Freshwater mussels are among the most imperiled animals on Earth, and Asia’s Unionidae family is their beating heart. Worldwide, the order Unionida comprises six families, 214 genera and 998 recent species, and more than 30 percent of all described species belong to the Unionidae, which reach their greatest diversity in Asia. China, as the center of freshwater mussel distribution in East Asia, harbors an exceptionally high number of endemic species. Yet the true scale of that diversity has been seriously underestimated, particularly in river basins that have received little scientific attention. The recent flood of new species descriptions from the Pearl River Basin and the upper Yangtze River Basin suggests that entire lineages have been sitting unrecognized in muddy riverbeds, waiting for someone to look closely.

The genus Cristaria is one of the most recognizable names in this group. These are large, flat, triangular mussels, often equipped with a well-developed post-dorsal wing along the shell’s rear edge, and their hinge teeth are vestigial or absent. The genus is the type taxon of the subtribe Cristariina within the family Unionidae and is widely distributed across East, Northeast and Southeast Asia, occurring in almost all river basins of eastern China. One member, Cristaria plicata, carries real economic weight: it is one of the main producers of freshwater pearls and is commercially known in China’s aquaculture industry as the freshwater pearl mussel. Until now, taxonomic work on the genus has focused heavily on the Far East, and four species were recognized in China: C. beirensis, C. plicata, C. radiata and C. truncata, the latter newly recorded from the Pearl River Basin in Guangxi.

That count has now changed. During surveys conducted in March and June of 2025, researchers collected living mussels from the Tongbo River in Qingning Town, Tongchuan District, Dazhou City, in Sichuan Province. The shells were peculiar: medium-sized, thin and fragile, ranging in outline from an elongated triangle to an irregular rectangle, with a small and low post-dorsal wing rather than the grand wing typical of the genus. Specimens were frozen at minus 20 degrees Celsius for 24 hours and thawed to facilitate dissection of the soft parts, which were fixed in 70 percent ethanol, while empty shells were cleaned, dried and preserved. The holotype, catalogued as JXAU2503001 at Jiangxi Agricultural University, measures 95.1 millimeters in length, 54.9 millimeters in height and 42.0 millimeters in width, with paratypes spanning 62.5 to 105.9 millimeters in shell length.

To test whether the odd shells represented a genuinely distinct lineage, the team turned to DNA. Genomic material was extracted from foot tissue using a commercial kit, and two gene fragments were amplified and sequenced: the mitochondrial cytochrome c oxidase subunit-I gene, or COI, and the nuclear 28S ribosomal RNA gene. The polymerase chain reactions were run in 25 microliter mixtures with a thermal profile beginning at 98 degrees Celsius, followed by 35 cycles of denaturation, annealing at 50 degrees Celsius and extension. The resulting sequences, deposited in GenBank, fed into two datasets: a COI alignment of 36 sequences spanning 615 base pairs, and a concatenated COI plus 28S dataset of 49 sequences totaling 1,023 base pairs before trimming. Best-fit substitution models were selected with PartitionFinder2 under the corrected Akaike Information Criterion, yielding GTR+I+G for the first and second codon positions of COI and GTR+G for the third position and the 28S gene.

The phylogenetic results were unambiguous. Maximum likelihood analyses performed in IQ-TREE and Bayesian inference in MrBayes, run for 10 million generations across four independent Markov Chain Monte Carlo chains, produced largely identical topologies. Cristaria emerged as monophyletic with maximal support, and within it, the Sichuan specimens formed a distinct taxon recovered as sister to all other members of the genus, with bootstrap support of 100 in the maximum likelihood tree. Pairwise uncorrected COI p-distances between the new species and its congeners ranged from 10.10 percent, against C. truncata, to 13.17 percent, against C. ‘beirensis’. Those figures dwarf the widely used 3 percent threshold for interspecific divergence in animal DNA barcoding, leaving little doubt that the Sichuan mussels belong to a separate evolutionary lineage. The species was named Cristaria wangyucheni in honor of Yu-Chen Wang, who first discovered it, and carries the vernacular Chinese name wang shi guan bang.

