Mussel of the Month

According to Article 5 of the International Code of Zoological Nomenclature, the species Anodonta californiensis needs to have a genus in its name, and it does: Anodonta. But it is unclear if the usage of “Anodonta” conveys information about classification (as it ideally would) or if it just satisfies the requirement that some generic nomen fill that slot. Anodonta californiensis (as well as A. kennerlyi, A. nuttalliana, and A. oregonensis) is found in the Pacific drainages (west of the Rocky Mountains) in North America, whereas the type species of the genus, Anodonta cygnea, is widely disjunct in western Europe. This Mussel of the Month post will shine a light on the taxonomy of the species of Anodonta and the weird biogeography of the genus — “weird” in the sense that the classification of those Nearctic species in Anodonta is either nuts or really interesting. As David St. Hubbins and Nigel Tufnel said, “It’s such a fine line between stupid and clever.”

Early in the 19th century, the genus Anodonta was widely applied to freshwater mussel species lacking hinge teeth. It is fair to say that, with a few notable exceptions (e.g., Castalia, Etheria), until Simpson (1900) transitioned us to the era of modern genera, most freshwater mussels were placed in Anodonta (without hinge teeth) or Unio (with hinge teeth). Over the course of the ensuing centuries, the scope of the name Anodonta narrowed. Today, Anodonta as we know it is restricted to Northern Eurasia (including Palearctic Africa), western North America, and, apparently, Central America (Graf & Cummings, 2021). All of the former Anodonta spp. of eastern North America and eastern Asia have been reassigned to genera in respective clades largely endemic to those regions. The Alasmidontina in eastern North America has taxa like Pyganodon, Utterbackia, and Utterbackiana, and genera like Cristaria, Sinanodonta, and Beringiana of eastern Asia are classified as Cristariina. This is the classification we use on this web site.

In June of 2021, the question remains: Is Anodonta californiensis (and related species) classified in Anodonta because that reflects their phylogenetic affinities? Or, is A. californiensis just sitting in the spot Isaac Lea put it in 1852, waiting to be revised?*

What is even more confounding is how much phylogenetic work has been done on the Pacific North American Anodontas to still not have a clearer answer. The first definitive cladistic study of Anodonta sensu lato was Hoeh (1990; re-analyzed by Zanatta et al., 2007). That analysis included representatives of eastern North American “Anodontas,” A. kennerlyi from western North America, and A. cygnea from Europe. Based on parsimony analysis and allozymes, that was the work that discovered that eastern North American genera like Utterbackia and Pyganodon are distinct from other Anodontas, but retained A. cygnea, A. kennerlyi, and the species we now call Utterbackiana in Anodonta. Though a major step forward, the Hoeh (1990) analysis was flawed by rooting the trees with Lasmigona. Lasmigona is part of the Alasmidontina (like Pyganodon, Utterbackia, and Utterbackiana), and so the topology of the Anodonta species in that paper was unnaturally contorted.

After a strong start, little progress has been made. There have been multiple phylogenetic studies that included a single western North American Anodonta species with broad enough taxon sampling to discover where it might hang on the freshwater mussel tree, but the results have been inconclusive. Some analyses recovered a topology that supported a clade of European and Pacific American Anodonta (Lopes-Lima et al., 2017, 2020; Huang et al., 2019), but some did not (Campbell et al., 2005; Bolotov et al., 2017; Riccardi et al., 2020). There have been a few studies with broad sampling of western North American Anodontas, but those lacked sufficient resolution and/or sampling of other Anodontas (Mock et al., 2004; Gustafson & Iwamoto, 2005; Chong et al., 2008), so they are not useful for the question at hand. Inoue et al. (2018) analyzed an extensive set of western and eastern North American anodontines, but they didn’t label the terminals on that part of their cladogram (it wasn’t the focus of their analysis).

The one Pacific Nearctic anodontine that has been well studied is Beringiana beringiana. It used to be classified with the rest of the western North American Anodontas, but it has been consistently placed in the Cristariina (with Sinanodonta and Cristaria), distinct from Anodonta cygnea of Europe, A. californiensis of western North America, or the Alasmidontina of eastern North America (Soroka, 2010; Klishko et al., 2014; Sayenko et al., 2017; Froufe et al., 2017 #3802; Bolotov et al., 2020; Lopes-Lima et al., 2020).

On this web site and in our recent checklist (Graf & Cummings, 2021), we include A. californiensis (and the other Pacific North American Anodonta spp.) in the genus Anodonta because so far, we don’t know of a better place to put it.

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* It turns out, a separate genus has been introduced for A. californiensis. Hannibal (1912) introduced Arnoldina for Anodonta deject, currently regarded as a junior synonym of A. californiensis (Williams et al., 2017). Thus, Arnoldina is at present a junior synonym of Anodonta.