I like Cumberland Gap. Daniel Boone discovered this hidden doorway through the Cumberland scarp in 1775, and decided to build a road through it, and found a new state on the other side, which he named Kentucky, in honor of his favorite recipe for fried chicken.
The Gap assumed tremendous strategic importance during the War Between the States, changing hands four times. Confederate forces under the command of Gen. Felix Zollicoffer abandoned it to Union forces under Gen. George W. Morgan in June of 1862, who was himself forced out by Gen. Edmund Kirby Smith’s confederates three months later. Elements of Smith’s army held the gap 12 months, surrendering it a second time in September of 1863.
|Detail from Capt. Lyon's Map |
It was during the first northern incursion of 1862 that Captain Sidney S. Lyon of the US Topographical Engineers arrived at Cumberland Gap, uninvited. Captain Lyon was immediately ordered by General Morgan to draft a map “showing the location of the works constructed by the enemy and those erected by the forces of the United States.” And at some point during the discharge of those duties Capt. Lyon happened to pass along Gap Creek, a lovely little stream of cold, clear water emerging from a spring above the town of Cumberland Gap, TN, and coursing freshly through its precincts at about point A. And there he alertly stooped to capture an entire squadron of pleurocerid snails, without firing a shot.
These rebel pleurocerids he dutifully posted back behind the lines to Dr. Isaac Lea at the Academy of Natural Sciences in Philadelphia. And in May of 1863, a scant nine months later, Lea  described four new species of Goniobasis “sent to me from Gap Creek and Spring by Capt. S. S. Lyon, U.S. Army,” as follows: Goniobasis aterina, G. cumberlandensis, G. porrecta, and G. vittatella.
The municipality of Cumberland Gap, Tennessee, certainly must have been a busy and exciting place during those years. But in 1889 the first of several railroad tunnels was blasted out of the mountains above the town, and an automobile tunnel added alongside in 1996, difficult though these engineering feats certainly are to envision, for those who have not seen them. So the pretty little town is today located down in a deep hole about a half mile below all modern arteries of commerce, as thousands of vehicles pass through tunnels high above, and nobody stops to consider the possibility that anybody might be living way down in there.
|P. "aterina" at Gap Creek|
I first visited the town of Cumberland Gap in 2006, in connection with a small grant from the Virginia Department of Game and Inland Fisheries to study several potentially endangered pleurocerids in Southwest Virginia. I found the rocks of Gap Creek covered with a strikingly high density of small, dark, eroded pleurocerids matching Lea’s figures of aterina, mixed with a smattering of small, dark, eroded pleurocerids matching Lea’s figures of porrecta and vittatella .
The allozyme data collected by John Robinson and myself  strongly suggested that Lea’s aterina was a (chubby, dwarfed) local population of the widespread Goniobasis (now Pleurocera) simplex, and that Lea’s nomina porrecta and vittatella were attached to a (not quite as chubby, but still dwarfed) local population of the widespread Goniobasis arachnoidea (now Pleurocera troostiana).
And in fact, had I sampled Gap Creek further downstream into Tennessee in 2006, the elaborate population genetic analysis undertaken by John Robinson and myself might well have been unnecessary. The figure below compares samples taken at Cumberland Gap (A) to samples taken from both populations at site B, approximately 5 km south at the state route 63 bridge. Here Gap Creek has slowed, and warmed, and taken on a richer character more typical of the East Tennessee Ridge and Valley Province. The populations of both P. simplex and P. troostiana under that bridge bear larger, more gracile shells of completely typical shell morphology.
The pleurocerid populations of Gap Creek display the phenomenon for which the term “cryptic phenotypic plasticity” (“CPP”) was coined in 2013. They demonstrate intrapopulation morphological variance so extreme as to prompt a (erroneous) hypothesis of speciation. Isaac Lea (and George Tryon right behind him, and Goodrich, and Burch) all thought that the eroded, dwarfed pleurocerid populations in the cold, clear, high-velocity headwaters of Gap Creek were different species than the populations in the richer waters downstream.
|CPP in P. simplex and P. troostiana of Gap Ck.|
In recent years the phenomenon of cryptic phenotypic plasticity has been shown very-nearly universal in the pleurocerid populations of the Eastern United States . Here in the columns of this blog I have documented CPP in Pleurocera clavaeformis, Pleurocera canaliculata, Pleurocera semicarinata, and Pleurocera laqueata . In the next several essays, I will extend such studies to include two of the most widespread pleurocerids in the southeast, P. simplex and P. troostiana. And perhaps lighten the burden with a few stories along the way? Stay tuned.
 Map of Cumberland-Gap and Vicinity laid down from Surveys, made by Capt. Sidney S. Lyon, acting Topographical Engineer, under Order of Genl. G. W. Morgan, commd'g. 7th Div., Army of the Ohio. Showing the location of the works constructed by the enemy and those erected by the forces of the United States. I myself have highlighted Gap Creek in blue.
 Lea, Isaac (1863) Descriptions of fourteen new species of Melanidae and one Paludina. Proceedings of the Academy of Natural Sciences of Philadelphia 15: 154 – 156.
 There are no pleurocerids matching Lea’s figure of cumberlandensis inhabiting Gap Creek as far upstream as Cumberland Gap today. Tryon  synonymized cumberlandensis under Goniobasis adusta. Goodrich  synonymized both adusta and cumberlandensis under the widespread Goniobasis (now Pleurocera) clavaeformis. And indeed, Gap Creek downstream at site B is inhabited by a P. clavaeformis population of typical shell morphology, as well as the P. simplex and P. troostiana populations that are the subject of the present essay.
 Tryon, G. W., Jr. 1873. Land and Freshwater Shells of North America. Part IV, Strepomatidae. Smithsonian Miscellaneous Collections 253, 435 pp. Washington, D.C
 Goodrich, C. 1940. The Pleuroceridae of the Ohio River system. Occasional Papers of the Museum of Zoology, University of Michigan 417:1-21.
 Dillon, R. T. & J. D. Robinson (2007a) The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey. Report to the Virginia Division of Game and Inland Fisheries. 25 pp. [pdf]
 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]
Dillon, R. T., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata. Hydrobiologia 709: 117-127. [pdf]
Dillon, R. T. (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata. Zoological Studies 53:31. [pdf]
 The most convenient entrance into this rather extensive literature would be to read essays 4, 12, 13, 16, 18 and 19 in: Dillon, R.T., Jr. (2019c) Essays on The Prosobranchs. Freshwater Gastropods of North America, Volume 3. FWGNA Press [html]. Or, if you’d prefer to click your way through it piecemeal:
- Goodrichian taxon shift [20Feb07]
- Mobile Basin III: Pleurocera puzzles [12Oct09]
- Pleurocera acuta is Pleurocera canaliculata [3June13]
- Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
- Pleurocera alveare: Another case of CPP? [7Aug18]
- Is Gyrotoma Extinct? [5Sept18]