Reclassification of Lamprotula rochechouartii as Margaritifera rochechouartiicomb. nov. (Bivalvia: Margaritiferidae) revealed by time-calibrated multi-locus phylogenetic analyses and mitochondrial phylogenomics of Unionoida

Unveiling the identity of Diaurora Cockerell, 1903 (Bivalvia, Unionidae): morphology, molecular phylogenetics, and the description of a new species

The monotypic freshwater mussel genus Diaurora Cockerell, 1903 has long been enigmatic due to its rarity and morphological confusion with Acuticosta. In this study, we comprehensively redescribed Diauroraaurorea (Heude, 1883) through a detailed analysis of shell morphology and molecular phylogenetics of recently collected specimens. Moreover, a new species, Diauroralaevesp. nov., was identified from the Fuyishui River, a tributary of the Zishui River in Shaoyang County, Shaoyang City, Hunan Province, China. Molecular phylogenetic analyses showed that D.aurorea and D.laevesp. nov. were reciprocally monophyletic and formed a clade as sister to Schistodesmus. Our study underscores the necessity of further exploring the diversity of freshwater mussels in understudied small tributaries throughout China.

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

DNA barcoding, multilocus phylogeny, and morphometry reveal phenotypic plasticity in the Chinese freshwater mussel Lamprotula caveata (Bivalvia: Unionidae)

Accurate species identification is crucial for developing conservation strategies for freshwater mussels, one of the most imperiled faunas in the world. Traditionally, mussel species description primarily relied on conchological characters. However, shell morphology has great variability, which leads to the complexity of species delimitation. As endemic species to China, Lamprotula caveata was originally described by Heude (1877). Lamprotula quadrangulosus and Lamprotula contritus were considered for synonymization of L. caveata based on shell variants in the early 20th century, which has been long debated due to lack of rigorous molecular analysis. Moreover, great morphological variation caused doubt whether there are cryptic species. In this study, we used a combined phylogenetic and morphometric approach to verify the validity of the synonymization of L. caveata. The results of molecular species delimitation showed that two molecular operational taxonomic units (MOTUs) were identified in Lamprotula spp., including the L. leaii lineage and the complex lineage (L. quadrangulosa, L. cornuumlunae, L. contritus, and L. caveata). Phylogenetic analyses revealed that L. cornuumlunae formed a basal monophyletic clade, whose divergence time was relatively recent (4.26 Ma [95% HPD = 1.91-7.22 Ma]), and L. contritus, L. caveata, and L. quadrangulosa formed a large polytomy group with very shallow branches. In the previous study, we have demonstrated the validity of L. cornuumlunae. The molecular evidences supported that the complex (L. quadrangulosa + L. contritus + L. caveata) was a valid species; L. quadrangulosa and L. contritus were synonyms of L. caveata. In addition, three morphospecies (L. quadrangulosa, L. contritus, and L. caveata) were aggregated without clear differentiation based on shell morphometric analysis. We confirmed multiple phenotypes in L. caveata for species identification and presumed that the phenotypic plasticity was a response to specific habitats. This study clarified the diversity and phylogeny of the Lamprotula group, which is a crucial step for developing new conservation and management strategies for this imperiled group.

Complete maternal mitochondrial genome of the freshwater mussel Cuneopsis celtiformis (Bivalvia: Unionidae)

The genus Cuneopsis Simpson, 1900 comprises seven valid species, and Cuneopsis celtiformis (Heude, 1874) is the type species of this genus. Previous phylogenetic studies using complete mitochondrial genomes showed that Cuneopsis was not monophyletic, but the result was hampered by incomplete species sampling and lack of the type species of this genus. In this study, we collected C. celtiformis from the type locality and determined its complete maternal mitochondrial genome. This mitogenome is 15,922 bp in length and contains 14 protein-coding genes (including one F-orf), two rRNA genes, 22 tRNA genes, and 1 putative control region. Our mitochondrial phylogenomic analysis confirms that currently recognized genus Cuneopsis is polyphyletic, and C. celtiformis is the closest to C. heudei with high maximum likelihood bootstrap support value. Comprehensive sampling of all Cuneopsis species is needed for phylogenetic analysis to erect new genera in future studies.

