Dr. Rob Dillon, Coordinator

Tuesday, October 4, 2022

The SNHTHICACBW Marstonia 5: scalariformis

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  The SNHTHICACBW Marstonia III: scalariformis.  Pp 243 – 252 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

Our modern understanding of the North American hydrobiid genus Marstonia is entirely to the credit of two men, as similar as they were different, Dr. Fred Thompson and Dr. Bob Hershler.  I featured the former colleague, who passed away five years ago, in a pair of essays posted on this blog in 2017 [15Feb17] [14Mar17].  I profiled the latter colleague, now retired, in an essay I published in July [5July22].  And to appreciate their contributions to our understanding of the most enigmatic group of gastropods inhabiting the fresh waters of the American East, let’s back up and get a running start at it.

F. C. Baker [1] described the subgenus Marstonia in 1926 to distinguish Henry Pilsbry’s familiar and widespread Amnicola lustrica [2] from Thomas Say's equally familiar and widespread Amnicola limosaBaker made this distinction on the basis of shell and radula, taking no notice of penial morphology at the time.  He recognized seven species in his new subgenus Marstonia inhabiting the lakes of Wisconsin, all of which would either be synonymized under lustrica, or subsequently split out to Lyogyrus, leaving his subgenus Marstonia effectively monotypic.

M. scalariformis, Cherrytree, AL

It was Fred Thompson who, in his seminal 1977 monograph [3], raised Marstonia to the genus level, highlighting a distinction that Berry [4] had made in the penial morphologies of limosa and lustrica.  Thompson included three previously described species in Marstonia: Pilsbry’s (1890) lustrica, Pilsbry’s (1895) olivacea [5] and his own (1969) agarhecta.  To this short list he added five new species: arga, castor, halcyon, pachyta, and ogmorhaphe [6].  This eight-species model of Marstonia was the understanding brought down from the mountain by Jack Burch in 1982 [7].

In his 1977 closing remarks on “species of questionable status,” however, Thompson suggested that four species “currently placed in Pyrgulopsis [8]” might “actually belong in Marstonia.”  Here he listed scalariformis (Wolf 1869), letsoni (Walker 1901), wabashensis (Hinkley 1908) and ozarkensis (Hinkley 1915), noting “these are Small Narrow Hydrobiids That Have In Common A Carinate Body Whorl.”  Let us henceforth refer to this subgroup as the “SNHTHICACBW” Marstonia.

The broad-brush similarity in penial morphology between his eight newly minted Marstonia species and those four SNHTHICACBW species, which since 1886 had been allocated to Pyrgulopsis, seems to have worked on Thompson’s mind, however.  And I speculate that a rising young star in the constellation of hydrobioid systematics also had something to do with it, as well.  Because in 1987 the dynamic duo of Bob Hershler and Fred Thompson united to synonymize Baker’s Marstonia under Call & Pilsbry’s Pyrgulopsis [9].

The next advance in the inexorable march of tiny-snail science came in 1994, with Bob Hershler’s masterful “Review of the North American Freshwater Snail Genus Pyrgulopsis[10].  As we mentioned back in July, my buddy Bob counted 54 “Western American Species” and 11 “Eastern American Species” in that work, including Fred Thompson’s original eight plus three of Thompson’s SNHTHICACBW: letsoni, ozarkensis and scalariformis.

Marstonia penis morphology [10]
And a mere eight years later, as if to validate their dynamic credentials before the Judgement Seat of Malacology Eternal, the duo of Thompson and Hershler [11] resurrected Marstonia back to full genus level on the basis of female reproductive anatomy [12], reallocating Hershler’s list of 11 eastern species into it once again, plus comalensis of Pilsbry & Ferriss 1906.

The systematic status of the SNHTHICACBW hydrobioids was the dasher in 76 years of taxonomic churn.  We have previously featured two elements of this enigmatic group in a pair of essays each, reviewing the mighty efforts of previous workers to understand the tangled evolutionary relationships among them.  In [19Jan16] and [5Feb16] we reviewed the situation with Marstonia letsoni, and in [10Feb20] and [16Mar20] we covered Marstonia ozarkensis.  For the remainder of this essay, which has actually turned out to be the fifth installment of a series I did not know I was starting back in 2016, we will focus on the original member of the group, M. scalariformis, together with its synonyms and forms.

