Mussel of the Month

When Simpson (1900) classified freshwater mussel species as Lampsilis, he had a much broader conception that we use today. Based mostly on conchological extrapolation, he divided the genus into four subgenera. After a century of augmenting Ortmann’s (1912, etc.) studies of freshwater mussel soft anatomy and the molecular phylogenetics of the last 30 years, Simpson’s single genus has been split into at least 14 modern genera. We say “at least” because the correspondence between Simpson’s classification and the one we use here isn’t as clean as the table below might suggest. Many of the modern genera have species that were classified by Simpson among different subgenera of Lampsilis sensu lato. And some of the species that Simpson classified as “Lampsilis” aren’t even regarded as lampsilines anymore. We have more and better data than Simpson did to see through the homoplasy of shell characters!

As of this writing, we have logged 64 published cladograms that have analyzed at least two species of Lampsilis. And, the thing is, most of the time Lampsilis — as recognized in the modern literature (e.g., Graf & Cummings, 2021; FMCS species list right now) — has not been recovered as monophyletic. This is documented on the Lampsilis Cladomics page.

Take for example, the cladogram by Inoue et al. (2020, fig. 4), taken from the Lampsilis Cladomics page.

There are four different clades of species classified as Lampsilis: the Lampsilis cardium clade (the type species of Lampsilis), the Lampsilis teres clade, the Ortmanniana clade that includes L. higginsii, and the L. radiata clade (which includes Lampsilis virescens).

Another way to look at this, based just on this cladogram, is that the monophyly of Lampsilis would be achieved if Ortmanniana ligamentina is classified as Lampsilis. But just looking at one cladogram doesn’t take into account the diversity of results from previous studies. If you scroll through the Lampsilis Cladomics page, you will discover that clades representing the four listed above are relatively consistent among analyses, but the relationships among those clades are not. It may turn out that when the dust settles, our current concept of Lampsilis gets broken into multiple genera — and L. higginsii gets reclassified as Ortmanniana (Frierson, 1927; Haas, 1969)!

It is odd that, given the current mania for revising freshwater genera based strictly on molecular phylogenetic results, Lampsilis non-monophyly hasn’t already been sorted. Lampsilis has been recognizable and taxonomically stable for a long time. This may explain why there are so few available genus names available to apply in a new classification. One genus-group name that is available is Simpsonunio Starobogatov, 1970 for Lampsilis teres (+ anodontoides). In anticipation of future genus-splitting, we have selected Lampsilis (Simpsonunio) teres as the Mussel of the Month.