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Research Article
Molecular phylogeny reveals a new genus and species of freshwater mussel (Bivalvia, Unionidae, Gonideinae) from Jiangxi, China
expand article infoYu-Ting Dai‡, Zhong-Guang Chen‡, Yu-Zhuo Cheng‡, Xiao-Chen Huang‡, Shan Ouyang‡, Feng-Yue Shu§, Xiao-Ping Wu‡
‡ Nanchang University, Nanchang, China
§ Qufu Normal University, Qufu, China

Corresponding author: Feng-Yue Shu ( shfyue01@163.com )

Corresponding author: Xiao-Ping Wu ( xpwu@ncu.edu.cn )

Academic editor: Matthias Glaubrecht
© 2024 Yu-Ting Dai, Zhong-Guang Chen, Yu-Zhuo Cheng, Xiao-Chen Huang, Shan Ouyang, Feng-Yue Shu, Xiao-Ping Wu.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Dai Y-T, Chen Z-G, Cheng Y-Z, Huang X-C, Ouyang S, Shu F-Y, Wu X-P (2024) Molecular phylogeny reveals a new genus and species of freshwater mussel (Bivalvia, Unionidae, Gonideinae) from Jiangxi, China. Zoosystematics and Evolution 100(4): 1419-1429. https://doi.org/10.3897/zse.100.135217
ZooBank: urn:lsid:zoobank.org:pub:6BD143C1-F9E5-4685-9B80-857CDDB788AD
Open Access

Abstract

Freshwater mussels of the tribe Gonideini (Bivalvia: Unionidae: Gonideinae) constitute one of the most taxonomically diverse groups and serve as keystone species in riverine and lacustrine ecosystems across East Asia. A new genus and species of Gonideini (Bivalvia: Unionidae) is described from Jiangxi, China, as Pseudopostolata angula gen. et sp. nov. based on an integrative analysis of shell morphology and molecular data. The validity of the new genus and species is supported by distinct conchological features: a short, rounded anterior; a long, wide posterior; a slightly downward-curved dorsal margin; and a distinctly obtuse angle at the center of the posterior margin. The multi-locus (COI + 16S rRNA + 28S rRNA) phylogeny showed that the species formed a monophyletic group in the tribe Gonideini of the subfamily Gonideinae. Pseudopostolata angula gen. et sp. nov. is identified as the sister group to a clade comprising the genera Postolata, Cosmopseudodon, Obovalis, Ptychorhynchus, Parvasolenaia, and Koreosolenaia. We emphasize the significant morphological convergence in freshwater mussels, particularly within Gonideini, highlighting the necessity of an integrated taxonomic approach for accurate generic classification of this group.

Key Words

Gonideini, molecular systematics, morphological characters, taxonomy

Introduction

The Unionida, a group of freshwater mussels, is the most species-diverse among the freshwater bivalve, comprising approximately 192 genera and 958 of the more than 1,200 existing species (Graf and Cummings 2021). North America, as well as eastern and southeastern Asia, represent two major hotspots of unionid bivalve diversity (Haag 2012; Zieritz et al. 2018). Recent research efforts have concentrated on the Unionida fauna of China, encompassing extensive specimen collection and DNA sequencing. These endeavors have led to a notable expansion in the documented diversity of Unionida species in the region, along with the identification of numerous new taxa (Wu et al. 2022, 2023, 2024; Chen et al. 2023; Dai et al. 2023, 2024a, 2024b; Liu et al. 2024). The majority of these taxonomic changes were from Southern China. This indicates that the diversity of species in South China may be significantly underestimated, particularly in creeks that have not been previously studied (Dai et al. 2023). Many freshwater mussel populations have rapidly declined because of different factors such as pollution, water quality degradation, habitat destruction or alteration, and invasive species (Haag 2012; Aldridge et al. 2022; Sousa et al. 2022). Consequently, research and conservation efforts for this community are receiving greater attention, underscoring the imperative to describe species diversity and systematics in previously unstudied areas (Huang et al. 2019; Chen et al. 2023; Dai et al. 2023; Zieritz et al. 2024).