Pyrgula scalariformis was described by Wolf (1869) from a single empty shell collected along the Illinois River in Tazewell County, Illinois [13].  And the original rationale that Call & Pilsbry offered for their description of the new genus, Pyrgulopsis, back in 1886 was to contain scalariformis and their own virtually-identical species from the Illinois bank of the Mississippi river at Rock Island, P. mississippiensis [8].  Hershler [10] considered both mississippiensis and wabashensis (Hinkley 1908) synonyms of scalariformis, the latter described from the Wabash River in Indiana, perhaps 200 miles southeast.  All three nominal species have been considered extinct from their type localities by multiple authorities.  Hershler made his anatomical observations from a population collected in the Meramec River of Missouri, perhaps 200 miles south of Wolf’s type locality.

Hershler’s [10] figure 53b showed an overall penial morphology really quite different from the peculiar blade-shaped structure mounted by the western Pyrgulopsis we beat to death last month, or indeed typical eastern Marstonia such as M. lustrica, which we figured back on [5Feb16].  The M. scalariformis penis actually looks like a penis, with a prominent hose-shaped pointy-thing and a single reduced gland sticking out the side, which Hershler simply called a “terminal gland.”  It is very reminiscent of the penis of the SNHTHICACBW Marstonia letsoni that E. G. Berry [4] figured from up in Michigan, which we also reprinted in our essay of [5Feb16].  I’ve pasted Hershler’s figure 52e of the letsoni penis next to his 53b of the scalariformis above, to refresh your memory.  The only difference seems to be a larger terminal gland in letsoni.

Turning now to the shell morphology.  Some significant fraction of Hershler’s Missouri sample did indeed bear shells every bit as “scalariform” as Wolf’s type specimen from Illinois.  This is the ACBW predicate of Thompson’s SNHTHICACBW, which Hershler called a “peripheral keel.”  But that carination or keel varied “from weak to well-developed.”  And strangely, although Hinkley’s [14] description of P. wabashensis emphasized the absence of a carination/keel, Hershler’s reexamination of Hinkley’s Indiana type material also revealed that “many of the paratypes from the type locality (the Wabash) are carinate and very closely resemble some scalariformis.”

Shimek's P. mississippiensis [15]

The situation with the other scalariformis synonym, Call & Pilsbry’s mississippiensis, is identical.  In 1892 the University of Iowa Professor Bohumil Shimek [15] published an excellent paper in an obscure journal [16] exhaustively cataloguing shell variation in “more than 1500” subfossil SNHTHICACBW collected from Pilsbry’s type locality on the bank of the Mississippi River.  Quoting Shimek verbatim:

“The series is an unbroken one, the species varying from the ecarinate forms which are scarcely angled at the periphery of the body-whorl, and in which the suture is not impressed, to forms in which the carina is elevated and extends quite to the apex, and in which the suture is deeply impressed.”

Importantly, three different early-twentieth century authors reported SNHTHICACBW species identified as either mississippiensis or scalariformis from Shoal Creek, a tributary of the Tennessee River in North Alabama.  The initial collection, identified as Pyrgula mississippiensis, was made by Hinkley and confirmed by Bryant Walker [17].  F. C. Baker [18] dissected a fresh sample he identified as Pygrulopsis scalariformis from Shoal Creek, figuring shell, operculum, and radula, agreeing with Shimek that mississippiensis should be synonymized underneath it.  Baker did not offer us any anatomical data on his Shoal Creek sample, alas.

The special importance of this record, which I cannot find any reason to doubt, is that it extends the range of scalariformis 500 km south from Indiana into drainages of the Southern Interior.  But alack, the same fate seems to have befallen the Shoal Creek population as has been suffered by Wolf’s original scalariformis, Pilsbry’s mississippiensis, and Hinkley’s wabashensis.  Repeated efforts by Thompson and Hershler failed to rediscover any living specimens.  Shoal Creek is today inhabited by a similar species better adapted to trashy, lentic environments, Marstonia arga.

