Anchored hybrid enrichment, Leptoxis lessons, and the advice of Queen Elsa

Editor’s Note – This is the third installment of a three-part series reviewing the 2022 paper by Whelan and colleagues [1] on “Prodigious polyphyly in the Pleuroceridae.”  We recommend that you back up and read both our posts of September [23Sept25] and October [14Oct25] if that material isn’t fresh in your mind.  You will also find our posts of [6Apr23] and [9May23] helpful as background for the essay that follows, if you want to dig more deeply into the subject matter itself.

Back in early September I enjoyed a wonderful two-week field trip to the Ozark/Ouachita Highlands of Arkansas, southern Missouri, and eastern Oklahoma, logging 3,700 miles on my pickup and 20 miles on my kayak, collecting 59 sites.  And I am looking forward to sharing a lot of stories about my adventures in that biogeographically fascinating part of the world with you all in future posts.  Some of the observations I made on Leptoxis arkansensis during that memorable excursion are, however, especially relevant to the “prodigious polyphyly” discovered by Nathan Whelan and his colleagues [1] in the AHE phylogenomic study we are currently reviewing.

Populations of Leptoxis arkansensis are uncommon and widely scattered in the region, primarily inhabiting the rocky riffles of small rivers with good water quality, always (as far as I can tell) co-occurring with large and nearly-omnipresent populations of Pleurocera potosiensis.  From all evidence, the two are distinct biological species.  The shells born by L. arkansensis demonstrate a larger body whorl than those of P. potosiensis, consistently and reliably, from birth to maturity.  But their grossly similar morphology in overall shell form can render the two species difficult to distinguish in the field.  There is not a shadow of doubt in my mind that they are “sister” species, one evolved from the other.

The strongest hypothesis, I would suggest, is that the original population of Leptoxis arkansensis evolved from a Pleurocera potosiensis population specialized for life in the riffles at midstream, and that the larger body whorl reflects a relatively larger foot, adapted for clinging to rocks in rapid currents.  Might that same adaptation have evolved more than once in the hundred-million-year history of the North American Pleuroceridae?  Who among my vast and erudite readership could imagine any answer to my rhetorical question other than the affirmative [2]?

The most loyal and patient among you might remember a (rather personal) essay I posted back in [6Apr23] entitled, “Growing Up With Periwinkles.”  In that post I reviewed my own 60+ years of experience with Leptoxis in the Southern Appalachians, from childhood through college and into my professional career, including several allozyme studies and even one (rather atypical, for me) sequence study as well.  I concluded that essay as follows: “If you had asked me ten years ago, I would have listed six valid species of Leptoxis in North America: carinata, praerosa, crassa, picta, plicata, and maybe that weirdo way out in the Ozarks, Leptoxis arkansensis (Hinkley 1915), I have no reason to doubt.”

Then, in the essay that followed [9May23], I “tested the periwinkle hypothesis” with the four-gene phylogeny (CO1, 16S, 28S, H3) that Nathan Whelan developed for his (2013) dissertation [3], finding solid agreement with my six-species model.  The main trunk of Nathan’s gene tree showed L. praerosa, L. crassa, L. carinata and L. picta distinct as expected.  Leptoxis plicata and L. arkansensis were also distinct, but depicted way off in the distant foliage with a variety of pleurocerids representing other genera.

There was also one surprise – evidence of a cryptic species sympatric with Leptoxis praerosa through much of their shared range in East Tennessee and North Alabama.  Nathan identified that cryptic species, quite problematically, as Leptoxis virgata. At the end of my [9May23] essay I had renamed my hypothesis for the genus Leptoxis in North America the 6* Species Model, adding that asterisk for the possibility of a seventh, cryptic species.

Figure 4 of Whelan et al [1]

Both of those 2023 essays were focused entirely on evolutionary relationships at the species level and below.  I wouldn’t say that I myself am entirely disinterested in evolution at higher levels, it just seems to me that we ought to work out some plausible hypotheses for the populations and the species first.  So now, with the 6* Species Model firmly supported both by Nathan’s 4-gene dissertation and by my own 60+ years of experience in field and laboratory, we are in a position to understand what Nathan’s more recent AHE phylogenomic analysis is telling us about the evolution of Leptoxis as a genus.  Then starting at three o’clock, and proceeding clockwise:

Leptoxis picta.  Way back in 1998, our good buddy Chuck Lydeard and I published a paper [4] showing that the allozyme divergence among six populations of three nominal species of Alabama Leptoxis: L. picta, L. ampla, and L. taeniata (subsequently [5] renamed coosaensis), was no greater than the divergence among control Leptoxis populations sampled from the Tennessee drainage, which nobody had ever suggested were anything other than L. praerosa.  We suggested that those three Mobile Basin nomina be united under the oldest name, Leptoxis picta (Conrad 1834).

Since 1998, an impressive body of research has been published supporting that hypothesis.  Nathan’s four-gene dissertation [3] returned negligible divergence among picta, ampla, taeniata/coosaensis, and foremani (7 populations, 18 individuals).  And both of Nathan’s (really quite lovely) subsequent studies of interpopulation divergence in “Leptoxis ampla,” his (2016) study of mitochondrial superheterogeneity [6] and his (2019) RADseq study [7], recovered strikingly high levels of interpopulation divergence, which easily extrapolate to include Mobile Basin populations historically identified as picta, taeniata/coosaensis, and foremani.

Now open before us we have a fifth study, estimating levels of genetic divergence among 192 individual pleurocerid snails over their entire single-copy genomes using the cutting-edge technique of Anchored Hybrid Enrichment, showing exactly the same result as references [3], [4], [6], and [7].  The nomina ampla, coosaensis, foremani and taeniata are all obsolete, junior synonyms of L. picta.  And to find the branches of a 21st century phylogenomic tree cavalierly decorated all about its entire periphery with 19th century superstition is an embarrassment to our entire profession.

Leptoxis carinata.  Quite independently of the evolution of Leptoxis picta in the Mobile Basin of Alabama, Nathan’s AHE phylogenomic analysis suggests a second origin of the genus Leptoxis in the southern Appalachians.  This origin is likely much older.

The sequencing studies I published with John Robinson in 2009 confirmed that Ohio drainage populations identified as Leptoxis dilatata (Conrad 1835) are conspecific with Atlantic drainage populations identified (since 1792!) as Leptoxis carinata, and suggested that the pleurocerid fauna of this ancient region might have evolved at the (Paleozoic) Appalachian orogeny [8].  Both the conspecific status of the two nomina, and the origin of Leptoxis carinata (now more broadly and properly understood) independent of any other Leptoxis species, were confirmed by Nathan’s dissertation [3], and have now been triple-checked by the AHE phylogenomic analysis under review here.

Leptoxis crassa and praerosa.  Again dovetailing nicely with his (2013) dissertation research, Nathan’s big AHE phylogenomic analysis confirms that Leptoxis crassa (identified here under the obsolete [9] synonym “Athearnia anthonyi”) is related to, but specifically distinct from, Leptoxis praerosa.  And the larger set of crassa + praerosa together apparently represents a third independent evolution of the shell morphology we associate today with the genus Leptoxis.  This third origin seems to have occurred in the Tennessee/Cumberland region, I should guess after Leptoxis carinata evolved further east, but before L. picta to the south.