Morphologically, the new species is a genuine outlier. Where most Cristaria species grow enormous shells reaching 150 to 500 millimeters, C. wangyucheni tops out around 100 millimeters. Its shell is inflated, very thin and fragile, glossy and opaque, with a yellowish green to brown periostracum marked by irregular concentric growth lines and a shallow sulcus in the middle of the dorsal margin. The umbo is inflated and slightly higher than the hinge line, positioned at a quarter of the dorsal margin, and the beak cavities are shallow and open. Internally, the pseudocardinal teeth are completely degenerated, the lateral teeth are short and thin, and the nacre is light white. The soft anatomy follows suit: a light brown mantle with black aperture margins, highly vestigial papillae on the excurrent aperture, and inner gills slightly longer and wider than the outer gills. Together, these traits expand the known range of shell variation within a genus previously defined by its large, winged, triangular shells.

The molecular work also raised a puzzle about two of the genus’s established species. In the phylogenetic trees, sequences labeled C. ‘beirensis’ and C. radiata cluster together in the same clade, a result inconsistent with earlier species delimitations based on COI. The authors note that the specimen previously identified as C. radiata may have been misidentified despite its morphological similarity, and they point out a geographic complication: C. beirensis was originally described from Beier Lake in northern China, whereas the sequences in question came from specimens collected in Sichuan in the south. Insufficient sampling in the phylogenetic reconstruction could also explain the discrepancy. Resolving the question will require fresh specimens of C. radiata from its type locality, a reminder that even familiar species in this group may hide taxonomic surprises.

Ecologically, C. wangyucheni is known only from its type locality, where it lives in cold, flowing tributaries with pebble bottoms and seasonal floods, sharing the habitat with other mussels of the genera Pseudocuneopsis, Nodularia and Ptychorhynchus. Multiple surveys revealed a narrow distribution and a scarcity of live specimens, suggesting a small population. That fragility is set against a troubling regional backdrop. The Jialing River, the first major tributary of the Yangtze to be fully channelized, has undergone profound hydrological alteration, with many sections transformed from free-flowing lotic habitats to slow-flowing or still lentic waters. Compared with survey results from the 1980s, the new investigation suggests the basin’s mussels are at risk of extinction: the researchers could not locate live individuals of several undescribed species, and in much of the basin only empty shells remain.

The upper Yangtze’s hidden diversity is only beginning to come into focus. In addition to C. wangyucheni, recent years have yielded Pseudocuneopsis sichuanensis, Acudonta baitiaoensis, Pseudocuneopsis perflora and Tongchuania mingiana, the last two also discovered in Dazhou City, and the team has identified three further mussel species from the region that remain undescribed. Each new name is a race against habitat degradation, because shell morphology in freshwater mussels is famously plastic, adapting to local environments and complicating classification, while molecular tools are only now catching up with the diversity on the riverbed. The authors argue that systematic research and targeted conservation efforts in the Jialing River Basin are urgently needed. A species that spent millennia unrecognized in a Sichuan tributary has finally been given a name; whether it survives to keep it depends on what happens to its river next.

Subject of Research: Description of a new freshwater mussel species, Cristaria wangyucheni, from the upper Yangtze River basin in Sichuan, China, based on morphology and molecular phylogeny

Article Title: A new species of Cristaria Schumacher, 1817 ( Bivalvia , Unionidae ) from Sichuan, China

Article References: Dai, Y.-T., Chen, Z.-G., Chen, H., Niu, B.-W., Jia, D.-C., Xie, T.-X., Xiong, L.-F., Wu, X.-P., & Peng, M. (2026). A new species of Cristaria Schumacher, 1817 (Bivalvia, Unionidae) from Sichuan, China. Zoosystematics and Evolution, 102(1), 87-95. https://doi.org/10.3897/zse.102.173182

Image Credits: AI Generated

DOI: 10.3897/zse.102.173182

Keywords: freshwater mussels, Cristaria wangyucheni, Unionidae, new species, Sichuan, Yangtze River, COI barcoding, molecular phylogeny, biodiversity, conservation, Jialing River, taxonomy