The complete mitochondrial genome of freshwater mussel Pronodularia japanensis (Gonideinae, Unionidae, Unionida) from Tochigi Prefecture, Japan, and its phylogenetic analysis

We have sequenced the female-type (F-type) complete mitochondrial genome of Pronodularia japanensis (Gonideinae, Unionidae, Unionida, Bivalvia) from Tochigi Prefecture, Japan. The complete F-type mitochondrial genome (16,803 bp; LC505454) contains 13 protein-coding genes, 2 rRNA genes, and 22 tRNA genes. Molecular phylogenetic analyses using complete F-type mitochondrial genomes of 56 Unionida species revealed the phylogenetic position of P. japanensis in Unionidae. This study should be basic data to investigate the genetic diversity in this species.

Freshwater mussels (Bivalvia: Unionidae) from the rising sun (Far East Asia): phylogeny, systematics, and distribution

Freshwater mussels (Bivalvia: Unionidae) is a diverse family with around 700 species being widespread in the Northern Hemisphere and Africa. These animals fulfill key ecological functions and provide important services to humans. Unfortunately, populations have declined dramatically over the last century, rendering Unionidae one of the world's most imperiled taxonomic groups. In Far East Asia (comprising Japan, Korea, and Eastern Russia), conservation actions have been hindered by a lack of basic information on the number, identity, distribution and phylogenetic relationships of species. Available knowledge is restricted to studies on national and sub-national levels. The present study aims to resolve the diversity, biogeography and evolutionary relationships of the Far East Asian Unionidae in a globally comprehensive phylogenetic and systematic context. We reassessed the systematics of all Unionidae species in the region, including newly collected specimens from across Japan, South Korea, and Russia, based on molecular (including molecular species delineation and a COI + 28S phylogeny) and comparative morphological analyses. Biogeographical patterns were then assessed based on available species distribution data from the authors and previous reference works. We revealed that Unionidae species richness in Far East Asia is 30% higher than previously assumed, counting 43 species (41 native + 2 alien) within two Unionidae subfamilies, the Unioninae (32 + 1) and Gonideinae (9 + 1). Four of these species are new to science, i.e. Beringiana gosannensissp. nov., Beringiana fukuharaisp. nov., Buldowskia kamiyaisp. nov., and Koreosolenaia sitgyensisgen. & sp. nov. We also propose a replacement name for Nodularia sinulata, i.e. Nodularia breviconchanom. nov. and describe a new tribe (Middendorffinaiini tribe nov.) within the Unioninae subfamily. Biogeographical patterns indicate that this fauna is related to that from China south to Vietnam until the Mekong River basin. The Japanese islands of Honshu, Shikoku, Kyushu, Hokkaido, and the Korean Peninsula were identified as areas of particularly high conservation value, owing to high rates of endemism, diversity and habitat loss. The genetically unique species within the genera Amuranodonta, Obovalis, Koreosolenaiagen. nov., and Middendorffinaia are of high conservation concern.

Phylogenetic position of Aculamprotula polysticta, comb. res. (Bivalvia : Unionidae) inferred from phylogenetic relationships in Unionida

Accurate phylogenies are important for understanding the evolutionary histories of organisms, their reproductive traits and ecological habits. The freshwater mussel order Unionida is currently thought to include six families. However, assignment of particular species to these families has been unstable, particularly for species that have been described solely on conchological characters. Unio polystictus Heude, 1877 represents such a species. Based on DNA sequence data from five genes (COI, 16S rRNA, 18S rRNA, 28S rRNA and histone H3) and complete mitochondrial genomes, we investigated the phylogenetic position and generic affinities of U. polystictus using various analytical methods. Both the five-gene and mitogenome datasets strongly supported transferring U. polystictus from Margaritiferidae to Unionidae as Aculamprotula polysticta, comb. res. Our results also supported the following intrageneric relationships: (Aculamprotula tortuosa, ((Aculamprotula polysticta, Aculamprotula scripta), (Aculamprotula fibrosa, Aculamprotula tientsinersis))). In addition, by comparing the morphological features of Aculamprotula (Unionidae, Unioninae), Lamprotula (Unionidae, Gonideinae) and Gibbosula (Margaritiferidae, Gibbosulinae) species, potential issues of relying solely on shell morphology for high-level classification of freshwater mussels are highlighted. Confirmation of classification position and genetic relationship for Aculamprotula polysticta will helpful to understand the ecological characteristics, reproductive strategies and host-fish requirements, which can be inferred from closely related taxa.

