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8 January 2020

Mussel of the Month

The 2020 Mussels of the Month, so far ...

January 2020

UtterbackianaUtterbackiana suborbiculata (Unionidae, North America)

The January 2020 Mussel of the Month is Utterbackiana suborbiculata. Utterbackiana is a genus of five species from eastern North America.

The genus Utterbackiana was described by Frierson (1927), but it was largely forgotten until Williams et al. (2017) brought it back for U. suborbciulata (type sp.), U. couperiana, U. hartfieldorum, U. heardi, and U. implicata. There has been no classic revision (evaluating synonyms, etc.), but the use of Utterbackiana is supported by recent molecular phylogenetic work.

(This is a long post with lots of Dillon-esque footnotes, so you might want to pause to fill up your coffee before you continue.)

It is worth reviewing how we got to this generic revision for a group of relatively well-studied North American mussels. The story technically begins at the end of the 18th century when Lamarck (1799) described the genus Anodonta, and the name was widely applied around the world. Then a bunch of stuff happened ... blah blah blah ... and Hoeh (1990) took up the problem of Anodonta sensu lato in Europe, western North America, and eastern North America. Using allozyme data (reanalyzed by Zanatta et al., 2007), it was reported that those species belonged to three separate clades that were recognized as genera: Pyganodon (like P. grandis) and Utterbackia (e.g., U. imbecillis) in eastern North America, and Anodonta in Europe (e.g., A. cygnea), western North America (A. kennerlyi), and Eastern North America (“A.” suborbiculata and “A.” couperiana). This new taxonomy was soon taken up by the freshwater mussel illuminati (e.g., Cummings & Mayer, 1992; Williams et al., 1993; Turgeon et al., 1998), and that taxonomy carried us into the 21st century.*

By the early 2000’s, hindsight had revealed that the Hoeh (1990; Zanatta et al., 2007) analyses were flawed. The ingroup consisted solely of species from Europe and North America then classified as Anodonta (like the species listed above), and Lasmigona (from North America) comprised the outgroup. As now understood, the eastern North American anodontines (including Pyganodon, Utterbackia, and Lasmigona) form a clade (Graf & Ó Foighil, 2000; Chase et al., 2018). Thus, a monophyletic Anodonta that includes species from eastern and western North America and Europe was only achieved by biased taxon sampling and a misplaced root on the cladograms.** Biogeographically, that kind of a distribution for Anodonta isn't well supported either since few genus-level clades are known to be shared across both sides of the Rocky Mountains in North America and Europe.***

This brings us to 2007 and the Graf & Cummings checklist (like so many of these tales often do). When considering only the species of North America — as Williams et al. (1993) and Turgeon et al. (1998) did — the difficulties with the taxonomy and biogeography of the local Anodonta species are somewhat smoothed over.**** But taking on the scope of all freshwater mussels and new knowledge gained from phylogenetic studies, it was evident to us (Graf & Cummings, 2007) that the “Anodonta” of eastern North America shared a more recent common ancestor with mussels in eastern North America than with Anodonta species in either the American West or Europe.

Fortunately, previous taxonomic opinions provided a solution. Hoeh (1990) didn’t invent the genus-level taxonomy of the former Anodontas. Rather, those names had been used previously as subgenera (Johnson, 1972, 1980; Heard, 1975) — e.g., A. (Pyganodon) grandis, A. (Utterbackia) imbecillis, A. (Utterbackia) couperiana, and A. (Utterbackia) suborbiculata. Putting U. couperiana and U. suborbiculata in Utterbackia solved the nonsensical eastern Anodonta problem, allowing the taxonomy to reflect a clade of eastern North American anodontines (Alasmidontina), although the relative phylogenetic positions of U. imbecillis and U. suborbiculata were not well understood at the time.

The relative positions of U. imbecillis and U. suborbiculata are better understood now: Utterbackia imbecillis, U. peggyae, and U. peninsularis comprise a clade (frequently sister to Pyganodon), and Utterbackiana suborbiculata and the other species represent a distinct clade (Breton et al., 2011; Smith et al. 2018). And thus explains the resurrection of Frierson’s (1927) long unused genus name.*****

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* This discussion is limited to the North American taxa. The name Anodonta has been, until recently, applied to Anemina and Sinanodonta in East Asia.

** For example, if Quadrula had been used to root the tree and more eastern North American anodontine genera were included (e.g., Strophitus, Alasmidonta),† then we would expect the “Anodonta” species of eastern North America to be in a clade with Pyganodon, Utterbackia, and Lasmigona rather than European Anodonta. But that is a lot of “ifs.”

† And, assuming we could expect allozymes to behave like DNA to recover phylogeny.

*** Margaritifera being, perhaps, the main exception. Check out our discussion of Gibbosula for that whole other story.

**** The story of Ortmanniana and Actinonaias is analogous. “Actinonaias” ligamentina makes sense if you ignore the Central American species.

***** This post is long enough, so we won’t get into the curious coincidence of F.C. Baker (1927) and Frierson (1927) simultaneously naming new, closely related genera for William Utterback.

 
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