Ecologica Montenegrina 39: 1-6 (2021) This journal is available online at: www.biotaxa.org/em http://dx.doi.org/10.37828/em.2021.39.1 https://zoobank.org/urn:lsid:zoobank.org:pub:DDA69FE1-8F86-483D-B824-E2D109A41936 A new genus of ultra-elongate freshwater mussels from Vietnam and eastern China (Bivalvia: Unionidae) IVAN N. BOLOTOV1, ALEXANDER V. KONDAKOV1, EKATERINA S. KONOPLEVA1,* & ILYA V. VIKHREV1 1N. Laverov Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences, Arkhangelsk, Russia *Corresponding author. E-mail: [email protected] Received: 10 December 2020│ Accepted by V. Pešić: 30 December 2020 │ Published online: 4 January 2021. Taxa with an ultra-elongate shell are widespread among marine and freshwater bivalves, being an iconic illustration of convergent evolution, e.g. Solen Linnaeus, 1758 (Solenidae), Mycetopoda d’Orbigny, 1835 (Mycetopodidae), and Lanceolaria Conrad, 1853 (Unionidae) (Anderson 2014). The genus Solenaia Conrad, 1869 (Unionidae: Gonideinae) was considered a compact group of freshwater mussels having an elongate shell and ranging from the Mekong Basin in Thailand through northern Vietnam and eastern China to South Korea (Haas 1969; Graf & Cummings 2007; Zieritz et al. 2018). However, a growing body of modern phylogenetic research revealed that this genus is not monophyletic despite the fact that its members are very similar conchologically (Huang et al. 2019; Pfeiffer et al. 2019, 2020; Lopes-Lima et al. 2020). Indeed, it is one more remarkable example of shell convergence in unionids together with the former genera Trapezoideus Simpson, 1900 sensu lato and Oxynaia Haas, 1911 sensu lato, each of which was found to be an amalgam of several phylogenetically distant but conchologically similar lineages (Konopleva et al. 2017, 2019; Bolotov et al. 2018). The shell convergence within the Unionidae may be caused by different factors such as similar environments (e.g. type of substrate and flow rate), behavioral features or biochemical requirements. In the case of ultra-elongate bivalves, the common way of deep penetration into substrate may determine their shell shape (Anderson 2014). First, the new genus Parvasolenaia Huang & Wu, 2019 was erected for three species that were formerly placed within Solenaia, i.e. P. neotriangularis (He & Zhuang, 2013), P. rivularis (Heude, 1877), and P. triangularis (Heude, 1885) (Huang et al. 2019). Second, it was found that a population of what was thought to be Parvasolenaia triangularis from South Korea belongs to a new genus and species, Koreosolenaia sitgyensis Lee, Kim, Lopes-Lima & Bogan, 2020 (Lopes-Lima et al. 2020). Both Parvasolenaia and Koreosolenaia are members of the tribe Gonideini and represent sister but phylogenetically distant groups (Huang et al. 2019; Lopes-Lima et al. 2020). Third, Solenaia emarginata (Lea, 1860) from Thailand, the type species of this genus, was found to be a member of the tribe Contradentini based on a nuclear genomic dataset and COI barcode data (Pfeiffer et al. 2019, 2020). There are two additional examples of species with an ultra-elongate shell among the Contradentini, i.e. Yaukthwa Ecologica Montenegrina, 39, 2021, 1-6 NEW GENUS OF ULTRA-ELONGATE FRESHWATER MUSSELS FROM VIETNAM AND CHINA baniensis (Bolotov et al., 2020) and Y. elongatula Bolotov et al., 2019 from Myanmar (Bolotov et al. 2019, 2020; Pfeiffer et al. 2020). Finally, the rest of Solenaia species such as S. carinata (Heude, 1877) and S. oleivora (Heude, 1877) (Fig. 1A-C) belongs to the tribe Gonideini (Lopes-Lima et al. 2020). Once again, these novel findings highlight strong differences between freshwater mussel assemblages of the Sundaland and East Asian freshwater biogeographic subregions (Bolotov et al. 2018, 2020; Konopleva et al. 2019; Pfeiffer et al. 2020). In the absence of available name, we here introduce a new genus for the former Solenaia clade from East Asia that belongs to the tribe Gonideini but does not relate to Parvasolenaia and Koreosolenaia. Furthermore, we provide a brief overview of species within this new genus and propose four new combinations for these taxa. Family Unionidae Rafinesque, 1820 Subfamily Gonideinae Ortmann, 1916 Tribe Contradentini Modell, 1942 Genus Solenaia Conrad, 1869 Type species: Mycetopus emarginatus Lea, 1860 [holotype USNM 86787; deposited in the National Museum of Natural History, Washington, USA; type locality: “Siam”] (by original designation) Comments: Solenaia khwaenoiensis Panha & Deein, 2004 [holotype PRDC 10001; deposited in the Museum of Phitsanulok Inland Fisheries Research and Development Center, Phitsanulok, Thailand; type locality: “Khwae Noi River at Ban Ta Ngam, Wat Bot District, Phitsanulok Province, Thailand”] was considered a synonym of S. emarginata (Lea, 