New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia

Taxonomic revision of two species in the genus Ptychorhynchus Simpson, 1900 (Bivalvia: Unionidae: Gonideinae), with description of a new species

Accurate identification and precise classification of freshwater mussel species that are among the most threatened freshwater taxa in the world, play a crucial role in informing conservation and management efforts for these organisms. However, due to the variability in shell morphology, relying solely on shell characteristics for species taxonomy poses significant challenges, thereby impeding effective conservation planning and management. The freshwater mussel genus Ptychorhynchus Simpson, 1900 is one such group in need of study. We integrate molecular phylogeny, shell morphology and soft-body anatomy to examine the classification of Ptychorhynchus denserugata (Haas, 1910) and Ptychorhynchus resupinatus (von Martens, 1902). The COI barcoding data support the clustering of P. denserugata and Nodularia douglasiae within a single clade, and P. denserugata shares the diagnostic feature of the genus Nodularia , i.e. knobs or bumps on the inner mantle surface in the excurrent aperture. Therefore, by integrating molecular data and anatomical characteristics, we confirm that the nominal species P. denserugata syn. nov. is a new synonym for N. douglasiae . The multi-locus (COI + ND1 + 16S rRNA + 18S rRNA + 28S rRNA ) phylogeny and mitochondrial phylogenomics support the transfer of P. resupinatus from Ptychorhynchus to the newly elevated genus Cosmopseudodon stat. rev., as Cosmopseudodon resupinatus stat. rev. that is still considered the designated type species. We also describe a new species based on integrative taxonomy, i.e. Cosmopseudodon wenshanensis sp. nov. The comprehensive understanding of the taxonomy and diversity of the revised Cosmopseudodon species, and shell heteromorphism of N. douglasiae (=P. denserugata syn. nov.), will serve as a crucial foundation for further scientific assessment and conservation strategies pertaining to these taxa. ZooBank: urn:lsid:zoobank.org:pub:E48968B1-DF0F-42AD-8F31-B8C95F23CE57.

Multilocus and mitogenomic phylogenetic analyses reveal a new genus and species of freshwater mussel (Bivalvia: Unionidae) from Guangxi, China

Freshwater mussels are essential for the integrity of freshwater ecosystems but numbers of these organisms are declining rapidly at regional and global scales. The phylogenetic and biogeographic aspects of the rich unionoid fauna of the Indo-Burma region are becoming increasingly well understood. Guangxi is part of the Chinese portion of the Indo-Burma biodiversity hotspot but regional studies of the freshwater mussel diversity are scarce. In this study, we report a new genus and species of freshwater mussel from Guangxi, China. Genetic datasets including three genes (COI, 16S rRNA and 28S rRNA) and complete maternal mitogenomes were compiled to infer the phylogenetic history of the group. Molecular phylogenetic analyses showed that the new species formed a monophyletic group and was closely related to Obovalis and Ptychorhynchus in the tribe Gonideini of the subfamily Gonideinae. Morphological and molecular evidence supported that these specimens represent an undescribed genus and species that we describe as Postolata guangxiensis gen. nov., sp. nov. The discovery of this new taxon adds to the known level of endemism of freshwater mussels in Guangxi and a detailed survey of uncharted areas should reveal new diversity in the future. We also suggest that complete mitogenomes or even genome-scale nuclear data should be used for phylogenetic reconstructions when proposing major taxonomic changes. ZooBank: urn:lsid:zoobank.org:pub:76FC5A1D-7507-4F26-A12C-EC08AB333274

Freshwater mussel conservation: A global horizon scan of emerging threats and opportunities

We identified 14 emerging and poorly understood threats and opportunities for addressing the global conservation of freshwater mussels over the next decade. A panel of 17 researchers and stakeholders from six continents submitted a total of 56 topics that were ranked and prioritized using a consensus-building Delphi technique. Our 14 priority topics fell into five broad themes (autecology, population dynamics, global stressors, global diversity, and ecosystem services) and included understanding diets throughout mussel life history; identifying the drivers of population declines; defining metrics for quantifying mussel health; assessing the role of predators, parasites, and disease; informed guidance on the risks and opportunities for captive breeding and translocations; the loss of mussel-fish co-evolutionary relationships; assessing the effects of increasing surface water changes; understanding the effects of sand and aggregate mining; understanding the effects of drug pollution and other emerging contaminants such as nanomaterials; appreciating the threats and opportunities arising from river restoration; conserving understudied hotspots by building local capacity through the principles of decolonization; identifying appropriate taxonomic units for conservation; improved quantification of the ecosystem services provided by mussels; and understanding how many mussels are enough to provide these services. Solutions for addressing the topics ranged from ecological studies to technological advances and socio-political engagement. Prioritization of our topics can help to drive a proactive approach to the conservation of this declining group which provides a multitude of important ecosystem services.