Gonideini Ortmann, 1916, within the subfamily Gonideinae of the family Unionidae, represents one of the most taxonomically diverse groups of freshwater mussels in East Asia. At least 12 genera of 33 valid species are recognized, with more than half distributed in China (MUSSEL Project Database, see http://mussel-project.uwsp.edu/). Notably, Postolata Dai, Huang, Guo & Wu, 2023 is endemic to China. Gonideini species are distinguished by their trapezoidal to rectangular shells, the absence or presence of only vestigial hinge teeth, and a tetragenous brooding type (Lopes-Lima et al. 2017; Froufe et al. 2020). Nevertheless, the shell morphology exhibits significant phenotypic plasticity and convergence, making it difficult to classify based solely on morphological traits reliably (Zieritz and Aldride 2009; Inoue et al. 2013). This is particularly pronounced in Gonideini. In the tribe Gonideini, the four species of the genus Sinosolenaia, except Sinosolenaia carinata (Heude, 1877), exhibit a high degree of convergence. Moreover, the morphology of Ptychorhynchus Simpson, 1900, Postolata Dai, Huang, Guo, and Wu, and Obovalis Simpson, 1900, displays notable similarities (Dai et al. 2023). An integrative approach, encompassing both morphological and molecular characterization, is a relatively straightforward and precise method for classifying freshwater mussels (Smith et al. 2019; Bolotov et al. 2020a, 2023).

In the present study, we found a unique freshwater mussel species from Wujiang River, Ji’an City, China. Morphological and molecular evidence supported these loach specimens as a new genus and a new species in the tribe Gonideini of the subfamily Gonideinae. Hence, the new genus and species Pseudopostolata angula gen. et sp. nov. are described herein. Furthermore, the phylogenetic relationships within Gonideini are discussed.

Materials and methods

Specimen sampling, identification, and deposition

In December 2023, eight specimens were collected from the Wujiang River (27°03'37"N, 115°42'17"E) in Ji’an City, Jiangxi Province, China (Figs 1, 2, 4). A digital vernier caliper with an accuracy of ± 0.01 mm was used to measure the length, height, and width of the type series for the new taxa. Live specimens were euthanized with 100% ethanol and then separated into soft tissue and shell. The adductor muscle was used for DNA extraction, while the remaining soft tissue was preserved at −80 °C. All voucher specimens were deposited in the Museum of Biology, Nanchang University (NCUMB), China.

Figure 1. 

Distribution map of Pseudopostolata angula gen. et sp. nov.

Figure 2. 

The type locality and shell morphology of Pseudopostolata angula gen. et sp. nov. A, B. General view of the type locality, Wujiang River, Ji’an City, Jiangxi Province, China; C, D. Shell morphology of Pseudopostolata angula.

Molecular phylogenetic analyses

The Qiagen Genomic DNA Kit (Qiagen, Hilden, Germany) was employed to extract total genomic DNA from the excised tissue following the instructions provided by the manufacturer. The quality and concentration of the DNA were checked on 1% agarose gel electrophoresis and NanoDrop 2000 (Thermo Scientific, USA). We amplified and sequenced fragments from the mitochondrial cytochrome c oxidase subunit-I gene (COI) (LCO22me2 + HCO700dy2) (Walker et al. 2007), 16S small ribosomal RNA gene (16S) (16sar-L-myt + 16sbr-H-myt) (Bolotov et al. 2018), and nuclear 28S ribosomal RNA gene (28S) (D23F + D4RB) (Park and Foighil 2000). The polymerase chain reaction (PCR) was conducted using a 25 µL mixture of 2 × Taq Plus Master MixII (Vazyme, China) (12.5 µL), ddH2O (9.5 µL), 10 µM primers (1 µL each), and genomic DNA (1 µL, about 100 ng/μL). Thermal cycling was started at 98 °C for 10 s, followed by 35 cycles of 94 °C for 1 min, annealing at 50 °C for 1 min, extension at 72 °C for 1 min, and then a final extension at 72 °C for 7 min. The PCR products were sequenced commercially by Sangon Biotech (Shanghai, China). The newly obtained sequences have been deposited in GenBank (Tables 1, 2).

Table 1.
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List of sequences used in genetic distances (*, sequenced in this study).

