The April 2026 Mussel of the Month is Brachyanodon impura
Finally: the genus Anodonta is only for species of Anodonta
One of the last the slivers of Victorian Era freshwater mussel taxonomy has been swept away with the reclassification of 5 Recent species of Pacific/western North American Anodonta species to Brachyanodon.
Mussel of the Month Brachyanodon impura is the culmination of two seismic shifts in freshwater mussel systematics precipitated by the availability of new data and novel philosophies. At the beginning of the 20th century, the First Wave followed from the discovery of valuable comparative data in the soft-anatomy and life histories. Those data made it possible to construct a hierarchical classification that reflected freshwater mussel evolution rather than merely cataloging conchological diversity. The molecular phylogenetic revolution at the other end of the 20th century kicked off the Second Wave, further improving the resolution of the evolutionary tree of freshwater mussels.
The First Wave: Soft-Parts & Larvae
In 1900, our Mussel of the Month was classified as Anodonta impura.
Anodonta is a nicely descriptive genus name referring to the absence of hinge teeth. Back in the 19th century, just about any freshwater mussel anywhere on Earth that lacked hinge teeth was called Anodonta. Simpson’s (1900, 1914) major global revision of the order narrowed the concept of Anodonta down to the edentulous (i.e., toothless) mussels known or assumed to have a shell outline and soft-parts like Anodonta cygnea, the type species from Europe.
Hold on to your butt, gentle-reader: I am going to get a little technical for a paragraph. Mussels like A. cygnea have large, triangular hooked-type glochidia (larvae) that are brooded by their mothers in marsupia occupying the whole of their outer pairs of gill demibranchs. Moreover, gravid marsupial water-tubes (i.e., the vertical spaces within the gills in which developing larvae are stacked) have a diagnostic “tripartite” configuration. Each water-tube is partitioned into three parallel compartments by lateral septa, with glochidia occupying the middle one. The two auxiliary compartments channel the free(er) movement of water through the gills during periods of parental care. A. cygnea and related species and genera are long-term brooders (i.e., bradytictic), and tripartite water-tubes are hypothesized to be an adaptation to support that behavior. For discussions of the nature and taxonomic value of those characters, checkout Ortmann (1912) and Cummings & Graf (2010).
That suite of conchological, anatomical, and reproductive characters is the hallmark of the taxon we know today as the tribe Anodontini (subfamily Unioninae, Family Unionidae). In Simpson’s day, that meant segregating from Anodonta the species lacking the diagnostic triangular hooked-type glochidia and tripartite water-tubes. That carved out the species now classified as Anodontites (Family Mycetopodidae) in South and Central America and Aspatharia and Chambardia (Iridinidae) from Africa, as well as many other edentulous species — e.g., Gonidea angulata (Unionidae: Gonideinae).
However, the process of whittling down Anodonta to its modern conception was just getting going. Simpson’s (1900, 1914) comprehensive revision provided the theoretical skeleton that was fleshed out by the comparative anatomical work of Ortmann (1912), Haas (1924), and many others. While the modern concept of the Anodontini has been recognized for a century (most often as the “Anodontinae”), for most of that time, the genus Anodonta was still a hodge-podge of species widely spread across North America, Europe, and Asia. As shown in the table below, the authoritative, global classifications of Simpson (1900, 1914), Thiele (1934), and Haas (1969) regarded Anodonta to included species we now classify in three different subtribes of the Anodontini: Anodontina, Alasmidontina, and Cristariina. The old Anodonta couldn’t stand up against the next wave of molecular phylogenetic data.
The Second Wave: DNA & Trees
The molecular revolution that reclassified Mussel of the Month Brachyanodon impura wasn’t incited with the nucleotides and probabilistic phylogenetic tree reconstructions that are predominant today. Before PCR and computing clusters, Davis & Fuller (1981) produced tree-ish branching ordination diagrams of species based on immunoelectrophoretic data. That article was the “Ah ha!” that opened the door to a new way to discover and describe freshwater mussel diversity, and their biochemical data confirmed that anodontines represented a cluster of species distinct from other clusters of freshwater mussel species. Davis & Fuller’s analysis included four species then classified as Anodonta, each of which would eventually end up in their own genus — including Brachyanodon, but their initial taxon sampling of only North American species and the vagaries of their analyses couldn’t resolve the problem of Anodonta smeared across three different clades.
Subtribe ALASMIDONTINA
Hoeh (1990, fig. 4) was among the first to show that North American mussels then named Anodonta grandis and A. imbecillis shouldn’t be classified as Anodonta. Based on parsimony analysis of allozyme data, Hoeh recovered topologies that supported elevating the subgenera Pyganodon and Utterbackia to full genera. Other North American species from the east (e.g., A. suborbiculata) and west (e.g., A. kennerlyi) remained congeneric with European Anodonta anatina. With broader taxon sampling and the greater precision of nucleotides, subsequent analyses found that the eastern North American Anodonta species were more closely related to Nearctic genera like Alasmidonta and Lasmigona than with European Anodonta (Lopes-Lima et al., 2017). Simpson’s eastern North American Anodonta are now classified in Pyganodon (e.g., P. grandis), Utterbackia (e.g., U. imbecillis), and Utterbackiana (e.g., U. suborbiculata), and the clade of eastern North American anodontines is the Alasmidontina. The available phylogenetic results are summarized on the Alasmidontina Cladomics page from the MUSSEL Project Database, and the Hoeh (1990) topology is illustrated below.
