While a growing body of modern phylogenetic research reveals that the Western indochina represents a separate biogeographic subregion having a largely endemic freshwater fauna, the boundaries of this subregion are still unclear. We use freshwater mussels (Unionidae) as a model to reconstruct spatial patterns of freshwater biogeographic divides throughout Asia. Here, we present an updated freshwater biogeographic division of mainland Southeast Asia and describe 12 species and 4 genera of freshwater mussels new to science. We show that the Isthmus of Kra represents a significant southern biogeographic barrier between freshwater mussel faunas of the Western indochina and Sundaland subregions, while the Indian and Western Indochina subregions are separated by the Naga Hills, Chin Hills, and Rakhine Yoma mountain ranges. Our findings highlight that the freshwater bivalve fauna of Southeast Asia primarily originated within three evolutionary hotspots (Western Indochina, Sundaland, and east Asian) supplemented by ancient immigrants from the indian Subcontinent. Freshwater mussels (Unionida) are an economically and environmentally important group of aquatic animals having a broad distribution on all continents except Antarctica 1,2. Southeast Asia houses one of the richest endemic faunas of freshwater mussels globally 3-7. Unfortunately, freshwater mussels are among the most endangered animal groups at the global scale, with numerous local extinctions triggered by multiple anthropogenic impacts and climate changes 8-11. Human-mediated degradation of natural habitats, e.g. water pollution, river damming, and irrigation practices, appears to be the most influential factor causing the decline and local extinctions of freshwater mussels 11-13. It was shown that even a prehistoric decline in freshwater mussels corresponds to the early development of agricultural techniques 14. Alien species may represent a significant threat to native freshwater mussel assemblages in Southeast Asia 13 and other regions 15. For example, the tropical lineage of Sinanodonta woodiana (Lea, 1834) is widely spread throughout Malaysia, the Indonesian Archipelago, and the Philippines 16-18 , while the temperate lineage of this taxon was found in Myanmar 19. Recent advances in mitogenomic 20 and multi-locus nuclear 21 phylogenetic modeling reveal that two widespread Southeast Asian subfamilies of the Unionidae, i.e. Pseudodontinae 22 and Rectidentinae 22,23 , represent tribes within the monophyletic Gonideinae. The genus-and species-level taxonomy of freshwater mussels in Southeast Asia is still poorly known 3 , but several integrative studies performed in Myanmar, Thailand, Laos, and Malaysia have recently improved our knowledge about the diversity and biogeographic patterns in the region.

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The systematics of Oriental freshwater mussels (Bivalvia: Unionidae) is poorly known. Here, we present an integrative revision of the genus Trapezoideus Simpson, 1900 to further understanding of freshwater mussel diversity in the region. We demonstrate that Trapezoideus as currently circumscribed is non-monophyletic, with its former species belonging to six other genera, one of which is new to science and described here. We recognize Trapezoideus as a monotypic genus, comprised of the type species, T. foliaceus. Trapezoideus comptus, T. misellus, T. pallegoixi, and T. peninsularis are transferred to the genus Contradens, T. subclathratus is moved to Indonaia, and T. theca is transferred to Lamellidens. Trapezoideus prashadi is found to be a junior synonym of Arcidopsis footei. Trapezoideus dallianus, T. nesemanni, T. panhai, T. peguensis, and two species new to science are placed in Yaukthwa gen. nov. This genus appears to be endemic of the Western Indochina Subregion. The two new species, Yaukthwa paiensis sp. nov. and Y. inlenensis sp. nov., are both endemic to the Salween River basin. Our results highlight that Southeast Asia is a species-rich freshwater mussel diversity hotspot with numerous local endemic species, which are in need of special conservation efforts.

Freshwater mussels (Unionida) are one of the most imperiled animal groups worldwide, revealing the fastest rates of extinction. Habitat degradation, river pollution and climate change are the primary causes of global decline. However, biological threats for freshwater mussels are still poorly known. Here, we describe a diverse ecological group of leeches (Hirudinea: Glossiphoniidae) inhabiting the mantle cavity of freshwater mussels. So far, examples of mussel-associated leech species are recorded from East Asia, Southeast Asia, India and Nepal, Africa, and North America. This group comprises a dozen glossiphoniid species with a hidden life style inside the mantle cavity of their hosts largely overlooked by researchers. We show that the association with freshwater mussels evolved independently in three leech clades, i.e. Batracobdelloides, Hemiclepsis, and Placobdella, at least since the Miocene. Seven mussel-associated leech species and two additional free-living taxa are described here as new to science.

Freshwater mussels are sensitive to habitat and water quality, revealing the fastest rates of human-mediated global extinction among aquatic animals. These animals are especially diverse in tropical Asia, the faunas of which are characterized by high levels of endemism. Here we describe four new species and four new subspecies of freshwater mussels from Myanmar. Leoparreysia whitteni sp. nov ., the smallest representative of this genus, was discovered from the Ayeyarwady and Chindwin rivers. Radiatula myitthanensis sp. nov . and R. chindwinensis sp. nov . were recorded from the Chindwin Basin, and R. mouhoti haungthayawensis ssp. nov . has been discovered from the Haungthayaw River. Indochinella pugio has been revised with a description of three subspecies: I. pugio viridissima ssp. nov . from the Sittaung, Bilin and Bago rivers, I. pugio daweiensis ssp. nov . from the Dawei River, and I. pugio paradoxa ssp. nov . from the Haungthayaw River. Yaukthwa elongatula sp. nov ., a peculiar species, conchologically resembling representatives of the genus Solenaia (Gonideinae) with ultra-elongated shell was found in the Chindwin Basin. Our records highlight that tropical Asia harbors numerous, but still overlooked local endemic lineages of freshwater bivalves, which may be on the brink of extinction due to the high anthropogenic and climate change impacts.