Dr. Rob Dillon, Coordinator

Thursday, November 3, 2022

The SNHTHICACBW Marstonia 6: pachyta

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  The SNHTHICACBW Marstonia IV: pachyta.  Pp 253 – 260 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

Last time we reviewed the minor hydrobioid taxon Marstonia from its origin as a subgenus in 1926 through its 1977 promotion by Fred Thompson, its 1994 death at the hands of Bob Hershler, and its 2002 resurrection by the dynamic duo of Thompson and Hershler working together.  And we focused on a subset of Marstonia that Thompson called, “small narrow hydrobiids that have in common a carinate body whorl,” abbreviated “SNHTHICACBW.”  This subset includes M. letsoni from way up north, which we reviewed in [19Jan16] and [5Feb16], and which in retrospect, might have been numbered 1 and 2 in this series.  And the subset also includes M. ozarkensis, which we reviewed in [10Feb20] and [16Mar20], which in retrospect might have been numbered SNHTHICACBW installments 3 and 4.

Then after laying all that groundwork, which took almost 700 words, which is approaching my target length for an entire essay, I blathered on another 1,400 words about the original member of the SNHTHICACBW group, Marstonia scalariformis.  We focused especially on the ACBW part of the formula, “a carinate body whorl,” dwelling at length on the variability that carination can demonstrate.  And at the eagerly anticipated end of last month’s essay, we concluded that the ranges of these little hydrobiids can be vast.  The range of M. scalariformis seems to extend from Illinois to Alabama.  And that they are obscure.  You will not find a population of SNHTHICACBW Marstonia unless you look sharp and employ special techniques.

So, when last we left our intrepid malacologist, he was standing knee-deep in the Flint River at Cherrytree, Alabama, washing stones into a sawed-off trash can (Site CT on the map way down below).  He was indeed finding Marstonia scalariformis.  But that was not all.

The four hydrobioids of the Flint River

There are four hydrobioids inhabiting the dark recesses of rocks and organic debris at Cherrytree, as figured above.  The Marstonia scalariformis we beat to death last month.  We’ll feature the lithoglyphid Somatogyrus populations in a pair of essays to be posted later this winter.  Maybe we’ll come back to Marstonia arga at some time further in the future [1].  What is that fourth hydrobiid?   What is Marstonia pachyta?

About three paragraphs into the first 700 words of blather I published last month I mentioned that when Thompson [2] initially elevated Marstonia to the genus level in 1977 he described five new species.  Among these was a snail “known to occur only in Limestone Creek and Piney Creek, Limestone County, Alabama” that “is readily identified by characteristics of both its verge (penis) and shell.”  That new species was a not-especially-small, not-particularly-narrow hydrobiid without a notably-carinate body whorl that he called Marstonia pachyta.

Thompson figured the M. pachyta penis as demonstrating a typical spatulate or bladelike form featuring three glandular areas, which I have labelled using Bob Hershler’s system as a pair of terminal glands (Tg) and a ventral gland (Vg) in the figure below.  (Thompson also figured another M. pachyta penis missing a second Tg. Bookmark that for later.)  Bob Hershler [3] came behind Thompson in 1994 and re-drew the essentially identical figure I have reproduced in the bottom half of the figure as well.

M. pachyta penis [4]
Regarding the shell morphology of his Marstonia pachyta, Thompson described the shape as “ovate conical,” and gave the adult length as ranging 3.3 – 4.0 mm.  The little sample figured below came from the type locality in Limestone Creek, east of Mooresville, AL (marked MV on the map way down below).

And regarding the range.  Although at the 1977 writing of his description Thompson was only aware of M. pachyta populations in Limestone and Piney Creeks, on 15Aug2000 he collected a sample from Bradford Creek at Martin Road [5] about 7 miles east (marked MR below), and on 16Aug2000 he collected a sample from Round Island Creek at Nuclear Plant Road [6], about 10 miles west (marked NP below).

From Bradford Creek it is just 18 miles further east to the Flint River, on the other side of Huntsville.  But here is yet another peculiar lapse in the long, strange career of Fred Thompson.  As far as I can tell, Thompson only spent one, single day collecting in the entire 500 square mile Flint River subdrainage, during which time he never recorded a single hydrobioid.  The freshwater gastropod collection of the FLMNH holds exactly N = 17 lots collected by Fred Thompson from anywhere in the Flint River or its tributaries, from seven sites he visited on Saturday, 27Sept69.  Among these lots are 16 of pleurocerids and 1 of Laevapex fuscus.  Zero hydrobiids of any species [8].

