Dr. Rob Dillon, Coordinator





Wednesday, April 15, 2020

Huntsville Hunt

Editor’s Note – After a couple months of digression into other topics, today we return to phenotypic variety and taxonomic chaos in the pleurocerid fauna of the Tennessee/Cumberland.  I have written six essays in the series thus far, from August 2019 to January 2020.  But I’d especially recommend that you review my December and January posts on P. troostiana before going forward.

Variation in the number and strength of shell plicae (or costae), those little scallop-shaped folds wrapped around the whorls of diverse gastropods worldwide, no different from variance in every other character of the phenotype of every other creature that walks, creeps, swims or flies on this earth, has both genetic and environmental components [1].  It is always important to remember both.

Calvin Goodrich dedicated Number III in his “Studies of the gastropod family Pleuroceridae” to the phenomenon of shell plication in 1934 [2].  He documented variation in an impressive list of 21 species of Goniobasis, including G. arachnoidea (a junior synonym of troostiana), generally correlating plicate shell sculpture with upstream/downstream environmental gradients or with broadly-regional pattern. 

Misako Urabe [3] reported evidence that the strength of the shell costation (plication) developed by sibships of Korean Semisulcospira was a function both of the phenotype of the mother and the coarseness of the substrate upon which her offspring were raised, sand promoting costation more than cobble.  Taking inspiration from the work of both Goodrich and Urabe, I have argued that populations of Pleurocera catenaria from Atlantic drainages of The Carolinas varying dramatically in their shell plication be accorded subspecific status, regardless of the origin of the trait [4].
 
Melania perstriata [7]
In any case, none of the populations of P. troostiana we discussed in January, under any synonym of that species, bore plicate/costate shells, under any synonym of that shell character.  Nor indeed, does any population of any pleurocerid species anywhere in East Tennessee demonstrate any shell plication whatsoever [5].  If you set off on a voyage down the Tennessee River from headwaters in Virginia to heart in Alabama, you would pass thousands of populations of pleurocerid snails, comprising 16 species.  But the first plications you would see on the shell of any pleurocerid waving at you as you passed would pop up around the westward bend of the river at Chattanooga, where its dip begins into North Alabama.  Those would be the easternmost populations of Pleurocera laqueata [6], a chunky-shelled inhabitant of small rivers and mid-sized streams, widespread in Middle Tennessee but strangely absent further eastward.

And for some reason – perhaps regional substrate – the pleurocerid populations bearing slender, striate shells inhabiting the smallest tributaries of the Tennessee west of Chattanooga also begin to demonstrate plication, only where their range overlaps the characteristically-plicate P. laqueata.

So in 1853 Isaac Lea described and figured a pleurocerid from Alabama which he called Melania perstriata [7].  He focused on its striate shell, acutely conical with elevated spire, drawing no distinction between his new species and any of the pleurocerid species bearing striate shells and elevated spires he had previously described in East Tennessee.  Indeed, he observed that his new perstriata was “strongly allied” to his striatula (1841/43), which we discussed in January.  Although Lea did not mention any plication on the shell of perstriata in his description, his figure (reproduced above) shows light but distinct plicae on the upper whorls.

Lea gave the habitat of his Melania perstriata as “Coosa River, Alabama, Prof. Brumby.  Huntsville, Tenn., Mr. J. Clark.”  I feel certain that Lea meant Huntsville, Alabama, not Huntsville, Tennessee, for reasons that will become obvious shortly.  Tryon [8] passed Lea’s figure and description along uncritically as “Goniobasis” perstriata.

One of Calvin Goodrich’s best least-known works was his (1930) “Goniobases of the vicinity of Muscle Shoals” [9], by which he meant North Alabama, generally.  Here are the first two sentences of that important contribution: 
“Nine species and two subspecies of Goniobasis are recognized in this paper as inhabiting the vicinity of Muscle Shoals, Alabama.  I have not had the heart to count the names it has been thought necessary to throw into synonymy, always a slough of despond in the case of the Pleuroceridae.” 
Amen, Brother Calvin!  That second sentence could summarize much of my professional career.