Species COI GenBank accession no. Locality
Pseudopostolata angula gen. et sp. nov. PQ189757 * China: Jiangxi, Ji’an, Wujiang River
Pseudopostolata angula gen. et sp. nov. PQ189758 * China: Jiangxi, Ji’an, Wujiang River
Obovalis omiensis MT020684 Japan
Obovalis omiensis LC518995 Japan: Gifu
Obovalis omiensis LC518996 Japan: Kyoto
Obovalis omiensis LC518997 Japan: Shiga
Ptychorhynchus pfisteri MG463036 China: Jiangxi, Gan River
Ptychorhynchus pfisteri MG463034 China: Jiangxi, Gan River
Ptychorhynchus pfisteri MG463035 China: Hunan, Xiangyin
Ptychorhynchus pfisteri MG933729 Dongting Lake, China
Ptychorhynchus pfisteri MG933730 Dongting Lake, China
Ptychorhynchus pfisteri KY067440 China
Parvasolenaia rivularis MG463100 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463101 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463098 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463103 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463102 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463099 China: Jiangxi, Gan River
Parvasolenaia rivularis MG463104 China: Jiangxi, Gan River
Koreosolenaia sitgyensis MT020682 South Korea
Koreosolenaia sitgyensis MT020683 South Korea
Postolata guangxiensis OP009379 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009380 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009381 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009382 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009383 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009384 China: Guangxi, Guilin, Luo Qing River
Postolata guangxiensis OP009385 China: Guangxi, Guilin, Luo Qing River
Postolata longjiangensis PP786557 * China: Guangxi, Hechi, Longjiang River
Postolata longjiangensis PP786557 * China: Guangxi, Hechi, Longjiang River
Cosmopseudodon wenshanensis PP079444 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079445 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079446 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079447 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079448 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079449 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079450 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon wenshanensis PP079451 China: Yunnan, Wenshan, Panlong River
Cosmopseudodon resupinatus PP079436 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079437 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079438 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079439 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079440 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079441 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079442 China: Guangxi, Hechi, Taohua River
Cosmopseudodon resupinatus PP079443 China: Guangxi, Hechi, Taohua River
Table 2.
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List of sequences used in multi-locus phylogenetic analyses (*, sequenced in this study).

