Dr. Rob Dillon, Coordinator





Friday, June 5, 2020

What is Melania edgariana?


If you follow the FWGNA blog on a regular basis, bless your heart, it is possible, indeed likely, that a terrible revelation has by now dawned upon you.  You have become ensnared in a series of essays on shell morphological variation in the pleurocerid fauna of the Tennessee/Cumberland that began back in August of 2019, eight episodes ago, with no end in sight.

And even as this soggy mat of dubious science, arcane history, and overwrought pontification has unrolled these many months, the supercilious weaver who labors today to augment its warp and weft expects you to remember threads now gone, and appreciate patterns as yet emerging. 

But he will deign to refresh your memory, briefly.  In August we heard the story of Captain S. S. Lyon, who in 1862 sent a sample of pleurocerid snails collected from Cumberland Gap to the eminent scientist Dr. Isaac Lea in Philadelphia.  Although Lea gave four new names to this collection of snails, the evidence available to us today suggests that only two species were actually present, best identified as Pleurocera simplex (Say 1825) and P. troostiana (Lea 1838).  And in Gap Creek, at least, their shell morphology seemed to vary together.  Both populations were dwarfed upstream and typical downstream, demonstrating the phenomenon we abbreviate CPP, for “cryptic phenotypic plasticity,”

In September and October, we learned that populations of P. simplex range throughout Tennessee and Kentucky, varying strikingly in their shell morphology, likely as a consequence of CPP.   And in December, January, April and May, we documented a similar but even more extreme phenomenon in populations of Pleurocera troostiana, inhabiting a big swath of the Tennessee drainage from SW Virginia through East Tennessee into North Alabama. Everywhere in this vast region we have seen simplex and troostiana living side-by-side, varying together.

Melania edgariana [1]
Now the main theme of our September post was this.  In the Cumberland River drainages of Middle Tennessee and Kentucky, populations of Pleurocera simplex inhabit larger rivers than they do in the Tennessee River drainages further east.  And correlated with this range extension, they bear shells that are larger, broader, and heavier than the P. simplex populations of East Tennessee drainages.  The middle Tennessee populations are so different from populations of East Tennessee that they have traditionally been identified by a different specific nomen, ebenum (Lea, 1841), and in some cases even a different genus, Lithasia.

Might the range of P. troostiana also extend through the Cumberland drainages of Middle Tennessee into Kentucky as well?  Perhaps shifting shell morphology in parallel with P. simplex, demonstrating similar levels of CPP?

Yes.  In 1841 Isaac Lea published a brief, Latinate description of “Melania edgariana,” with full description and figure in 1843, sent to him from “Cany Fork, Tenn.” by Mr. S. M. Edgar [1].  This must be a misspelling of Caney Fork, the tributary of the Cumberland River from which we extracted the Pleurocera simplex populations we studied in September.   Lea described the shell of edgariana as “spire elevated,” and enthused “it is remarkable for being folded and transversely striate on all the whorls.”

In 1873 G. W. Tryon [2] synonymized edgariana under Goniobasis nassula, which T. A. Conrad had described from the Big Spring at Tuscumbia, Alabama, in 1834 [3].  Tryon said, and I quote verbatim: “Mr. Lea agrees with me that his Edgariana is a synonym of nassula.”  Lea despised Conrad [4].  Wouldn’t you have loved to be a fly on the wall when Lea and Tryon had their conversation about edgariana?

Goodrich disagreed with Tryon, however, removing Goniobasis edgariana from under the synonymy of Conrad’s G. nassula, suggesting for the range of edgariana “Streams of Cumberland, Duck and Elk Rivers, Tennessee” and listing 13 synonyms underneath it [5].

And as usual, I’m with Goodrich.  I really think that the population of pleurocerids inhabiting Tuscumbia’s Big Spring is something else.  The biological situation at Tuscumbia’s Big Spring is even more fascinating than the situation at Huntsville’s Big Spring, and I feel sure we will come back to Tuscumbia in some future post.  But for now, we will stipulate that Lea’s edgariana and Conrad’s nassula are two entirely different things, and that’s all I’ve got to say about that.

So back to the Caney/Collins River system.  It will be remembered from our September post that populations of Pleurocera simplex become progressively more robust and heavily-shelled from headwater tributaries on the west slope of the Cumberland Plateau north toward the confluence of the Caney with the Cumberland River at Carthage, TN.  I’ve taken a slice from the map I published in September and expanded it below, retaining simplex sites J, K, and L.  

(J) Collins River, (I) Center Hill Lake, (H) Caney Fork
The cryptic phenotypic plasticity demonstrated by populations of P. simplex in the Caney/Collins is especially important for the argument I’m preparing to advance below.  So if my September post is not vivid in your memory, I’d be gratified if you opened this link [4Sept19] in a separate window and reviewed the material featured therein.  We will wait for you.

Are you back?  Good.  Going forward now.

Crawling around on the rocks with P. simplex in the Collins River at site J (35.5874, -85.6994) is a population of pleurocerids bearing shells with elevated spires, folded and transversely striate, as depicted at far left in the figure below.

This must be Isaac Lea’s Melania edgariana.  The snails bearing shells such as depicted in figure (J) below are occupying the same habitat as topotypic troostiana in East Tennessee, and all of those troostiana synonyms we reviewed in January, and topotypic perstriata in North Alabama, and all of those perstriata synonyms we reviewed in May.  In the company of P. simplex, they are grazing on the rocks of a small stream coursing down off the Cumberland Plateau.

