Pseudocuneopsis yangshuoensis Wu & Liu, 2023

Wu, Ruiwen, Liu, Lili, Zhang, Liping, Jia, Junli, Jin, Dandong, Wu, Xiaoping & Liu, Xiongjun, 2023, New species of the genus Pseudocuneopsis Huang, Dai, Chen & Wu, 2022 (Bivalvia, Unionidae) from Guangxi Province, China, ZooKeys 1166, pp. 261-270 : 261

Pseudocuneopsis yangshuoensis Wu & Liu sp. nov.

Fig. 1

Type specimens.

Holotype: China • Guangxi Province, Yangshuo County (阳朔县), Li River (24.90099°N, 110.52585°E), 11 December 2022, coll. Dandong Jin (SXNU22121104). Paratypes: same data as holotype (SXNU22121105 - SXNU22121108).

Diagnosis.

Pseudocuneopsis yangshuoensis sp. nov. is morphologically distinct from the other two recognized species within the genus by shell shape, beak position and surface sculpture (Table 2 ). Diagnostic characteristics: shell wedge-shaped; the ventral margin nearly straight or slightly concave; the umbo situated 1/3 of shell length, and obviously lower than the dorsal margin; nacre orange.

Description.

Shell wedge-shaped, medium thickness; anterior margin regularly rounded and inflated; ventral margin nearly straight or slightly concave; umbo located at 1/3 of shell length and obviously lower than the dorsal margin; umbo sculptured unknown due to severe erosion; posterior slope formed by the ventral margin and dorsal margin low, blunt, approximately 60°; epidermis brownish-black covered with concentric ridges; anterior adductor muscle scars elliptical, deep and unrough; posterior adductor muscle scars long elliptical, shallow and smooth; anterior and posterior retractor muscle scars obvious, with the anterior and posterior irregularly elliptical; mantle muscle scars obvious; left valve with two separate pseudocardinal teeth and two later teeth; the outer and inner pseudocardinal teeth are the same length and projected outward at the same level; right valve with one pseudocardinal tooth and one lateral tooth; lateral and pseudocardinal teeth usually curved; nacre orange in colour.

Length 41.39-50.51 mm, height 15.34-19.40 mm.

Etymology.

The specific epithet is derived from the type locality Yangshuo County. For the common name, we recommend "Yangshuo Wedged Mussel" (English) and "Yang Shuo Wei Xie Bang" (阳朔伪楔蚌) (Chinese).

Distribution.

Li River, Guangxi, China (Fig. 2 ).

Anatomical characteristics.

The soft tissue morphology reveals elongated papillae arranged in two to three rows within the incurrent aperture, with stocky papillae forming the outer row; notable pigmentation and small sarcomas are present along the outer margin of the excurrent aperture; and the size of inner gills exceeds that of outer gills (Fig. 3 ).

Molecular analyses.

Pairwise COI sequence divergences from Pseudocuneopsis yangshuoensis sp. nov., Pseudocuneopsis capitata , and Pseudocuneopsis sichuanensis were conducted in MEGA 7.0 with the uncorrected p -distance model. The intraspecific divergence of Pseudocuneopsis yangshuoensis sp. nov. was 0%. The interspecific divergence between Pseudocuneopsis yangshuoensis sp. nov. and P. sichuanensis was 8%, and 9% with P. capitata . Both BI and ML trees obtained a consistent topology (Fig. 4 ). In the phylogenetic trees, Pseudocuneopsis yangshuoensis sp. nov. formed a well-supported sister-group relationship with P. sichuanensis (BS = 100; PP = 1.00, Fig. 4 ). Cuneopsis and Tchangsinaia were each separated from Pseudocuneopsis belonging to three different clades (Fig. 4 ).

Remarks.

Species delineation can be problematic in the presence of morphological ambiguities due to phenotypic plasticity and convergence (e.g., cryptic species), especially in mollusks ( Zieritz et al. 2010; Inoue et al. 2013). The use of molecular genetics can aid species delineation in the case of phenotypic plasticity and/or convergence ( Pieri et al. 2018; Wu et al. 2022b). Pseudocuneopsis yangshuoensis sp. nov. can be distinguished from congeneric species based on the diagnostic characteristics of shells. We also analyze the interspecific divergence among P. sichuanensis , P. capitata and P. yangshuoensis sp. nov. based on the COI barcode. The results show that the average interspecific divergence between the two species was 8%-9%, which is much higher than intraspecific divergences. Genetic analysis conducted in this study supports P. yangshuoensis sp. nov. as a valid species, which can be easily distinguished by COI barcode.

Figures

Figure 1. Photographs of Pseudocuneopsis yangshuoensis sp. nov. A holotype SXNU 22121104 B-E paratype SXNU 22121105 - SXNU 22121108. Sacle bar: 1 cm.

Figure 2. Photograph of the sampling site of Pseudocuneopsis yangshuoensis sp. nov. at the Li River, Guangxi in China.

Figure 3. Anatomical features of Pseudocuneopsis yangshuoensis sp. nov. with left valve removed. Abbreviations: aam, anterior adductor muscle; pam, posterior adductor muscle; exa, excurrent aperture; ia, incurrent aperture; f, foot; ig, inner gill; og, outer gill; lp, labial palps; m, mantle; p ia, papillae in incurrent aperture; pg exa, pigmentation of excurrent aperture.

Figure 4. Phylogenetic tree of freshwater mussels inferred from maximum likelihood (ML) and Bayesian inference (BI) analyses of COI barcode. Support values above the branches are bootstrap support (BS) / posterior probabilities (PP). The red font indicates the species from this study.