Burrowing more deeply into that lovely lavender subcluster, we find further confirmation of Nathan’s 2013 results suggesting that umbilicata (Weatherby 1876) and subglobosa (Say 1825) are junior synonyms of L. praerosa (Say 1821).  Somewhat surprisingly, however, Nathan’s AHE analysis did not return evidence of any significant genetic distinction between two individuals he identified as L. virgata (only one of which is depicted below) and the larger L. praerosa cluster.

You might recall from my [9May23] review of Nathan’s dissertation that he did not specify any morphological trait by which his L. virgata could be distinguished from sympatric populations of L. praerosa, and that the genetic difference was apparently only mitochondrial, not nuclear.  Here ten years later, we are left wondering whether Nathan might have misidentified both of his nominal virgata specimens in 2022, or if the evidence he adduced in 2013 that a second biological species might be cryptic under our old friend Leptoxis praerosa might have been an atypically widespread case of mitochondrial superheterogeneity [11].

 

Leptoxis arkansensis.  We reviewed this branch of Nathan's AHE phylogenomic tree at the top of this essay.  The independent origin of Leptoxis arkansensis way out west in the Ozark highlands seems to be yet a fourth evolution of the shell morphology we associate today with the genus Leptoxis. 

 

Leptoxis plicata.  Even as the 1998 allozyme study of Dillon & Lydeard [4] was returning conspecific levels of genetic divergence among Alabama populations of Leptoxis picta, ampla, and taeniata/coosaensis, a fourth nominal species of Leptoxis from the state of Alabama, Leptoxis plicata, was revealed to be genetically distinct.  The single population of L. plicata known at that time, inhabiting the Locust Fork of the Black Warrior River, bore shells with a characteristically high apex and weak but distinctive plication on the upper whorls. 

 

From Whelan et al [1]

Additional evidence for the validity of L. plicata as a biological species was brought to light in 2013, with the publication of Nathan Whelan’s dissertation [3]. Nathan’s four-gene phylogeny depicted L. plicata as quite distantly removed from the other Alabama Leptoxis species, distant indeed from (almost) all other populations ever identified as Leptoxis anywhere, way off on a branch with clusters of Pleurocera and Lithasia.

Regarding that parenthetical modifier, “almost.”  I did not mention it at the time of my [9May23] review, but it is certainly worthy of note today, that in 2013 Nathan seems to have found very little sequence divergence between Leptoxis plicata and a newly-rediscovered Leptoxis population inhabiting the Cahaba River about 50 km south of Locust Fork, which he identified as Leptoxis compacta [13].  Here’s the copy of Nathan’s gigantic Baysian tree that I first made available for download from my 2023 review, with the little plicata/compacta cluster encircled in red:

[Whelan 2013 Figure 5.4, modified]

To objectively evaluate the significance of the evolutionary relationship between L. plicata and L. compacta, perhaps among the first data sets of interest would be their percent sequence divergence.  Alas, Nathan never uploaded any of the sequence data from his massive dissertation to GenBank.  But holding Nathan’s gigantic 2013 gene tree at arm’s length and scanning for an overall impression, L. plicata and L. compacta appear conspecific, the former nomen (Conrad 1834) having priority over the latter (Anthony 1854).

Now here ten years later, we find Leptoxis plicata and L. compacta again depicted as conspecific in Nathan’s AHE phylogenomic tree.  I myself have no biological insight to contribute, my experience with the legendary pleurocerid fauna of the Cahaba River being limited to song, story, and musty chronicle.  The descriptions I have read and the figures I have seen of the L. compacta shell do not mention shell plication, but do show the same unusually high spire characteristic of L. plicata.

In any case, I am sure we can all agree that the shell morphology borne by the Locust Fork / Cahaba pleurocerid populations together, clustered as both are upon the far-flung branches of every gene tree they’ve ever hung, suggests yet another separate origin of the genus Leptoxis.

 

So, to summarize the first 71% of the present essay.  Nathan’s AHE phylogenomic analysis confirms the 6-Species Model for the genus Leptoxis, without an asterisk.  The rocky shoals of North American rivers are inhabited by, in order of their description, Leptoxis carinata (Brug. 1792), L. praerosa (Say 1821), L. picta (Conrad 1834), L. plicata (Conrad 1834), L. crassa (Hald. 1841), and L. arkansensis (Hinkley 1915).  And the shell morphology that has historically served to unite these six biological species into the genus Leptoxis has evolved five separate times.

 

The first genus-level name to be proposed for the freshwater cerithiaceans of North America was, apparently [14], Pleurocera. And it was the eccentric French naturalist Constantine Rafinesque who did the proposing, way back in 1818.  The shell that he seems to have had in his hand at that time was elongate, with a high spire and small body whorl, best-guess-identified today as Pleurocera canaliculata [15].

 

And the second genus-level name proposed for the North American freshwater cerithiaceans was like unto the first, also proposed by Rafinesque, one year later.  That genus name was Leptoxis.  And the shell that Rafinesque seems to have had in his hand at that juncture was rotund and obovate, with a low spire and a large body whorl, best-guess-identified today as Leptoxis praerosa [15].

 

Thus, it seems that even prior to the birth of American malacology, when Frenchmen were still running around loose in our backwoods with butterfly nets, everybody who has ever pulled more than one pleurocerid snail out of our crystal-clear waters has divided out a subset bearing shells that are elongate and a subset bearing shells that are obovate.

Rafinesque (1783-1840)

Additional subsets were carved out as the nominal species accumulated, of course.  Generally following Goodrich, Burch [10], suggested a seven-genus system for the North American Pleuroceridae, most of those seven genera with subgenera, and listed an additional 25 genus names in synonymy.  But that elongate/obovate dichotomy has always been the most fundamental, and the Pleurocera/Leptoxis division the first.

So today we sit at our desks, a remarkably plausible reconstruction of the evolutionary history of the North American Pleuroceridae blossoming before us in lovely pastel shades of yellow, green, blue, and lavender.  And it seems quite appallingly evident that the very first taxonomic distinction made by our forefathers, around which 200 years of taxonomy subsequently developed, has no evolutionary basis.  Shall we then discard the entire richly historic but scientifically obsolete classification system of the Pleuroceridae?

 

No.  The binomial system of nomenclature was not first proposed by Carl Linnaeus in 1758 for any evolutionary purpose whatsoever.  It was first a tool for information storage and retrieval, and it served that function alone for its first 100 years of application, carrying Darwin toward the Theory of Evolution just as surely as The Beagle.  And that information-retrieval function continues to be the primary utility of biological taxonomy even to the present day.

 

I absolutely understand that the names of organisms must be changed to reflect scientific advance, and have certainly done my fair share of the changing.  But when we change a name, we pay the price of losing some of the information that the previous name historically transmitted.

 

The reason I synonymized Goniobasis and Elimia under Pleurocera in 2011 [16] was my discovery that single randomly-breeding populations of freshwater gastropods were being classified into three genera – not just in one case, but repeatedly, throughout the North American pleurocerid fauna.  The cost of the misinformation being generated by the Pleurocera/Goniobasis/Elimia confusion outweighed, in my judgement, the cost of the good information lost by the combination of those three names.

 

And I would invite Nathan Whelan and all our mutual colleagues, once again, to follow suit on that.  Look at that tangle of blue and yellow branches in the GHIJKL quadrant of your tree, colleagues!  Your own analysis shows that there is no evolutionary basis for the genus name “Elimia.” 

