Diversity, morphology, and phylogeny of freshwater mussels of the genus Nodularia (Bivalvia: Unionidae) from China, with descriptions of four new species

Freshwater bivalves (Bivalvia, Unionida) are one of the most threatened groups of animals in the world. Defining species boundaries and understanding the phylogeny and genetic diversity of these species is key to guiding their conservation and management. However, the presence of significant phenotypic plasticity and convergence within this group complicates species delimitation. This includes the freshwater mussel genus Nodularia, endemic to East Asia, for which a comprehensive understanding of species diversity and phylogenetic relationships remains elusive due to inadequate sampling in previous studies, particularly in China, a widely recognized biodiversity hotspot for freshwater mussels. Here, we conduct comprehensive taxonomic and phylogenetic analyses of Nodularia species based on extensive sampling across 23 provinces in China and multiple data sources, including shell morphology, soft body anatomy, six‐gene (COI + ND1 + 16S + 18S + 28S + histone H3) and mitogenome datasets. The integrative systematics approach used here reveals 10 distinct species in this genus, four of which are new to science, i.e. Nodularia hanensis sp. nov., Nodularia huana sp. nov., Nodularia fusiformans sp. nov., Nodularia dualobtusus sp. nov. and two of which are new records for China, i.e. Nodularia dorri (Wattebled [Journal de Conchyliologie, 34, 1886, 54]) and Nodularia micheloti (Morlet [Journal de Conchyliologie, 34, 1886, 75]). We also propose that the nominal species Nodularia jourdyi (Morlet [Journal de Conchyliologie, 34, 1886, 75]) syn. nov. is a new synonym for Nodularia douglasiae (Griffith & Pidgeon, 1833) based on molecular data. BI, ML, and BEAST analyses based on the six‐gene dataset and mitochondrial phylogenomics consistently support the following phylogenetic relationships: (N. dorri + (N. hanensis sp. nov. + N. micheloti)) + (N. breviconcha + (N. huana sp. nov. + (N. fusiformans sp. nov. + ((N. nuxpersicae + N. nipponensis) + (N. dualobtusus sp. nov. + N. douglasiae))))). The molecular clock with fossil calibration indicates that Nodularia originated in the Late Cretaceous period (ca. 73.78 Mya). It then diverged into two independent clades during the Middle Paleogene (ca. 45.01 Mya), followed by a rapid burst of extant speciation during the Neogene (mean age 28.28 to 4.79 Mya). Nodularia breviconcha is the earliest differentiated taxon among the 10 Nodularia taxa, appearing during the Paleogene‐Neogene transition (28.28 Mya; 95% HPD = 14.35–48.44 Mya). Taken together, we provide a robust systematic framework for Nodularia species, addressing phylogenetic relationships, taxonomy, and evolutionary history of this group.