Taxon COI 16S rRNA 28S rRNA
UNIONIDAE Rafinesque, 1820
Gonodeinae Ortmann, 1916
Gonideini Ortmann, 1916
Obovalis omiensis MT020684 LC223994 MT020830
Obovalis omiensis LC518995 LC223994 MT020830
Obovalis omiensis LC518996 LC223995 LC519064
Obovalis omiensis LC518997 LC519045 LC519065
Ptychorhynchus pfisteri MG463036 KY067440 MG595564
Ptychorhynchus pfisteri MG463034 KY067440 MG595563
Ptychorhynchus pfisteri MG463035 KY067440 MG595562
Parvasolenaia rivularis MG463100 KX966393 MG595626
Sinosolenaia carinata MG463087 NC_023250 MG595616
Sinosolenaia oleivora MG463090 NC_022701 MG595617
Sinosolenaia iridinea MG463091 MT477834 MG595618
Sinosolenaia recognita MG463092 KY561653 MG595619
Leguminaia wheatleyi MN402614 MN396725 MN396721
Microcondylaea bonellii KX822652 KT966473 KX822609
Gonidea angulata MN402615 MN396726 MN396722
Koreosolenaia sitgyensis MT020682 GQ451859 MT020817
Postolata guangxiensis OP009379 OP020466 OP020470
Postolata guangxiensis OP009380 OP020467 OP020470
Postolata guangxiensis OP009381 OP020468 OP020470
Postolata guangxiensis OP009382 OP020469 OP020471
Postolata guangxiensis OP009383 OP020467 OP020472
Postolata guangxiensis OP009384 OP020468 OP020470
Postolata guangxiensis OP009385 OP020469 OP020471
Postolata longjiangensis* PP786557 PP786405 PP786407
Postolata longjiangensis* PP786557 PP786406 PP786407
Postolata longjiangensis* PP786558 PP786405 PP786407
Postolata longjiangensis* PP786558 PP786406 PP786407
Pseudopostolata angula gen. et sp. nov.* PQ189757 PQ201945 PQ201943
Pseudopostolata angula gen. et sp. nov.* PQ189757 PQ201945 PQ201944
Pseudopostolata angula gen. et sp. nov.* PQ189758 PQ201945 PQ201943
Pseudopostolata angula gen. et sp. nov.* PQ189758 PQ201945 PQ201944
Cosmopseudodon resupinatus PP079436 PP079964 PP080006
Cosmopseudodon wenshanensis PP079444 PP079972 PP080014
Pseudodontni Frierson, 1927
Pseudodon mekongi KX865861 KX865632 KX865733
Pseudodon vondembuschianus KP795029 KP795052 MZ684028
Pseudodon cambodjensis KP795028 NC_044112 KP795011
Bineurus loeiensis KX865879 KX865650 KX865750
Bineurus mouhotii KX865876 KX865647 KX865747
Sundadontina tanintharyiensis MN275057 MN307248 MN307189
Sundadontina brandti MN275058 MN307249 MN307190
Pilsbryoconcha exilis KP795024 NC_044124 KP795007
Pilsbryoconcha compressa KX865875 KX865646 KX865746
Thaiconcha callifera KX865862 KX865633 KX865734
Thaiconcha munelliptica MN275063 MN307252 MN307193
Nyeinchanconcha nyeinchani KP795025 KP795050 KP795008
Lamprotulini Modell, 1942
Lamprotula caveata MG462991 NC_030336 MG595518
Lamprotula leaii MN402616 MN396727 MN396723
Potomida littoralis MN402617 MN396728 MN396724
Pronodularia japanensis KX822659 AB055625 KX822615
Chamberlainiini Bogan, Froufe & Lopes-Lima in Lopes-Lima et al., 2017
Sinohyriopsis schlegelii MT020706 EF507846 MT020836
Sinohyriopsis cumingii MG463086 NC011763 MG595613
Chamberlainia hainesiana KX822635 NC_044110 KX822592
Rectidentini Modell, 1942
Hyriopsis bialata KX051274 MT993644 MT993697
Ensidens ingallsianus MT993541 MT993687 MT993739
Contradentini Modell, 1942
Lens contradens MG581991 MT993693 MT993745
Lens comptus KX865928 KX865682 KX865799
Physunio superbus MG582020 MT993689 MT993741
Schepmaniini Lopes-Lima, Pfeiffer & Zieritz, 2021
Schepmania sp. MZ678755 MZ684082 MZ684035
Ctenodesmini Pfeiffer, Zieritz, Rahim & Lopes-Lima, 2021
Khairuloconcha lunbawangorum MN900790 MZ684078 MN902294
Khairuloconcha sahanae MZ678752 MZ684079 MZ684024
Unioninae Rafinesque, 1820
Anemina arcaeformis NC_026674 NC_026674 MG595457
Cristaria plicata NC_012716 NC_012716 MG595484
Sinanodonta woodiana HQ283346 HQ283346 MG595604
MARGARITIFERIDAE Henderson, 1929
Gibbosula laosensis JX497731 KC845943 KT343741
Margaritifera margaritifera KX550089 KX550091 KX550093

Two datasets were constructed in this study: (i) the COI dataset (46 sequences; 600 bp) (Table 1); (ii) the three-gene dataset (containing COI, 16S, and 28S; 60 sequences; 1,482 bp) (Table 2).

The COI sequences were codon-aligned by MUSCLE ver. 3.6 (Edgar 2004) implemented in MEGA ver. 10.1.6 (Kumar et al. 2018), whereas 16S rRNA and 28S rRNA were aligned in MAFFT ver. 7 (Katoh et al. 2019) using the Q-INS-i algorithm. We used Gblocks ver. 0.91b (Castresana 2000) to exclude ambiguous areas of the alignment for each gene. DnaSP ver. 6 (Rozas et al. 2017) was used to calculate the number of haplotypes. The best-fit model for each gene and gene partition was calculated by PartitionFinder2 ver. 2.3.4 (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 four 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.

Inter- and intra-specific distances based on the COI dataset were calculated in MEGA X using the uncorrected p-distance. Standard error estimates were obtained by 1000 bootstrapping replicates.