Subtribe CRISTARIINA
Before Second Wave molecular phylogenetics wrangled the East Asian clade of anodontines into the Cristariina (Lopes-Lima et al., 2017; the Cristariina Cladomics Page), there were a few that rebelled against the Simpson-Haas concept of Anodonta. Both Modell (1964) and Starobogatov (1970) classified Anodonta arcaeformis and Anodonta woodiana in the genera Anemina (a replacement name for Haasiella) and Sinanodonta, respectively. However, the idea of splitting East Asia Anodonta didn’t catch on. Modell and Starobogatov independently proposed ultra-split genus- and family-group level classifications for the world that were too far-out from the prevailing paradigm. The Soviet Comparatory School of Taxonomy pushed the mania even further, concocting split Communist genera that basically corresponded to the lumped Capitalist species of the West (Graf, 2007).
Then came the molecular phylogenies. Not only were East Asian Anodonta reclassified into Anemina and Sinanodonta, but those genera have been further split to include Buldowskia, Amuranodonta, and Beringiana. On the bright side, previously unappreciated Russian splitting bestowed available genus names that could be resurrected to reconcile phylogenies with taxonomy.
Subtribe ANODONTINA
With the taxonomy of the other two subtribes settled, that only left the problem of the Anodontina: Pseudanodonta from Europe, the five North Eurasian Anodonta species distributed from western Europe and northern Africa to Siberia, and a disjunct cluster of five more Anodonta species extending from Pacific North America south into Central America. The available phylogenetic data supporting the genera of the Anodontina and the subtribe itself are pretty flimsy to-date, as summarized on the Anodontina Cladomics Page. The Anodontina is basically the anodontine freshwater mussels that don’t belong with the Alasmidontina or Cristariina.
The recently published phylogeny of Campbell & Lydeard (2026, fig. 9) is representative and the basis for reclassifying the remaining American Anodonta species as Brachyanodon, including Mussel of the Month Brachyanodon impura. Sure, Brachyanodon wasn’t recovered as monophyletic. That’s true, but good ol’ Anodonta wasn’t either. This cladogram isn’t the slam dunk we would like it to be, but is there a better one?
Conclusion
It might freak out some of the taxonomic squares. However, until someone comes up with a more compelling revision, I am going to bask in the glow of finally having Anodonta constrained to just the North Eurasian species of Anodonta.
References Cited
Campbell, D.C. & C. Lydeard. 2026. Three decades of mussel mitochondria. Malacologia 68(1/2): 237-294.
Cummings, K.S. & D.L. Graf. 2010. Mollusca: Bivalvia. [in] Thorp & Covich, eds. Ecology and Classification of North American Freshwaster Invertebrates : 309-384.
Davis, G.M. & S.L.H. Fuller. 1981. Genetic relationships among Recent Unionacea (Bivalvia) of North America. Malacologia 20: 217-253.
Graf, D.L..2007. Palearctic freshwater mussel (Mollusca: Bivalvia: Unionoida) diversity and the Comparatory Method as a species concept. Proceedings of the Academy of Natural Sciences of Philadelphia 156: 71-88.
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. 1924. Beiträge zu einer Monographie der asiatischen Unioniden. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 38: 129-203.
Haas, F. 1969. Superfamilia Unionacea. Das Tierreich 88: 1-663.
Hoeh, W.R. 1990. Phylogenetic relationships among eastern North American Anodonta (Bivalvia: Unionidae). Malacological Review 23: 63-82.
Lopes-Lima, M., E. Froufe, V.T. Do, M. Ghamizi, K.E. Mock et al. 2017. Phylogeny of the most species-rich freshwater bivalve family (Bivalvia: Unionida: Unionidae): Defining modern subfamilies and tribes. Molecular Phylogenetics and Evolution 106: 174-191.
Modell, H. 1964. Das natürliche System der Najaden. 3. Archiv für Molluskenkunde 93: 71-126.
Ortmann, A.E. 1912. Notes upon the families and genera of the najades. Annals of the Carnegie Museum 8: 222-365.
Simpson, C.T. 1900. Synopsis of the naiades, or pearly fresh-water mussels. Proceedings of the United States National Museum 22: 501-1044.
Simpson, C.T. 1914. A descriptive catalogue of the naiades, or pearly fresh-water mussels. Parts I-III. 1540 pp.
Starobogatov, Ya.I. 1970. Fauna of molluscs and zoogeographic division of continental waterbodies of the globe. Nauka, Leningrad. 372 pp.
Tankersley, R.A. & R.V. Dimock, jr. 1992. Quantitative analysis of the structure and function of the marsupial gills of the freshwater mussel Anodonta cataracta. Biological Bulletin 182: 145-154.
Thiele, J. 1934. Handbuch der systematischen Weichtierkunde 3: 779-1022.