Sinomytilus liuliwanqing Chen & He, 2024

Wang, Chong Rui, He, Yue Ming, Xiang, Hong Quan & Chen, Hui, 2024, A new species of the genus Sinomytilus (Bivalvia: Mytilidae) from Lake Fuxian, Yunnan Province, China, Ecologica Montenegrina 72, pp. 81-87 : 83-87

Sinomytilus liuliwanqing Chen & He sp. nov.

( Fig. 2 )

Material examined. 4 ex. From Lake Fuxian in Yuxi City , Yunnan Province, China; 24°35'58" N, 102°50'33" E, 1723 m alt. 09.01. 2023, Yue-Ming He leg.

Holotype. Shell length 3.17 mm, width 1.64 mm, NNU YNFXS001 .

Paratypes. 3 ex. NNUYNFXS002–004, shell length 3.05 – 3.23 mm, width 1.38 – 1.71mm, same data as holotype .

Locus typicus. Lake Fuxian in Yuxi City, Yunnan Province, China; 24°35'58" N, 102°50'33" E, 1723 m alt.

Etymology. The species name is derived from the ancient alternative name of Lake Fuxian, "Liuliwanqing". We suggest the Chinese common name as " 琉•华Dzṃ ".

Description. Shell length 3.05 – 3.23 mm and width 1.38 – 1.71 mm length, small, thin, ovately triangular outline, orange yellow or orange red, with concentric growth lines; anterior margin bluntly rounded, ventral one little concave near the postero-ventral corner, dorsal margin straight and forming an obtuse angle 150° with the posterior one; hinge ligament short and flat, diamond; posterior adductor muscle scar and palial line scar very shallow, anterior adductor muscle scar oval shape not obvious, pearl layer thin, light silver.

Anatomy. Ctenidium white, cylindrical, tightly arranged, inner demibranch of ctenidium slender, outer demibranch of ctenidium stubby; labial palp thin and long, near umbo; mantle thin, white ( Fig. 3A ).

Differential diagnosis. In contrast to S. harmandi (Rochebrune, 1882) , the new species is extremely small, and its ventral side lacks the curvature observed in S. harmandi . Compared to S. morrisoni ( Brandt, 1974) , the umbo of S. morrisoni is highly prominent and the dorsal margin is uneven. In addition, the hinge ligament of S. liuliwanqing sp. nov. is flatter. Compared to S. swinhoei (Adams, 1870) , the ventral side of S. swinhoei is highly curved, resembling a crescent shape. Additionally, S. swinhoei is larger and exhibits longitudinal stripes.

Distribution. This new species has only been found along the shores of Lake Fuxian in Yuxi City, Yunnan Province, China.

Ecology. S. liuliwanqing sp. nov. mainly inhabits rocks in the shallow waters (0-3m) along the lakeshore ( Fig. 3B ).

Molecular results. A total of 15 mitochondrial COI sequences from 10 species were utilized in the phylogenetic analyses A 646 bp COI segment was aligned. The GTR+I+G4+F model was selected as the best-fit of nucleotide substitution by the BIC criterion. ML and BI analyses resulted in congruent tree topologies with some minor differences in the terminal lineages ( Fig. 4 ). Molecular phylogenies based on BI and ML analyses showed that S. liuliwanqing sp. nov. and S. harmandi were nested a monophyletic group with strong support (BPP = 1, BS = 98%,). The average COI genetic distance (uncorrected p-distance) between the S. liuliwanqing sp. nov. and S. harmandi is 16.4%.

Discussion

The Sinomytilus genus has received little attention in China, with only one species, S. swinhoei , has been recorded, in the Yangtze River between Sichuan and Hubei provinces. Furthermore, in an area of Lake Fuxian with a depth of approximately 80 meters, Mytilidae gen. sp. have been documented ( Cui et al., 2008). During several surveys of Lake Fuxian from July 2021 to January 2024, we found some large, Mytilidae-like fragments in areas slightly further from the shore, but no live individuals were discovered. Initially, we thought that S. liuliwanqing sp. nov. might represent the juvenile form of these shells; however, we did not find larger individuals near the rocks where S. liuliwanqing sp. nov. was attached. After observing S. liuliwanqing sp. nov. in the same area of Lake Fuxian for several years, we did not find any large individuals in these areas. There was also no evidence of significant growth observed in individuals from the previous area. Therefore, we can only conclude that the adult size of S. liuliwanqing sp. nov. is extremely small. We also conducted surveys of the rivers surrounding Lake Fuxian for the time being, however, no S. liuliwanqing sp. nov. have been found. Therefore, it is highly likely that this species is exclusively distributed in Lake Fuxian.

In recent years, Lake Fuxian has experienced a certain degree of pollution ( Feng et al., 2008), and the development of tourism has caused serious damage to the ecological environment along the lake shores ( Peng & Li, 2021). Many areas along the shores have become increasingly difficult to find S. liuliwanqing sp. nov. In the January 2024 survey, no live S. liuliwanqing sp. nov. were found on the northern shore of Lake Fuxian, and the numbers of previously discovered freshwater snails have also decreased. This survey shows freshwater ecosystems biodiversity and highlights the necessity to promote the conservation of Lake Fuxian.

Acknowledgements

We are thanks to Chong-Ye Li for assistance with the collection of samples.

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Figures

Figure 2. Sinomytilus liuliwanqing sp. nov. (A–H) Holotype, NNU YNFXS001; (I, J) Paratype, NNU YNFXS002. Scale bar = 1mm.

Figure 3. (A)Dissection with labelled structures, Abbreviations: ID, inner demibranch of ctenidium; OD, outer demibranch of ctenidium; (B)Natural habitat Photographs by Hong-Quan Xiang 1 September 2023, Lake Fuxian, Yunnan Province, China.

Figure 4. A Bayesian inference (BI) tree based on the COI sequence. Values at nodes represent support values for BI and maximum likelihood (ML). Only values> 0.5 or 70 are shown.