Lepidodesma (Simpson, 1896)

Wu, Ruiwen, Liu, Lili, Zhang, Liping, Jin, Dandong, Wu, Xiaoping, Liu, Xiongjun, Xie, Zhicai & Li, Zhengfei, 2024, Molecular phylogeny and comparative morphology reveal the species validity and systematic position of Lepidodesma (Bivalvia: Unionidae), Organisms Diversity & Evolution (New York, N. Y.) 24 (1), pp. 51-66 : 61-62

Taxonomy position of Lepidodesma

Unionidae is the most prevalent group among freshwater mussels ( Graf & Cummings, 2007), but the subfamily classification has been discordant due to uncertainties surrounding early evolution and lineage genesis ( Bolotov et al., 2017; Lopes-lima et al., 2017a; Pfeiffer & Graf, 2015; Whelan et al., 2011). Recently, Pfeiffer et al. (2019) proposed a taxonomy system of Unionidae consisting of five subfamilies based on genomic data, among which Modellnaiinae was excluded due to the lack of molecular data for Modellnaia siamensis limited to Thailand. In this study, we obtained completely consistent phylogenetic relationships at the subfamily level based on comprehensive mitogenomes, e.g., Ambleminae +( Gonideinae + ( Unioninae + Parreysiinae )).

Although the taxonomic position of the genus Lepidodesma at the subfamily level is definite, its classification at the tribe level remains controversial. According to the published literature on morphological characteristics, Graf and Cummings (2007) classified Unioninae into two tribes: Unionini and Anodontini , and assigned the genus Lepideodesma to Unionini . With the application of molecular techniques, the taxonomic position of Lepidodesma species at the subfamily level has been determined based on mitochondrial genome data ( Huang et al., 2002; Wen et al., 2017; Zhou et al., 2016). However, due to insufficient molecular data, inadequate taxon sampling, and low node support, the phylogenetic position of Lepidodesma in the infra-subfamily remains unresolved. Based on the mitochondrial genome, Huang et al. (2019) considered that the genus Lepidodesma could not be classified into any known tribe of Unioninae , thus necessitating the description of a new tribe, namely Lepidodesmini Huang and Wu, 2019 . Subsequently, Lopes-Lima et al. (2020) further supported the classification of the genus Lepidodesma into Lepidodesmini within Unioninae based on (COI + 28S rRNA) dataset. However, the aforementioned study solely focused on a single species within Lepidodesma , and the phylogenetic results at the tribe level of Unioninae were not robustly supported. Recently, Graf and Cumming (2021) reaffirmed the classification of Lepidodesma within Unionini .

Compared to previous studies, our research utilizes the most comprehensive data by selecting all published complete mitogenomes in Unionidae , combined with two new mitochondrial genomes of Lepidodesma aligera and Lepidodesma languilati , to construct five phylogenetic trees using multiple datasets and methods. The result demonstrates that Lepidodesma is the sister species and occupies a basal position in Unioninae (BS = 100%, PP = 1.00). Therefore, it is our recommendation that the genus Lepidodesma be reclassified as Lepidodesmini rather than Unionini .

Endangered status and conservation implications

Freshwater ecosystems are highly vulnerable due to the impact of biotic and abiotic factors, making them one of the most endangered ecosystems on Earth ( Dudgeon et al., 2006; Sutherland et al., 2022; Vörösmarty et al., 2010). In particular, the escalation of anthropogenic activities in recent decades has resulted in alterations and depletion of freshwater mussel habitats ( Aldridge et al., 2022; Lopes-Lima et al., 2018b; Strayer & Dudgeon, 2010). It is imperative to conduct species conservation status assessments in order to determine the priority for conserving and protecting freshwater mussels.

Lepidodesma , a rare endemic freshwater mussel in China ( Liu et al., 1979), is distributed in Jiangxi, Hunan, and Jiangsu for Lepidodesma aligera ( Guo, 2022; Hu, 2005), and Anhui, Zhejiang, Jiangsu, Jiangxi, Hubei, and Hunan for Lepidodesma languilati ( Liu et al., 1979) . Recently, Liu et al. (2020) assessed L. languilati as the third priority according to the QASCP. However, the scarcity of L. aligera in its distribution area makes it extremely challenging to collect specimens, thereby hindering the assessment of its endangered status. Fortunately, we accidentally obtained two live specimens and performed molecular phylogenetic analysis along with an examination of soft-body morphology. According to the conservation assessment method proposed by Liu et al. (2020), our preliminary evaluation ranks L. aligera as the third priority, alongside L. languilati . As a consequence, both species are facing endangerment and require immediate protection. It is imperative that we establish a range of protective measures for these freshwater mussels, such as promoting their significance in conservation efforts and enhancing the quality of their aquatic habitats. Ultimately, this study enhances our comprehension of Lepidodesma species, which will aid in managing and safeguarding resources for these rare organisms.