Fish hosts, glochidia features and life cycle of the endemic freshwater pearl mussel Margaritifera dahurica from the Amur Basin

Margaritiferidae is a small freshwater bivalve family with 16 species. In spite of a small number of taxa and long-term history of research, several gaps in our knowledge on the freshwater pearl mussels still exist. Here we present the discovery of host fishes for Margaritifera dahurica, i.e. Lower Amur grayling, sharp-snouted lenok, and blunt-snouted lenok. The host fishes were studied in rivers of the Ussuri Basin. The identification of glochidia and fish hosts was confirmed by DNA analysis. The life cycle of M. dahurica and its glochidia are described for the first time. The SEM study of glochidia revealed that the rounded, unhooked Margaritifera dahurica larvae are similar to those of the other Margaritiferidae. Margaritifera dahurica is a tachytictic breeder, the larvae of which attach to fish gills during the Late August – September and finish the metamorphosis in June. Ancestral host reconstruction and a review of the salmonid - pearl mussel coevolution suggest that the ancestral host of the Margaritiferidae was a non-salmonid fish, while that of the genus Margaritifera most likely was an early salmonid species or their stem lineage. The overfishing of lenoks and graylings appears to be the most significant threat for this rare mussel species.

Analysis of mitochondrial genomes resolves the phylogenetic position of Chinese freshwater mussels (Bivalvia, Unionidae)

The Yangtze River basin is one of the most species-rich regions for freshwater mussels on Earth, but is gravely threatened by anthropogenic activities. However, conservation planning and management of mussel species has been hindered by a number of taxonomic uncertainties. In order to clarify the taxonomic status and phylogenetic position of these species, mitochondrial genomes of four species (Acuticostachinensis, Schistodesmuslampreyanus, Cuneopsisheudei and Cuneopsiscapitatus) were generated and analyzed along with data from 43 other mitogenomes. The complete F-type mitogenomes of A.chinensis, S.lampreyanus, C.heudei, and C.capitatus are 15652 bp, 15855 bp, 15892 bp, and 15844 bp, respectively, and all four F-type mitogenomes have the same pattern of gene arrangement. ML and BI trees based on the mitogenome dataset are completely congruent, and indicate that the included Unionidae belong to three subfamilies with high bootstrap and posterior probabilities, i.e., Unioninae (Aculamprotula, Cuneopsis, Nodularia, and Schistodesmus), Anodontinae (Cristaria, Arconaia, Acuticosta, Lanceolaria, Anemina, and Sinoanodonta), and Gonideinae (Ptychorhynchus, Solenaia, Lamprotula, and Sinohyriopsis). Results also indicate that A.chinensis has affinities with Arconaialanceolata and Lanceolariagrayii and is a member of the subfamily Anodontinae.

Towards a global phylogeny of freshwater mussels (Bivalvia: Unionida): Species delimitation of Chinese taxa, mitochondrial phylogenomics, and diversification patterns

The Yangtze River Basin in China is one of the global hotspots of freshwater mussel (order Unionida) diversity with 68 nominal species. Few studies have tested the validity of these nominal species. Some taxa from the Yangtze unionid fauna have not been adequately examined using molecular data and well-positioned phylogenetically with respect to the global Unionida. We evaluated species boundaries of Chinese freshwater mussels, and disentangled their phylogenetic relationships within the context of the global freshwater mussels based on the multi-locus data and complete mitochondrial genomes. Moreover, we produced the time-calibrated phylogeny of Unionida and explored patterns of diversification. COI barcode data suggested the existence of 41 phylogenetic distinct species from our sampled 40 nominal taxa inhabiting the middle and lower reaches of the Yangtze River. Maximum likelihood and Bayesian inference analyses on three loci (COI, 16S, and 28S) and complete mitochondrial genomes showed that the subfamily Unioninae sensu stricto was paraphyletic, and the subfamily Anodontinae should be subsumed under Unioninae. In addition, we described two new tribes (Aculamprotulini tribe nov. and Lepidodesmini tribe nov.) in the subfamily Unioninae and one new genus (Parvasolenaiagen. nov.) in the subfamily Gonideinae. Molecular dating analysis suggested freshwater mussels diversified at 346.1 Mya (HPD = 286.6-409.9). The global diversification rate for Unionida was estimated to be 0.025 species/Myr. Our study found only a single well-supported rate shift in Unionida diversification, occurring at the base of the subfamily Ambleminae. The evolution of active host-attraction may have triggered the burst of speciation in Ambleminae, and the environment and geography of the Mississippi River Basin likely sustained this radiation.