For comparison, Thompson collected 90 freshwater gastropod lots (48 hydrobiids) from Limestone Creek over his long career.  He then seems to have travelled 25 miles west to the Flint, crossed it barely wetting a boot toe, travelled another 10 miles further west, and collected 39 freshwater gastropod lots (8 hydrobiids) from the Paint Rock River.  Does it seem a bit irresponsible to make statements of the form “known to occur only in Limestone Creek and Piney Creek” when you haven’t even looked in the (biologically very similar) Flint River 25 miles away?  I don’t know.  I’ve probably done the same sort of thing myself.

Marstonia pachyta from Limestone Ck, AL

All of which brings us back, yet a third time, to yours truly standing knee-deep in the Flint River at Cherrytree, AL (site CT).  As my readership most certainly will have been able to divine by this point, what I was finding in the bottom of my sawed-off trash can was important.  Yes, a population of M. pachyta does inhabit the Flint River, matching the shell and penial morphology of the Limestone Creek type population in all respects.  And most of the shells are indeed “ovate-conical” as described by Thompson in 1977.  They are a bit smaller-bodied, however, and some of them are beginning to show a little bit of carination on the body whorl.  Yes, you heard me right.  They are beginning to look like SNHTHICACBW Marstonia.

Ten miles east of the Flint, the next south-draining tributary of the Tennessee River deep enough to wet your mule is the Paint Rock River.  It was from the Paint Rock 0.7 mi east of Cedar Point (CP) that Fred Thompson in 2005 described a new SNHTHICACBW species, Marstonia angulobasis [9].  Thompson distinguished his M. angulobasis by its shell of “minute size” (adulthood only 2.5 mm SL), bearing flattened whorls bordered at the periphery by a distinct angle or cord.  He characterized the penis as bearing “a terminal small apocrine gland.”  That’s what Hershler would have called a “terminal gland” and abbreviated “Tg.”

Tennessee drainages of North Alabama

So about seven paragraphs above, I asked you to “bookmark” the tidbit that Fred Thompson also “figured one M. pachyta penis missing a second Tg.”  The dorsal and ventral aspects of that pachyta penis (his Figs 13C and 13D) are reproduced in the top half of the figure below, compared to his figure of the penis of M. angulobasis below.  And let me ask you a rhetorical question.  If the M. pachyta penis can have 2Tg+1Vg, or 1Tg+1Vg, could it also have 1Tg+0Vg [10]?

Regarding the shell morphology of the Marstonia population inhabiting the Paint Rock River, see figures A and B below.  Thompson collected his type lot on a canoe trip, at a spot not readily accessible.  But he also listed “other specimens examined” from Butler Mill, about 2 miles downstream.  My observations at Butler Mill (BM) suggest a Marstonia population bearing shells quite variable in their “distinct angle or cord,” or carination, or keel, or whatever anybody would like to call it.  Some of them (like B below) show one, and some of them (like A below) do not.

In my September post [7Sept22] I went to great lengths to demonstrate that the distribution of glands on the Marstonia penis shows a great deal of intrapopulation variability.  And in last month’s post [4Oct22] I went to great lengths to demonstrate intrapopulation variation in the shell carination.  The weight of the evidence before us does not suggest that Thompson’s M. angulobasis is specifically distinct from his M. pachyta.

pachyta & angulobasis [11]

But let us save Thompson’s nomen “angulobasis” at the subspecific level, shall we?  Let us henceforth refer to populations of M. pachyta bearing carinate body whorls as “Marstonia pachyta angulobasis Thompson 2005."  And let us remember, as we do, that the FWGNA Project has adopted the definition of that term as it has been understood since the birth of the modern synthesis, “populations of the same species in different geographic locations, with one or more distinguishing traits,” which means exactly what it says, neither more nor less [12].

Now let me go back and pick up those other themes from last month’s post, that business about SNHTHICACBW Marstonia being widespread and obscure.  Although it will always be difficult for field biologists to find populations of such obscure little creatures looking for them, one might not be surprised to find them pop up in quantitative macrobenthic samples.

Over the last few years, I have been blessed to develop a professional relationship Ms. Debbie Arnwine, Ms. Patricia Alicea, and Ms. Carrie Perry of the Tennessee DWR in Nashville who, in the course of their routine duties, collect and sort huge numbers of quantitative macrobenthic samples collected from all over the Volunteer State.  These they hold for some years, but ultimately discard.