Diversity and Phylogenetics of Freshwater Mussels (Unionidae) from Southern Thailand with the Description of One New Genus and Five New Species-Group Taxa

Southern Thailand represents a region of unique freshwater biodiversity with many endemic taxa, including a number of freshwater mussel species (Bivalvia: Unionidae). In this study, we recognize 13 taxa in the tribes Contradentini, Rectidentini, Pseudodontini (subfamily Gonideinae), and Indochinellini (subfamily Parreysiinae) that inhabit different localities in the Songkhla Lake, Tapi River, and Tha Taphao River basins. Based on the results of morphological and phylogenetic analyses, we discovered among these mussels six taxa new to science, including one genus, three species, and two subspecies. New taxonomic names are introduced here as follows: Songkhlanaiagen. nov.; S. tamodienicagen. & sp. nov.; Sundadontina plugpomenicasp. nov.; Monodontina vondembuschiana tapienicassp. nov.; M. vondembuschiana thasaenicassp. nov. (Pseudodontini); and Trapezoideus thachiadensissp. nov. (Contradentini). These new taxa confirm the high conservation priority of the Southern Thai freshwater mussel fauna.

Phylogeny and biogeography of Indochinese freshwater mussels in the genus Pilsbryoconcha Simpson, 1900 (Bivalvia: Unionidae) with descriptions of four new species

The body of knowledge regarding the classification and evolution of freshwater mussels in the family Unionidae (Bivalvia) in Indochina has recently increased. However, the taxonomic revision of all extant taxa in the region is still ongoing. In this study, the genus Pilsbryoconcha was revised based on an integrative analysis of shell morphology, biogeography, and molecular data. Multi-locus phylogeny indicated the availability of eight species within the genus. Four previously recognized species are P. exilis (Lea, 1838), P. schomburgki (Martens, 1860) stat. rev., P. linguaeformis (Morelet, 1875), and P. carinifera (Conrad, 1837), while four other species are described herein as P. acuta sp. nov., P. mekongiana sp. nov., P. kittitati sp. nov., and P. hoikaab sp. nov. In addition, the neotype of P. carinifera is also designated to clarify its long taxonomic ambiguity. Divergent time estimation and historical biogeography analysis revealed that Pilsbryoconcha originated in the area now called the Khorat Plateau around the middle of the Eocene (mean age = 43.12 Mya), before its range was expanded across Indochina through a series of complex geomorphological changes of river systems, which also led to diversification of the genus.

Integrated taxonomy reveals new threatened freshwater mussels (Bivalvia: Hyriidae: Westralunio) from southwestern Australia

The freshwater mussel Westralunio carteri (Iredale, 1934) has long been considered the sole Westralunio species in Australia, limited to the Southwest and listed as vulnerable on the IUCN Red List and under Australian legislation. Here, we used species delimitation models based on COI mtDNA sequences to confirm existence of three evolutionarily significant units (ESUs) within this taxon and conducted morphometric analyses to investigate whether shell shape differed significantly among these ESUs. "W. carteri" I was found to be significantly larger and more elongated than "W. carteri" II and "W. carteri" II + III combined, but not different from "W. carteri" III alone. We recognise and redescribe "W. carteri" I as Westralunio carteri (Iredale, 1934) from western coastal drainages and describe "W. carteri" II and "W. carteri" III as Westralunio inbisi sp. nov. from southern and lower southwestern drainages. Two subspecies are further delineated: "W. carteri" II is formally described as Westralunio inbisi inbisi subsp. nov. from southern coastal drainages, and "W. carteri" III as Westralunio inbisi meridiemus subsp. nov. from the southwestern corner. Because this study profoundly compresses the range of Westralunio carteri northward and introduces additional southern and southwestern taxa with restricted distributions, new threatened species nominations are necessary.

