Mussel of the Month

This post will delve into the implicit debate over the classification of Lasmigona and how the data available on the MUSSEL Project web site can be used to resolve it. To peek ahead to the end of this post, we treat the traditional concept of Lasmigona as three genera: Lasmigona, Platynaias, and Alasminota. But, let’s start at the beginning…

More information can be found in the previous MotM posts for Lasmigona, Platynaias, and Alasminota.

As ever, the stories we tell about freshwater mussel classification are divided into the thinking through the 20th century and the subsequent revisions following the molecular phylogenetic revolution. If we can admit that classification before hypothesis-testing via cladistic analyses was just authoritarian essays, why don’t we just call the past null and void and move on? Why can’t we keep the baby and throw out the bathwater? A sufficiently short answer can focus on two reasons: 1) not every part of freshwater mussel tree of life has yet received sufficient phylogenetic scrutiny, and 2) taxonomic nomenclature relies on the Principle of Priority.

Although the community of freshwater mussel systematists has achieved an astounding level of progress in the 21st century, there are still a lot of unanswered questions about the evolutionary relationships among unionoid family-, genus, and species-group taxa. The old, non-scientific-though-often-close-enough, pre-cladistic classification provides a familiar taxonomic scaffold while new analyses shift branches around as new data become available. Moreover, even if every node and branch of the bivalve evolutionary tree was known with confidence, what we would call those clades (i.e., entire branches of the tree) corresponding to the taxa we want to talk about would depend on what name had properly been applied first. Because of the Principle of Priority, modern systematics still requires us to know the history of zoological nomenclature going back to Linnaeus and the 10th edition of Systema Naturae in 1758! We need the baby and the bathwater.

For example, consider the freshwater mussel genus Lasmigona as we inherited it from the pre-cladistic tradition. Our first global checklist (Graf & Cummings, 2007) followed the detailed revisionary work of Clarke (1985). Clarke’s work was the basis for the “official” late 20th century names-lists (e.g., Williams et al. 1993; Turgeon et al., 1998) that reported the same six species (+ 1 subspecies) shown in the table below. It is important for this story that most subsequent authorities did not bother with the subgenera. Everything was just classified as Lasmigona. In our most recent, published checklist (Graf & Cummings, 2021), we followed subsequent revisions that elevated L. alabamensis to a species and that recognized L. etowaensis as valid rather than a synonym of L. holsonia. But, all eight species were still classified under one genus, Lasmigona. As of this typing that is how the species appear on the FMCS Bivalve Name List (2023).

However, a quick perusal of all the available published cladograms demonstrates that this concept of Lasmigona is flawed. Before I move on to explaining the “flaw” in Lasmigona sensu lato, I want to point at the “quick perusal” of the previous sentence. Even with our modern Internet search engines that let us ignore the Zoological Record of 1900s, synthesizing a comprehensive bibliography of all published phylogenies that include at least some set of those eight species would take some work. But, the thing is, the MUSSEL Project Database has already done that. For each rank, from genus up to superfamily, a Cladomics page summarizes the available phylogenetic information for each taxon.

The Cladomics page for the subtribe Alasmidontina shows the phylogenetic data for Lasmigona and related genera. The top of the page reports some summary statistics about the subtribe and the number of genera it represents. This is followed with some charts dealing with pairwise comparisons among the different cladograms on the page. For this post, it needs to be sufficient to say the Cladomics FARQ is available to explain those.

What I want to talk about are the cladograms on the page, sorted in order of how informative they might be (e.g., breadth of taxon sampling). What makes these side-by-each comparisons of cladograms informative is that they collate the various published topologies but uniformly formatted. For example, terminal taxon labels reflect the current taxonomy in the MUSSEL Project Database. For example, if taxonomy has been revised or the specimens have been re-identified since publication, the terminals are labeled with the current name.

The original cladogram is also formatted with the terminals of the ingroup collapsed to the lowest appropriate subtaxon, when possible. For example, for a Cladomic analysis of a family, the ingroup taxa would be collapsed to subfamilies. Or, for a subtribe, like the Alasmidontina, the ingroup is represented by genera — if they can be. If the specific cladogram didn’t recover the subtaxon as monophyletic, then that non-monophyly is depicted as such.

Looking at an actual example might make all that clear (or, at least, clearer). Look at the Alasmidontina cladogram from Whelan et al. (2023). The terminals of the ingroup — the genera of the subtribe — are shown in black and highlighted with double asterisks (**). The original species-tree of the entire cladogram can be seen by clicking the cladogram link (i.e., “fig. 2”). Many of the ingroup genera were represented by multiple species (or even specimens), but they are collapsed to genera for clarity. However, Lasmigona complanata and Lasmigona costata are listed as species because this cladogram didn’t depict Lasmigona as monophyletic. The outgroup terminals (in gray) show the relationships of the ingroup clade. Feel free to scroll down the page to look at the available cladomic data on the Alasmidontina.

Figure 2 from Whelan et al. (2023) was based on a phylogenetic analysis of mitochondrial DNA (COI and ND1) of alasmidontine species, including six Lasmigona sensu lato: L. (Lasmigona) complanata, L. (Lasmigona) costata, L. (Platynaias) compressa, L. (Platynaias) subviridis, L. (Alasminota) hostonia, and L. (Alasminota) etowaensis. To-date, this cladogram has the most comprehensive taxon sampling for the Alasmidontina. That’s why it is listed first in the Cladomics report. Lasmigona s.l. was not recovered as monophyletic, and these results are broadly consistent with other published analyses — just scroll down the Alasmidontina Cladomics page and see. The cladograms there already reflect the updated taxonomy in the MUSSELpdb: we simply elevated the subgenera to genera. These are marked on the cladogram depicted above.

Revision of the taxonomy of Lasmigona sensu lato obtains organically from the expectation that genera should be monophyletic and the rules of the International Code of Zoological Nomenclature. The former subgenera were laundered into legitimacy in the pre-cladistic era by Clarke (1985) and others going back to Walker (1918). The available genus-group names and their type species have been understood for more than 100 years, and the Principle of Priority (ICZN, Article 23) prescribes which names are applied to which circumscriptions of species. Elevating those subgenera to genera is a change in nomenclature, but the taxa remain unambiguously aligned with their previous groupings. The genus names are available and have been applied without confusion for a century, so we will keep using them without shackling them to a flawed hypothesis of Lasmigona s.l. Instead of one genus with three subgenera, we simply have three genera: Lasmigona sensu stricto, Platynaias, and Alasminota.

This case of Lasmigona was easy to resolve because all the necessary nomenclature already existed. A similar situation has worked itself out by splitting Prolasmidonta and Pressodonta from Alasmidonta. For other genera, revising the taxonomy would necessitate describing new taxa. For an example, the February 2023 Mussel of the Month explains the unresolved situation with the non-monophyly of the genus Lampsilis.

The Cladomics tools on the MUSSEL Project Web Site are a useful recourse for discovering the low-hanging freshwater mussel phylogeny fruit!

Visit https://musselproject.substack.com/ to subscribe and get the MUSSEL Project Mussel of the Month in your inbox!

Episode 2025-1 of The Whoo! on the MUSSELp YouTube Channel is discussion about Lasmigona & the Cladomics Functionality of the MUSSEL Project Database.