Freshwater mussels cannot spread through oceanic barriers and represent a suitable model to test the continental drift patterns. Here, we reconstruct the diversification of Oriental freshwater mussels (Unionidae) and revise their taxonomy. We show that the Indian Subcontinent harbors a rather taxonomically poor fauna, containing 25 freshwater mussel species from one subfamily (Parreysiinae). This subfamily most likely originated in East Gondwana in the Jurassic and its representatives arrived to Asia on two Gondwanan fragments (Indian Plate and Burma Terrane). We propose that the Burma Terrane was connected with the Indian Plate through the Greater India up to the terminal Cretaceous. Later on, during the entire Paleogene epoch, these blocks have served as isolated evolutionary hotspots for freshwater mussels. The Burma Terrane collided with mainland Asia in the Late Eocene, leading to the origin of the Mekong’s Indochinellini radiation. Our findings indicate that the Burma Terrane had played a major role as a Gondwanan “biotic ferry” alongside with the Indian Plate.

The Oriental Region harbours the second richest fauna of freshwater bivalves in the world, including many endangered endemic taxa. However, the Oriental fauna of the Unionidae have been very poorly studied using an integrative taxonomic approach, which may provide reasonable revisions of complicated (cryptic) taxa based on morphological, molecular, biogeographic and ecological evidence. Here, we present the first example of an integrative taxonomic revision concerning the status of Unio exolescens Gould ( 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ), a nominal mussel taxon that was accepted as a valid species within the genus Trapezoideus Simpson (1900). Currently, Trapezoideus exolescens is considered the type of the genus as far as the originally designated type species, U. foliaceus Gould ( 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ), was considered to be a synonym of T. exolescens. Using nucleotide sequences obtained from mitochondrial (COI and 16S rRNA) and nuclear (28S rDNA) genes, we found that the topotypes of Unio exolescens Gould ( 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ) cluster together with representatives of another mussel genus, Lamellidens Simpson (1900). Based on these results and on morphological data, we transfer Unio exolescens Gould ( 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ) from Trapezoideus to Lamellidens and propose Lamellidens exolescens (Gould, 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ) comb. nov. In addition, we revisited the status of Unio foliaceus Gould ( 1843 Gould, A.A. (1843). Dr. Gould had examined the shells not long since announced as having been received from the Rev. Francis Mason, missionary at Tavoy, in British Burmah. Proceedings of the Boston Society of Natural History, 1, 139–141. [Google Scholar] ) as a valid species and the type of the genus Trapezoideus based on the morphological study of the type specimen, although a question concerning the true position of this taxon is still open because its molecular sequences are not available. Our findings highlight that an integrative taxonomic approach is an important tool, particularly when dealing with such species-rich Unionidae fauna as those of the Oriental Realm.

Freshwater mussels belonging to the tribe Pseudodontini (Bivalvia: Unionidae: Gonideinae) play a keystone role in riverine and lacustrine environments throughout Southeast Asia. Many of them are narrowly endemic and habitat specialists, which need special conservation efforts. Unfortunately, the systematics of this group is rather poorly understood. Here, we show that Pseudodon inoscularis, the type species of the genus Pseudodon, belongs to the eastern clade of the tribe (Southeast Asia, east of the Salween Basin), and, more certainly, clusters with members of the genus Monodontina. Hence, the latter genus becomes a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. In turn, the genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. must be applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We present a revision of the genus Indopseudodon with an updated synonymy and a description of a new species, I. indawgyiensis sp. nov. We also link four species-level genetic lineages from the Mekong to available taxonomic names in the genera Pseudodon and Sundadontina (Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata). Based on morphological features, we revise the taxonomic position of several recent and fossil species that were traditionally placed in the Pseudodontini and transfer them to the genera Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini).

The Mekong and Chao Phraya rivers harbor a species-rich freshwater mussel assemblage containing a large radiation of the Pseudodontini species. Members of the genera Bineurus Simpson 1900 and Thaiconcha Bolotov et al., 2020 primarily inhabit small and medium-sized tributaries of these rivers. Here, we present an integrative taxonomic review of these genus-level clades. We show that Bineurus contains four species: B. mouhotii (Lea, 1863), B. exilis (Morelet, 1866) stat. rev., B. anodontinum (Rochebrune, 1882) stat. rev., and B. loeiensis sp. nov. In its turn, Thaiconcha comprises three species: T. callifera (Martens, 1860), T. munelliptica sp. nov., and T. thaiensis sp. nov. Two species, Pseudodon ovalis Morlet, 1889 and P. thomsoni Morlet, 1884, are considered here as questionable taxa. These findings further highlight that Southeast Asia represents a significant evolutionary hotspot of freshwater mussels, which requires further international collaborative research and conservation efforts.