Resolving what ended up becoming the well-supported subfamily Parreysiinae of the family Unionidae has been a focus of freshwater mussel molecular systematics for more than two decades. Before the 1990s, “Parreysiinae” was a name that was sort of “in use” but poorly defined: basically, freshwater mussels from India to Southeast Asia, often with zig-zag umbo sculpture (Brandt, 1974; Subba Rao, 1989). Then, around 2000 CE, malacology pivoted to cladistic analyses to test the hypotheses of the evolutionary relationships that underpin classification and taxonomy — as opposed to relying on authoritarian essays. (Don’t get me started…) The whole system of freshwater mussel classification has been in flux ever since, and the “Parreysiinae” was one of those untested names that was, in the most noncommittal of nomenclatural terms, “available.”
In the 2000s, a good chunk of the classification of freshwater mussels from other-than-North-America was left “under construction.” So many of the 20th century taxa had crumbled under the slightest weight of phylogenetic examination that we couldn’t bear to prop up traditional subfamilies just because no one had yet given them a hard look. In our earliest global assessments, we just fudged most “Old World” unionids into incertae sedis geographical assemblages (Graf & Cummings, 2006, 2007).
Though I don’t think anyone knew it at the time, the search for the real Parreysiinae began with a study by Bogan & Hoeh (2000). That was the first broad taxon sampling of the Unionoida for a phylogenetic analysis representing all 6 Recent families. The most species-rich family, the Unionidae, was represented by all the major North American lineages (i.e., the modern tribes), 3 European species, and Coelatura aegyptiaca from Egypt. Among the several interesting results from Bogan & Hoeh (2000) was the recovery of C. aegyptiaca as sister to a clade of the rest of the Unionidae + Margaritiferidae — e.g., i.e., WTF, the Unionidae was not monophyletic! It was quite a rude discovery that the most speciose, best-studied freshwater mussel family might not even be a natural lineage. Today’s taxonomists prefer our formal taxa to represent all the descendent species from a common ancestor. This result placed the Margaritiferidae branch in between at least Coelatura and the rest of the Unionidae on the freshwater mussel tree of life. It was a whole “thing” in the literature that couldn’t be solved by reanalyzing the same old data (the saga was reviewed in Graf & Cummings, 2010).
To get to the bottom of the Coelatura aegyptiaca problem, we needed some fresh sampling. We went to Egypt to get more C. aegyptiaca (shukran, Gamil Soliman and Kohar Garo!), but we also went to tropical Africa to get other African mussel genera (thank you, Alex Chilala!). And, we lucked into a haul of species of several genera from Myanmar (thanks to Jeff Wilkinson and Christina Piotrowski!). What we discovered with fuller taxon and character sampling was that Coelatura and the other African species were nested within a clade of Indo-Burmese species with zig-zag umbo sculpture. The Parreysiinae was a nice, natural clade, but it included most African unionid species as well as the traditional bunch from southern Asia. We had found good support for a monophyletic Parreysiinae within a monophyletic Unionidae (Whelan et al., 2011).
More recently, Ivan Bolotov, John Pfeiffer, and others have densely sampled and analyzed the phylogenetic relationships of the species of the Parreysiinae (Pfeiffer et al., 2018; Bolotov et al., 2018, 2024). The subfamily is divided among five tribes, 23 genera, and 108 species — including Assamnaia involuta, our Mussel of the Month. The recent revisionary record of the genera and species is too complex to fully cite in this post, but the table below tallies the current taxonomy alongside how they were classified back when The West Wing was still on TV. To get into the technical literature on the phylogenetic relationships within and among the taxa of the Parreysiinae, the Parreysiinae Cladomics Page on the MUSSEL Project Web Site can help you find what you need.
Having established the monophyly of the Parreysiinae, the next problem is its sister-group. Where does the subfamily fit in the wider freshwater mussel tree? Most phylogenetic analyses of “regular” phylogenetic data (i.e., nucleotides from COI, 16S, 28S, etc.) with sufficient taxon sampling have recovered the Parreysiinae as sister to the rest of the Unionidae (Whelan et al., 2011; Lopes-Lima et al., 2017), but the genomic, “Unioverse” approach of Pfeiffer et al. (2019) gave a different result. But, we will have to pick up that thread in a future Mussel of the Month…
Visit https://musselproject.substack.com/ to subscribe and get the MUSSEL Project Mussel of the Month in your inbox!
The table below depicts compares the current classification of the Parreysiinae with that of Graf & Cummings (2007). Click the image to make it larger, or click here to get it as a PDF.
 |
