The March 2026 Mussel of the Month is Globunio mirificus
A new species in a new genus in a new subtribe
There are only two things you can count on in a life of freshwater mussel biology: death and taxonomic revision.
The net species flux in freshwater mussels over the last 10 years has been overwhelmingly positive, with many new species (and genera., etc.) The time is ripe to take stock of our consensus of global freshwater bivalve diversity — with the expectation of continued change.
Mussel of the Month Globunio mirificus is one of many recently discovered and described species and genera of freshwater mussels. A quick-look dashboard of freshwater mussel genus and species richness on the home page of the MUSSEL Project Website tracks the rapid increase in taxa since 2007. That year, we (Graf & Cummings, 2007) published our first checklist of Recent global freshwater mussel species, genera, and families. Back then, we tallied 840 species in 161 genera by accounting for the then-known named species and tracking how they had been classified and synonymized in more than two centuries worth of revisions. That is much more than the around 800 species contemporaneously estimated by Bogan (2008) but barely changed from the 837 valid species treated almost 40 years earlier by Haas (1969). As I recently described when Mutela dubia was mussel of the month, the task then was sorting through all the available freshwater mussel names to figure out which were valid and which could be “lumped.” The MUSSEL Project Database (MUSSELpdb) was created for that purpose.
When we first took on the mission of the MUSSELp, a global synthesis had been long overdue. The situation was mostly a stagnant mess waiting for someone to clean it up, like Hercules and the Augean Stables. (I know I have used that metaphor before but it just works.) These days, the challenge of tracking freshwater mussel diversity is the constant flux of names in to and out of validity (mostly in). Since 2017, the MUSSELp dashboard tickers of species and genus richness have been decidedly bullish. In 2021, we published an updated global checklist (Graf & Cummings, 2021) just as the pace of discovery accelerated. When we submitted the manuscript in November 2019, we clocked the species and genus richness at 915 and 182, respectively. By the time we were done with reviews and revisions in June 2020, those values had accelerated to 958 and 192! The ascent continues through the present day. The MUSSELpdb currently stands at 1006 species in 219 genera.
In lieu of publishing another article (that would likely be out of date frustratingly quickly), let’s just take a few hundred words right now to plant a flag and describe the current state of global freshwater mussel affairs. To facilitate this discussion and offer a little mussel taxonomy SWAG, please accept the following colorfully formatted data in the Google Sheets document: Unionida cum Grano Salis 2026 (“Freshwater Mussels with a Grain of Salt.” That was the original title of the 2007 checklist, nixed by the editor. I’m bringing it back!)
There are two sheets with data. The global diversity patterns sheet presents the full current classification of the Unionida (freshwater mussels) from the order down to the genera, with tallies of species and genus richness. Taxa are further subdivided among biogeographical regions. The checklists sheet shows the correspondence among the genera and species from our two previous publications (2007 and 2021) and the MUSSELpdb right now. The first column lists the relevant nominal species. Not a complete synonymy, but the ones sufficient to indicate the alignment among taxa. Color-coding is intended to highlight both genera and species as well as changes from list to list.
The regions and subregions we use in the MUSSELpdb to summarize global diversity patterns have been tailored here to highlight two additional areas of endemism. North America is divided into Eastern and Western North America by the Rocky Mountains. Western North America corresponds to the Pacific subregion in the MUSSELpdb, and Eastern North America is a combination of the remaining subregions draining to the Atlantic. Most of the subregions of East Asia are combined as Eastern Asia, but the India-Myanmar subregion is singled out as Southern Asia in these analyses. Central America (and the Caribbean), South America, the Afrotropics, North Eurasia, and Australasia represent the traditional biogeographical realms. Graf & Cummings (2021: Table 1) provided a detailed delineation of the freshwater regions and subregions as applied in the MUSSELpdb, including their correspondence to the freshwater ecoregions of the world.