 

I understand your fervent desire to preserve obsolete nomina for the information they transmit.  I would point out, however, that “Goniobasis” contained even more information than “Elimia” when Burch [10] high-handedly dumped Goniobasis for an obscure point of taxonomic priority in 1980, yet our discipline survived.  And I would argue that the profits we all stand to gain by correcting the misinformation being promulgated even unto the present day by the entirely artificial distinction [17, 18] between Elimia and Pleurocera far outweigh the loss of good information we will suffer synonymizing the former under the latter.

 

But as to the other Burch/Goodrich genera – Lithasia, Io, Gyrotoma and especially Leptoxis, I believe that we have now reached consensus.  Can we all agree to take Queen Elsa’s advice, henceforth?  Let it go.

Notes:

 

[1] Whelan, N. V., Johnson, P. D., Garner, J. T., Garrison, N. L., & Strong, E. E. (2022). Prodigious polyphyly in Pleuroceridae (Gastropoda: Cerithioidea). Bulletin of the Society of Systematic Biologists, 1(2). https://doi.org/10.18061/bssb.v1i2.8419

 

[2] Here I have answered a rhetorical question with a second rhetorical question.  And this isn’t even rhetoric, it is prose.  Have I violated some ancient and hoary guideline?  If so, I plead ignorance, and apologize.

 

[3] Whelan, Nathan V. (2013) Conservation, life history and systematics of Leptoxis Rafinesque 1819 (Gastropoda: Cerithioidea: Pleuroceridae).  PhD Dissertation, University of Alabama, Tuscaloosa.  179 pp.  For a review, see:

• Testing the periwinkle hypothesis [9May23]

[4] Dillon, R.T., and C. Lydeard (1998) Divergence among Mobile Basin populations of the pleurocerid snail genus, Leptoxis, estimated by allozyme electrophoresis.  Malacologia. 39: 111-119. [pdf]

 

[5] Whelan, Nathan V.; Johnson, Paul D.; Garner, Jeffrey T.; Strong, Ellen E. (2017). On the identity of Leptoxis taeniata – a misapplied name for the threatened Painted Rocksnail (Cerithioidea, Pleuroceridae). ZooKeys (697): 21–36. https://zookeys.pensoft.net/article/14060/

 

[6] Whelan, N.V. & E. E. Strong (2016) Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.  For a review, see:

• Mitochondrial superheterogeneity and speciation [3May16]

[7] Whelan, N.V., M.P. Galaska, B.N. Sipley, J.M. Weber, P.D. Johnson, K.M. Halanych, and B.S. Helms (2019)  Riverscape genetic variation, migration patterns, and morphological variation of the threatened Round Rocksnail, Leptoxis ampla.  Molecular Ecology 28: 1593 – 1610.  For a review, see:

• Intrapopulation gene flow, the Leptoxis of the Cahaba, and the striking of matches [2Nov21]

[8] Dillon, R T. and J. D. Robinson (2009) The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11. [pdf] For a review, see:

• The snails the dinosaurs saw [16Mar09] 

[9] Jack Burch [10] lowered “Athearnia” to subgeneric status under Leptoxis way back in 1980, as he lowered “anthonyi” to subspecific status under crassa.  The relentlessly archaic taxonomy to which Nathan Whelan and his colleagues cling is not even current to the 20th century, much less the 21st.

 

[10] 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).

 

[11] The phenomenon of mitochondrial superheterogeneity was first documented in the pleurocerid snails by Dillon & Frankis [12] and made famous in 2009 by Dillon & Robinson [8].  The term was not actually coined until 2016, however, in a series of posts on this blog.  See:

• Mitochondrial superheterogeneity: What we know [15Mar16]
• Mitochondrial superheterogeneity: What it means [6Apr16]
• Mitochondrial superheterogeneity and speciation [3May16] 

[12] Dillon, R. T., and R. C. Frankis. (2004) High levels of DNA sequence divergence in isolated populations of the freshwater snail, Goniobasis.  American Malacological Bulletin 19: 69 - 77 [pdf].

 

[13] Whelan NV, Johnson PD, Harris PM (2012) Rediscovery of Leptoxis compacta (Anthony, 1854) (Gastropoda: Cerithioidea: Pleuroceridae). PLoS ONE 7(8): e42499. https://doi.org/10.1371/journal.pone.0042499

 

[14] The actual meaning of Rafinesque’s 1818 nomen “Pleurocera” was shrouded in mystery and roiled by controversy for many years.  For a review, see:

• Joe Morrison and the great Pleurocera controversy [10Nov10] 

[15] Both Pleurocera canaliculata and Leptoxis praerosa were described by Thomas Say in 1821.  Yes, it is possible to describe a genus before its type species.

 

[16] Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277 [pdf]. For a review, see:

• Goodbye Goniobasis, Farewell Elimia [23Mar11] 

[17] For a demonstration of the artificiality of the genus nomen Elimia, see:

• Goniobasis and Elimia [28Sept04] 

[18] And for a demonstration of the artificiality of the genus nomen Goniobasis, see:

• A House Divided [10May20]
  

Anchored Hybrid Enrichment and the prodigious clavaeformis confusion

Editor’s Note – This is the second installment of a projected three-part series reviewing the 2022 paper by Whelan and colleagues [1] on “Prodigious polyphyly in the Pleuroceridae.”  We recommend that you back up and read last month’s post [23Sept25] before proceeding onward, if that material isn’t fresh in your mind.  You will find a review of our August post [20Aug25] helpful, as well.  And in fact, refreshing your memory on our posts of September – November of 2016 [2] will also fill in a lot of the relevant background.  But that’s probably too much to ask.

Last month we summarized, to some modest level of detail, the fancy new technique called “Anchored Hybrid Enrichment” (AHE), recently wielded by a team of our colleagues headed up by Dr. Nathan V. Whelan of Auburn University to construct a phylogenomic tree of the North American Pleuroceridae.  AHE analysis begins with the development of “anchors,” DNA probes designed to target and enrich highly conserved regions within the genomes of the big set of organisms under study.  The slang term for these highly-conserved anchor regions is “bait.”

So, this month, let’s talk about bait.  Nathan and his team selected five probe species to “maximize phylogenetic diversity” across the Pleuroceridae, extracted mRNA from that small subset, and ultimately developed from the transcriptomes thereof 742 baits, requiring that they be present in at least four of the five probe species and larger than 120 bp in length.

Upon further review, I would not fuss about the identifications of three of those five probe species: Leptoxis “ampla” [3], “Elimia” crenatella [4], and Lithasia geniculata [5].  I could not find images of those three shells in the supplementary materials provided (USNM 1638590, 1638624, and 1638596, respectively) but I could find images of other nominally conspecific individuals bearing the same locality data, and they looked OK.

Pleurocera clavaeformis in Pistol Creek [6]

Let me set aside Nathan’s fourth probe species, “Pleurocera prasinata,” and come back to that situation later in this essay.  With regard to the fifth individual snail that the Whelan team selected for the generation of their AHE bait set, which they identified as "Elimia clavaeformis," what a mess.