There are two differences between edgariana of the Cumberland drainage and troostiana of the Tennessee drainage.  One is that, just as is the case with simplex, the edgariana populations of the Caney/Collins system extend much further downstream than the troostiana of East Tennessee and North Alabama.  My biological intuition suggests to me that a combination of current speed, substrate, temperature, and oxygenation is involved, such that the Caney/Collins, and many other rivers of Middle Tennessee and Kentucky, retain more of their upstream character downstream.  They look like big trout streams, not bass rivers.

In response to common environmental conditions, the shell morphology demonstrated by freshwater gastropod populations may vary together.  One of the more interesting papers to pass across my desk in recent years reports a shell morphological study conducted by Kistner and Dybdahl [6] in the Snake River of Idaho [7].  The authors demonstrated that generalized Procrustes variance in populations of the (native) Pyrgulopsis robusta and the (introduced) Potamypyrgus antipodarium varied in parallel, apparently as a function of current speed.

(J) Collins R, (I) Center Hill Lake, (H) Caney Fork
So the only other distinction between the edgariana of the Cumberland drainage and the troostiana of the Tennessee is the degree of shell costation – strong for edgariana, weak for subspecies perstriata in North Alabama, and absent for the typical subspecies in East Tennessee.  We devoted some considerable fraction of our April post to research on shell costation or plication.  Intraspecific variation in that trait has been well-documented – sometimes apparently heritable, sometimes not – but intraspecific, in any case. 

I cannot find any evidence counter to the hypothesis that Melania edgariana (Lea 1841) of the Cumberland is a junior synonym of M. troostiana (Lea 1838) of the Tennessee.  But let us save Lea’s name “edgariana” as a subspecies to describe populations of P. troostiana bearing strongly costate shells, shall we?  Again, I am duty-bound to remind my readership that we here in the FWGNA project define the term “subspecies” in its original, classical sense, and to point you to my essays of February and March, 2014, for elaboration [8].

I do not want to nominate the population of edgariana at site J as topotypic, however, since it inhabits the Collins River, and Lea specified “Cany” Fork.  So further downstream, in 1948 the US Army Corps of Engineers built Center Hill Dam near Smithville, and impounded the Caney Fork upstream 64 miles, almost to its junction with the Collins River.  Most unexpectedly, to me in any case, several pleurocerid populations, including edgariana, have survived the impoundment [9].  The shell labelled (I) in the figure above was collected during the winter draw-down from a population inhabiting the sandy shallows of Center Hill Lake at the Floating Mill Recreation Area (36.0460, -85.7620).

But again, I hate to restrict an 1841 type locality to a 1948 impoundment.  So the Caney Fork picks up again below the Center Hill Dam and flows another 25 miles to its mouth at The Cumberland River.  The shell labelled (H) in the figure above was sampled at the TN 264 bridge (36.1816, -85.9081), a spot I suggest as the type locality for Pleurocera troostiana edgariana by virtue of ease of access, as well as historical fidelity.

As Goodrich [5] suggested, Pleurocera troostiana edgariana populations seem to range widely across the Cumberland drainage of TN/KY, including the Stones, Harpeth, and Obey Rivers as well as the Caney/Collins.  And indeed, several of the larger tributaries of the Tennessee River in Middle Tennessee also host edgariana populations, including the Duck [10] and the Elk.  We depicted a specimen from the Elk River at Kelso (site “G”) last month, as a teaser for this month’s essay.

Well, I do appreciate the forbearance of my rapidly-dwindling readership as yet another soggy length of malacological tapestry has rolled off the FWGNA loom, dripping with arcana and arrogance.  Next month we’ll finish up where we started, back in Kentucky with Captain Lyon, and summarize, at long last, I promise.


Notes:

[1] These were the same publications in which Lea described the M. ebenum about which we obsessed in October, and the M. teres and M. strigosa we mentioned in January:
  • Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings o the American Philosophical Society 2: 11 – 15.
  • Lea, Isaac (1843)  Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society 8: 163 – 250.
[2] Tryon, G. W. (1873)  Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

[3] Conrad, T.A. (1834)  New fresh water shells of the United States, with coloured illustrations, and a monograph of the genus Anculotus of Say.  Also a synopsis of the American naiades.  Judah Dobson, Philadelphia.  76 pp.

[4] For all we know about the Lea/Conrad relationship, see:
  • Isaac Lea Drives Me Nuts [5Nov19]
[5] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.

[6] Kistner, E.J., and M.F. Dybdahl.  2014.  Parallel variation among populations in the shell morphology between sympatric native and invasive aquatic snails.  Biological Invasions  16(12):2615-2626.

[7] We devoted a significant number of column inches to the Pyrgulopsis robusta populations of the Snake River some years ago.  See:
  • Idaho Springsnail Showdown [28Apr05]
  • Idaho Springsnail Panel Report [23Dec05]
  • When pigs fly in Idaho [30Jan06]
  • FWS Finding on the Idaho Springsnail [4Oct06]
[8] Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits.  For elaboration, see:
[9]  In addition to P. troostiana edgariana, the pleurocerid species inhabiting the margins of Center Hill Reservoir include P. canaliculata, which does not surprise me, and P.laqueata alveare, which is really rather interesting.  But no, I have not seen any P. simplex in the shallows of Center Hill Lake.  Why not?  We may come back to this.

[10] According to van der Schalie (Sterkiana 52: 45 - 56), in 1931 Goodrich recorded Goniobasis edgariana way up the Duck River at Manchester.  He did not apparently find edgariana, or any pleurocerid populations that looked anything like troostiana, anywhere else in the Duck drainage.  But I am  not sure how well Goodrich surveyed the smaller tributaries.