Reticulate Evolution in the North American Pleuroceridae

Last month [15Oct24] we reviewed the evidence that populations of two pleurocerids widespread in the Greater Ohio Drainage, P. laqueata and P. troostiana, hybridize extensively in the rivers and streams of Middle Tennessee.  And our most useful genetic marker was shell plication, a scallop-shaped ridging pattern characteristic of P. laqueata, absent from P. troostiana outside the laqueata range, but variably present in troostiana populations overlapping with laqueata.

Sharing most of those same rivers and streams with both laqueata and troostiana are populations of a third pleurocerid species, P. simplex, our old friend familiar from five previous essays, see footnote [1] below to refresh your memory.  The FWGNA Project recognizes two subspecies of simplex: the typical form found in small streams throughout the greater Tennessee/Cumberland region and a paler, more heavily shelled form common in larger streams of the Cumberland drainage, extending into Central Kentucky, Pleurocera simplex ebenum.

In 1934, Calvin Goodrich [2] published #3 in his “Studies on the gastropod family Pleuroceridae” series, focusing on shell plication.  Here is a verbatim quote from page 5:

“G. ebenum (Lea), commonly a smooth species, occurs in the Cumberland River drainage basin. In the upper part of the drainage, material containing plicate shells has been taken. The only lot at hand that can be accepted as a “pure" race of these forms is from New River, Scott County, Tennessee. Of 46 shells from Straight Creek at Pineville, Bell County, Kentucky, 54.4 per cent are plicate. In the Cumberland River a few miles below Pineville, 18 per cent of 72 shells are so sculptured; 74 shells of ebenum taken just above the falls of the Cumberland are 14.8 per cent plicate. The only specimens from the river below the falls which have been seen, taken at Smith's Shoals near Burnside, Pulaski County, Kentucky, are all smooth; so also are shells of all lots of the species ranging as far to the west as streams of Dickson County, Tennessee.”

Yes, all of that is true.  I myself have confirmed at least seven populations of P. simplex ebenum bearing lightly plicate shells scattered about Middle Tennessee, in minor tributaries of the Cumberland, the Harpeth, the Red, and the Duck.  All these populations co-occur with populations of P. simplex bearing normal, smooth shells and populations of (you guessed it) P. laqueata.  The first three shells figured at left below were collected from the backs of pleurocerids inhabiting Brush Creek, a tributary of the Red-Cumberland in Robertson County, NW of Nashville (36.4342, -87.0662): an apparently pure P. simplex, an apparently pure P. laqueata, and what most certainly appears to be a simplex/laqueata hybrid (“castanea”), almost entirely smooth but bearing tiny plications around the apex.

Reticulate evolution in the Pleuroceridae

Exactly as is the case with P. troostiana, P simplex populations inhabiting East Tennessee, where P. laqueata does not occur, never bear plicate shells.  Only where the ranges of P. simplex and P. laqueata overlap in Middle Tennessee does one find pleurocerid populations bearing fat, pear-shaped simplex-looking shells with tiny apical plications.

There is not a shadow of doubt in my mind that P. simplex hybridizes with P. laqueata, just as P. laqueata hybridizes with P. troostiana.  The two shells at right were sampled from Spring Creek east of Nashville, carried over from last month: an apparently pure P. troostiana and a laqueata/troostiana hybrid (“perstriata.”)  This is reticulate evolution.

Digging back through the classic literature, it turns out that Isaac Lea described a Melania castanea in 1841, the shell of which appears to be a perfect match for the simplex/laqueata hybrid populations I have been referring to here.  Lea’s brief Latinate description appeared in that same early work that featured such notables as clavaeformis, ebenum, and edgariana [3], with a longer English description and figure following in 1843 [4].  Lea’s type locality, “Maury County, Tenn.” is in the upper Duck River drainage, where simplex and laqueata are both common.  Calvin Goodrich [5] lowered Lea’s nomen castanea to subspecific status under Goniobasis laqueata in 1940, giving its range as “Headwaters of the Duck River, Tennessee.”

Melania castanea [4]

OK, fine.  Given that we have recognized three subspecific names for laqueata/troostiana hybrids, I suppose it is only fair to recognize a subspecific name for hybrids between P. laqueata and P. simplex.  So, this week I have added a new (sub)species page to the FWGNA website for Pleurocera laqueata castanea (Lea 1841), with corresponding entries in the gallery and dichotomous key for the Tennessee/Cumberland [6].  This is the 135th species or subspecies of freshwater gastropod we have recognized as valid in our 21-state study region.

