New taxa of freshwater mussels (Unionidae) from a species-rich but overlooked evolutionary hotspot in Southeast Asia

www.nature.com/scientificreports OPEN Received: 17 July 2017 Accepted: 1 September 2017 Published: xx xx xxxx New taxa of freshwater mussels (Unionidae) from a species-rich but overlooked evolutionary hotspot in Southeast Asia Ivan N. Bolotov 1,2, Ilya V. Vikhrev1,2, Alexander V. Kondakov1,2, Ekaterina S. Konopleva1,2, Mikhail Yu. Gofarov1,2, Olga V. Aksenova1,2 & Sakboworn Tumpeesuwan3 Southeast Asia harbors a unique and diverse freshwater fauna of Mesozoic origin, which is under severe threat of extinction because of rapid economic development and urbanization. The largest freshwater basins of the region are certainly the primary evolutionary hotspots and they attract the most attention as key biodiversity areas for conservation. In contrast, medium-sized rivers are considered lowimportance areas with secondary biodiversity, whose faunas originated via founder events from larger basins during the Pleistocene, although such a scenario has never been tested by using a phylogenetic approach. In this investigation, we used freshwater mussels (Unionidae) as a model to estimate the levels of endemism within the Sittaung, a little-known remote basin in Myanmar, compared with the surrounding larger rivers (Irrawaddy, Salween and Mekong). We discovered that the Sittaung represents an exceptional evolutionary hotspot with numerous endemic taxa of freshwater mussels. On the basis of our extensive dataset, we describe two new tribes, two genera, seven species and a subspecies of Unionidae. Our results highlight that medium-sized basins may represent separate evolutionary hotspots that harbor a number of endemic lineages. These basins should therefore be a focus of special conservation efforts alongside the largest Southeast Asian rivers. In the modern period of the sixth mass extinction (Anthropocene), freshwater biodiversity is under severe threat because of increasing anthropogenic pressure, which leads to habitat degradation, water pollution, keystone species declines, and the homogenization of faunas1–6. Climate change may increase the effects of human impacts, especially for taxa with low abundance and restricted ranges, and may trigger multiple local extinctions7, 8. Our understanding of spatial biodiversity patterns across freshwater basins is very limited because the systematics of many groups are not developed, including the important invertebrates such as bivalves and gastropods9–13. In Southeast Asia, the lack of reliable taxonomic information precludes producing the national maps of freshwater biodiversity hotspots, which is a task of great importance for conservation planning9. Here, we use mussels in the family Unionidae, or naiads, as a model group for the assessment of spatial patterns of freshwater biodiversity across western Indo-China. This is the most species-rich bivalve family, with ~620–680 extant species14–17. The Unionidae most likely originated in Southeast and East Asia in the Jurassic, with subsequent expansions into other landmasses9. In several major Asian river systems (e.g., Mekong and Yangtze), exceptional intra-basin radiations of the Unionidae were discovered, which suggests that these basins may be considered ancient (long-lived) rivers that have existed throughout the Cenozoic9, 18. However, the freshwater mussel faunas of Asia have attracted little attention from scientists compared with those from Europe and North America10, 17. Although the importance of freshwater mussels in tropical ecosystems is still poorly known, they could play an essential role as biofilters in polluted water bodies19. Several species are successful invaders, and have spread beyond their native ranges together with the introduction of their host fishes and may threaten native communities20, 21. Finally, freshwater mussels are important objects for the ornamental pet trade, pearl cultivation and food markets across Asian countries22–24. 1 Northern Arctic Federal University, Arkhangelsk, Russian Federation. 2Federal Center for Integrated Arctic Research, Russian Academy of Sciences, Arkhangelsk, Russian Federation. 3Department of Biology, Faculty of Science, Mahasarakham University, Maha Sarakham, Thailand. Correspondence and requests for materials should be addressed to I.N.B. (email: ) SCIeNTIFIC ReporTS | 7: 11573 | DOI:10.1038/s41598-017-11957-9 1 www.nature.com/scientificreports/ The taxonomy of freshwater mussels of Southeast Asia is complicated, because many nominal taxa were described from this region on the basis of conchological features, including small differences in shell shape9, 10, 21, 25. In western Indo-China, most historical samples were collected from the Irrawaddy, Salween, Pegu, Tavoy and Great Tenasserim river catchments10, 24–48. In contrast, nominal taxa from the Sittaung, a medium-sized river basin in Myanmar, were not described. In the 1880s, Leonardo Fea, an Italian naturalist and traveler, collected mussels from several tributaries of the Sittaung49. Fea’s collection was deposited in the Museo Civico di Storia Naturale di Genova (MSNG, Genoa, Italy). Since then, none of the freshwater mussels have been collected from the basin. Moreover, for some enigmatic reason Fea’s samples from the Sittaung were not used for the description of any unionid taxon39, 40, 43, 50. From the comprehensive phylogenetic study of the Unionidae across the primary basins of the Oriental Region, Bolotov et al.9 showed that each large river system of this region is a separate evolutionary hotspot harboring a unique endemic naiad fauna. In the present investigation, we expand this sample to estimate the levels of endemism within the Sittaung compared with the surrounding larger rivers (Irrawaddy, Salween, and Mekong). From the phylogenetic and morphological studies of the mussel samples, we describe seven new species and a single subspecies, which are ancient endemic lineages of the Sittaung. Additionally, we found two new tribes and two new genera, which are described herein. Our findings highlight that the medium-sized basins may represent separate evolutionary hotspots that should be considered in future conservation planning for freshwater biodiversity in Southeast Asia. Results Phylogenetic and species delimitation analyses. Our multi-locus phylogeny (COI + 16S rRNA + 28S rDNA) contains 469 specimens of Unionidae, including 403 specimens from the Oriental Region (Supplementary Tables 1 and 2, Supplementary Figs 1 and 2). The Oriental sample includes 256 unique haplotypes belonging to 80 putative species (Fig. 1). The Bayesian Poisson tree processes (bPTP) model supports the majority of these possible species-level units (Fig. 1). The results inferred from the single-rate PTP (sPTP) model were generally similar to that of bPTP model, with a few exceptions (Fig. 1). The molecular operational taxonomic units (MOTUs) of the multi-rate PTP (mPTP) model were also comparable to those of the two other models, but in some cases, the mPTP revealed larger clusters (...truncated)