Mussel of the

Page last updated
6 March 2024

Mussel of the Month

The March 2024 Mussel of the Month is Pustulosa infucata. Pustulosa is a genus of nine species from eastern North America.

Pustulosa infucata
USNM 84037 Macon Co. Georgia, USA.

If you have been paying attention to freshwater mussel taxonomy for as long as we have, then you know that this month’s mussel has been on a long journey to get to the name Pustulosa infucata. It has been a part of so many of consequential events in the systematics of Nearctic freshwater mussels that P. infucata might as well be called Forrest Gump.

Unio infucata Conrad, 1834 was described in the genus Unio. As currently conceived, the native range of Unio is mostly confined to Europe, Africa, and Asia. But, in the olden days, almost every freshwater mussel with hinge teeth was called Unio.

Simpson (1900) was the first to meaningfully break-up the catch-all genera Unio and Anodonta. By “meaningful” we mean his revisions stuck and were the basis for the system of classification that emerged during the 20th century. Simpson (1900) classified "Quadrula" infucata in Quadrula. Back then Quadrula was worldwide in distribution and the bin for all thick-shelled bumpy freshwater mussels. Nowadays, Simpson’s Quadrula sensu lato includes not only Quadrula, Pustulosa and other genera from North America but also, Aculamprotula, Lamprotula, and Gibbosula from East Asia.

Ortmann & Walker (1922) tried to reconcile Simpson’s global system with the growing body of anatomic data for North American mussels. In general, they found that Simpson’s generic splitting didn’t go far enough — more genera were necessary to account for the diversity of soft-parts inside the shells. Ortmann & Walker (1922) created Quincuncina for Q. burkei and included "Q." infucata in that genus. Quincuncina was set aside to recognize the species that were tetragenous (brooding in all four demibranchs) with “subcylindrical placentæ” (conglutinates) like Fusconaia but with sculptured shells like Quadrula (which they viewed as having lanceolate or compressed conglutinates. Based on that combination of characters, "Quincuncina" infucata was the prevailing name for our Mussel of the Month for the next 80 years.

While Quincuncina passed largely unchanged from Ortmann & Walker to Frierson to Haas to Burch, the science of taxonomy was being revolutionized thanks to Watson & Crick, Frederick Sanger, Kary Mullis, Willi Hennig, and David Swofford. Those two beams of knowledge collided in Lydeard et al. (2000) and Campbell et al. (2005), and Quincuncina was broken apart. Some of it, including the type species, went to Fusconaia, and other species like "Q." infucata landed among “the quadrulas.”

The next two decades was spent trying to figure out the taxonomy of “the quadrulas.” Based on the phylogeny by Serb et al. (2003), there was a push for a single genus for all the quadrulas — i.e., Quadrula. As can be seen on the relevant Cladomics pages on this web site, these mussels have received more than their share of phylogenetic attention. In the most recent classification (Neemuchwala et al. 2023), we landed on four genera classified in the subtribe QUADRULINA: Quadrula (including the old Tritogonia), Theliderma, Cyclonaias, and Pustulosa.

Both the road to Pustulosa has been windy. It is evident from the recent taxonomic history of Pustulosa infucata that it took a while to figure out what its genus-level clade should be called. Quincuncina? Quadrula? Rotundaria? Amphinaias? Cyclonaias? Nope. For now anyway, it is Pustulosa infucata.


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily UNIONOIDEA Rafinesque, 1820
Family UNIONIDAE s.s.
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe QUADRULINI Ihering, 1901
Subtribe QUADRULINA s.s.

Genus Pustulosa Frierson, 1927

Species Pustulosa infucata (Conrad, 1834)

To find out more about the classification of Pustulosa and the phylogenetics of the Quadrulina, check out:

current tally

NSF icon MUSSEL icon
"Making the world a better place, one mollusk at a time."