A new genus and species of freshwater mussels, Guiunio rarus Chen, Li, Dai & Wu, gen. et sp. nov., is described from the Pearl River Basin in China based on comparative morphology and molecular phylogeny. The new taxon belongs to the tribe Gonideini and can be distinguished from other genera by a unique combination of characteristics: shell medium-sized, moderately thick, flat, long, sub-glossy, opaque; anterior small, rounded and short; posterior extremely expanded, wide and long, with an indistinct obtuse angle in the middle of posterior margin; papillae on flap margin highly degraded. The validity of it is further supported by the molecular phylogenetic analysis based on COI, 16S and 28S sequences.

Biodiversity, molluscs, phylogeny, taxonomy

Freshwater mussel is a group of large benthic animal that play an important role in freshwater ecosystems (Graf and Cummings 2007; Huang et al. 2019; Liu et al. 2022). China is a diversity hotspot of freshwater mussels, with about 100 accepted species recorded and new taxa continuing to be discovered (Heude 1875, 1877a, 1877b, 1878, 1879, 1880a, 1880b, 1881, 1883, 1885; Simpson 1900; He and Zhuang 2013; Graf and Cummings 2021, 2025; Guo 2022; Liu et al. 2022, 2023, 2024; Wu et al. 2021, 2022, 2024; Chen et al. 2023; Dai et al. 2023, 2024a, 2024b, 2024c, 2024d; MolluscaBase eds. 2025). The majority of studies on Chinese freshwater mussels are concentrated in the Changjiang River Basin (Yangtze) (Heude 1875, 1877a, 1877b, 1878, 1879, 1880a, 1880b, 1881, 1883, 1885; Zeng et al. 1981, 1985; Zeng and Liu 1989; Liu and Wu 1991; Wu et al. 1994, 1999, 2000, 2017, 2018, 2021, 2022; Shu and Wu 2004; Ouyang et al. 2011; Xiong et al. 2011, 2012; Xiao et al. 2012; Chen et al. 2023; Dai et al. 2024a, 2024d), with very few focusing on the Pearl River Basin (Dai et al. 2023, 2024b, 2024c; Liu et al. 2023, 2024; Wu et al. 2024). The Pearl River is the southernmost large river in China, located between the Changjiang River and the Red River. The recurrence of connectivity events throughout the history of the region has resulted in the fish community of the Pearl River Basin exhibited by both the local and the Changjiang River Basin’s characteristics (Huang et al. 2017; Sun et al. 2022). However, the freshwater mussel community here remains a subject of limited knowledge due to the absence of systematic studies. Some sporadic studies have revealed the unique composition of freshwater mussels and several as yet undescribed species in the region (Dai et al. 2023, 2024b, 2024c; Liu et al. 2023, 2024; Hou et al. 2025). There are substantial regions here that have not yet been the subject of exploration and research.

Subfamily Gonideinae Ortmann, 1916, encompasses small to large freshwater mussels from East Europe, West Asia, East Asia, Southeast Asia, and North America (Graf and Cummings 2025; MolluscaBase eds. 2025). The shell morphology of the subfamily exhibits significant phenotypic plasticity and convergence, making it difficult to classify based solely on morphology reliably (Zieritz and Aldride 2009; Inoue et al. 2013). China represents a diversity hotspot of the subfamily, harboring about 10 genera and 30 accepted species (Graf and Cummings 2025; MolluscaBase eds. 2025), and new taxa being discovered continuously (Wu et al. 2021; Dai et al. 2023, 2024a).

During the surveys in 2024, we discovered a group of freshwater mussel specimens with the special expanded posterior that did not resemble any known species and were challenging to place in any genus. Based on a combination of morphology and molecular phylogeny, we describe it as a new genus and species of subfamily Gonideinae. The discovery improved the diversity of freshwater mussels in the Pearl River Basin, leading to a more comprehensive understanding of the endemic species in the region.

Specimens were collected from Guaangxi in China in 2024. Living specimens were initially frozen at -20 °C for 24 hours and subsequently thawed at room temperature for 2 hours to facilitate the extraction of soft parts. The soft parts were then fixed in 70% ethanol. Empty shells were cleaned, dried, and preserved at room temperature. Photographs were taken by camera and edited with Adobe Photoshop CC 2015 (Adobe, San Jose, US). Maps were made in ArcGIS Pro (Esri, Redlands, US).

