The November 2025 Mussel of the Month is Schistodesmus xinyuensis
The old Schistodesmus lampreyanus (Unionidae) from China has been split-up based on molecular phylogenetic results
The way freshwater mussel systematists approach their work has changed over the last two decades, and the MUSSEL Project Database has adapted to those changing needs. Hou et al. (2025) recently published a revision of the genus Schistodesmus Simpson, 1900, including naming and describing our Mussel of the Month, Schistodesmus xinyuensis Hou & Wu in Hou et al., 2025. Their novel data and conclusions precipitated a broader update of the MUSSEL Project Database to reflect current knowledge.
When we (Kevin Cummings and I) started the MUSSEL Project back in 2002, we (systematic malacologists, generally) went about our jobs as we had in the previous century. Our primary data for discovering, describing, and explaining the diversity of freshwater mussels were found in collections and libraries. The collections we used are assemblages of voucher specimens — typically shells, in our case — as records of the occurrence of organisms with particular traits at specific places, and the libraries curated the literature of taxon descriptions and revisions. The intersection of those two data streams is the type specimens: the vouchers on which species descriptions are based. We had to (got to) travel the world to sample rivers in exotic places and visit natural history museums in the major cultural centers of the world. It’s a pretty sweet gig if you can get it.
That protocol (more of a lifestyle, really) for doing biodiversity research was expensive, time-consuming, and largely limited to the few researchers stationed at major natural history museums. We (Kevin and I) made the MUSSEL Project Web Site to digitally document and organize the freshwater mussel taxonomy, specimen, and literature data that we collected so that more people had access (at least virtually) to the shells and information that the National Science Foundation was funding us to study. The MUSSEL Project was like Bob Dylan at the 1965 Newport Folk Festival: we electrified the old way of doing things.
The MUSSEL Project Web Site has been serving the MUSSEL Project Database (MUSSELpdb) to provide access to those traditional data. The most novel aspect is the access to what we call “taxonomic histories.” For genera and species, we provide a summary of what each taxon has been called by different workers over the years. This is a handy tool when reading the historical literature since species often went by different names in the past. For example, our Mussel of the Month Schistodesmus xinyuensis has been generally confused for a related species, Schistodesmus lampreyanus (Baird & Adams, 1867).
The practices of freshwater mussel systematics have been further streamlined by other comprehensive digitization efforts and the new prominence of molecular systematics. Because of efforts like GBIF, iDigBio, and the Biodiversity Heritage Library, more research collections and libraries are available to everyone. Many important historical works, the back-catalogs of major journals, and virtually every modern journal article are accessible through the world wide web. MolluscaBase provides a similar digital infrastructure for general molluscan taxonomy — including freshwater mussels. The MUSSELpdb provides links to those works.
The MUSSELpdb has also expanded to accommodate new-fangled modes of taxonomic information, like nucleotide sequences and cladogram topologies. The first phylogenetic analysis of freshwater mussel evolutionary relationships was published in 1989, and by 2002, there were 30 publications with freshwater mussel trees. The total number of such papers doubled by 2008 and doubled again by 2016 and again by 2023. Today, there are more than 300 such works, more than half of which have been published since 2017. While those cladograms provide a wealth of information on the validity of genus- and family-group level taxa, those data don’t automatically align with the traditional classification hierarchies and synonymies that systematists need to apply the International Code of Zoological Nomenclature. A gene-tree or species-tree may depict the monophyly and sister-relationships among various higher taxa, but if the authors didn’t specifically indicate all the applicable genera, families, subfamilies, tribes, and subtribes, how is anyone supposed to recognize that work as a relevant source?
That is where the Cladomics functionality of the MUSSEL Project Database comes in. For each freshwater mussel genus- and family-group taxon in our database hierarchy, a Cladomics page lists the cladogram topologies involving that taxon. For example, on the Schistodesmus Cladomics Page, the relevant branch of a phylogeny by Hou et al. (2025) is presented. In addition, there are 86 more published cladograms on that page with at least one Schistodesmus terminal taxon.
