Alpine freshwater ecosystems in southwestern China, particularly those exhibiting sky-island characteristics (geographically isolated high-elevation habitats surrounded by contrasting lowlands), represent critical yet understudied hotspots for aquatic biodiversity evolution. The minute gastropod genus Erhaia (<3 mm shell height) exemplifies this pattern, with species distributed across fragmented high-elevation streams from the Himalayas to eastern China. Despite 20 described species, the group’s diversity is likely underestimated due to their small size, collection neglect, and the lack of recent records—including the type species, E. daliensis, which has not been recollected since its original description. During field surveys conducted in 2023–2024, we rediscovered E. daliensis at its type locality in Yunnan, Chinas, and identified three new species: E. xiaoboqiani H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov., Erhaia guizhouensis H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov., and Erhaia bailong H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov. from mountain streams in Yunnan and Guizhou. Combining morphological and molecular evidence (mitochondrial COI, 16S; nuclear 18S, 28S), we formally describe these taxa and discuss their biogeographic implications for understanding diversification in sky-island-like freshwater systems.

Asian uplift, biogeography, Erhaiidae, molecular phylogeny, sky islands, species diversification

Alpine freshwater ecosystems, particularly those situated in topographically isolated mountain regions, often exhibit “sky island” dynamics, making them critical hotspots for biodiversity evolution (Heald 1951; Moore et al. 2013; He and Jiang 2014; Zhang et al. 2019). The genus Erhaia provides a compelling case study of such isolation in montane freshwater systems. These minute gastropods (typically < 3 mm shell height) (Davis et al. 1985; Gittenberger et al. 2017; Gittenberger et al. 2020; Gittenberger et al. 2022) inhabit high-elevation streams across the Himalayas (Bhutan, Nepal) to eastern China (Yunnan, Guizhou, Hubei, and Fujian), displaying a classic insular distribution pattern in which populations are isolated by arid lowland valleys (Gittenberger et al. 2022; Gyeltshen et al. 2023; Sitnikova et al. 2023). Although 21 species have been formally described, the group’s diversity remains severely underestimated due to their inconspicuous size leading to collection neglect, the type species E. daliensis and most congeners not being recollected since their original descriptions, and a consequent research paucity (Gittenberger et al. 2022; Sitnikova et al. 2023). This knowledge gap persists despite their significance for understanding montane freshwater biogeography and sky island diversification processes.

During field surveys conducted between 2023 and 2024, we collected live specimens of Erhaia daliensis from its type locality, a montane tributary stream of Lake Erhai in Dali City, Yunnan Province, China. Additionally, we discovered three new Erhaia species: E. bailong H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov., E. guizhouensis H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov., and E. xiaoboqiani H. Chen, Y.-M. He, H.-Q. Xiang & X.-P. Wu, sp. nov. from Yunnan and Guizhou provinces. Herein, we formally describe these new species based on morphological and molecular evidence and discuss the role of Erhaia in understanding freshwater sky island biogeography in Asia.

The disjunct montane distribution of Erhaia species may reflect a freshwater “sky island” biogeographic pattern, in which high-elevation stream habitats function as isolated patches separated by uninhabitable lowland barriers. Species of this genus exhibit strict habitat specificity, occurring exclusively in mountain streams while being conspicuously absent from connected rivers at lower elevations. This pronounced habitat specificity, likely facilitated by the continued uplift of the Himalayas (Tada et al. 2016), appears to promote geographic isolation among Erhaia populations. The Wuyi Mountains, straddling the Jiangxi–Fujian border, constitute one of southeastern China’s most significant biogeographic barriers. Often referred to as the “Roof of Eastern China,” this mountain range exerts particularly strong isolating effects on freshwater species dispersal and diversification (Shih et al. 2011; Gao et al. 2019; Zhou et al. 2023). The uplift of the Wuyi Mountains (Zhou et al. 2006; Ni et al. 2021) played a pivotal role in driving the divergence of the two major clades within Erhaiidae. Notably, our current support for the sky island model derives primarily from the distributional concordance between Erhaia’s exclusive mountain stream occupancy and the physical isolation of these habitats by uninhabitable lowlands. Future studies incorporating broader sampling and more comprehensive analyses will be essential to substantiate these findings.

The taxonomy of Erhaia species presents dual challenges due to highly conserved shell morphology with limited diagnostic characters and a critical lack of molecular data for key taxa. This morphological uniformity has contributed to persistent uncertainties regarding the genus’ monophyly, which remains unresolved despite recent taxonomic revisions. Multiple molecular phylogenetic studies have consistently demonstrated that Erhaia does not form a monophyletic group, with E. jianouensis showing closer affinity to Akiyoshia kobayashii than to other Erhaia congeners (Gittenberger et al. 2017; Gittenberger et al. 2020; Gittenberger et al. 2022). These results fundamentally challenge the genus-level classification and necessitate a re-evaluation of diagnostic characters in light of molecular evidence. Notably, on the phylogenetic tree, Erhaia species with a columellar tooth do not form a distinct cluster. Compounding these taxonomic difficulties is the critical lack of available nucleotide sequences for several nominal species, leaving their true phylogenetic positions and taxonomic validity impossible to properly assess. Future studies should prioritize comprehensive sampling, multi-locus molecular data, and comparative microanatomical investigations to resolve these taxonomic uncertainties.

Despite their fascinating distribution patterns and morphological adaptations, Erhaia species face significant conservation challenges. In this study, some Erhaia species were found living under unfavorable conditions. For example, during the search for E. kunmingensis Davis & Kuo, 1985, in Kunming, Yunnan Province, China, the streams and springs recorded in the literature had either dried completely or been converted into artificial ornamental fish ponds. No specimens were found, underscoring the urgent need for further research to identify potential distribution areas and reassess its conservation status. As specialized inhabitants of montane stream ecosystems, these snails are particularly vulnerable to environmental disturbances due to their strict habitat requirements and naturally fragmented distributions. Our discovery of three new Erhaia species on the Yunnan–Guizhou Plateau highlights the region’s remarkable yet threatened freshwater gastropod diversity. Erhaia may be among the first casualties of climate change and anthropogenic impacts in these fragile sky-island habitats. Our findings provide a critical foundation for understanding and conserving these ecologically vulnerable yet often overlooked freshwater microgastropods.

We are deeply grateful to Editor Dr. Thomas von Rintelen, Subject Editor Dr. Le-Jia Zhang, and the two anonymous reviewers for their time and constructive suggestions, which significantly improved the scientific rigor and clarity of this manuscript.