An overlooked genus-level clade illuminates the origin and tectonic-driven diversification of a freshwater radiation in Asian mytiloid mussels (Bivalvia: Mytiloidea: Modiolidae)

Abstract Mytiloid mussels are a notable bivalve group that includes several economically important and invasive species. Although most are marine or estuarine dwellers, a few lineages occur in fresh waters. The origin, diversification, and taxonomic richness of freshwater mytiloids have been long-standing subjects of discussion. Here, we describe Shanomytilus gen. nov. (Bivalvia: Mytiloidea: Modiolidae), comprising three species: Shanomytilus mortoni gen. & sp. nov. (Shan Plateau, Myanmar), Shanomytilus liuliwanqing (Chen & He, 2024) comb. nov. (Yunnan, China), and Shanomytilus supoti (Brandt, 1974) comb. nov. (Thailand). These dwarf species exhibit internal fertilization and larval brooding within the mantle cavity, which are likely adaptations for dwelling in fast-flowing rivers, waterfalls, and lacustrine surf zones. Our phylogenetic reconstructions suggest that the freshwater Limnoperninae clade originated in the Early Oligocene and that its diversification coincided with periods of tectonically driven river rearrangement in Indochina. In addition, we describe Neoprososthenia davisi sp. nov. (Gastropoda: Littorinimorpha: Pomatiopsidae), a small snail with a neritiform shell, which co-occurs with Shanomytilus mortoni. Our findings emphasize that ‘white-water’ river sections in tropical Asia host unique, highly specialized lineages of Mollusca, which merit further research and conservation efforts. Moreover, the degradation of critical riffle habitats risks triggering extinction cascades among endemic freshwater invertebrates.