Maximum likelihood (ML) analyses were performed in raxmlGUI ver. 2.0 (Edler et al. 2020) with the ML + rapid bootstrapping method and 1000 replicates. Bayesian inference (BI) analyses were conducted in MrBayes ver. 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 1000 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.

Results

Phylogenetic analyses

A total of two COI haplotypes, one 16S haplotype, and two 28S haplotypes were identified in the eight sequenced specimens from Ji’an, Jiangxi. The COI dataset had an aligned length of 600 characters, with 164 variable sites and 162 parsimony informative sites. After trimming and concatenation, the three-gene dataset consisted of 1,482 characters, including 603 bp from COI, 467 bp from 16S, and 412 bp from 28S. This combined dataset contained 600 variable sites and 538 parsimony-informative sites.

The ML and BI trees based on the three-gene dataset exhibited largely congruent topologies, except for the phylogenetic relationships within Lamprotulini (Fig. 3). Both the ML and BI trees display a node with polytomies in the tribe Gonideini due to low nodal support. In our multilocus phylogenetic analysis, all eight recognized tribes within the subfamily Gonideinae were recovered as monophyletic groups. Specimens from Jiangxi formed a robust monophyletic clade (BS/BPP = 97/1.00) in the tribe Gonideini that did not belong to any previously known species or genera in the subfamily Gonideinae (Fig. 3). This species was identified as the sister group to the focal clade comprising the genera Postolata, Cosmopseudodon, Obovalis, Ptychorhynchus, Parvasolenaia, and Koreosolenaia. In this clade of Gonideini, the pairwise uncorrected COI p-distance ranged from 11.69% (between this species and Ptychorhynchus pfisteri (Heude, 1874)) to 13.58% (between this species and Koreosolenaia sitgyensis Lee, Kim, Lopes-Lima & Bogan, 2020)) (Table 3), providing compelling evidence for the founding of the new genus (Jeratthitikul et al. 2021; Wu et al. 2022; Dai et al. 2023). Furthermore, this species shows unique morphological characteristics distinguishable from other genera. Therefore, it is described herein as Pseudopostolata angula gen. et sp. nov.

Figure 3. 

Maximum likelihood (ML) and Bayesian inference (BI) trees of Unionidae based on the three-gene dataset. Gibbosula laosensis and Margaritifera margaritifera from the family Margaritiferidae were used as outgroup taxa. Maximum likelihood bootstrap support less than 50% or Bayesian posterior probability less than 0.5 were not shown. Taxa marked with an asterisk (*) were sequenced in this study.

Table 3.
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Average intraspecific (bold) and interspecific uncorrected p-distance (% ± S.E.) for COI sequences of species in Gonideini.

Taxa 1 2 3 4 5 6 7 8 9
1. Pseudopostolata angula gen. et sp. nov. 0.17 ± 0.17
2. Postolata guangxiensis 12.01 ± 1.31 0.59 ± 0.20
3. Postolata longjiangensis 12.33 ± 1.32 7.94 ± 1.10 0.17 ± 0.17
4. Cosmopseudodon wenshanensis 12.92 ± 1.40 10.64 ± 1.28 11.08 ± 1.31 0 ± 0
5. Cosmopseudodon resupinatus 13.35 ± 1.42 11.41 ± 1.27 11.35 ± 1.31 1.94 ± 0.55 0.09 ± 0.09
6. Koreosolenaia sitgyensis 13.58 ± 1.40 12.14 ± 1.34 12.50 ± 1.39 11.50 ± 1.29 11.50 ± 1.29 0.17 ± 0.16
7. Obovalis omiensis 12.38 ± 1.33 9.76 ± 1.17 10.67 ± 1.21 10.25 ± 1.23 10.44 ± 1.20 12.96 ± 1.35 1.61 ± 0.36
8. Parvasolenaia rivularis 12.11 ± 1.29 11.71 ± 1.25 12.65 ± 1.35 11.40 ± 1.33 11.65 ± 1.35 11.08 ± 1.24 12.48 ± 1.27 1.23 ± 0.27
9. Ptychorhynchus pfisteri 11.69 ± 1.31 10.43 ± 1.26 11.53 ± 1.33 8.56 ± 1.14 8.60 ± 1.12 11.61 ± 1.30 9.22 ± 1.16 10.23 ± 1.23 0.59 ± 0.19

Taxonomy

Family Unionidae Rafinesque, 1820

Subfamily Gonideinae Ortmann, 1916

Tribe Gonideini Ortmann, 1916

Pseudopostolata Dai, Chen, Huang & Wu, gen. nov.

https://zoobank.org/4B4557A9-93D2-408E-AC1B-12CF9BDA2D2F
Figs 2C, D, 4

Type species

Pseudopostolata angula Chen, Dai, Huang & Wu, sp. nov.