I am every bit as certain that P. simplex hybridizes with P. semicarinata in Kentucky and Tennessee, although I have no genetic data or photos to enter into evidence.  The two species are only distinguishable by subtle differences in shell shape, the former bearing fatter shells with a larger body whorl, neither demonstrating any sort of shell sculpture (beyond a carinate upper whorl) that might serve as a discrete marker.  The range of P. semicarinata semicarinata overlaps that of P. simplex broadly in the Cumberland, Green, and Kentucky Rivers, and extends much further north, up into Wisconsin, Michigan and New York, where chubby-shelled populations are referred to the subspecies P. semicarinata livescens.

And I am still amazed [7] by the 1994 allozyme study of Bianchi and colleagues [8] demonstrating hybridization between Great Lakes P. semicarinata livescens and the Hudson River population of Pleurocera virginica through the Erie Canal.  Those two species bear strikingly different shell morphologies, have entirely distinct ranges, and could not have shared a common ancestor in many, many millions of years.  Perhaps since the Appalachian Orogeny?

Hybridizing? [9]

Yes, that is my next point.  The architects of the Modern Synthesis generally seem to have considered hybrid zones an unstable and transitory step toward speciation [11].  I am sympathetic with the Darwinian rationale for such an hypothesis, and admit it could certainly hold in many cases.  But more recently the research emphasis seems to have shifted toward hybrid zones that give evidence of stability and permanence [12].

The photo below comes from the 8Mar24 issue of Science [13].  Here’s the caption: “This fish is the hybrid offspring of an alligator gar and a spotted gar – members of genera that last shared a common ancestor at least 100 million years ago.”

The paper reviewed, by Brownstein and colleagues [14], detailed the results of a survey of 1,105 exons over 481 vertebrate species, demonstrating exceptionally slow rates of molecular evolution in gars and sturgeons.  Yet gar species last sharing a common ancestor no later than the Cretaceous still hybridize naturally in the greater Ohio and southern Mississippi drainages today.

Could some cranky, washed-up old crackpot wading those same rivers and streams, throwing snails into a bucket and measuring them with rusty calipers, achieve the same results as an international team of eight scientists from six different institutions with “massive” DNA data sets and ten different sources of funding?

The distribution of pleurocerid snails in the rivers and streams of North America is whispering a story to us in a language that we do not understand.  It is an ancient story of colliding continents and earthquakes and mountains 10,000 feet high, eroding and shifting and washing into the sea.  Most of the pleurocerids of the Greater Ohio drainage, including P. simplex and P. troostiana, range across the entirety of the state of Tennessee, as well as into Kentucky and North Alabama and even into SW Virginia.  Then why are populations of P. laqueata absent East of Chattanooga?  Is their dispersal capability so much poorer than P. simplex and P. troostiana that they are unable to penetrate Walden’s Ridge?  I simply do not think so.  Here is the story that I hear the pleurocerids whispering to me.

The story I hear is that the crest of the ancient Appalachians, at some point in the millions of years of their orogeny, was approximately where Walden’s Ridge lies today, at the eastern edge of the Cumberland Plateau.  Pleurocera laqueata evolved on the west side of that crest, while P. troostiana and P. simplex evolved on the East.  Then the mountains eroded such that the divide shifted east, opening a hole at Chattanooga, switching the flow of the rivers in which troostiana and simplex evolved from east to west, bringing those pleurocerid populations into secondary contact with laqueata.

I have said it many times [15], but I will say it again.  A step off the creek bank in the Southern Appalachians is a step back millions of years.  Look around you, colleagues, look!  Those banks are covered with mosses and liverworts, horsetails and ferns.  The waters team with dragonflies and stoneflies, gars and hellbenders.  And pleurocerid snails jostle each other to graze across every square inch of substrate.

Why does this entire ecosystem seem frozen in time?  My hypothesis calls on three independent sets of factors: environmental, genetic and historical.

First, the freshwater environment is more stable than that of the land.  Water temperatures lag behind and buffer air temperatures.  That buffer is not just seasonal, it is climatological.  The temperature in smaller streams, in particular, typically remains very close to that of the ground, 10 – 15 degrees Centigrade year round.  Such environments are not simply protected from hot Julys, they are protected from ice ages.  And the lower the temperature of the environment, I might add, the slower the generation times of its poikilothermic biota.

Rock Island State Park, TN [16]

Rainfall and storm are similarly buffered.  Droughts obviously have less effect on rivers than on the surrounding land, ditto wind and fire.  The ecosystems of many (especially smaller) bodies of water are based on allochthonous input, rather than primary productivity, and life could more easily survive (let us say) a cometary impact, and a period of worldwide darkness.

