Editor’s
Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera alveare: Another case of CPP? Pp 109 - 112 in The Freshwater Gastropods of North America Volume 3, Essays on the
Prosobranchs. FWGNA Press, Charleston.
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 [5] 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 [6].
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 [2].
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
[7], 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.
Notes
[1] 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]
[7] For more on the subspecies concept:
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