1860) (Deein et al. 2004; Pfeiffer et al. 2020). Hence, it is currently a monotypic genus. Tribe Gonideini Ortmann, 1916 Genus Sinosolenaia gen. nov. https://zoobank.org/urn:lsid:zoobank.org:act:F6329FCD-887E-4104-8ECA-0268B89A3E53 Type species: Micetopus [sic!] recognitus Heude, 1877. Etymology: The name of this genus is compiled using words “Sino” (i.e. Chinese) and “Solenaia” (the former generic name for this clade). Differential diagnosis: The new genus can be distinguished from the sister taxon Solenaia by more elongated concentric ridges on shell surface, less developed umbo, and narrower hinge plate. Description: This genus was found to be a separate clade within the tribe Gonideini. Its members share an ultra-elongate, thin shell, tapering anteriorly, with a narrow anterior margin. Posterior slope triangularly shaped, pointed at the end ventrally and posteriorly. Gonial ridge prominent. Umbo not prominent, with weak sculpture of waved, concentric ridges. Hinge plate narrow, with traces of lateral teeth. Distribution: Red (Hồng Hà) River basin in northern Vietnam to the Yangtze River basin in eastern China. Comments: This group appears to contain not less than four valid species, which are listed below. Among them, Sinosolenaia carinata (Heude, 1877) comb. nov. can be distinguished from other species by having the narrowest, sometimes pointed anterior margin (vs. broader and rounded) (Fig. 1C). The other three species can be joined into the Sinosolenaia iridinea species complex and could not clearly be separated based on morphological criteria (Fig. 1A-B). Sinosolenaia recognita (Heude, 1877) gen. & comb. nov. =Micetopus [sic!] recognitus Heude (1877): pl. 22, fig. 47. Type: Syntype MNHN MP 3423; labelled: “Ngan Noue, Chine”; deposited in the Muséum National d'Histoire Naturelle, Paris, France. Type locality: “La Hoai supérieure, département de Ing-tch'eou” [Upper Huai River, Yangtze River basin, Anhui Province, China] (Heude 1877). 2 BOLOTOV ET AL. Reference DNA sequence data: Mitogenome: not available; COI barcode: KY561639 (Bolotov et al. 2017: as Solenaia sp.) and MG463092 (Huang et al. 2019: as Solenaia oleivora). Distribution: Red (Hồng Hà) River basin in northern Vietnam to the Yangtze River basin in eastern China. Comments: Two distant lineages of this species are known to occur in East Asia (COI p-distance = 2.9%), i.e. those from Vietnam (Bolotov et al. 2017) and eastern China (Huang et al. 2019; Lopes-Lima et al. 2020). Based on conchological similarity of our sample from Vietnam with the nominal taxon recognitus Heude, 1877, we tentatively assign the latter name to this species. It shares a slightly broader anterior margin compared with other species in the genus (Fig. 1A). Sinosolenaia carinata (Heude, 1877) comb. nov. =Micetopus [sic!] carinatus Heude (1877): pl. 21, fig. 45. Type: Whereabouts unknown; probably deposited in the Muséum National d'Histoire Naturelle, Paris, France. Type locality: “probablement des lacs de Mien-iang-tch'eou (Hou-pé); se trouve au Kiang-si, riviere de Foutch'eou?” [probably some lakes on a river near Fuzhou, south of Nanchang, Yangtze River basin, Jiangxi Province, China] (Heude 1877). Reference DNA sequence data: Mitogenome: NC_023250 (Wu et al. 2019; Froufe et al. 2020); COI barcode: MG463087, MG463088, MG933744, MG742248, and KX822669 (Lopes-Lima et al. 2017; Wu et al. 2018; Huang et al. 2019). Distribution: Yangtze River basin in eastern China (Huang et al. 2019). Comments: This species was unambiguously identified based on morphological features, i.e. the specific shell shape with very narrow, somewhat pointed anterior margin (Lopes-Lima et al. 2017; Wu et al. 2018; Huang et al. 2019). Sinosolenaia iridinea (Heude, 1874) comb. nov. =Mycetopus iridineus Heude (1874): 117. Type: Syntype USNM 126494; labelled: “Ngan-hue, China”; deposited in the National Museum of Natural History, Washington, USA. Type locality: “Rivieres de Tai-ping-fou et de Fou-tcheou-fou” [rivers near Nanchang and Fuzhou, Yangtze River basin, Jiangxi Province, China] (Heude 1874). Reference DNA sequence data: Mitogenome: MT477834 (Chen et al. 2020: as Solenaia oleivora); COI barcode: MG463089, MG463091, MG463094, and MG463097 (Huang et al. 2019: as Solenaia oleivora). Distribution: Yangtze River basin in eastern China (Huang et al. 2019). Comments: The DNA sequences and a mitogenome of this species were published under the name Solenaia oleivora (Huang et al. 2019; Chen et al. 2020). Conversely, it is distant phylogenetically from Solenaia oleivora identified in several papers (Lopes-Lima et al. 2017; Huang et al. 2019; Wu et al. 2019; Froufe et al. 2020). Here, we tentatively assign the available name iridineus Heude, 1874 to this species. Sinosolenaia oleivora (Heude,