Genomic DNA was extracted from foot tissues preserved in 70% ethanol using a TIANamp Marine Animals DNA Kit (Tiangen Biotech, China). The quality and concentration of the DNA were checked on 1% agarose gel electrophoresis and NanoDrop 2000 (Thermo Scientific, USA). Partial cytochrome c oxidase subunit 1 (COI), 16S ribosomal RNA (16S) and 28S ribosomal RNA (28S) were amplified and sequenced for molecular phylogenetic analyses. Polymerase chain reaction (PCR) systems, conditions and primer pairs were followed Dai et al. (2024a). The COI sequences were aligned using MEGA v. 6.0 (Tamura et al. 2013), and the 16S and 28S sequences were aligned using MAFFT v. 7 (Katoh et al. 2019) by the Q-INS-i algorithm. The accession numbers of other species and newly obtained sequences are given in Table 1.

Phylogenies reconstructed by the dataset combined three genes using Maximum Likelihood (ML) and Bayesian Inference (BI). Five species of Unioninae Rafinesque, 1820 and Margaritiferidae Henderson, 1929 were used as the outgroups for rooting the trees. The best-fit model for each gene and gene partition was calculated by PartitionFinder2 v. 1.1 (Lanfear et al. 2017), based on the corrected Akaike Information Criterion (AICc) and using a heuristic search algorithm. The program proposed the division of the concatenated dataset into three partitions, comprising partitions for the 16S and 28S genes and each of the three codon positions of the COI gene. The best-fit model was determined to be GTR+I+G for the first and second codon positions of COI, as well as for 16S and 28S, while GTR+G was selected for the third position of COI. ML analyses were performed in IQ-TREE v. 1.6.12 (Minh et al. 2013) using the Ultrafast bootstrap approach (Minh, et al. 2013) with 10,000 iterations. Bayesian inference (BI) analysis was conducted in MrBayes v. 3.2.6 (Ronquist et al. 2012). Four simultaneous runs with four independent Markov Chain Monte Carlo (MCMC) were implemented for 10 million generations, and trees were sampled every 10,000 generations with a burn-in of 25%. The convergence was checked with the average standard deviation of split frequencies <0.01 and the potential scale reduction factor (PSRF) ~ 1. Trees were visualized in FigTree v.1.4.3 (http://tree.bio.ed.ac.uk/software/figtree/).

Abbreviations: NCU_XPWU: Laboratory of Xiao-Ping Wu, Nanchang University (Nanchang, Jiangxi, China); aam: anterior adductor muscle; pam: posterior adductor muscle; ea: excurrent aperture; ia: incurrent aperture; pia: papillae of the incurrent aperture; pea: papillae of the excurrent aperture; ig: inner gills; og: outer gills; m: mantle; lp: labial palps; vm: visceral mass; f: foot.

Despite the fact that only three type specimens were collected after an extended period of collection, molecular phylogeny and morphological comparison offer robust support for its independence. Molecular phylogenetic results supported placing the new genus in tribe Gonideini Ortmann, 1916. Within the tribe, the new genus is only similar to Parvasolenaia Huang & Wu, 2019 by having an extremely expanded posterior, but differs by the thicker, opaque and sub-glossy shell (vs. thinner, semi-transparent and glossy shell), and the highly degraded papillae on flap margin (vs. with developed papillae on flap margin). The new genus was sistered with Cosmopseudodon in the phylogenetic tree, but differs by the extremely expanded posterior (vs. narrowed), the absence of an incision at the posterior (vs. presence) and the highly degraded papillae on flap margin (vs. with distinct papillae on flap margin). Within the subfamily, the new genus was similar to Trapezoideus Simpson, 1900, Yaukthwa Bolotov, Konopleva, Vikhrev, Lopes-Lima, Bogan, Lunn, Chan, Win, Aksenova, Gofarov, Tomilova & Kondakov, 2019, and Lens Simpson, 1900 of tribe Contradentini Modell, 1942 by the similar shell shape, but differs by the oblique dorsal margin without developed wing, the more blunt posterior margin and the different distribution (southern China vs. Indochina). The convergent evolution of shell morphology among different groups in Unionida is a common phenomenon (Huang et al. 2018; Wu et al. 2020, 2022; Dai et al. 2024b), which may be attributable to adaptation to similar habitat environments.

The distribution of both the new species and a yet-to-be-described species of Rhodeus is confined to the same river, representing the second documented instance of sympatric occurrence between a new freshwater mussel and a new bitterling (Wu et al. 2022). Southern China is a hotspot for the both freshwater mussels and bitterlings (Guo 2022; Li 2025), and the special mutually beneficial symbiotic relationship between them may have driven their coevolution.

This study was supported by the National Natural Science Foundation of China under Grant No.32360132, No.31772412, and the Laboratory of Ecological Security and Biodiversity Conservation of Cities on the Yangtze River Delta, Shanghai Science and Technology Museum.