Sorting through hundreds of old samples released to us by the TNDEC-DWR, Bob Winters and I have discovered SNHTHICACBW hydrobiids identifiable as Marstonia pachyta angulobasis in three tributaries of the Cumberland River, perhaps 100 miles north of the North Alabama focus of the present essay: Smith Fork of the Caney System (C, below), the West Fork Stones River south of Nashville (D, below) and in Spring Creek of the Red River system almost to the Kentucky line (unfigured).  In fact, it seems possible to us that a single, enigmatic SNHTHICACBW Marstonia shell recovered by our colleague Ryan Evans from the bank of the Elkhorn Creek north of Frankfort, KY, might represent M. pachyta angulobasis, rather than M. letsoni as we have tentatively identified it [13].

A,B = Paint Rock River, C = Smith Fork, D = W.Fk. Stones River

So now has come the time to sum up, over all six of the essays I have contributed on the SNHTHICACBW Marstonia.  In 1977 Fred Thompson recognized a group he called, “small narrow hydrobiids that have in common a carinate body whorl,” comprising four specific nomina: scalariformis, letsoni, wabashensis, and ozarkensis.  Since that date wabashensis has been synonymized, while pachyta and angulobasis added.

Populations identified by two of those specific nomina, scalariformis and pachyta, demonstrate reproductive isolation where the co-occur in the Flint River at Cherrytree, Alabama.  Their shells are distinctive, those of the former bearing a strong carination extending higher than the body whorl, those of the latter occasionally bearing weak carination on the body whorl only.  Their penial morphology is also distinctive, that of scalariformis quite slender, that of pachyta bladelike.  These are two good biological species.

Evidence presented here suggests that angulobasis is a subspecies of pachyta.  The data on penial morphology we reviewed back in 2016 suggests that letsoni has affinities with scalariformis.  The evidence we reviewed in 2020 was too fragmentary to offer any hypothesis whatsoever on ozarkensis.

And finally.  All of these tiny little snails are widespread and obscure.  Their ranges can extend over many, many states.  They are not endemic to anywhere; they are epidemic everywhere across most of the eastern interior.  Populations come, and populations go.  You cannot find them.  Opening my thesaurus and dumping it wholesale onto the computer screen flickering before you, the SNHTHICACBW Marstonia are shadowy, secretive, enigmatic, mysterious, and obscure.


[1] Thompson [2] described Marstonia arga from Guntersville Reservoir in 1977, but it has since spread up the impounded Tennessee to the vicinity of Knoxville, and throughout the impounded Cumberland River as well.  It’s an evolutionary winner!  Seems unfair to ding the TVA for extincting some species without crediting them for carp, kudzu, and M. arga, doesn’t it?

[2] Thompson, F.G. (1977) The hydrobiid snail genus Marstonia.  Bulletin of the Florida State Museum 21(3):113-158.

[3] Hershler, R. (1994)  A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae).  Smithsonian Contributions to Zoology 554: 1 - 115.

[4] Above, Marstonia pachyta penis modified from Thompson [2] figure 13A and 13B.  Below, Marstonia pachyta penis from Hershler [3] figure 53a.  Dorsal on left, ventral on right. Tg = terminal glands, Vg = ventral gland, P = penial filament.

[5] FLMNH catalog 279921, collected by FGT on 8/15/2000 from Bradford Creek at Martin Road, 2 miles south of Madison, AL.

[6] FLMNH 279628, collected by FGT on 8/16/2000 from Round Island Creek at County Road 25, 3 miles west of Jones Crossroads, AL.  Haggerty & Garner [7] were not able to confirm this record in their exhaustive survey of 2008, however.

[7] Haggerty, T.M. & J.T. Garner (2008)  Distribution of the armored snail (Marstonia pachyta) and slender Campeloma (Campeloma decampi) in Limestone, Piney, and Round Island Creeks, Alabama.  Southeastern Naturalist 7: 729 – 736.

[8] And in fact, the entire Flint River (AL) catalog at the FLMNH is a disappointment.  Just N = 50 freshwater gastropod records total from all collectors, the 33 lots not collected by Thompson undated and obviously ancient.

[9] Thompson, F.G. (2005)  Two new species of hydrobiid snails of the genus Marstonia from Alabama and Georgia.  The Veliger 47: 175 – 182.

[10] Yes.

[11] Above, Marstonia pachyta penis modified from Thompson [2] figure 13C and 13D.  Below, Marstonia angulobasis penis from Thompson [9] figure 18a and 18b. Dorsal on left, ventral on right. Tg = terminal gland, Vg = ventral gland, P = penial filament.

[12] For more on the subspecies concept as applied by the FWGNA Project, see:

  • What is a subspecies [4Feb14]
  • What subspecies are not [5Mar14]

[13] For more about that single, enigmatic SNHTHICACBW shell from Kentucky, see:

  • Is Marstonia ozarkensis extinct? [16Mar20]