Mussel of the Month

Tritogonia verrucosa as the Mussel of the Month ends a long-running discussion we have had on this web site regarding how to revise the genus-level classification of the species formerly classified under the genus Quadrula sensu lato.

Back in the day (e.g., Williams et al., 1993), there were there three genera in this story: Quadrula (several spp.), Cyclonaias (1 sp.), and Tritogonia (1 sp.). But in 2003, it was discovered that T. verrucosa was actually nested within a clade otherwise composed of Quadrula s.l. species (Serb et al., 2003). One way to solve this problem of “taxonomic-load” was by simply classifying T. verrucosa as a species of Quadrula s.l. (Willams et al., 2008).

In our global checklist (Graf & Cummings, 2007), we went another direction. Serb et al. (2003) had recovered clades of the traditional subgenera used to divide up Quadrula s.l. Rather than omitting all that phylogenetic information from the genus-level classification, we simply elevated the long recognized (e.g., Frierson, 1927; Thiele, 1934) subgenera to genera: Quadrula s.s., (type: Q. quadrula), Amphinaias (type: A. couchiana), and Theliderma (type: T. metanevra). This left Tritogonia intact as a monotypic genus.

Our checklist revision wasn’t perfect. Since we were endeavoring to base our taxonomy on the published record and not make novel revisions, we followed Frierson (1927), Haas (1969), and others to circumscribe species into genera. Unio couchianus Lea, 1860 was considered by those authors to be in the same genus as what had been known as Quadrula pustulosa, so the Pustulosa-group became Amphinaias. However, in reality, Q. couchiana seems to be affiliated with Q. quadrula (although no phylogenetic work has been done). The genus-level names Pustulosa and Bullata are available for that group of species.

But then there was a new wrinkle. Campbell & Lydeard (2012) and others showed that the monotypic Cyclonaias belongs among the Pustulosa-group, and reclassification of C. tuberculata to the old Amphinaias secured the name Cyclonaias for the whole clade.

Recently, the four-genus system — Quadrula, s.s., Cyclonaias, Theliderma, and Tritogonia — was codified by Williams et al. (2017), and we have updated the classification on this web site. Now we just need to solve the problem of Uniomerus being a quadrula...