All of which brings us to the present day, and the first person.  Many have been the sleepless nights I have passed in recent years, combing through the yellowed pages of dusty journals, preparing for the expansion of the FWGNA Project through the Tennessee drainages of North Alabama [19].  And many have been the Latin binomina assigned to the hydrobioid fauna of those rich waters.  And many have been the hours I have waded in the shallows of Shoal Creek, wiping rocks in a sawed-off trashcan, hunting for M. scalariformis in the drainage of The Tennessee. To no avail.

The SNHTHICACBW Marstonia are really, really hard to find.  At an AMU meeting many years ago I asked Fred Thompson how he did it.  And seven of the twelve words he ever spoke to me were these: “I wash rocks in a white bucket.”

Washing vegetation into a white bucket

I have personally found the walls of standard pickle-dimension buckets too deep to effectively inspect and recover little 3 mm items from the bottom of.  So, a few years ago I bought a white kitchen trashcan from the Walmart and sawed it off at about six inches deep, yielding a vessel that is more like a very-deep tray.  And I pull rocks up from the rapids – especially rocks covered with Podostemum or bryophytes – and wipe them off into the vessel.  I also target larger items of organic debris – sticks and so forth, and the stems of emergent vegetation.  And then I slosh the bottom mess around and slide it off like I was panning for gold, which in the malacological world, I suppose I am.

I have found this technique effective for surveying the diverse hydrobioid fauna of North Alabama, about which we will have much more to say in coming months.  But as for Marstonia scalariformis in Shoal Creek – goose egg, bagel, O-fer.

Ah, but!  Not all of the reference materials to which I have turned these sleepless nights have been yellowed tomes.  Reference to the Global Biodiversity Information Facility [20] returns 62 records of Marstonia (or Pyrgulopsis) scalariformis (or mississippiensis) held in national collections, 16 of which are from North Alabama.  Almost all of those 16 records are old, historic lots from Shoal Creek.  But late one night I was pleased to find in a GBIF data download [21] three recent records of Marstonia scalariformis (1 USNM, 2 NCSM) collected by Jeff Garner, working in the Flint River just east of Huntsville, about 70 miles east of Shoal Creek.

And with that intelligence on a clipboard riding on the passenger seat of my little Mazda pickup, in 2021 I was indeed able to confirm a population of Marstonia scalariformis deep in the bryophytes and periphyton covering the rocks in rapids of the Flint River at Cherrytree, AL.  Witness the figure that opened this essay, way up above.

To summarize.  The essay you have just suffered through, discursive as it most certainly has been, has nevertheless managed to touch three points.  First, the ranges of SNHTHICACBW Marstonia can be vast.  The straight-line distance from the bank of the Mississippi River at Rock Island, Illinois, to the shoals of the Flint River at Cherrytree, Alabama is 900 km.  Second, populations of M. scalariformis can vary strikingly in their shell morphology, especially with regard to carination.  And third, those doggone things are tough to find.  Populations come, and populations go.  It is hard to imagine how an itinerant malacologist, such as yours truly, might find a Marstonia scalariformis population anywhere, ever, unless he is consciously looking for one.  Special techniques are required.

Or alternatively, I suppose, biologists who are NOT looking for them might turn them up in quantitative macrobenthic samples, perhaps?  In any case.  The SNHTHICACBW Marstonia are, in three words, widespread, polymorphic, and obscure.  Carry those three words forward, and we’ll see you all next month.


[1] Baker, F. C. (1926) Nomenclatural notes on American fresh water Mollusca. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters 22:193-205.

[2] Of course, a freshwater gastropod as common and widespread as what we identify today as Marstonia lustrica was well known before Pilsbry described it (or re-described it) in 1890.  The taxonomic situation was complicated by uncertainty over the identity of Thomas Say’s (1821) Paludina lustrica.  See:

  • Baker, H. B. (1960)  Lustrica (Paludina) Say, 1821 (Gastropoda): Proposed suppression under the plenary powers.  Z.N.(S.) 730.  Bulletin of Zoological Nomenclature 18: 146 – 148.

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

[4] Berry, E. G. (1943) The Amnicolidae of Michigan: distribution, taxonomy and ecology.  Miscellaneous Publications of the Museum of Zoology, University of Michigan 57: 1 – 68.

[5]  Thompson considered Pilsbry’s M. olivacea “of uncertain status and probably extinct.”  We will have much more to say about this in a future post.