Back in August [20Aug25] we reviewed the paper I published in 2011 [6] vividly demonstrating cryptic phenotypic plasticity in populations of that species, best identified today as Pleurocera clavaeformis, as they range across East Tennessee.  The shell morphology demonstrated by many upstream populations, historically identified as “Goniobasis (or Elimia) acutocarinata” is gracile with a striking peripheral carination.  The shell morphology of populations inhabiting mid-sized rivers and streams is what everybody has always associated with typical Goniobasis (or “Elimia”) clavaeformis.  And the shell morphology of big river populations, historically identified as “Pleurocera unciale,” is smoothly-rounded and robust.

Now look at Nathan Whelan’s Figure 5, as reproduced below.  Even given a healthy appreciation of the dramatic phenotypic variation in shell morphology that can be demonstrated by the pleurocerid populations of North America, do the four shells depicted across the top row of that figure look like the same species as the four shells depicted on the bottom row?  Who among my vast and far-flung readership cannot immediately distinguish two subsets of shells in Nathan’s figure: an ABCD group and an EFGH group?  Contact me immediately!  Your FWGNA credentials are in dire jeopardy.

The four shells in the EFGH group on the bottom row of Nathan’s figure below all belonged to individual Pleurocera clavaeformis acutocarinata, fine, as we just spent the entirety of our August post laboring over, no problem there.  But the pair of smooth, squat-looking shells in the upper right, labeled C and D, belonged to two Pleurocera simplex of very typical morphology.  And the pair of smooth, slender-looking shells in the upper left, labeled A and B, belonged to its sibling species, Pleurocera gabbiana.

“Figure 5. Shell morphology of Elimia clavaeformis s.l.” [1]

My readership can be forgiven for confusing simplex and gabbiana.  They co-occur in mixed populations throughout much of East Tennessee, and I myself was only able to distinguish them after rather tedious allozyme and morphometric studies, which I published in the FMCS newsletter Ellipsaria in 2016 [7], and relayed in three posts to this blog later that same year [2].

But alas, I bear bad news. A team of five professional malacologists, headed by our own colleague Nathan Whelan, identified all eight of the shells shown in Figure 5 above as belonging to “Elimia clavaeformis s.l.”

It is difficult for me to understand how the Whelan team missed my 2016 simplex/gabbiana series in Ellipsaria [8], near impossible for me to understand how they missed my 2011 paper on clavaeformis published in Malacologia, and inconceivable to me that they could confuse clavaeformis with simplex.  Yet that is exactly what they did.

Both shell A and shell F [9] were collected sympatrically at Turkey Creek, about 20 km W of Knoxville.  Both C and G were also collected sympatrically from Pistol Creek at Maryville, 25 km S of Knoxville, one of the four streams I myself sampled for my 2011 clavaeformis study, and also where I conducted my simplex/gabbiana study in 2016.  The reproductive isolation between the clavaeformis and simplex populations inhabiting those creeks is writ vivid on the shells of the animals that bear them, entirely distinctive, 100% sortable at streambank.  For five of our colleagues to confuse P. clavaeformis with P. simplex/gabbiana four separate times, cavalierly dismissive or abysmally ignorant of previously published research, and advertise their abject confusion in a full-page montage in an international scientific journal, is the most inconceivable concatenation of error upon error since Vizzini drank the goblet.

Now here is some good news.  Nathan’s supplementary material included a photo of the shell on the back of the individual snail from which he developed his AHE bait set, and it turns out to have indeed belonged to a Pleurocera clavaeformis acutocarinata.  So, his initial identification was correct.

And here is a second item of even better news.  Even if that snail had been misidentified, it would not have mattered.  To generate a good AHE bait set, all that really mattered to the Whelan team was that the five probe species they selected cover the range of genetic divergence across the 194 snails they intended to troll with them.  Which four of the five probe species certainly did.  Which almost brings us back to probe species #4 (of 5), which Nathan identified as “Pleurocera prasinata.”  But let’s put that issue off again.  And continue to move forward.

Seven of the eight snails depicted in Figure 5 above were included among the 194 test individuals in Nathan’s overall AHE analysis, and those seven did appear hanging among the branches in the big set of huge phylogenomic trees (generated under various assumptions) that he made available in his supplementary material.  Nathan only selected a couple of the Figure 5 shells to depict on the pretty tree he ultimately published as Figure 4 in the journal article, reproduced below.  But the bottom line is this.

Figure 4 of Whelan and colleagues [1]

Snails A and B clustered in the group I have labeled Pleurocera gabbiana and snails C and D clustered in the group I have labeled Pleurocera simplex.  Snail G (Pistol Creek) clustered in another big group a bit further around clockwise, with taxa Nathan labeled prasinata, acuta, pyrenella, attenuata, and so forth.  And snails E (Town Creek, not depicted) and F (Turkey Creek, depicted) clustered in a group much further around on the other side of the tree, between a snail Nathan labeled arachnoidea and a snail he labeled edgariana.

Knowing what we know about the biology of these remarkable organisms, the placement of the Pistol Creek individual is correct.  Look back up at the top of this essay, to that detail from Figure 2 of my 2011 paper.  As I mentioned above, the Pistol Creek P. clavaeformis population was one of the four I selected for allozyme analysis in that research.  My sample CA2 was taken from the exact spot that Nathan collected his individual Pistol Creek snail, an upstream reach running through a lovely little park in downtown Maryville.  I also sampled the Pistol Creek P. clavaeformis population 8 km further downstream, where it demonstrated the typical shell morphology depicted as C2.  And the shell labeled P2 was collected another 6 km downstream and out into the Little River.  That robust P2 shell phenotype, historically identified as “Pleurocera unciale” throughout East Tennessee, is indistinguishable from the shells Nathan has identified as “prasinata” and “aff. acuta” in his Figure 4 above.

And now at last, we see the problem with Nathan’s selection of both “Elimia clavaeformis” and “Pleurocera prasinata” as probe species from which to develop his AHE baits.  Far from “maximizing phylogenetic diversity,” those two nominal species appear so genetically similar as to be conspecific [10].

So now let’s turn our attention to that pair of P. clavaeformis acutocarinata (only one of which was figured) collected from Turkey Creek and Town Creek clustered way off in the upper left quadrant of Nathan’s tree with arachnoidea and edgariana.  Science is the construction of testable hypotheses about the natural world.  Let’s do some science.

In a series of essays posted on this blog late last year I reviewed what we know about interspecific hybridization in the North American Pleuroceridae, documented widespread hybridization between Pleurocera laqueata and P. troostiana in Middle Tennessee, North Alabama, and Kentucky, and suggested that reticulate evolution may have played a significant role in the diversification of the family across eastern North America generally [11].

The nomen “Elimia arachnoidea” depicted immediately above the Town/Turkey acutocarinata in Nathan’s Figure 4 is a junior synonym of Pleurocera troostiana, and the nomen “Elimia edgariana” depicted immediately below is a troostiana/laqueata hybrid [12].  The “Elimia sp. Red River” below edgariana is unidentifiable, but next below Red River is “Elimia curreyana,” a junior synonym of Pleurocera laqueata.  My hypothesis is that both of the individual snails sampled from Town Creek and Turkey Creek are clavaeformis/troostiana hybrids, a possibility I mentioned at the end of my essay back in August [20Aug25].

Or, the misplacement of the Turkey and Town Creek individuals might be attributable to a couple mislabeled sample tubes.  Or a spreadsheet error.  Any one of a million things could have gone wrong, who knows?  The larger point I am making here is that, because we bring an understanding of the evolutionary relationships of these animals to Nathan’s elaborate phylogenetic analysis, we can recognize error when it occurs.  Those two snails should have clustered with the conspecific Pistol Creek individual below.