Diagnosis

Shell medium size, moderately thick, flat, long, glossy, black without any color rays. Anterior rounded, short, posterior long, and wide. Dorsal margin slightly curved downwards and truncated behind, with a distinct obtuse angle in the middle of posterior margin. Shell surface with a low secondary posterior ridge end in the angle on the posterior margin. Both left and right valves with one or two pseudocardinal teeth.

Etymology

The specific name pseudopostolata is made from the Latin pseudo for false and postolata for a unionid genus, alluding to their similar shell morphology.

Vernacular name

‘Pseudorear-wide mussel genus’ (English) and ‘Ni Hou Ju Bang Shu’ (拟后矩蚌属) (Chinese).

Distribution

Wujiang River in the Yongfeng section of Jiangxi Province, China.

Remarks

The new genus belongs to the tribe Gonideini of the subfamily Gonideinae, which currently consists of only one species. The new genus exhibits morphological similarities with Postolata guangxiensis Dai, Huang, Guo & Wu, 2023. However, it differs in terms of its elongated shell and the more distinct angle on the posterior margin.

Figure 4. 

Eight specimens of Pseudopostolata angula gen. et sp. nov. were collected from the Wujiang River in Ji’an City, Jiangxi, China. A is the holotype; A-H responds to specimen vouchers: 24_NCU_XPWU_PA01- 24_NCU_XPWU_PH08.

Pseudopostolata angula Chen, Dai, Huang & Wu, sp. nov.

https://zoobank.org/B1F2A547-13E7-47CF-A80B-FF54B9404AB5
Figs 2C, D, 4

Material examined

Holotype • 24_NCU_XPWU_PA01, Wujiang River [乌江], Yongfeng County [永丰县], Ji’an City [吉安市], Jiangxi Province [江西省], China, 27°03'37"N, 115°42'17"E, collected by Zhong-Guang Chen in December 2023; Paratypes China • 7 shells; same collection data as for holotype; specimen vouchers were shown in Table 4.

Table 4.
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Shell measurements of Pseudopostolata angula gen. et sp. nov. Measurements in millimeters (mm).

Status of specimen Specimen voucher Shell length Shell width Shell height
Holotype 24_NCU_XPWU_PA01 63.29 19.05 37.77
Paratype 24_NCU_XPWU_PA02 52.16 16.80 32.40
Paratype 24_NCU_XPWU_PA03 84.18 25.95 49.16
Paratype 24_NCU_XPWU_PA04 83.55 24.92 49.53
Paratype 24_NCU_XPWU_PA05 80.64 25.38 46.28
Paratype 24_NCU_XPWU_PA06 75.27 22.15 45.68
Paratype 24_NCU_XPWU_PA07 76.67 22.6 45.19
Paratype 24_NCU_XPWU_PA08 62.94 19.31 37.60

Diagnosis

See the diagnosis of the genus.

Description

Shell medium size, moderately thick, flat, long, glossy. Length 52.16–84.18 mm, width 16.80–25.95 mm, height 32.40–49.53 mm (Table 4). Anterior rounded, short, posterior long, and wide. Dorsal margin slightly curved downwards and truncated in behind, with a distinct obtuse angle in the middle of posterior margin; ventral margin weakly curved. Umbo inflated, below or even with the hinge line, located at 1/3 of the dorsal margin, and often eroded. Periostracum black with thin growth lines. Posterior slope with a low secondary posterior ridge end in the angle on the posterior margin. Growth lines arranged in irregular concentric circles. Hinge long. Ligament short and strong. Beak cavities shallow, open. Mantle attachment scars on the pallial line obvious. Anterior adductor muscle scars irregularly oval, deep, rough; posterior adductor muscle scars long, oval, smooth. Anterior retractor muscle scar deep; posterior adduster muscle scar shallow, Located obliquely above posterior adductor muscle scars. Left valve with one or two pseudocardinal teeth; anterior tooth elevated pyramidal or degenerated; posterior tooth thick and pyramidal. Right valve also with one or two pseudocardinal teeth; anterior tooth small or elevated pyramidal; posterior tooth low triangular or degenerated. Lateral teeth of both valves long and thick. Nacre light orangish.