Cryptic genetic divergence of the red dwarf rasbora, Microrasbora rubescens, in and around Inle Lake: implications for the origin of endemicity in the ancient lake in Myanmar

Inle Lake, an ancient lake located in the Shan Plateau of Myanmar, is a biogeographically attractive region with high fish endemism. Some endemic species inhabit the lake as well as the surrounding areas. The genetic and ecological relationships between populations in the lake and surrounding areas provide important insights into the process underlying ichthyofaunal formation in Inle Lake. In this study, the authors focused on red dwarf rasbora Microrasbora rubescens, an endemic genus and species in this region, and estimated its population structure and evolutionary scenario based on genome-wide polymorphism, mtDNA and geometric morphometric analyses using samples from Inle Lake and three areas surrounding the lake. The results showed that M. rubescens comprises at least three genetically divergent lineages (Inle, Heho and Hopong) with distinct geographic structures consistent with nuclear and mtDNA data. In contrast, there was no clear regional differentiation in morphology. The divergence time estimation based on mtDNA suggests that the Hopong lineage diverged at 2.7 Ma and the Inle and Heho lineages diverged at 1.9 Ma - consistent with the nuclear DNA results. The deep divergence observed in the endemic species supports the ancient history of ichthyofaunal development in this region. The distinct regional differentiation and morphological conservatism of this species might have been shaped by niche conservatism in stagnant water environments that limit dispersal and morphological diversification. Future comprehensive genetic and morphological analyses and comparisons for other native species should reveal the geographic and ecological processes that shaped the ichthyofauna in this region.

New Molecular-Based Phylogeny of Mussel-Associated Mites Reveals a New Subgenus and Three New Species Representing an Example of a Host-Driven Radiation in Indochina and Confirms the Concept of Division of the Genus Unionicola Haldeman, 1842 (Acari: Unionicolidae) into Numerous Subgenera

Here we describe a new subgenus and three new species of parasitic water mites in the genus Unionicola (Acari: Hydrachnidia) from Myanmar: Myanmaratax subgen. nov., Unionicola (Myanmaratax) savadiensis subgen. and sp. nov. (hosts: Lamellidens savadiensis and L. generosus), U. (My.) generosa sp. nov. (the same hosts), and U. (My.) trapezidenssp. nov. (hosts: Trapezidens dolichorhynchus and T. angustior). These taxa were identified based on a two-gene phylogenetic analysis (COI + 28S), which also confirms the division of the genus Unionicola into numerous subgenera. The new species are cryptic species, which are morphologically indistinguishable but strongly resemble U. (Prasadatax) brandti Vidrine, 1985 described from Thailand (hosts: Lens spp. and Ensidens spp.). We also transfer the latter taxon from Prasadatax to Myanmaratax based on a set of morphological evidence and propose U. (My.) brandti comb. nov. The new subgenus contains a total of five species, one of which needs future sampling efforts and will be described elsewhere. Additionally, 56 valid subgenera, which were placed in the synonymy of the genus and in one case raised to the genus level, are restored here until robust phylogenetic evidence on their taxonomic status is available. Our results also confirm that Unionicola mites are narrow host specialists that are associated with either one or a few closely related freshwater mussel species belonging to one or two sister genera.

Resolving species-level diversity of Beringiana and Sinanodonta mussels (Bivalvia: Unionidae) in the Japanese archipelago using genome-wide data

Accurate species identification is of primary importance in ecology and evolutionary biology. For a long time, the unionid mussels Beringiana and Sinanodonta have puzzled researchers trying to unravel their diversity because of their poorly discernible morphologies. A recent study conducted species delineation of unionid mussels based on mitochondrial DNA variation, opening up a new avenue to grasp species diversity of the mussels. However, mtDNA-based classification may not align with species boundaries because mtDNA is prone to introgression and incomplete lineage sorting that cause discordance between species affiliation and gene phylogeny. In this study, we evaluated the validity of the mtDNA-based classification of unionid mussels Beringiana and Sinanodonta in Japan using mitochondrial sequence data, double digest restriction site-associated DNA library (ddRAD) sequencing, and morphological data. We found significant inconsistencies in the mitochondrial and nuclear DNA phylogenies, casting doubt on the reliability of the mtDNA-based classification in this group. In addition, nuclear DNA phylogeny revealed that there are at least two unionid lineages hidden in the mtDNA phylogeny. Although molecular dating technique indicates that Beringiana and Sinanodonta diverged >35 million years ago, their shell morphologies are often indistinguishable. Specifically, morphological analyses exhibited the parallel appearance of nearly identical ball-like shell forms in the two genera in Lake Biwa, which further complicates species identification and the morphological evolution of unionid mussels. Our study adds to a growing body of literature that accurate species identification of unionid mussels is difficult when using morphological characters alone. Although mtDNA-based classification is a simple and convenient way to classify unionid mussels, considerable caution is warranted for its application in ecological and evolutionary studies.

