CPP Diary: The spurious Lithasia of Caney Fork

The Duck River is our Galapagos, and Calvin Goodrich our Darwin.  Or possibly our inverse-Galapagos, and our anti-Darwin, I’m not sure.  For the brilliant evolutionary insight that Goodrich glimpsed in 1934 through the lens of this rich, fresh waterway coursing through the heart of middle Tennessee was not more species, but less.

It’s an old, old story [1], but let’s tell it again.  Prior to the dawn of the modern synthesis, North American freshwater malacology recognized at least 12 – 15 species of pleurocerids in the Duck River, probably more.  The lower reaches of the Duck (e.g., Wright Bend site E) were inhabited by heavily-shelled, “obese” populations identified as Lithasia geniculata, distinguished by their shells with prominently-shouldered whorls.  The slightly-less-obese, smooth-shouldered populations of the middle reaches (e.g., US41A, at site C) were identified as Lithasia fuliginosa.  And the headwaters of the Duck River (e.g., Old Fort site B) were inhabited by Anculosa pinguis, lightly-shelled populations without any shoulders on their whorls at all.

CPP in the Lithasia geniculata of the Duck River

In 1934, Goodrich published his “Studies of the Gastropod Family Pleuroceridae I,” in which he synonymized all these forms as subspecies under L. geniculata [2].  He just did it, at the top of the section, without making any sort of declaration, or using any form of the noun “synonym,” as though his unique insight were already an article of established malacological doctrine [3].  He then meticulously documented, town to town and bridge to bridge down the length of the Duck, the gradual transition of the three subspecies from one form to the next.

Even to me, his latter-day apostle, Goodrich’s 1934 intuition about the plasticity of shell phenotype in freshwater gastropods was startlingly profound.  His “Studies in the Gastropod Family Pleuroceridae” series inspired me to coin the term “Goodrichian taxon shift” in his honor in 2007, subsequently generalized to cryptic phenotypic plasticity (CPP) by Dillon, Jacquemin and Pyron in 2013 [4].

So in 1940, Goodrich reviewed the taxonomy of the pleurocerid fauna of the Ohio River drainage in its entirety, not just the Lithasia of the Duck River but all species in all seven genera in a vast region touching 14 states [5].  And this is the range he gave for Lithasia geniculata pinguis: “Caney Fork and branches; Duck River, Coffee County, Tennessee.”  That range was transferred verbatim by Burch [6] onto page 160 of his North American Freshwater Snails and entered the gospel.

Goodrich’s 1940 suggestion that Lithasia geniculata of the pinguis form inhabits Caney Fork as well as the Duck River certainly seemed plausible.  The Caney (and its primary tributary, the Collins River) is physically quite similar to the upper regions of the Duck, the headwaters of the two systems interdigitating west of McMinnville.  The Caney/Collins then flows north to the Cumberland, as the Duck flows west to the Tennessee.

So in 2003 our colleagues Russ Minton and Chuck Lydeard undertook to construct a gene tree for the North American genus Lithasia [7].  And they did a good job rounding up samples from 11 of the species and subspecies of Lithasia listed by Goodrich/Burch, 25 populations in all, sequencing in some cases as many as 6 individuals per population.  From the Duck River Russ and Chuck sequenced one population of L geniculata geniculata (1 individual), three populations of L. geniculata fuliginosa (1, 3, and 3 individuals), and one population of L. geniculata pinguis (6 individuals).  And they also included 2 individuals of nominal Lithasia geniculata pinguis from a Caney/Collins population.  And here’s their gene tree:

Minton & Lydeard [7] Figure 3, modified.

By now my readership will understand gene trees are dependent variables, not independent variables [8].  You cannot work out the evolution of a set of organisms from a gene tree.  But if you have developed an evolutionary hypothesis from good solid data of some broader sort, you may be able to understand what a gene tree is telling you.

To completely unpack the message being telegraphed to us by the enigmatic arboreal specimen figured above would require at least 6 – 8 blog posts of standard length [9].  But for the present let us focus on just the two little branches labelled “geniculata pinguis” that I have circled in red.  The two sequences obtained from the 6 individuals sampled from the Duck River, D1 and D2, cluster with all the other Lithasia.  And the two sequences obtained from the Caney/Collins system, C1 and C2, are way off with pleurocerids of other genera.  To quote Minton & Lydeard verbatim: “Further work needs to be undertaken to determine the identity and placement of the Collins River taxa.”

Thanks, Captain Obvious!  Let’s back up about six decades.  If Calvin Goodrich had enjoyed access to collections from the Caney/Collins system of the same quality and detail that he enjoyed for the Duck, he would have found a gradual phenotypic progression in the direction illustrated by the figure below.

