Mussel of the Month

Back in May 2011, the Mussel of the Month was Obovaria unicolor. After 10 years, it is worth revisiting Obovaria again.

The thing is, it is unclear if Obovaria is a natural genus — that is, monophyletic. The genus has been consistently regarded as valid since Simpson (1900), and Ortmann (1912) gave his stamp of approval based on the combination of shell and soft-anatomical characters.

In this millennium, there has been some shuffling of species. Specifically, the former “Obovaria” rotulata was shifted briefly to Fusconaia before Reginaia was invented for it and R. ebenus (Campbell & Lydeard, 2012), the old “Villosa” arkansasensis was discovered to be a senior synonym of what we used to call Obovaria jacksoniana (Inoue et al., 2013), and what were “Lampsilis” haddletoni and “Villosa” choctawensis have joined Obovaria as well (Williams et al., 2008, 2011). Our most recent checklist is up-to-date on Obovaria and documents these recent changes (Graf & Cummings, 2021).

The new “Cladomics” feature on the MUSSELp database web pages, however, spotlights the shaky phylogenetic foundation for Obovaria, even with all this recent attention. According to the Obovaria Cladomics page, there have been 25 cladograms published in 14 articles that include a terminal taxon classified as Obovaria. In those works, Obovaria is recovered as monophyletic only 56% of the time! This, even though the mean number of ingroup terminals is less than 3 — i.e., even when a small number of species are sampled, those classified as Obovaria may not have been recovered as a clade!

The Obovaria Cladomics page lists the top studies with regard to ingroup sampling. In some, Obovaria is not consistently resolved as monophyletic because Venustaconcha gets between Obovaria olivaria and the other species of Obovaria (including the type, O. retusa) (Campbell et al., 2005; Zanatta & Murphy, 2006). But other studies that did recover O. olivaria in a clade with the rest of the Obovaria species didn’t include Venustaconcha in their analyses (Inoue et al., 2013; Porto-Hannes et al., 2019). In the absence of a consistent alternative classification, Obovaria stands.

The problematic species for Obovaria is our current Mussel of the Month, Obovaria olivaria. If someday someone decides that that species really doesn’t belong in the same genus as Obovaria retusa, we are in luck — the genus name Pseudoon Simpson, 1900 is available! (Never mind that it looks like a typo for Pseudodon.)