In any case, regardless.  One might pray that, at the conclusion of his complex and intricate study, with his voluminous set of AHE results unfolded across the desk in front of him, showing the eight snails he had initially identified as “Elimia clavaeformis” splattered into four clusters across three quadrants of his colorful and richly-detailed Figure 4, Nathan Whelan might have been able to diagnose his prodigious clavaeformis confusion.  Alas, no. Ultimately, Nathan identified the incongruous collection of pleurocerids depicted in his Figure 5 as “two distinct lineages of E. clavaeformis and of its putative synonym, E. acutocarinata.”  Of such is prodigious polyphyly born.

I have said it, by actual count, 73 times over the 28-year history of this blog, but I am going to say it again, number 74, count ‘em all.  Gene trees are dependent variables, not independent variables.  They can be useful to test an hypothesis brought to them; they cannot yield an hypothesis to test.  They are the last chapter of your dissertation, not the first. Only if you have developed an understanding of the evolutionary relationships of a set of organisms by finer, independent means, can you interpret what a gene tree is telling you.

But let us finish this month’s essay on the same positive note with which we ended in September.  Bringing a finer understanding of the evolutionary relationships among the North American pleurocerid snails with us to the analysis, Nathan Whelan’s AHE gene tree really does have something important to tell us about the entire pleurocerid forest.  Which we will interpret together, next time.

Notes:

[1] Whelan, N. V., Johnson, P. D., Garner, J. T., Garrison, N. L., & Strong, E. E. (2022). Prodigious polyphyly in Pleuroceridae (Gastropoda: Cerithioidea). Bulletin of the Society of Systematic Biologists, 1(2). https://doi.org/10.18061/bssb.v1i2.8419

[2] My blog posts on the Pleurocera simplex/gabbiana sibling species pair:

• 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]

[3] Well, I suppose I might fuss a bit about identifying any population of pleurocerids as Leptoxis “ampla.”  At least three good studies have been published in recent years strongly suggesting that Anthony’s (1855) nomen ampla is a junior synonym of Conrad’s (1834) picta.  More next month.

[4] I don’t have any insight to offer regarding the evolutionary relationships between pleurocerid populations identified as “Elimia crenatella” in Alabama and any of the other scores of nominal pleurocerid species inhabiting the malacologically benighted Mobile Basin.  This is because I have very little field experience in the region to draw upon.  Still waiting on my invitation, long time coming.

 

[5] For more about the fascinating evolutionary biology of Lithasia geniculata, see:

• Intrapopulation gene flow: Lithasia geniculata in the Duck River [7Dec21]
• What is character phase disequilibrium? [4Jan22]
• The third-most amazing research results ever published for the genetics of a freshwater gastropod population, and the fourth-most amazing, too. [3Mar22]

[6] Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [pdf]

 

[7] My series of published notes on the simplex/gabbiana sibling species pair:

• Dillon, R. T. (2016) Two reproductively isolated populations cryptic under Pleurocera simplex (Say, 1825) inhabiting Pistol Creek in Maryville, Tennessee.  Ellipsaria 18(2): 15-16. [pdf]
• Dillon, R. T. & J. D. Robinson (2016) The identity of the "fat simplex" population inhabiting Pistol Creek in Maryville, Tennessee.  Ellipsaria 18(2): 16-18. [pdf]
• Dillon, R. T. (2016)  Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825).  Ellipsaria 18(3): 10 - 12.  [pdf]

[8] Some not-insubstantial irony here.  From 2016 - 2020 I regularly published my technical results as notes in the FMCS Newsletter, “Ellipsaria.”  That stopped in 2020, when it was called to the attention of editor John Jenkinson that Ellipsaria contents were being indexed by Google Scholar, and hence that results such as mine might be reaching a wider audience.  According to the minutes of the FMCS Board Meeting of November 2, 2020, the complaint that prompted Dr. Jenkinson’s decision was lodged by one Dr. Nathan V. Whelan.

[9] I caught a small error in the caption of Nathan’s Figure 5.  Shells A and F were collected from Turkey Creek, not A and E.

 

[10] I both elaborated and belabored the genetic similarity between East Tennessee populations of Pleurocera clavaeformis and Alabama populations identified as “Pleurocera prasinata” back in 2016, here:

• Pleurocera clavaeformis in the Mobile Basin? [12July16]

[11] For a review of intraspecific hybridization in the North American Pleuroceridae, see:

• Widespread hybridization between Pleurocera laqueata and P. troostiana in streams of the Tennessee/Cumberland [15Oct24]
• Reticulate evolution in the North American Pleuroceridae [12Nov24]

[12] And for a review of the taxonomic consequences of widespread hybridization in pleurocerid populations referred to arachnoidea, edgariana, curreyana, and 23 other specific nomina, see:

• Taxonomy of the Pleurocera laqueata/troostiana complex. Part I [10Dec24]
• Taxonomy of the Pleurocera laqueata/troostiana complex, Part II [14Jan25]

Anchored hybrid enrichment, Pleurocerid evolution, and the obovata confirmation

Review: Whelan, N. V., Johnson, P. D., Garner, J. T., Garrison, N. L., & Strong, E. E. (2022). Prodigious polyphyly in Pleuroceridae (Gastropoda: Cerithioidea). Bulletin of the Society of Systematic Biologists, 1(2). https://doi.org/10.18061/bssb.v1i2.8419

Although I myself have never harbored any aspiration to reconstruct a phylogenetic tree [1], I can understand the intellectual appeal of the fancy new whole-genome technique called “Anchored Hybrid Enrichment” (AHE) to the not-insubstantial fraction of my professional colleagues who feel differently.  AHE reminds me of an obsolete technique called single-copy DNA hybridization, which I greatly admired in the 1980s, where pioneering researchers like Sibley & Ahlquist [3] extracted the entire genomic DNA of bird #1 and bird #2, chopped it, melted it to single strands, then cooled it to the temperature that repetitive DNA would reanneal and stick in a hydroxyapatite column, eluting just the single-copy DNA out the bottom.  They would then mark the single-copy DNA of bird #1 with P-32, mix it with a large excess of single-copy DNA of bird #2, melt the mixture again, and gradually elute the mixture back through the hydroxyapatite, cooling slowly, to estimate the similarity of bird #1 and bird #2 across all their single-copy genes together.

The terrible flaw in the technique of single-copy DNA hybridization, as practiced in the 1980s, was that it yielded a matrix of overall genetic similarities across pairs in a set of study individuals, committing the mortal sin of being phenetic.  The Sibley & Ahlquist hypothesis for the evolution of birds disappeared into the footnotes many years ago [4].  If only there were a technique to do exactly the same thing with a bazillion single-character nucleotide differences, that could be sanctioned as cladistic, am I right?

Anchored Hybrid Enrichment [6]

In recent years, the coupling of mind-numbing computer power with high-throughput DNA sequencing technology has spawned a variety of whole-genome phylogenetic techniques (“phylogenomics”) that harken back to the days of Sibley & Ahlquist.  As the name implies, anchored hybrid enrichment [5] depends on the development of “anchors,” DNA probes to target and enrich highly conserved regions within the whole genomes of a set of organisms under study.  The idea is that the regions flanking such probe areas – not the probe areas themselves – are likely to be less conserved, and hence have some utility in phylogenetic construction.