On the Cladomics page for each taxon, only a focused segment of the original cladogram topology is shown. In the example above, the ingroup of Schistodesmus species are shown in bold, black font and highlighted with two asterisks. The outgroup taxa in gray are those taxa of the same rank as the ingroup taxon (i.e., genera) that are classified in the same tribe (in this case, Unionini). These are what is known in our Cladomics parlance as “rigorous outgroup subtaxa” (see the Cladomics FARQ for more information).
Since the molecular phylogenetics mania has taken off worldwide among freshwater mussel systematists, the community has come to value specimens with Genbank accessions over those with only shell and/or soft-part anatomy. Rather than relying solely on the phenotypes of traditional voucher specimens to identify and revise species, mussels are genetically “barcoded” with molecular markers like cytochrome c oxidase subunit I (COI). Species today are routinely recognized from the clustering of sequences in molecular phylogenetic trees.
The phylogenetic analyses and revisionary work by Hou et al. (2025) provide an interesting case study to showcase how links to molecular data are curated in the MUSSELpdb. For a century, Schistodesmus was stable and widely regarded to be composed of two species, S. lampreyanus and S. spinosus (Simpson, 1900). But, as DNA sequences accumulated, that stability began to crack. For example, in the major molecular phylogeny of Chinese freshwater mussels by Huang et al. (2019), their sample of four Schistodesmus specimens suggested more diversity than just S. spinosus and S. lampreyanus. Their Schistodesmus clade even included a frequently used Genbank accession misidentified as Lamprotula gottschei(Martens, 1894) [= L. leaii (Griffith & Pidgeon, 1833] from a different subfamily! The Unionini branch of the Huang et al. (2019: Figure 2) tree is shown below.
Hou et al. (2025) sampled new specimens from throughout the range of Schistodesmus in China and re-identified the previous DNA sequences published by Huang et al. (2019) and others. It turned out that the true Schistodesmus lampreyanus in the previous studies was the material that had been erroneously attributed to Lamprotula gottschei, and the two terminal branches that had been labeled “Schistodesmus lampreyanus” represented heretofore undescribed species, S. tongpenensis Hou & Wu in Hou et al., 2025 from the southern part of the Yangtze Basin and Mussel of the Month S. xinyuensis from the vicinity of Poyang Lake.
Based on the results of Hou et al. (2025), the Genbank accessions and museum specimens in our database were assigned new identifications. This not only led to new maps being drawn for Schistodesmus species as vouchers moved from one to another, but it also meant that published trees on the Schistodesmus Cladomics page were improved. Every previous cladogram with terminal species based on those re-identified Genbank accessions has been corrected to reflect the current classification in the MUSSELpdb. The cladogram below shows the current topology based on the Huang et al. tree above, with today’s taxonomy applied.
The history of identifications for those Genbank accessions is also served by the MUSSELpdb. The screenshot below is for one of the COI sequences identified by Huang et al. (2019) as S. lampreyanus but that we now know as S. xinyuensis. You can find the list of available Genbank voucher specimens at the bottom of a valid species specimens page, such as for S. xinyuensis.
Freshwater mussel systematists build datasets for phylogenies by incorporating previously published DNA sequences. The MUSSEL Project Database has lots of tools to find the most up-to-date information about specimens, phylogenies, and Genbank accessions.
References Cited
Hou, K., X. Wang, J. Jia, X. Liu, X. Wu et al. 2025. Cryptic species, mitochondrial phylogenomics and historical biogeography in the endemic genus Schistodesmus Schistodesmus (Bivalvia, Unionidae) from China. Invertebrate Systematics 39: 1-17, corrigendum.
Huang, X.-C., J.-H. Su, J.-X. Ouyang, S. Ouyang, C.-H. Zhou & X.-P. Wu. 2019. Towards a global phylogeny of freshwater mussels (Bivalivia: Unionida): Species delimitation of Chinese taxa, mitochondrial phylogenomics, and diversification patterns. Molecular Phylogenetics and Evolution 130: 45-59.