Rhombuniopsis superstes Neumayr, 1899

Xiang, Hong Quan, He, Yue Ming, Zhang, Le Jia, Gao, Han, Guo, Liang, Lu, Yi Zhi, Fan, Shu Yun & Chen, Hui, 2024, A revision of Rhombuniopsis Haas, 1920 (Unionida, Unionidae) endemic to the ancient lakes of Yunnan, China, with descriptions of two new species, Zootaxa 5481 (2), pp. 273-289 : 280-282

Rhombuniopsis superstes Neumayr, 1899

Fig. 6A–F

Unio superstes Neumayr (1899): 643 , pl. 1, fig. 3 [‘Lake Tali’ = Lake Erhai, Dali City, Yunnan Province, China]; Simpson (1914): 722 –723.

Rhombuniopsis superstes Haas (1920): 146 –151; Haas (1924): 186–187; Haas (1969): 270; He & Zhuang (2013): 101, Figs 237–238; Zieritz et al. (2018): 29 –44.

Leoparreysia superstes Bolotov (2022): 12 .

Material examined. Newly collected material. Adult shell: Lake Erhai , Haidong town (DZ东Dz), 25.728939°N, 100.232401°E, 2023.7 ( KIZ1904301 – KIZ1904305 ), collected by Hong Quan Xiang and Yue Ming He; Lake Erhai , Haidong town (DZ东Dz), 25.663176°N, 100.276812°E, 2023.7 ( KIZ1904306 – KIZ1904310 ), collected by Hong Quan Xiang.; Lake Erhai, 2024.4 ( KIZ1904311 ), collected by Ren Xin Gao.

Diagnosis. Shell ovate, a little obese, periostracum dark brown to black; left valve pseudocardinal tooth triangular.

Description. Shell ( Fig.6 A–D ). Length 30–60 mm. Shell ovate, of medium thickness, unequal, a little obese, backwards weakly keeled, very little detruncated; umbo in the anterior third of the shell, weakly protruding, yellowish-green, sometimes sculptured with wavy wrinkles, often eroded. Ligament strong; pseudocardinal tooth of the right valve triangular, equal height, pseudocardinal tooth of the left valve well developed; lateral teeth long; anterior adductor muscle scar strong and deep, posterior adductor muscle scar smaller and significantly weaker. Periostracum olive black. Nacre white.

Comparison. This species can be differentiated from the most similar species R. heres by having incisions on its pseudocardinal teeth but lacking a distinct closed concavity in the middle of its pseudocardinal tooth of the right valve area.

Taxonomic remarks. Neumayr (1899) described Unio superstes and Unio heres based on shells collected from Lake Erhai in Yunnan. Haas (1920) established a new genus: Rhombuniopsi s, and considered that Rhombuniopsis superstes and Rhombuniopsis heres were the same species. Graf & Cummings (2007), He & Zhuang (2013), Zieritz et al. (2018), Graf & Cummings (2021), and Liu et al. (2022) all agreed with Haas’s conclusion. However, there is a significant issue with this. Each species studied by Haas (1920) has only one incomplete specimen (only one damaged left valve for U. superstes and one right valve for U. heres ). The limited number of specimens poses a challenge, and most subsequent scholars have relied solely on Haas's descriptions and the extremely limited images provided in the original literature to support their arguments, without collecting additional samples for comparison. Only Moskvicheva & Starobogatov (1973) compared a larger number of specimens and considered both species to be valid. The population of Rhombuniopsis in Lake Erhai is small, and the available specimens obtained through surveys are limited. However, the specimen images from Neumayr (1899) and Moskvicheva & Starobogatov (1973) are sufficient to fulfill the research objectives.

Based on our morphological analyses, we concur with the conclusion of Moskvicheva & Starobogatov (1973). Haas (1920) acknowledged the morphological differences in teeth, but he did not support the presence of two species from the same genus within a single lake. Based on our research, one lake can indeed harbor several species of this genus ( Table 1 ). For instance, both R. tauriformis and R. fultoni can be found in Lake Dianchi and Lake Lulianghai. In the southern and eastern regions of Lake Erhai, there are several ancient lakes that have dried up in recent times. These lakes used to be interconnected by rivers, and within the Lake Erhai water system, a few species from this genus coexist. Based on the comparison of the hinge teeth structure, we have observed that R. heres has a distinct closed concavity in the middle of its pseudocardinal tooth area (in right valve), while R. superstes lacks this feature and exhibits cracks in its pseudocardinal tooth region (in right valve). We did not find any shell having an intermediate pattern to support the idea that they are variations of the same species. Hence, we believe that R. superstes and R. heres are two distinct species.

Bolotov et al. (2022) suggests that R. superstes should be classified under Leoparreysia , but we disagree with this viewpoint. As we have explained previously, there are significant differences between these two genera, and the unique environment of the plateau ancient lakes makes it unlikely for any mussel species to adapt to such high-altitude conditions. There is no evidence to suggest that Leoparreysia can survive in such high-altitude environments, and there is no record of any species belonging to the Leoparreysia genus in China.

Habitat and distribution. Lives on muddy to sandy substrates, only known from Lake Erhai.

Figures

FIGURE 6. Shells of Rhombuniopsis superstes. A. Holotype of R. superstes, collected from Tali-fu, Dali, Yunnan (from Neumayr, 1899). B. Specimen, collected from Tali-fu, Dali, Yunnan (from Moskvicheva & Starobogatov, 1973). C. Specimen collected from Lake Erhai, KIZ1904301, shell length 28.8 mm. D. Specimen collected from Lake Erhai, KIZ1904306, shell length 44.1 mm. E. Specimen collected from Lake Erhai, KIZ1904311, 39.0 mm. Scale bars: 1 cm.

Tables

TABLE 1. Checklist of freshwater mussels (Unionidae) of Yunnan. X indicates that the lake has a corresponding distribution of species belonging to the genus Rhombuniopsis.

Taxa	Yangtze Basin			Pearl Basin					Lankang-Mekong Basin
	Dianchi	Babuhai	Chenghai	Fuxian	Xingyun	Lulianghai	Qilu	Yilong	Erhai	Jianhu
Rhombuniopsis tauriformis ( Fulton, 1906)	X					X
Rhombuniopsis fultoni Moskvicheva & Starobogatov, 1973	X					X
Rhombuniopsis superstes ( Neumayr, 1899)									X
Rhombuniopsis heres Neumayr, 1899			X						X	X
Rhombuniopsis linan Xiang, He & Zhang sp. nov.				X	X		X	X
Rhombuniopsis songmeng Xiang, He & Zhang sp. nov.		X				X