Partitioning the world’s waterways this way optimizes the distinctions among the various regional freshwater mussel assemblages. Most genera are endemic to only a single region, but a few are distributed across those boundaries — e.g., Unio occurs natively in North Eurasia and the Afrotropics (see figure below). The genera of each family-group level taxon are listed by region, with those genera reported in “extra” regions listed parenthetically. The parenthesis convention is repeated in the column tallying the number of genera to indicate which have already been counted. Non-native genera introduced to a subregion are marked with a hash (#). For example, two species of Unio have been introduced to Eastern Asia.
The only species listed by name in that table/sheet are those that occur in multiple regions, and those are few. Most represent introduced species, but others like Beringiana beringiana and Margaritifera margaritifera natively straddle regional borders. Species double-counted in “extra” regions are reported in parentheses in the tallies.
Summarizing the data this way reveals some illuminating patterns. For example, if we compare regional patterns of species diversity from the two previous checklists and the MUSSELpdb right now, we can locate the hot spots of freshwater mussel diversity flux. The figure below lists the total current number of species in each of 9 biogeographical regions and the net percentage of species flux: how the tally of species has changed since 2007.
The region that has gained the most species in the last two decades is Eastern Asia: 117 (165 gained – 48 lost), representing a net +88% flux. As the color-coding of gains and losses in the graph shows, Eastern Asia was busy with taxonomic revision and discovery before our 2021 checklist and since. Southern Asia and South America both also show large, positive fluxes, but with more change attributed to the inter-checklist years than recently. North Eurasia is the region with the greatest proportional net flux in species: +150%, while the region with the greatest negative flux is Central America (-65%). The list of key references is too long for the word limits of a Substack post, but they are cited in the MUSSELpdb. The links in this paragraph and below point to previous Mussels of the Month that also cite a sample of revisionary works and provide additional discussion.
An assessment of species flux by families reveals the same patterns: taxa from Eastern Asia (Unioninae and Gonideinae) and South America (Hyriidae) show the greatest increases in species richness, while the Popenaiadini in Central America has lost the most.
The reason for all this taxonomic flux? The proximate cause has been the application of cladistic methods to freshwater mussels taxonomy outside of the United States. In North America, the molecular phylogenetic revolution began in the 1990s. In Europe and East Asia, vigorous application of nucleotide data and computational tree-searching algorithms didn’t pick up steam until the late 2010s. That initial momentum is still driving the discovery of freshwater mussel diversity today. Keep an eye on the MUSSELpdb to stay abreast of future flux and the data behind it.
AUTHOR’S NOTE: After I finished editing this post and the images, some new publications dropped that will push the numbers here out of date. I thought this article could stay current for at least week, but alas… I expect the overall patterns to hold true for a while, and the current counts of genus and species richness can always be verified with the MUSSEL Project Database.
References Cited
Bogan, A.E. 2008. Global diversity of freshwater mussels (Mollusca, Bivalvia) in freshwater. Hydrobiologia 595: 139-147.
Dai, Y.-T., Z.-G. Chen, S. Ouyang, X.-C. Huang & X.-P. Wu. 2025. A new tribe, genus, and species of freshwater mussel from the Changjiang River Basin in China (Bivlavia, Unionidae, Unioninae). Zoosystematics and Evolution 101(2): 779-790.
Graf, D.L. & K.S. Cummings. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Journal of Molluscan Studies 73: 291-314.
Graf, D.L. & K.S. Cummings. 2021. A ‘big data’ approach to global freshwater mussel diversity (Bivalvia: Unionoida), with an updated checklist of genera and species. Journal of Molluscan Studies 87(1): eyaa034 (36 pp.).
Haas, F. 1969. Superfamilia Unionacea. Das Tierreich, Leif. 88. Walter de Gruyter and Co., Berlin. 663 pp.
Google Gemini AI claimed that Guiunio was named Xenodomus. There is a fungal genus, regarded as a subjective synonym, by that name but no rstional reason to associate it with Guiunio.