So, in June of 2022 a research group of our own colleagues, headed by Nathan Whelan of Auburn University, published the first application of AHE technology to the reconstruction of a molluscan phylogeny, as far as I know [7].  And it was to the North American Pleuroceridae that their attention was drawn.

Bless their hearts, all five members of the team.  I mean that sincerely.  A lot of what I am getting ready to write over the next couple essays will be critical.  Such is science.  But I do not mean to minimize their innovative spirit, their commendable effort, or their motives, which were the highest.  We really do need a lot more research like this, and a lot more researchers like Nathan Whelan and his colleagues to do it.  Better understanding of the basic evolutionary biology of their living, breathing study animals, before chopping them into massively-parallel chum and high-through-putting them off the stern of a next-generation sauceboat, is all that is wanted.  That, plus some nod toward calibration, for a change.  Oh, and a passing familiarity with the recently published scientific literature on the subject matter would have been really helpful, too.

OK, first.  Regarding the generation of the AHE probes (or “baits”).  Rather than extracting total genomic DNA, the Whelan team extracted mRNA from five pleurocerid species “chosen to maximize phylogenetic diversity,” which we will call “Probe Species A – E.”  Those five mRNA samples were mailed off to Rockville, Maryland, for the generation of transcriptomes, that subset of the genome actually doing something.  I like that.  That’s a good way to factor out junk DNA before stepping up to the starting block.  Ultimately, our colleagues identified 742 baits “present in at least 4 of 5 pleurocerid transcriptomes and larger than 120 bp in length.”

Figure 4 of Whelan et al [7]

These 742 baits were trolled through a school of 192 sharks – and by sharks, I mean the whole genomes of individual pleurocerid snails – representing 92 putative species, with two rays – and by rays, I mean the whole genomes of two individual Juga plicifera from the west coast – to serve as an outgroup.  And I do want to commend the Whelan group for the care and completeness they took in documenting their study sample.  The shells from each of those 194 individual snails were numbered, photographed, and deposited in the USNM.  And complete locality data for all 194 snails were made available in tabular form from the journal website, as well as a big folder of 194 jpeg images.  We who sail in your chum salute you, colleagues.

One could only wish that the actual fishing trip hadn’t turned so fraught with peril.  Total genomic DNA was extracted from those 194 individuals and sent off to Gainesville, Florida, for AHE library prep and sequencing.  Three different datasets were generated by this process, under three different assumptions (masked, probe, and full), and trees generated using two different algorithms (ML and ASTRAL).  The ASTRAL method also involved two assumptions (TaxMap yes or no), yielding N = 9 gigantic, eye-assaulting, headache-inducing phylogenomic trees.  Eight of these trees were consigned to the Supplementary Material.  And the tree that Whelan and colleagues liked best, probe + ASTRAL + TaxMap, was published in the journal article, in circular format, as though we evolutionary biologists were birds looking down upon a tree of sharks, which are actually snails, which the most spectacular mixture of metaphor I have muddled in recent memory.

But when I first laid eyes on Nathan Whelan’s favorite phylogenomic tree for the gastropod family we both so obviously love, I had to smile.  I honestly cannot remember when the result of an evolutionary study gave me more pleasure.

As (I imagine) my loyal readership will remember, in 2014 I published a paper in Zoological Studies [8] extending my research on cryptic phenotypic plasticity to the widespread set of pleurocerid populations variously identified using the specific nomina livescens, semicarinata, and obovata, variously allocated to the genera Pleurocera or Goniobasis or Elimia or Lithasia.  I showed that all of these populations, including those traditionally identified as Lithasia obovata and Elimia semicarinata, are conspecific, their evolutionary relationship obscured by extreme phenotypic plasticity of shell.  See my blog post of [11July14] for a complete review.

The power of a scientific hypothesis is its ability to predict, to yield new information, to give something back that was not put into it.  That Nathan Whelan and his team successfully recovered the close evolutionary relationship between N = 2 snails they identified as “Elimia semicarinata” from the Middle Fork Vermillion River in eastern Illinois and N = 2 snails they identified as “Lithasia obovata” from the Ohio River between Indiana and Kentucky is a demonstration that this new AHE technique can have great power.

Yes, the assumptions that brought us the Whelan et al. phylogenomic tree are as manifold and diverse as shells on the beach, and yes, the opportunities for experimental error were as countably-infinite as the nucleotides.  And yes, the human error introduced into the Whelan study most certainly did reach embarrassing levels at times, as we shall see.

But hiding in the abstract splatter of drab shells and gaily-colored tangle of branches above – yellow Elimia, blue Pleurocera, green Lithasia and purple Leptoxis – is genuinely important insight regarding the evolution of the North American pleurocerid snails.  There is sweet music to be heard in that noise, if one brings an understanding of the biology of those wonderful organisms to the concert.  Which we will do, next time.

 

Notes:

 

[1] By way of full disclosure, my colleagues and I did reconstruct a species tree for the Physidae in 2011 [2], which we compared to a gene tree previously published.  But please note that every other treelike diagram I have ever published in my entire career has been a nearest-neighbor or cluster analysis, intended to represent genetic similarities only.  Which I have always oriented sideways, to look as little like a tree as possible.

 

[2] Dillon, R. T., A. R. Wethington, and C. Lydeard (2011) The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa.  BMC Evolutionary Biology 11:144. [pdf]

 

[3] Sibley, C.G. & J.E. Ahlquist (1990) Phylogeny and Classification of birds.  Yale University Press, New Haven.

 

[4] Prum RO, Berv JS, Dornburg A, Field DJ, Townsend JP, Lemmon EM, and Lemmon AR (2015). A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing. Nature 526(7574):569-73. doi: 10.1038/nature15697.

 

[5] For more about Anchored Hybrid Enrichment, see:

• Lemmon, AR, SA Emme, and EM Lemmon (2012) Anchored hybrid enrichment for massively high-throughput phylogenomics.  Systematic Biology 61: 727 – 744.
• Lemmon, E.M., and A.R. Lemmon (2013) High-throughput genomic data in systematics and phylogenetics.  Annual Review of Ecology and Systematics 44: 99 – 121.
• Jones, M.R. and J.M. Good (2016) Targeted capture in evolutionary and ecological genomics.  Molecular Ecology 25: 185 – 202.

[6] This graphical summary is heavily modified from an original illustration by Bas Blankevoort in

Gravendeel, B., D. Bogarin, A. Dirks-Mulder, R. Kusuma Wati, and D. Pramanik (2018) The orchid genomic toolkit.  Cah. Soc. Fr. Orch. 9.

 

[7] Whelan, N. V., Johnson, P. D., Garner, J. T., Garrison, N. L., & Strong, E. E. (2022). Prodigious polyphyly in Pleuroceridae (Gastropoda: Cerithioidea). Bulletin of the Society of Systematic Biologists, 1(2). https://doi.org/10.18061/bssb.v1i2.8419

 

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

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

The birth, death, and resurrection of Melania acutocarinata

It was from the pen of our old buddy Isaac Lea [5Nov19], early on a frosty morning in 1841, that Melania acuto-carinata was born, in brief Latinate form, number 46 in a litter of 57 pleurocerid puppies [1].  Among its more famous siblings were #2 castanea [12Nov24], #7 ebenum [3Oct19], #10 clavaeformis [20Feb07, 12Oct09], #25 virgata [9May23], and #37 edgariana [5June20].  Lea followed with more complete English descriptions and figures of the entire litter in 1843 [2].

