Morphological Divergence Without Genetic Differentiation in Widely Distributed Chinese Cipangopaludina Species

The taxonomic status of Cipangopaludina chinensis and C. cathayensis remains controversial due to morphological overlap. We combined morphometrics (24 landmarks vs. 200 semilandmarks) with multi-locus phylogenetics (COI, 16S rRNA, H3, 28S rRNA) and haplotype networks on sympatric populations from Liuzhou and GenBank. Morphometrics completely separated C. chinensis_lz from C. cathayensis_lz in shell shape; key diagnostic traits include aperture size, spire height, and body whorl inflation. The high-density semilandmark method outperformed discrete landmarks in dimensionality reduction and fine-scale resolution, capturing subtle apex and lateral whorl variations. No sexual dimorphism occurred in C. cathayensis_lz, with only weak dimorphism in C. chinensis_lz. Molecular data were ambiguous: the species tree and mitochondrial gene tree recovered C. chinensis and C. cathayensis as monophyletic, but the nuclear gene tree showed mixing of C. chinensis_lz/C. cathayensis_lz with C. wisseli, C. chinensis, and C. cathayensis. Haplotype networks revealed haplotypes sharing between C. chinensis_lz and C. cathayensis_lz, yet neither population shared haplotype with GenBank sequences of the two nominal species. Morphology strongly supports C. chinensis_lz and C. cathayensis_lz as distinct species, whereas molecular evidence shows only low-level differentiation and discordant signals between mitochondrial and nuclear markers. We conclude they are likely valid species, yet mitonuclear discordance warrants further genomic investigation. Despite no evidence of microhabitat partitioning, the stable morphological divergence between these sympatric morphospecies supports separate management units to conserve phenotypic diversity and local adaptive potential. This study validates geometric morphometrics as an efficient frontline tool for biodiversity assessment in morphologically diverse yet genetically conserved freshwater snails, and reinforces the need for genome-wide data to resolve species boundaries within the Cipangopaludina complex.