Etymology

The specific name angula is made from the Latin angula for angled, alluding to the angle on the posterior margin of this species.

Vernacular name

‘Angulated pseudorear-wide mussel’ (English) and Ju Jiao Ni Hou Ju Bang (具角拟后矩蚌) (Chinese).

Distribution

Pseudopostolata angula sp. nov. is only known from the type locality (Fig. 1).

Habitat

This new species was found to occur in the muddy or pebbly substrate of the river together with Diaurora aurorea (Heude, 1883), Lanceolaria triformis (Heude, 1877), Lamprotula caveata (Heude, 1877), Nodularia douglasiae (Gray, 1833) and Pseudocuneopsis yangshuoensis Wu & Liu, 2023 (Fig. 2A, B). It is the occasional species in the habitat, accounting for 5% of the total density of freshwater mussels.

Discussion

This study integrates morphological and molecular analyses to describe a new genus and species within the Gonideini tribe, which is endemic to the Wujiang River, Jiangxi, China. The newly described genus and species, Pseudopostolata angula gen. et sp. nov., shares morphological features typical of Gonideini, including an irregularly rectangular shell (Lopes-Lima et al. 2017; Froufe et al. 2020; Dai et al. 2023). Given the similarities in shell shape, including a short, rounded anterior and a broad, elongated posterior, this species could easily be mistaken for belonging to the genus Postolata Dai, Huang, Guo & Wu, 2023 (Dai et al. 2023). However, further morphological examination revealed distinct characteristics specific to this species. The new genus differs from other genera in Gonideini by its slightly curved dorsal margin, truncated posteriorly with a distinct obtuse angle in the middle of the posterior margin (Fig. 2C, D, Table 5). Additionally, this species exhibits two types of pseudocardinal teeth, which is unique within Gonideini. The first type has two pseudocardinal teeth on both valves; in the left valve, the anterior pseudocardinal tooth is taller than the posterior one, while in the right valve, the anterior pseudocardinal tooth is smaller (Fig. 2C). The second type has a single tall triangular-conical pseudocardinal tooth on both valves; in the left valve, the anterior pseudocardinal tooth is reduced, and in the right valve, the posterior pseudocardinal tooth is reduced (Fig. 2D). The new genus is further distinguished from Postolata by its light orange-hued nacre, in contrast to the white nacre of Postolata, as well as by its longer shell (Table 5). Given the high prevalence of endemism among freshwater mussels, their taxonomic classification is closely tied to their geographic distribution (Bolotov et al. 2020b; Konopleva et al. 2023). The majority of Gonideini species are found in China, primarily in the Yangtze River Basin, with a few species also present in the Guangxi and Hainan regions (Graf and Cummings 2021; Liu et al. 2024). Pseudopostolata angula and Postolata are distributed across distinct geographical drainages. The former is endemic to the Yangtze River basin, while the latter, comprising two species (Postolata guangxiensis Dai, Huang, Guo & Wu, 2023 and Postolata longjiangensis Liu & Wu, 2024), is found in the Pearl River basin (Guangxi) (Dai et al. 2023; Liu et al. 2024). This distinct distribution facilitates a clear differentiation between the two genera.

Table 5.
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Analyzed conchological characters of Pseudopostolata, Postolata, Cosmopseudodon, Ptychorhynchus, Obovalis, Parvasolenaia, and Koreosolenaia.