The type locality of Melania acuto-carinata was vague, “Tenn., Dr Currey.”  We first met Dr. Richard Owen Currey (1816 – 1865), impeccably credentialed with Presbyterian heritage and doctorate from the University of Pennsylvania, back in [3Oct19].  Currey was born and raised in Nashville but moved to Knoxville in 1846 to become “the first person with an earned doctorate to teach science” at East Tennessee University, later the University of Tennessee [3].

Melania acuto-carinata [2]

Lea’s description of the acuto-carinata shell was almost as vague as its type locality: “It seems to be distinct in its large carina which extends over all the whorls, but it is scarcely distinct on the last.”  Nevertheless, the nomen was launched into malacological stardom in 1858 by the English naturalists Henry and Arthur Adams, in their justifiably maligned “Genera of Recent Mollusca” [4].

It seems highly unlikely to me that either of the Adams brothers ever laid eyes on a North American pleurocerid snail in their entire, justifiably-maligned lives.  Tryon [5] reprinted their classification verbatim in 1873, “in order that the insufficiency of their genera may become more apparent from the incongruous assemblage of shells of which they have composed them.” 

 

Among the 16 genera Henry and Arthur recognized in the Melanidae was Io (of Lea) “remarkable for the peculiar elongation of the axis anteriorly, and for the spinose nature of the last whorl.”  Fine.  Then, as a subgenus of Io (good grief!) the Adams brothers proposed, “Elimia” for pleurocerids bearing shells fusiformly ovate, “whorls reticulate or nodulose, carinate in the middle.”  And under Elimia they listed 16 apparently random pleurocerid specific nomina in alphabetical order, the first of which was “acuticarinata, Lea,” without the dash, misspelled, bearing a shell that is neither reticulate nor nodulose.

 

Tryon dismissed the Adams classification from further consideration and assigned acutocarinata to the genus Goniobasis, newly proposed by his mentor Isaac Lea in 1862 [6].  And pretty much the entire malacological community fell in line with Tryon and Lea, most notably Calvin Goodrich [23Jan07], toward whom our attention now turns.

 

In a 1939 paper that should be better known than it is, Goodrich [7] used Goniobasis acutocarinata as an example of “depauperization,” by which he meant “the outward manifestation of disease, accident or malnutrition or a reaction to inimical environment.”  He wrote: 

“In Goniobasis, very loose coiling appears to be a sign of depauperization.  Goniobasis acutocarinata, although described as a species, has not been found in pure colonies. Lea, its author, had only one specimen.  The shell occurs as a rare variant among G. clavaeformis in springs and spring branches of East Tennessee.” 

For that reason, in his influential review of 1940, Goodrich [8] synonymized Melania acutocarinata Lea 1841 under Goniobasis clavaeformis (Lea 1841).  And that should have been the end of it.  Pleurocerid nomina that did not survive Calvin Goodrich’s 1930 – 1950 piecemeal crusade to monograph the family were not, as a rule, listed or figured in the Burch Bible [9].  They have died, been buried, and forgotten, RIP.

 

P. clavaeformis from Burch [9]

And that is indeed what happened to 12 of the 13 specific nomina [10] synonymized under clavaeformis by Goodrich in 1940.  They disappeared, never to be seen again.  But not acutocarinata.

Although generally following Goodrich quite closely, in the 1980s Jack Burch [9] gave Goodrich’s system one great big kick and a bunch of little tiny anonymous tweaks [11].  Out of a sense obligation to the letter of the ICZN law [12], Burch elected to resurrect the Adams brothers’ obscure Elimia in place of the widely used Goniobasis, confounding the work of Lea, Tryon, Goodrich and everybody else over the previous hundred years.  Following Henry Pilsbry [26Jan21] and Pilsbry’s local collaborator Samuel Rhoads [13, 14], Burch selected acutocarinata as the type of the Adams polyglot genus, by virtue of its first place in the alphabetized list of 16 odd lot species included.

 

And among his tweaks, Burch saved Isaac Lea’s acutocarinata.  To be precise, what he did was exhume two acutocarinata specimens from the UMMZ collection (Figs 404 and 405 above) and figure them alongside a singleton specimen of typical clavaeformis (Fig 403), labeling both of the former “E. acutocarinata = ? E. clavaeformis.”  Why did Burch dump 12 of Goodrich’s clavaeformis synonyms and double-figure the thirteenth?  I don’t know.  If it was a coincidence that acutocarinata was the type of his resurrected Elimia, it was certainly a convenient one.  But for whatever reason, Isaac Lea’s 1841 acutocarinata was spared into a kind of taxonomic purgatory, a place of shadows, neither alive nor dead.

 

In retrospect, I myself should have rescued the nomen from the netherworld for use at the subspecies level in my 2011 paper on Goodrichian Taxon Shift [15], later renamed cryptic phenotypic plasticity (CPP) [16].  Longtime readers will remember that I selected three sites in East Tennessee (and one in North Georgia) inhabited upstream by what I was calling, at that time, Goniobasis acutocarinata, in the mid-reaches by what I was calling, at that time, Goniobasis clavaeformis, and downstream by what I was calling, at that time, Pleurocera unciale.  Those sites included the famous [17] Indian Creek tributary of the Powell on the VA/TN border (IC, map way down below) and the even more famous [19] Pistol Creek of Maryville (PC), as well as the (rather obscure) Lick Creek tributary (LC) of the Hiwassee.

 

My allozyme analysis showed that each acutocarinata population was more genetically similar to its downstream clavaeformis population than to any other acutocarinata, and that each unciale population was genetically more similar to its upstream clavaeformis population than to any other unciale.  On that basis I folded both acutocarinata (no surprise) and unciale (big surprise) under clavaeformis, and both Goniobasis and Elimia under Pleurocera.

 

P. clavaeformis in Indian Creek, Va - Tn

I elected, however, to save uncialis/unciale at the subspecific level, as Pleurocera clavaeformis unciale (Hald 1841), both because of its utility to describe a familiar shell form, and its indexing function to the older scientific literature.  Note that subspecific designation does not carry with it any assumption regarding the heritability of the distinguishing characteristics, much less genetic relationships [20].  It’s just useful.

By the same reasoning, I should also have elevated acutocarinata back up from taxonomic purgatory to the subspecific level, as Pleurocera clavaeformis acutocarinata (Lea 1841).  That should have happened back in 2011.  In my own defense, all I can offer is that unciale was recognized at the species level by Goodrich in 1940, and acutocarinata was not, and resurrecting old pleurocerid names from the nether regions is not the direction anybody wants to go.  But I’ve done it subsequently [21].  The indexing utility of Lea’s nomen acutocarinata is just as significant as that of Haldeman’s unciale/uncialis, probably greater, and the shell morphology just as distinctive.

 

Then let it be so, better late than never.  This month I have added a new Pleurocera clavaeformis acutocarinata page to the FWGNA website, and a new photo to the FWGTN gallery, and a new entry in the FWGTN dichotomous key – the second time I have found myself doing this in recent memory [22].  The number of freshwater gastropod species and subspecies now covered by the FWGNA web resource is 145 + 1 = 146. 