Most of the above, it must be admitted, could also be said for the marine environment as well as the freshwater.  This calls upon a second set of factors, which are population genetic.

In two words, marine populations are gigantic and panmictic.  Almost all the mollusks, for example, retain a planktonic larval stage lasting at least a couple weeks, facilitating dispersal over very long distances.  Here on the Atlantic side, the population of commercially important eastern quahogs (“cherrystone” or “littleneck” clams), demonstrates no significant allelic frequency differences at multiple allozyme-encoding loci from Canada to Florida [17].  Ditto oysters, ditto oyster drills, ditto whelks, ditto periwinkles [18].

Consequently, when a beneficial mutation arises in a marine population, it spreads quickly in evolutionary time.  Diseases, predators, and other riders of the apocalypse spread as quickly as the angels.  Speciation is quick, extinction is quick, evolution is quick.  The marine molluscan fauna of the Virginia Pliocene does not look like the marine molluscan fauna of the Virginia Recent.

But for better or worse, freshwater populations are small and fragmented.  Evolution does not stop, of course; the molecular clock keeps ticking [19].  But when adaptations evolve (such as reproductive isolation, for example) they do not spread [20, 21].  The outward appearances of such populations, then, will give the impression of morphological stasis.

So, freshwaters are more environmentally stable than the land, and the populations inhabiting those freshwaters more genetically stable than those inhabiting the sea.  There is a third factor.  History.

The land mass that we today identify as the “Appalachians,” together with the freshwaters that drain those mountains to the ocean, is really, really old.  It is clear that several orogenies have taken place, beginning with the Grenville over one billion years ago, proceeding through the Taconic (500 mybp) and the Acadian (400 mybp), culminating with the Alleghanian Orogeny at the formation of Pangaea 300 mypb.

Did Cerithiacean gastropods crawl from the sea at that time, evolve into the first pleurocerids, disperse and diverge across drainage systems as they existed in the ancient Appalachians hundreds of millions of years ago, and then sit in evolutionary stasis as the mountains wore down around them?  Yes, I think so.

Next month… taxonomic implications.

Notes:

[1] See the following essays for a review of the biology of Pleurocera simplex, its sibling gabbiana and its subspecies ebenum:

• The cryptic Pleurocera of Maryville [13Sept16]
• The fat simplex of Maryville matches type [14Oct16]
• CPP Diary: Yankees at The Gap [4Aug19]
• CPP Diary: What is Pleurocera ebenum? [3Oct19]
• CPP Diary: The spurious Lithasia of Caney Fork [4Sept19]

[2] Goodrich, C. (1934)  Studies of the gastropod family Pleuroceridae – III.  Occasional Papers of the Museum of Zoology, University of Michigan 300: 1 – 11.

[3] 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.

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

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

[6] Alas, Pleurocera laqueata castanea cannot be retroactively included in the hardcopy FWGNA Volume 5, which came off the presses in the fall of 2023.  In our next edition, however, castanea will be FWGNA species Number 103.2.

[7] See my essay of 3Mar22 for rankings of a broad selection of freshwater gastropod papers by international amazingness units. The paper of Bianchi et al [8] scored a whopping 93.2 iau, good for first place in the population genetics subdivision:

• The third-most amazing research results ever published for the genetics of a freshwater gastropod population. And the fourth-most amazing, too. [3Mar22]

[8] Bianchi, T. S., G. M. Davis, and D. Strayer 1994.  An apparent hybrid zone between freshwater gastropod species Elimia livescens and E. virginica (Gastropoda: Pleuroceridae).  Am. Malac. Bull. 11: 73 – 78.

[9] From left to right.  Pleurocera simplex simplex from Brush Creek, Robertson Co, TN (see above).  Pleurocera simplex ebenum from the Falls of The Cumberland, Whitley Co, KY [see 3Oct19].  Pleurocera semicarinata semicarinata from Harrison Ck, Nelson Co, KY [see 6Sept17]. Pleurocera semicarinata livescens from Portage Ck, Washtenaw Co, MI [10]. Pleurocera virginica, an especially chubby shell from Deer Ck, Harford Co, MD courtesy R. Aguliar.

[10] “Station 2” of Dazo, B. C. (1965)  The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae).  Malacologia 3: 1 – 80.

[11] Dobzhansky, T. (1940) Speciation as a stage in evolutionary divergence. American Naturalist 74: 312 – 321.

[12] Barton, N.H. and G.M. Hewitt (1985) Analysis of hybrid zones.  Annual Review of Ecology and Systematics 16: 113-148.

[13] Heidt, A. (2024) Gars truly are “living fossils,” massive DNA data set shows.  Science 383 (6687): 1041.