Conchological features Pseudopostolata Postolata Cosmopseudodon Obovalis Ptychorhynchus
Shell shape Irregularly rectangular Irregularly rectangular Elongate elliptical Elongate oval Elongate elliptical
Shell thickness Moderately thick Moderately thick Moderately thick Thin Thin-medium
Umbo Moderately inflated Inflated Moderately inflated Moderately inflated Low and flat
Posterior margin With a distinct obtuse angle in the middle of posterior margin Almost perpendicular to ventral margin Weakly curved Weakly curved Weakly curved
Surface sculpture With a low secondary posterior ridge end in the angle on the posterior margin One sulcus near the posterior dorsal margin Covered with multiple curved wrinkles and one sulcus Coarse nodules Posterior slope sculptured with strong ridges
Pseudocardinal teeth of the left valve One or two thick and pyramidal teeth Anterior tooth small, posterior tooth thick and pyramidal One tooth, thick and Obtuse One tooth, pyramidal Two teeth, rather stumpy and roughened
Pseudocardinal teeth of the left valve One or two thick and pyramidal teeth Anterior tooth well developed, posterior tooth reduced One tooth, thick and Obtuse One tooth, high and triangular One tooth, blunt
Lateral teeth One tooth on both valves, long and thick One tooth on both valves, small and short One tooth on both valves, weakly developed and short One tooth on both valves, small and short 1~2 granulous teeth on the left and slightly split up teeth on the right
Nacre colour Light orangish White Bluish-white with an orange umbo pocket Bluish-white White

Five genera within Gonideini, including Pseudopostolata, Postolata, Cosmopseudodon, Obovalis, and Ptychorhynchus, exhibit a high degree of convergence, with similarities in shell size, shape, and thickness (Table 4). For instance, Ptychorhynchus murinum (Heude, 1883) shares a similar shell shape with Postolata longjiangensis Liu & Wu, 2024. However, the absence of molecular data for Ptychorhynchus murinum precludes a molecular comparison between the two species. Morphologically, Postolata longjiangensis is distinguished by fine and dense growth lines on the posterior dorsal, a feature that clearly differentiates it from Ptychorhynchus murinum. Additionally, their distinct distributions—Ptychorhynchus murinum in the Yangtze River basin and Postolata longjiangensis in the Pearl River basin (Guangxi)—further support their differentiation (Graf and Cummings 2021; Liu et al. 2024). Consequently, accurate species identification requires not only useful distinguishing characteristics but also consideration of their distribution, habitat, and a combination of molecular data (Pieri et al. 2018; Jeratthitikul et al. 2022; Bolotov et al. 2023; Dai et al. 2024b).

The use of molecular data for DNA taxonomy has shown great promise in expediting the process of species discovery (Huang et al. 2019; Chen et al. 2023). The molecular data results are consistent with morphological analysis. In our multi-locus trees, Pseudopostolata angula formed a well-supported monophyletic clade in the tribe Gonideini that did not belong to any previously known species or genera (Fig. 3, Table 3). Furthermore, the considerable genetic divergence from other genera lends additional support to its classification as a distinct genus. (Jeratthitikul et al. 2021; Wu et al. 2022; Dai et al. 2023). The phylogenetic relationships of genera in the focal clade of Gonideini align with previous studies based on COI + 16S rRNA + 28S rRNA phylogenies (Dai et al. 2023; Liu et al. 2024), although our analysis did not recover the sister relationship between Ptychorhynchus pfisteri (Heude, 1874) and Obovalis omiensis (Heimburg, 1884) (Fig. 3). Moreover, the phylogenetic analysis revealed that Parvasolenaia rivularis (Heude, 1877) is sister to Koreosolenaia sitgyensis Lee, Kim, Lopes-Lima & Bogan, 2020, which contradicts the COI + ND1 + 16S rRNA + 18S rRNA + 28S rRNA phylogeny (Wu et al. 2024). The observed discrepancies between topologies are likely attributable to factors such as incomplete lineage sorting, insufficient taxon sampling, and varying rates of genome evolution and mutation (Perkins et al. 2017). The recognition of higher-level taxa based on poorly supported topologies can give rise to instability in classification systems (Pfeiffer et al. 2019). Phylogenomic analysis will be needed to provide a more robust understanding of the intergeneric relationships within this tribe.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (No. 32360132, No. 32100354) and the Jiangxi Provincial Natural Science Foundation (No. 20232BAB205067).

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