochondrial DNA sequence data. Molecular phylogenetic analyses based on cytochrome <i>c</i> oxidase subunit I (<i>cox1</i>) sequences revealed three well-supported genetic lineages within <i>H. nassa</i>, accompanied by high levels of pairwise genetic divergence. Morphological comparisons of shell, operculum, and radular characters further supported differentiation among these lineages, although some characters showed overlap. While <i>Hydrobioides</i> has previously been regarded as comprising a single morphologically defined species, our results demonstrate that <i>H. nassa</i> represents a complex of genetically distinct lineages with subtle but consistent morphological differences. This study highlights the importance of integrating molecular approaches with traditional morphological analyses to improve taxonomic resolution and to better understand biodiversity within freshwater snail groups exhibiting cryptic diversity.