Most of the headwaters of Caney Fork and its tributaries (e.g., site J) are inhabited by rather typical-looking populations of the widespread Pleurocera simplex simplex.  Pleurocera simplex ranges broadly all over East Tennessee, extending into SW Virginia and well up into Kentucky.  In drainages of the Tennessee River above Chattanooga, such as the Holston around Saltville from whence it was described by Thomas Say in 1825 [11] and Pistol Creek at Maryville [12] and Gap Creek at Cumberland Gap [13], darkly-pigmented populations of P. simplex bearing gracile, teardrop-shaped shells are quite commonly encountered in small creeks and streams of non-negligible groundwater content.  They do not, however, extend into larger rivers [14].

CPP in the P. simplex of Caney Fork

But in tributaries of the Cumberland, Kentucky, and Green Rivers, Pleurocera simplex populations often do extend into rivers of substantial size – as long as the rocky substrate they require does not entirely give way to mud.  Here their shells become heavier, chunkier, and more lightly-pigmented.  Goodrich [5] identified paler, heavier-shelled populations such as are found in the Collins River at site K as “Goniobasis ebenum (Lea 1841).”  And in the main Caney Fork (site L), Goodrich identified them as Lithasia geniculata pinguis.

Thus, the brilliant scientist who first recognized the phenomenon that we today call cryptic phenotypic plasticity in the Lithasia geniculata population of the Duck River in 1934 clean missed it 12 miles east in the Pleurocera simplex population of Caney Fork six years later.  The key, I think, is the sample coverage.  The Duck River Valley is rich farmland dotted with small towns networked by roads, while the Caney/Collins is more rugged and remote.  Goodrich simply did not have access to adequate collections from the Caney Fork Valley.

So our attention is now called to the enigmatic middle-taxon described by Isaac Lea in 1841, Pleurocera (aka Melania, aka Goniobasis, aka Elimia) ebenum.  Goodrich [5] identified ebenum populations through most of the Cumberland River drainage, from “Cumberland River above the falls” through “Smith’s Shoals, Pulaski County, Kentucky” west beyond Nashville to “springs and small streams” in Dickson County, Tennessee.  Are all these Pleurocera ebenum populations just pale, triangular, robustly-shelled P. simplex?  Stay tuned.

Notes

[1] The best entry into this literature would be to purchase FWGNA Volume 3 [html] and read pages 1 – 10 and 93 – 99.  Or you could click through it piecemeal:

• The Legacy of Calvin Goodrich [23Jan07]
• Goodrichian Taxon Shift [20Feb07]
• Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]

Both of those latter two essays feature scans of Goodrich’s Plate 1, showing the Duck River Lithasia.

[2] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae - I. Occasional Papers of the Museum of Zoology, University of Michigan 286:1 - 17.

[3] This is actually a bit frustrating.  Looking back on Goodrich’s body of work, there is almost never anything quotable – some “Aha moment” where the fullness of his vision is revealed.  Like Charles Darwin.

[4] 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]  For more, see:

• Pleurocera acuta is Pleurocera canaliculata [2June13]
• Pleurocera canaliculata and the process of scientific discovery [18June13]

[5] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system. Occasional Papers of the Museum of Zoology, University of Michigan 417: 1 - 21.

[6] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[7] Minton, R.L. & C. Lydeard (2003)  Phylogeny, taxonomy, genetics, and global heritage ranks of an imperiled, freshwater snail genus Lithasia (Pleuroceridae).  Molecular Ecology 12: 75 – 87.

[8] The best entry to this complex and long-running theme would be to read FWGNA Volume 2 [html] in its entirety.  Or for a quick lick at the problem, see my essays:

• Gene trees and species trees [15July08]
• What is a species tree? [12July11]

[9] Actually, I’ve already dedicated one blog post [10] to exegesis of the obovata1/obovata2 branch way down below the Caney Fork sequences.  And maybe in a few months we’ll come back to the geniculata/fuliginosa/duttoniana problem in the Duck River and look at that in more detail.

[10] Dillon, R. T. (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [pdf] For more, see:

• Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]

[11] Say, Thomas (1825)  Descriptions of some new species of freshwater and land snails of the United States.  Journal of the Academy of Natural Sciences of Philadelphia 5: 119 – 131.

[12] I explored the complex relationship between Pleurocera simplex and P. gabbiana in East Tennessee in a series of three blog posts in 2016:

• The cryptic Pleurocera of Maryville [13Sept16]
• The fat simplex of Maryville matches type [14Oct16]
• One Goodrich missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]

[13] We opened our series on CPP in Pleurocera simplex last month with:

• CPP Diary: Yankees at The Gap [4Aug19]

[14] Rocks and riffles in the mid-sized rivers in East Tennessee are often covered bank-to-bank by dense populations of Pleurocera clavaeformis and Leptoxis praerosa, neither of which ranges into drainages of the Cumberland.  I wonder if East Tennessee populations of P. simplex are restricted to smaller creeks and streams by grazing competition?