Goodrich began his treatment of Goniobasis perstriata with several paragraphs of detailed observations on the shell morphology of the population inhabiting Big Spring Creek in Huntsville, which he said is “apparently the type locality.”  I am confident that Goodrich was correct about this.  The “Mr. J. Clark” credited by Isaac Lea for the type collection was quite likely a gentleman named Joseph Clark, the President of Huntsville, Alabama from 1844 to 1849.  And where else would the President of Huntsville collect shells to pack off to the Eastern Scientific Establishment than the large and impressive spring around which his city was built, the historic site of one of the earliest waterworks in America?
Goodrich’s observations on the “average specimen” from Big Spring Creek [10] matched Lea’s description and figure very well, focusing on the “slender, delicate” shell with striking sculpture consisting of “low plicae crossed by strong revolving lines.  The plicae disappear on the spire and the striae continue to the base.”  But to my eye, the most intriguing of Goodrich’s observations about the Big Spring Creek population are not about the “average specimen,” but rather: 
“A study of the variation of perstriata takes one so far afield that some of the original characteristics seem altogether lost and to be replaced with new features.  In Big Spring Creek there have been taken specimens that are only microscopically striate, some that show no plicae and others that are smooth and shining upon the last whorl.” 
Ten years later, based on his observations of the North Alabama pleurocerid fauna generally, Goodrich [12] dropped Lea’s “Coosa River” suggestion and expanded Lea’s “Huntsville” suggestion to “Springs and small streams of North Alabama.”  He then recognized three subspecies, all from Alabama tributaries of the Tennessee River: the typical G. perstriata perstriata (Lea 1853), G. perstriata crispa (Lea 1862), and G. perstriata decampii (Lea 1863). 

“Elimia” perstriata was accorded “Priority 1” conservation concern in the state of Alabama by Garner and Johnson in the big 2017 review volume edited by Shelton-Nix [13].  As a locality Garner & Johnson suggested, “Extant only in a few streams in Madison and Lawrence counties.”  Yes, modern-day Huntsville does indeed sprawl across much of modern-day Madison County.
Madison Co, AL. See note [15] for locality data
So on Monday morning March 9 I launched an expedition to North Alabama, resolving to visit as broad a sample of springs, streams, and small tributaries of the Tennessee drainage across the ten-county region as practically possible, focusing especially upon the environs of Huntsville.  I had mighty good weather as a general thing, and nothing ever happened to me at all [14].

And on Tuesday and Wednesday I was able to document six populations of pleurocerids under the bridges of Madison County, Alabama [15], in both rural and urban settings, bearing shells “acutely conical with elevated spire,” varying rather strikingly in both their striation and their plication, as figured below.  The shells most closely matching Lea’s (1853) figure were indeed borne by the population inhabiting the waters of the Big Spring of Huntsville, marked (P) on the map above.

Today the Huntsville Big Spring emerges from the rocky face of a small hill in the middle of the city and runs about a half-mile through a recently-renovated park to join Pinhook Creek.  The first couple-hundred yards of the stream are somewhat green and shady; the remainder of the course is entroughed in massive concrete bulkheads, thoroughly urbanized, and infested with gigantic koi of elaborate morphology and voracious habit.  But right at the mouth of Big Spring Creek, where it cascades down to join Pinhook Creek, I found a population of pleurocerids inhabiting what I here offer as the type locality of Melania perstriata (Lea 1853).

Madison County P. troostiana populations [15]
The shells borne by the perstriata population at its type locality (P) are entirely striated, bearing light but distinct plicae that tend to become obsolete with growth.  That is generally true upstream in Pinhook Creek (V) and at Ashburn Spring south of Huntsville (Z) as well.  But the shells borne by populations in the Flint River (W) and at Burns Spring east of Huntsville (Y) are only weakly striate or plicate, on the upper whorls alone.  And in Limestone Creek (U) no striation nor plication is detectable on the shells whatsoever.  To borrow Goodrich’s poetic imagery, they were “smooth and shining upon the last whorl.”  Compare Limestone Creek shell morphology with that demonstrated by the P. troostiana population inhabiting Gap Creek, which kicked off this long series way back in August of 2019 [16].

I cannot find any biological evidence counter to the hypothesis that perstriata (Lea 1853) is a junior synonym of troostiana (Lea 1838).  But let’s save perstriata at the subspecific level to describe lightly-plicate/costate populations of Pleurocera troostiana, shall we?  Remember that we have defined the word “subspecies” to mean “populations of the same species in different geographic locations, with one or more distinguishing traits.”  Those words mean exactly what they say, nothing less and certainly nothing more [4].  The relationship between typical troostiana and its subspecies perstriata in Alabama is the same as that between typical catenaria and its subspecies dislocata in the Carolinas.  The FWGNA database will show that sites P, V, W, Y and Z are inhabited by Pleurocera troostiana perstriata, and while site U hosts typical P. troostiana troostiana.