Follow the Footsteps of Leonardo Fea: An Example of an Integrative Revision of Freshwater Mussel Taxa Described from the Former British Burma (Myanmar)

Leonardo Fea, an Italian explorer and traveler, sampled a comprehensive collection of continental Mollusca during his travels throughout the former British Burma (currently Myanmar) in 1885-1887. Cesare Maria Tapparone-Canefri, an Italian malacologist, studied this sample and published a paper with a description of numerous terrestrial and freshwater molluscan taxa new to science. This collection was partly deposited in the Museo Civico di Storia Naturale di Genova (MSNG), Italy and the Indian Museum (ZSI: Zoological Survey of India) in Kolkata. Here, we provide a re-analysis of C.M. Tapparone-Canefri’s Burmese Unionidae collection. Our study reveals that the type series of only four nominal taxa described by Tapparone-Canefri as new to science in 1889 are still available in the MSNG, i.e. Unio rectangularis, U. pulcher, U. protensus var. obtusatus, and U. marginalis var. subflabellata. The first taxon is a valid species belonging to the genus Yaukthwa, while U. pulcher and U. protensus var. obtusatus are considered here as junior synonyms of the widespread Lamellidens generosus, and the last nominal taxon corresponds to L. savadiensis. The MSNG collection also contains shell lots of Indochinella pugio pugio, I. pugio paradoxa, Indonaia andersoniana, Radiatula chaudhurii, R. mouhoti haungthayawensis, Lamellidens savadiensis, L. generosus, Yaukthwa nesemanni, and Y. zayleymanensis, most of which were listed in Tapparone-Canefri’s work under incorrect names. We revise all the freshwater mussel taxa listed by Tapparone-Canefri based on the original descriptions, available DNA sequences, morphological data, and biogeographic evidence. A freshwater mussel from the Haungthayaw River that was identified by Tapparone-Canefri as Unio exolescens is described here as Trapezoideus mitanensis sp. nov., a fourth species in this small Contradentini genus with a restricted range. Finally, new taxonomic opinions are proposed here for Leoparreysia tavoyensis, Trapezidens dolichorhynchus, Lamellidens generosus, and Lamellidens savadiensis.

Oriental freshwater mussels arose in East Gondwana and arrived to Asia on the Indian Plate and Burma Terrane

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.

Using Species Groups to Approach the Large and Taxonomically Unresolved Freshwater Fish Family Nemacheilidae (Teleostei: Cypriniformes)

Large animal families with unresolved taxonomy are notoriously difficult to handle with respect to their biodiversity, systematics, and evolutionary history. We approach a large and taxonomically unresolved family of freshwater fishes (Nemacheilidae, >600 species) by proposing, on the basis of morphologic data, a species group within the family and study its phylogeny with conclusions regarding its diversity, taxonomy, and biogeographic history. Phylogenetic analyses of two mitochondrial and three nuclear genes of 139 specimens, representing about 46 species (17 candidate species from the proposed species-group, plus 29 comparative species), revealed that the proposed species group does not form a distinct monophyletic lineage, but that the candidate and comparative species mixed in three different lineages. However, the results revealed more than 20% of undescribed species within the ingroup and showed that species do not cluster according to the presently recognised genera. At least one of the genetic clades shows signs of an eastward range expansion during the second half of Miocene from north India via Myanmar into Laos, western China, and western Thailand. We conclude that the approach of picking monophyletic lineages to study biodiversity, systematics, and evolutionary history helps to open the door to large animal families.