[14] Brownstein, Chase B, Daniel J MacGuigan, Daemin Kim, Oliver Orr, Liandong Yang, Solomon R David, Brian Kreiser, and Thomas J Near (2024) The genomic signatures of evolutionary stasis.  Evolution 78: 821 – 834. https://doi.org/10.1093/evolut/qpae028

[15] 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.  (Rosemary Mackay Award)  [pdf].  For a review, see:

• The snails the dinosaurs saw [16Mar09]

[16] The Caney/Collins River system, impounded below Rock Island State Park, was home to at least eight species of pleurocerid snails, including P. simplex [4Sept19], P. troostiana edgariana [5June20] and the pleurocerid megafauna hung in Cousin Bob Winter’s prehistoric necklace as depicted [5Apr22].

[17] The population genetic literature on Atlantic coastal bivalves is very large.  For a review of the Mercenaria case, see:

• Dillon, R.T. and J.J. Manzi (1992) Population genetics of the hard clam, Mercenaria mercenaria, at the northern limit of its range.  Canadian Journal of Fisheries and Aquatic Sciences 49:2574-2578. [pdf]

[18] For reviews of the genetics of marine gastropod populations on the Atlantic coast, see:

• Wise, J., M. G. Harasewych, and R. T. Dillon. (2004)  Population divergence in the sinistral Busycon whelks of North America, with special reference to the east Florida ecotone.  Marine Biology 145:1167-1179. [pdf]
• Dayan, N.S., and R.T. Dillon (1995) Florida as a biogeographic boundary: Evidence from the population genetics of Littorina irrorata. The Nautilus 108: 49-54. [pdf]

[19] An inexorable (but not especially clocklike) accumulation of neutral mutations yields the startlingly high levels of mtDNA sequence divergence often recorded among pleurocerid populations.  And the crazy distribution patterns of those crazy mtDNA sequence markers come from rare long-distance dispersal events which, given hundreds of millions of years of birds wading through these streams and flying off elsewhere, do happen.  For more about my Jetlagged Wildebeest Model of mitochondrial superheterogeneity, see:

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

[20] The absence of any correlation between genetic divergence and environmental difference in isolated populations of Pleurocera proxima, together with strong correlations between genetic divergence and geographic distance, supports this hypothesis.  See:

• Dillon, R.T. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82. [pdf]

[21] Evidence from Pleurocera proxima transplant experiments is also consistent with the hypothesis that beneficial genomes may be prevented from spread by the isolated character of southern Appalachian streams.  See:

• Dillon, R.T. (1988) Evolution from transplants between genetically distinct populations of freshwater snails. Genetica 76: 111-119. [pdf]

Widespread hybridization between Pleurocera laqueata and P. troostiana in streams of the Tennessee/Cumberland

Editor’s Note – This essay is considerably more data-heavy than we normally post in the columns of this blog.  We apologize in advance.  Readers with the fortitude to slog through a column yard of charts and graphs and regression analyses, however, will be rewarded with a real slam-bang finish next month, I promise you.  So, buck up.

In last month’s essay [18Sept24] we focused on Pleurocera laqueata, a widespread and common inhabitant of streams and rivers in Middle Tennessee, central Kentucky, and North Alabama.  The species was described by Thomas Say in 1829 [1] from specimens collected by Prof. Gerard Troost in the “Cumberland River,” an overly broad region which we ultimately restricted to Browns Creek, running through the state fairgrounds in Nashville.  The topotypic population of P. laqueata bears shells that are variably plicate but never striate, matching Say’s original description.

In direct contrast stands Pleurocera troostiana, also first collected by Prof. Troost [6Dec19] but described a bit later by Thomas Say’s successor at the ANSP, Isaac Lea around 1838 [2].  In its East Tennessee type locality, P. troostiana bears shells that are variably striate, but entirely without plication.  In a painfully detailed and ultimately exhausting series of six essays posted on this blog between December 2019 and July 2020, we reviewed the shell morphological variation demonstrated by populations of P. troostiana across Tennessee, Kentucky, and North Alabama, and the elaborate taxonomy that developed in the 19th century in an attempt to capture it.

So, if you have more than a casual interest in the taxonomy, systematics, and evolution of the North American Pleuroceridae, I would encourage you to go back and click through my 2019 - 20 series on P. troostiana from the links at footnote [3] below and download the pdf summary for your files.  Otherwise, here is a quick summary.