[6] Thompson spelled the specific nomen with two pees, “ogmorphaphe” at the heading of his 1977 description, but just the single pee “ogmorhaphe” enough times subsequently to make the one-pee spelling stick.

[7] 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 a stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[8] Call R. E. & Pilsbry H. A. (1886). On Pyrgulopsis, a new genus of rissoid mollusk, with description of two new forms. Proceedings Davenport Academy of Natural Sciences 5: 9-14.

[9] Hershler, R., and F.G. Thompson (1987)  North American Hydrobiidae (Gastropoda: Rissoacea): redescription and systematic relationships of Tryonia Stimpson, 1865 and Pyrgulopsis Call and Pilsbry, 1886. The Nautilus 101:25-32.

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

[11] Thompson, F. G. & R. Hershler (2002)  Two genera of North American freshwater snails: Marstonia Baker, 1926, resurrected to generic status, and Floridobia, new genus (Prosobranchia: Hydrobiidae: Nymphophilinae).  The Veliger 45: 269 - 271.

[12] The occurrence of a large extension of the albumen gland into the pallial roof is a unique Marstonia characteristic [10].  Molecular phylogenetic analyses have also supported the distinction between Marstonia and Pyrgulopsis, and the retention of both genera in the Hydrobiidae sensu strictu.  See:

  • Hershler,R., Liu,H.-P. and Thompson,F.G. (2003) Phylogenetic relationships of North American nymphophiline gastropods based on mitochondrial DNA sequences.  Zool. Scr. 32 (4): 357-366.
  • Liu, H., and R. Hershler (2005)  Molecular systematics and radiation of western North American nympholine gastropods. Molecular Phylogenetics and Evolution 34: 284-298.
  • Wilke T., Haase M., Hershler R., Liu H-P., Misof B., Ponder W. (2013)  Pushing short DNA fragments to the limit: Phylogenetic relationships of “hydrobioid” gastropods (Caenogastropoda: Rissooidea).  Molec. Phyl. Evol. 66: 715 – 736.

[13] Wolf, J. (1869)  Descriptions of three new species of shells.  American Journal of Conchology 5: 198.

[14] Hinkley, A. A. 1908. A new species of Pyrgulopsis. Nautilus 21: 117-118.

[15] Bohumil Shimek (1861 – 1937) should be better known in the malacological community today than he is.  Variously listed as a botanist, a zoologist, a geologist, a conservationist, and an engineer, he published widely in a variety of disciplines, including quite a few papers in The Nautilus 1888 – 1936.  There’s a photo of him on his Wikipedia page, if you are curious.

[16] I cannot help but think that, if Shimek’s conclusion had supported the validity of Pilsbry’s P. mississippiensis, this paper would have been published in The Nautilus:

  • Shimek, B. (1892) Pyrgulopsis scalariformis (Wolf) Call and Pilsbry.  Bulletin from the Laboratories of Natural History of the State University of Iowa 2: 168 – 174.

[17] Hinkley, AA (1906) Some shells of Mississippi and Alabama. Nautilus, 20:40-44.  Walker B (1906) New and little known species of Amnicolidae. Nautilus, 19:114-117.

[18] Baker, F.C. (1928) Freshwater Mollusca of Wisconsin, Part I, Gastropoda. Bull. Wisc. Geol. Natur. Hist. Survey, no. 70.  Madison: University of Wisconsin Press.  Baker simply wrote (pg 138) , “Animal: Not examined or described as far as known.”  Why my hero F. C. Baker didn’t seize the opportunity to describe the penial morphology of the fresh sample of M. scalariformis before him, I cannot say.  I admit that I am a bit disappointed.  With page 138, anyway.

[19] Grand opening announced this spring:

  • Freshwater Gastropods of the Tennessee/Cumberland [16May22]

[20] I could just as easily have referred to the IDigBio database and obtained the same results, I feel sure.  I don’t know why I chose GBIF over IDigBio.  See:

  • 20 Years of Progress in the Museums [22May19]

[21] Here is my direct download from GBIF, accessed 10Feb22.  Extract and open the text file called “occurrence” in excel, tab delimited:  https://doi.org/10.15468/dl.mqfb8b