Mussel of the Month

Gibbosula rochechouartii is a new genus for this freshwater mussel species. If you have been keeping track like we have, then a mussel species being reclassified from one genus to another won't blow your mind. Based on the data in the MUSSELpdb, each valid freshwater mussel species has, on average, been classified in more than 4 different genera (4.46 to be precise). Among the "champions"* in this regard is Ortmanniana ligamentina. Over the last couple hundred years, that species and its synonyms have been classified under 14 different genera (e.g., Unio, Obliquaria, Lampsilis, Nephronaias, Actinonaias, Ellipsaria, Venustaconcha, Ligumia) — and that number goes even higher if we consider subgenera.

The classification of G. rochechouartii had been pretty straight forward until very recently. It was originally described in Unio (like most freshwater mussels of the 19th century). Simpson (1914) classified it as Quadrula (Lamprotula), and then Haas (1969) moved it to Lamprotula.** And there it stayed for decades. "L." rochechouartii held its position even after Lamprotula was up-ended and split to move 7 species to Aculamprotula in a different subfamily because not all bumpy, thick-shelled Asian freshwater mussels necessarily belong to the same genus (Zhou et al., 2007; Pfeiffer et al., 2013).

But then last year, some fresh phylogenetic work by Huang et al. (2018) recovered our Mussel of the Month in the family Margaritiferidae, rather than Unionidae where it and the rest of Lamprotula (and Aculamprotula) had always been. Recent phylogenetic work had confirmed that all the Recent margaritiferids can be classified as Margaritifera (Graf & Cummings, 2007; Araujo et al., 2016; Bolotov et al., 2016), so our mussel became Margaritifera rochechouartii.

And then again last year, a more comprehensive analysis by Lopes-Lima, Bolotov et al. (2018) examined a wider array of taxa and data, and the classification of margaritiferids changed course. Not only did they advocate splitting the Recent Margaritiferidae back into multiple genera (as had been done by Smith, 2001 and almost everyone before), but we even have subfamilies now. "M." rochechouartii became Gibbosula rochechouartii in the subfamily Gibbosulinae. G. rochechouartii sat for more than 100 years in the genus (or subgenus) Lamprotula in the family Unionidae, and then WHAM, BAM, thank-you CLAM — two more different genera and a change in family.

Fortunately, the MUSSELpdb is available to help you keep score with freshwater bivalve taxonomy! As we have discussed in various posts over the last few months, freshwater mussel taxonomy has been changing at an almost unprecedented pace. This can be attributed to:

• the increase in the availability of molecular sequences,
• the increase in the number of people working on these questions, and
• a greater willingness to make post hoc revisions based on phylogenetic tree topologies — every node on a cladogram apparently gets a name now.

The table summarizes the history of the taxonomic shifts described in this post.

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* If confusion = winning.
** The MUSSELpdb currently holds no references to this species dating between 1914 and 1969. As we have recently noted, those years were slow ones for mussel taxonomy. There may be data out there, but we haven't captured it yet.