As of 2018 three cases of cryptic phenotypic plasticity (CPP) have been documented in the pleurocerid fauna of eastern interior drainages: the Pleurocera acutocarinata/clavaeformis/unciale group , the Pleurocera pyrenellum/acuta/canaliculata group , and the Pleurocera semicarinata/livescens/obovata group . We have also speculated on this blog  regarding a similar situation in the Pleurocera carinifera/vestitum/prasinatum/foremani populations of the Coosa River. A correlation between shell robustness and stream size seems quite general in pleurocerid populations throughout North America, does it not?
One of the main lines of evidence by which all these cases were recognized has been a concordance between the distributions of upstream and downstream taxon sets. So, for example, in the Tennessee drainage the distribution of (nominal) P. pyrenellum in small creeks corresponds closely with the distribution of P. canaliculata in the main river. The easternmost edge of the pyrenellum range is approximately the same as the easternmost edge of canaliculata, both right around Knoxville. Given the continent-scale distributions of all three taxa in the group, canaliculata, acuta, and pyrenellum, such concordance hardly seems like a coincidence.
Pleurocerid populations bearing shells with the robust, triangular, gnarly phenotype identified as “Pleurocera alveare (Conrad 1834)” are, or at least were, widespread throughout many of the largest rivers of the US interior. A quick search of the online Global Biodiversity Information Facility  returns 408 occurrences, of which 156 are georeferenced, primarily in the Cumberland, the middle Tennessee, the Green, and the main Ohio Rivers, plus the White River of Arkansas. GBIF records any younger than 1955 total just 11, however, almost entirely in the Cumberland. And I have exactly two modern records in my FWGO database. Pleurocera alveare is nowhere near as common today as the world thinks that it is .
Historically, big-river populations of Pleurocera alveare bore shells like figure A above, from an undated lot in the US National Museum, locality given simply as “Kentucky.” The shells look a bit like the ornately robust populations of Pleurocera canaliculata still widespread in big rivers of the Ohio drainage today, but the costae around the apex are distinctive. The specimen depicted in Figure B above was collected by our good friend Martin Kohl from the Cumberland impoundment known as Dale Hollow Lake in 2006.
Pleurocera laqueata (Say 1829) is a common inhabitant of creeks and small rivers of the lower Ohio river system through the Green, Cumberland, and Tennessee drainages of Kentucky, north Alabama, and Tennessee west of Chattanooga. The shells born by P. laqueata populations in small streams are slender and characterized by costae on the upper whorls (Figure D above), becoming broader and more robust in medium-sized rivers, such as the Duck (Fig C).
Sampling east across Tennessee, the observation that P. laqueata populations disappear from small streams at Chattanooga, exactly where P. alveare historically disappeared from the main Tennessee River, is striking. And the gradients in shell morphology demonstrated by P. laqueata populations inhabiting long, gradually-growing reaches such as that in the Duck River, and the Obey River of the Cumberland, remind me very much of the P. canaliculata gradient we documented from the Wabash in 2013 .
It seems quite likely to me that populations long identified as “Pleurocera alveare” constitute another demonstration of cryptic phenotypic plasticity, Say’s (1829) nomen laqueata taking priority over Conrad’s (1834). But following FWGNA conventions , let’s save Conrad’s (1834) alveare as a subspecies, shall we?
And no, let’s not send up any flares regarding the apparent rarity of P. laqueata alveare in the big rivers of the central and southern United States today. I simply do not think that robust, gnarly shell phenotype, no matter how striking, is heritable.
 Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [PDF] For more, see:
- Pleurocera acuta is Pleurocera canaliculata [3June13]
- Pleurocera canaliculata and the process of scientific discovery [18June13]
- Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
- Mobile Basin III: Pleurocera puzzles [12Oct09]
- Freshwater Gastropod Databases Go Global! [26May09]
 For more on the subspecies concept: