Mussel of the Month

Amblema plicata, a former Mussel of the Month from way back in August 2006, is the type species* of the genus Amblema. Amblema elliottii, this month’s mussel, is a closely related species endemic to the Coosa and Cahaba rivers (Mobile Basin) in the southern USA. A. neislerii is the third species of the genus, and the analyses that have tested the phylogeny of the genus Amblema have recovered it as monophyletic (Mulvey et al., 1997; Inoue et al., 2020, Patterson et al., 2021).

The current conception of the genus Amblema — limited to these three species — has changed over time. Simpson (1914) and Frierson (1927) included all the Nearctic species with plicate (i.e., pleated) shellstogether in a single genus, but these have since been split into genera that we now recognize as Megalonaias, Plectomerus, and Amblema. Haas (1930) included species of the Central American genus Psoronaias (+Psorula) in Amblema, and if we drag in all the species originally included by Rafinesque, we find amblemas scattered in Epioblasma, Reginaia, and Obovaria.

Amblema is the type genus of family-group taxa: the tribe Amblemini and the subfamily Ambleminae. The Ambleminae is by far the most species-rich subfamily of the Unionidae. It accounts for 41% of the species and 34% of the genera of the family. Moreover, almost all phylogenetic analyses have recovered the Ambleminae as monophyletic. The studies that didn’t tend to be ones with insufficient character sampling to fully resolve the relevant clades.

Whereas the subfamily Ambleminae rests on about as solid of a phylogenetic foundation as we could ask for, the tribe Amblemini is a phylogenetic house made of straw. The tribe consists of three genera — Amblema, Reginaia, and Plectomerus — and, their monophyly has largely been supported by phylogenomic approaches (Pfeiffer et al., 2019; Smith et al. 2020). Traditional molecular phylogenetic methods have given mixed results.

Even though Amblema is among the best studied freshwater mussel genera in the world, there is still more work to be done to nail down its relatives and the synapomorphies that illustrate its evolutionary history and diversification.

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* Actually, Amblema costata Rafinesque, 1820 is the type species of the genus Amblema, but A. costata is widely regarded as a junior synonym of Amblema plicata.