Evidence of cryptic diversity in freshwater Macrobrachium prawns from Indochinese riverine systems revealed by DNA barcode, species delimitation and phylogenetic approaches

The diversity of Indochinese prawns in genus Macrobrachium is enormous due to the habitat diversification and broad tributary networks of two river basins: the Chao Phraya and the Mekong. Despite long-standing interest in SE-Asian decapod diversity, the subregional Macrobrachium fauna is still not yet comprehensively clarified in terms of taxonomic identification or genetic diversification. In this study, integrative taxonomic approaches including morphological examination, DNA barcoding, and molecular species delimitation were used to emphasize the broad scale systematics of Macrobrachium prawns in Indochina. Twenty-seven nominal species were successfully re-verified by traditional and molecular taxonomy. Barcode gap analysis supported broad overlapping of species boundaries. Taxonomic ambiguity of several deposited samples in the public database is related to inter- and intraspecific genetic divergence as indicated by BOLD discordance. Diagnostic nucleotide positions were found in six Macrobrachium species. Eighteen additional putative lineages are herein assigned using the consensus of species delimitation methods. Genetic divergence indicates the possible existence of cryptic species in four morphologically complex and wide-ranging species: M. lanchesteri, M. niphanae, M. sintangense, and some members of the M. pilimanus group. The geographical distribution of some species supports the connections and barriers attributed to paleo-historical events of SE-Asian rivers and land masses. Results of this study show explicitly the importance of freshwater ecosystems in Indochinese subregions, especially for the Mekong River Basin due to its high genetic diversity and species composition found throughout its tributaries.

New freshwater mussels from two Southeast Asian genera Bineurus and Thaiconcha (Pseudodontini, Gonideinae, Unionidae)

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.

Taxonomic revision of a radiation of South-east Asian freshwater mussels (Unionidae : Gonideinae : Contradentini+Rectidentini)

The tribes Contradentini and Rectidentini (Unionidae) comprise a diverse clade of freshwater mussels endemic to South-east Asia. Our understanding of the diversity and phylogeny of this radiation has improved dramatically in recent years, but this systematic transformation has not yet benefited from comprehensive museum sampling or phylogenomic methods. A synthetic taxonomic revision of the Contradentini+Rectidentini that leverages these useful and accessible methods is needed. We set out to (1) generate a phylogenomic reconstruction of the supraspecific relationships of the Contradentini+Rectidentini using anchored hybrid enrichment, (2) revise the taxonomy and geographic boundaries of the generic and species-level diversity of the radiation, and (3) identify patterns of freshwater mussel diversity and distribution in this clade and discuss the processes that may have precipitated them. Our phylogenomic reconstruction using over 1600 loci, with a total alignment length of over a half a million nucleotides, recovers a well supported phylogeny of the clade that resolves four independent multispecies radiations endemic to the Mekong drainage. We examined, digitised, and imaged 1837 records from 15 natural history museums that provided the necessary data to document the morphological variation and geographic distributions of the focal taxa. We also analysed 860 COI sequences, 519 of which were generated in this study, to better understand the species boundaries and geographic distributions of the recovered clades. We recognise 54 valid species in the tribes Contradentini and Rectidentini, including 9 described herein as new to science. Out of this revision emerged several interesting biogeographic patterns that appear to have resulted from recent stream capture, historical confluence, and intradrainage barriers to dispersal. We hypothesise that these phenomena shaped the diversity and distribution of the Contradentini+Rectidentini, contributing to the formation of several characteristic freshwater mussel provinces in South-east Asia.

Mitogenomic phylogeny and fossil-calibrated mutation rates for all F- and M-type mtDNA genes of the largest freshwater mussel family, the Unionidae (Bivalvia)

The Unionidae represent an excellent model taxon for unravelling the drivers of freshwater diversity, but, phylogeographic studies on Southeast Asian taxa are hampered by lack of a comprehensive phylogeny and mutation rates for this fauna. We present complete female- (F) and male-type (M) mitogenomes of four genera of the Southeast Asian clade Contradentini+Rectidentini. We calculate substitution rates for the mitogenome, the 13 protein-coding genes, the two ribosomal units and three commonly used fragments (co1, nd1 and 16S) of both F- and M-mtDNA, based on a fossil-calibrated, mitogenomic phylogeny of the Unionidae. Phylogenetic analyses, including an M+F concatenated dataset, consistently recovers a monophyletic Gonideinae. Subfamily-level topology is congruent with that of a previous nuclear genomic study and with patterns in mitochondrial gene order, suggesting Unionidae F-type 2 as a synapomorphy of the Gonideinae. Our phylogeny indicates that the clades Contradentini+Rectidentini and Lamprotulini+Pseudodontini+Gonideini split in the early Cretaceous (~125 Mya), and that the crown group of Contradentini+Rectidentini originated in the late Cretaceous (~79 Mya). Most gonideine tribes originated during the early Palaeogene. Substitution rates were comparable to those previously published for F-type co1 and 16S for certain Unionidae and Margaritiferidae species (pairs).