 

And I have dug back through my old collections and re-identified 22 populations of P. clavaeformis from the typical subspecies to acutocarinata.  Almost all of these inhabit small streams south of Knoxville, primarily direct tributaries of the main river or of the Hiwassee.  The Indian Creek population, way up on the Virginia/Tennessee Line, appears to be an outlier.

 

In all my earlier writings on Goodrichian Taxon Shift and CPP, I have tended to favor explanations that are not heritably genetic.  Although I have always left room for natural selection, most of the column inches in the discussion sections of the papers I have published on this subject [23] focus on the (many and striking) laboratory demonstrations of ecophenotypic plasticity in freshwater gastropod shell morphology.

 

Reidentified acutocarinata in blue

Quite recently a third possible explanation has dawned on me, in the specific case of the apparent transition between P. clavaeformis populations bearing shells of the typical morphology, and those bearing shells of the acutocarinata morphology.  Hybridization [24].

Populations of P. clavaeformis reach maximum abundance in streams of moderate size – marginally wadeable, where you’d bait up for redeye bass and bream.  Populations certainly extend upstream into colder waters suitable for trout, however, where in East Tennessee they bump into big populations of P. troostiana [25].

 

It is conceivable to me that Pleurocera clavaeformis acutocarinata may be a clavaeformis x troostiana hybrid.  I don’t have any genetic evidence of that [26].  This is just a speculation, based on the acutocarinata shell, which appears morphologically intermediate between the two species, both in its carination and in its overall length-to-width, body-whorl-to-apex slenderness.

 

But to conclude, I must emphasize.  Whether the shell morphological distinction between the (now three) subspecies of P. clavaeformis results from adaptation, hybridization, or ecophenotypic plasticity is irrelevant to their subspecific identity.  Here we resurrect Lea’s 1841 nomen acutocarinata because it is a useful descriptor of shell form, and because it has appeared frequently in the published literature.  And will appear again.  Stay tuned.

Notes:

[1] Lea, Isaac (1841) Continuation of Mr. Lea's paper on New Fresh Water and Land Shells.  Proceedings of the American Philosophical Society 2: 11 – 15.

 

[2] Lea, Isaac (1843) Description of New Fresh Water and Land Shells.  Transactions of the American Philosophical Society (New Series) 8: 163 – 250.

 

[3] Quoting the Richard Owen Currey entry in the University of Tennessee “Volopedia” archives: “In 1861, Currey entered Confederate service as a chaplain-surgeon. In 1865, he started caring for Union prisoners in North Carolina. He died while working in a disease-infested hospital on February 17, 1865.” He must have been a wonderful man.  It would have been an honor to shake his hand.

 

[4] Adams, Henry and Arthur (1854 - 1858) The Genera of Recent Mollusca Arranged According to Their Organization. In Three Volumes.  London: J. Van Voorst.  The Melanidae is covered in Volume 1, pp 293 – 311.

 

[5] Tryon, G. W. (1873)  Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.

 

[6] Lea, Isaac (1862) Description of a new genus (Goniobasis) of the Family Melanidae and eighty-two new species. Proceedings of the Academy of Natural Science of Philadelphia 19: 262 – 272.

 

[7] Goodrich, Calvin (1939) Aspects of depauperization.  The Nautilus 52: 124 – 128.

 

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

 

[9] 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).

 

[10] Which I will not list, not even in the footnote of an obscure blog post.

 

[11] For the record, here is the complete text of Burch’s Supplemental Note #23: “Elimia H. and A. Adams (type species Melania acutocarinata Lea 1841 = Melania clavaeformis Lea 1841) is used in place of its better-known synonym Goniobasis Lea 1862 (type species Goniobasis osculata Lea 1862).  The classification of the genus Elimia presented here, and the distribution of the various recognized species and subspecies, is that of Goodrich (1930a, 1936, 1939d, e, 1940d, 1941a, b, c, 1942b, 1944d, 1945, 1950).  No attempt has been made to assess the taxonomic validity of the species and subspecies.”

 

[12] Burch, J. B. 2001. On the genus name Goniobasis (Elimia - Gastropoda: Pleuroceridae) and other recent nomenclatural inconsistencies. Walkerana 12:97-105.

 

[13] Pilsbry, H. and S. Rhoads. 1896. Contributions to the Zoology of Tennessee, Number 4, Mollusca. Proc. Acad. Nat. Sci. Phila. 1896:487-506.

 

[14] Note, however, that Pilsbry never used “Elimia” himself.  His imperial majesty continued to prefer Goniobasis in every paper he ever wrote or edited for The Nautilus.  And all the labels in the ANSP collection read “Goniobasis” to this day.  For example:

• Pilsbry, H. 1916. Goniobasis in western Pennsylvania. Nautilus 30:4-5.

[15] Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [pdf]

 

[16] For an entry into the rather extensive literature of CPP in North American pleurocerid snails, see:

• Goodrichian taxon shift [20Feb07]
• Mobile Basin III: Pleurocera puzzles [12Oct09]
• Pleurocera acuta is Pleurocera canaliculata [3June13]
• Pleurocera canaliculata and the process of scientific discovery [18June13]
• Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]

[17] I was wading in waters of Indian Creek, just like John, when the scales fell from my eyes.  John Robinson and I published a gray-literature report on those findings to the VDGIF in 2007 [18], and I coined the term “Goodrichian Taxon Shift” on this blog.  See:

• Goodrichian Taxon Shift [20Feb07]

[18] Dillon, R. T. & J. D. Robinson (2007b) The Goniobasis ("Elimia") of southwest Virginia, II. Shell morphological variation in Goniobasis clavaeformis. Report to the Virginia Division of Game and Inland Fisheries, contract 2006-9308. 12 pp. [pdf]

 

[19] It was at Pistol Creek that I ultimately discovered a forgotten pleurocerid species cryptic under P. simplex.  See:

• 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]

[20] Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits.  For an elaboration, see:

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

[21] I resurrected Isaac Lea’s (1862) lyonii, previously subsumed by Goodrich (1940) under laqueata, as a subspecies of P. troostiana in:

• The return of Captain Lyon [6July20]

[22]  I added a new Pleurocera laqueata castanea page to the FWGNA website last fall, with corresponding entries in the FWGTN species gallery and dichotomous key.  See:

• Reticulate evolution in the North American Pleuroceridae [12Nov24]

[23] Published papers on CPP:

• Dillon, R. T. (2011) Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [pdf]
• 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]
• Dillon, R. T. (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [pdf]

[24] For more on hybridization in pleurocerids, see:

• Widespread hybridization between Pleurocera laqueata and P. troostiana in streams of the Tennessee/Cumberland [15Oct24]
• Reticulate evolution in the North American Pleuroceridae [12Nov24]

[25] And simplex. And gabbiana.  But I don’t think that is relevant to my argument here.

 

[26] Just the opposite, actually.  In our VDGIF allozyme study [27], John Robinson and I did run gels on N = 31 G. clavaeformis (C1) and N = 30 G. “arachnoidea” (A1 = troostiana) from Indian Creek, finding a little bit of evidence of mixing at some loci, but fixed differences at three others.

 

[27] Dillon, R. T. & J. D. Robinson (2007a) The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey. Report to the Virginia Division of Game and Inland Fisheries, contract 2006-9308. 25 pp. [pdf]