Phylogeny and biogeography of Indochinese freshwater mussels in the genus Pilsbryoconcha Simpson, 1900 (Bivalvia: Unionidae) with descriptions of four new species

Abstract

The body of knowledge regarding the classification and evolution of freshwater mussels in the family Unionidae (Bivalvia) in Indochina has recently increased. However, the taxonomic revision of all extant taxa in the region is still ongoing. In this study, the genus Pilsbryoconcha was revised based on an integrative analysis of shell morphology, biogeography, and molecular data. Multi-locus phylogeny indicated the availability of eight species within the genus. Four previously recognized species are P. exilis (Lea, 1838), P. schomburgki (Martens, 1860) stat. rev., P. linguaeformis (Morelet, 1875), and P. carinifera (Conrad, 1837), while four other species are described herein as P. acuta sp. nov., P. mekongiana sp. nov., P. kittitati sp. nov., and P. hoikaab sp. nov. In addition, the neotype of P. carinifera is also designated to clarify its long taxonomic ambiguity. Divergent time estimation and historical biogeography analysis revealed that Pilsbryoconcha originated in the area now called the Khorat Plateau around the middle of the Eocene (mean age = 43.12 Mya), before its range was expanded across Indochina through a series of complex geomorphological changes of river systems, which also led to diversification of the genus.

Figure 1

Maximum likelihood tree based on 2216 bp concatenated alignment dataset of COI + 16S + 28S genes. The outgroup sample is not shown. Numbers on nodes indicate bootstrap values (bs) from maximum likelihood (ML) and bipartition posterior probability (bpp) from Bayesian inference analysis (BI), and are shown as ML/BI. Nodes marked with black circles were sufficiently supported by both ML (bs ≥ 70) and BI (bpp ≥ 0.95). Nodes with grey circles were supported only by ML, and white circles were supported only by BI.

Figure 2

Fossil-calibrated ultrametric tree of the tribe Pseudodontini generated from BEAST v2.6.1 based on the concatenated alignment dataset of COI + 16S + 28S genes. The outgroup sample is not shown. Sufficiently supported nodes (bpp ≥ 0.95) are marked with “*”. Numbers above branches are estimates of time since the most recent common ancestor (tMRCA). Node bar indicates 95% highest posterior density interval (HPD) of the node ages. Pie charts at nodes indicate probabilities of certain ancestral areas from the combined results calculated by the common area reconstruction. Border color of the pie charts indicates biogeographic events of dispersal (orange border) and vicariance (blue border) events. Map shows the boundaries of freshwater basins used in the ancestral area reconstruction analysis. Map was modified from the GIS shapefile from Freshwater Ecoregions of the World (https://www.feow.org). Geologic time scale according to the geological Society of America.

Figure 3

Shells of Pilsbryoconcha species. (a,b) P. exilis, (a) syntype USNM 86726, Java, (b) specimen MUMNH-UNI2792, Sa Kaeo, Thailand; (c,d) P. schomburgki, (c) holotype NHMUK 1859.5.23.8, Siam, (d) specimen MUMNH-UNI 2810, Nakhon Ratchasima, Thailand; (e,f) P. carinifera, (e) specimen ANSP 56519, ‘India’, (f) neotype MUMNH-UNI2823, Sa Kaeo, Thailand; (g,h) P. linguaeformis, (g) holotype NHMUK 1893.2.4.614, Battambang, Cambodia, (h) specimen MUMNH-UNI2636, Pursat, Cambodia; (i) P. mekongiana sp. nov. holotype MUMNH-UNI0843, Bueng Kan Thailand; (j) P. hoikaab sp. nov. holotype MUMNH-UNI0305, Nakhon Phanom, Thailand; (k) P. acuta sp. nov. holotype MUMNH-UNI1510, Ubon Ratchathani, Thailand; (l) P. kittitati sp. nov. holotype MUMNH-UNI0372, Udon Thani, Thailand. Scale bar = 10 mm. Image: USNM collection database [a], D. Graf and K Cummings, https://mussel-project.uwsp.edu/fmuotwaolcb/specimen_6006.html [c]; P. Callomon [e], and NHMUK collection database under a CC0 1.0 license [g].

Figure 4

(a,b) Map of mainland Southeast Asia showing the distribution of Pilsbryoconcha species and the main river systems. Maps were developed using QGIS v3.24.3 by compiling topographic base map of freshwater river basins from the Freshwater Ecoregions of the World (https://www.feow.org), river and lake topology from the HydroSHEDS database (https://www.hydrosheds.org), and map raster data from the NASA EARTHDATA (https://www.earthdata.nasa.gov/).