So what about those other species that Goodrich shifted underneath perstriata, Lea’s crispa of 1862 and decampii of 1863?  Indeed, weren’t doctors, lawyers, and Indian chiefs from all over small-town America, spurred by nineteenth-century civic pride, plucking gastropods from their local springs and packing them off to the great Isaac Lea in Philadelphia?  Why yes, they were.

But storm clouds were gathering over the fertile fields of American malacology.  In our next installment... Crisis!


Notes:

[1] For an in-depth review of the heritability of shell morphology, albeit in pulmonates, see:
  • The heritability of shell morphology in Physa h^2 = 0.819! [15Apr15]
[2] Goodrich, C. (1934)  Studies of the gastropod family Pleuroceridae – III.  Occasional Papers of the Museum of Zoology, University of Michigan 300: 1 – 11.

[3] Urabe, M. 2000. Phenotypic modulation by the substratum of shell sculpture in Semisulcospira reiniana (Prosobranchia: Pleuroceridae). J. Moll. Stud. 66: 53-59.  For more, see:
  • Semisulcospira research: A message from The East [6Jan08]
  • Semisulcospira II: A second message from The East [1Feb08]
[4] To refresh your memory on the situation with Pleurocera catenaria dislocata, and the concept of the subspecies as we use it in the FWGNA project, See:
  • What is a subspecies [4Feb14]
  • What subspecies are not [5Mar14]
[5]  Here’s a memo from the FWGNA Exception-that-Proves-the-Rule Department.  To be absolutely complete, we should note that the shells of the pleurocerid population inhabiting the Hiwassee River as it flows northwest down the mountains of East Tennessee do in fact bear dramatic costations.  This is actually a trans-Appalachian population of the Atlantic-drainage species Pleurocera catenaria.

[6] The degree of costation also varies dramatically along an upstream/downstream gradient in populations of Pleurocera laqueata as well.  See:
  • Pleurocera alveare: Another case of CPP? [8Aug18]
[7]  Melania perstriata was listed without description by Lea in 1852 (Proceedings of the American Philosophical Society Volume 5, page 252).  The species was formally described and figured the next year:
Lea, Isaac (1853)  Description of a new genus (Basistoma) of the Family Melaniana, together with some new species of American Melaniae.  Transactions of the American Philosophical Society (new series) 10: 295 – 302.

[8] Tryon, G. W. (1873)  Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.  For more about the relationship between Lea and Tryon, see:
  • Isaac Lea drives me nuts [5Nov19]
[9] Goodrich, C. (1930)  Goniobases of the vicinity of Muscle Shoals.  Occasional Papers of the Museum of Zoology, University of Michigan 209: 1 – 25.

[10] Goodrich’s observations were based both on the extensive collections of Dr. Bryant Walker [11] and on those of the Alabama Museum of Natural History.  He especially thanked “Mrs. Herbert H. Smith who made the selection of shells of the Alabama Museum, packed and dispatched them.”

[11] Walker’s collection was donated to the University of Michigan in 1936.  For a tribute, see:
  • Bryant Walker’s Sense of Fairness [9Nov12]
[12] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system.  Occasional Papers of the Museum of Zoology, University of Michigan  417: 1-21.

[13] Garner, J.T. & P. Johnson (2017) Freshwater Snails (Gastropods).  pp 7 – 42 in Shelton-Nix, E. (ed) Alabama Wildlife, Volume 5. The University of Alabama Press, Tuscaloosa. 355 pp.

[14] A tip of the straw hat to Huckleberry Finn, Chapter 12.

[15] Pleurocera troostiana populations in Madison County, AL:
  • P = Type locality of M. perstriata. Big Spring outfall, Huntsville 34.7246, -86.5915
  • U = Limestone Ck.  34.9199, -86.7645
  • V = Pinhook Ck. 34.7752, -86.5915
  • W = Flint R.  34.8228, -86.4832
  • Y = Burns Spring  34.7696, -86.4258
  • Z = Ashburn Spring  34.5249, -86.5132
[16] For a quick refresher on phenotypic plasticity in P. troostiana:
  • CPP Diary: Yankees at The Gap [4Aug19]

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