The range of variably-striate-but-never-plicate populations of P. troostiana, which we refer to as P. troostiana troostiana or P. troostiana sensu strictu (s.s.) is shown in blue above.  That is all you will find in East Tennessee.  West of Chattanooga, however, as the Tennessee River breaks through Walden Ridge into Alabama and Middle Tennessee, the range of P. troostiana begins to overlap with the range of P. laqueata, shown as a dashed line.  And populations of P. troostiana bearing shells that are both striate and plicate, variously identified under the subspecific nomina perstriata (yellow), edgariana (red), and lyonii (gray), begin to predominate.  This is not a coincidence.

I have hypothesized that P. troostiana hybridizes with P. laqueata several times previously on this blog, most prominently in my P. troostiana perstriata essay of [15Apr20].  But to show this, we will require a second, independent genetic character of some sort, beyond shell sculpture.  Let me back up a couple steps and refocus this entire essay away from shell sculpture, and toward shell shape.

Quite a few 19th century authorities remarked on the “spire elevation” or slenderness of the P. troostiana shell.  Isaac Lea, in his original description of 1838 [2], remarked that the shell of M. troostiana is “elevated.”  In 1841 Lea described M. teres (a troostiana synonym) as “remarkably elevated, spire much drawn out,” and ditto “spire drawn out” for a second troostiana synonym, M. strigosa [4].  John G. Anthony [5] described his M. arachnoidea (yet another troostiana synonym) as “rather thin, spire slender and much elevated” in 1854.

Now I daresay that no man nor beast who ever held a gastropod shell in hand, nor cracked it open with tooth, nor crushed it with claw, has ever in the history of this wide earth been more sensitive to that portion of the variance in shell shape that is not heritably genetic than the humble author of the present essay [6].  My filing cabinets bulge with papers vividly demonstrating ecophenotypic effects on gastropod shell morphology.  Bulge.  I cannot close them.  They remain ajar, to scar my wife’s shoulders should she dare enter the sanctum sanctorum wherein I lurk, writing quaint and curious blog posts such as this.

Shell shape and shell sculpture in pure populations.

But the heritable component of shell shape in gastropod mollusks is equally undeniable.  Working with Physa acuta in controlled conditions, I have estimated the heritability of simple shell length (SL) as h^2 = 0.429, and that of body whorl length (B) as h^2 = 0.321 [7].  In recent years I have favored the simple regression of shell width on shell length [8], or body whorl height (B) on apex height (SL), as a quick and reliable method of extracting the heritable component of shell morphological variance [9, 10] correcting for the age structure variance inevitable in wild populations.

So, last month I reported the collection of 29 topotypic P. laqueata from Browns Creek in Nashville, mapped as “L” at the top of this essay.  Of those, N = 25 were adults.  I measured total shell length for each (SL) and body whorl height (B), then calculated apex height as SL – B = A.   These data are plotted on the figure above.  The regression of B on A  was A = 0.70B – 1.19 (R = 0.77), a good fit.

I also measured N = 25 shells from a sample of P. troostiana troostiana collected from Steekee Creek at Loudon, Tennessee (35.7252, -84.3482), mapped as “T” way up above [11].  This is the type locality of J. G. Anthony’s (1854) Melania arachnoidea [5], synonymized under Isaac Lea’s Melania troostiana, see my essay of [7Jan20], FWGNA Circular 2 [pdf], or FWGNA Volume 6: 41 – 49 [publications].  The regression of body whorl height on apex height for troostiana was A = 0.98B – 1.11 (R = 0.90), an excellent fit.

Between the two elongated clusters of shell measurements, I have drawn a dashed line corresponding to the function A = 0.7B.  As a convenient approximation, it would appear that the two species can be distinguished by the ratio of shell apex height to body whorl height, greater than 0.7 for P. troostiana and less than 0.7 for P. laqueata.

Example Pleurocera from Spring Creek

So now let’s examine the Pleurocera in habiting Spring Creek (Wilson County, TN), a small tributary of the Cumberland River about 45 km east of the state fairgrounds in Nashville, mapped as “h” way up above (36.1800, -86.2411).  The Tennessee Department of Environment and Conservation took a quantitative sample of the Spring Creek macrobenthos back in August of 2014, using a kick net along three linear meters of creek bank to good effect, returning N = 185 Pleurocera [12].

I subsampled the N = 30 largest adults, measured their shells, and categorized the sculpture on their body whorl, ultimately recognizing (with some head-scratching) N = 9 striate (only), N = 8 plicate (only), and the remainder N = 13 as both striate and plicate.  The result is graphed below.

There is clearly a significant relationship between shell sculpture and shell shape in this sample of 30 pleurocerid snails, such that the fraction bearing smaller body whorls for their apex height (A > 0.7B) tend to bear striation (only) on their body whorl, and the fraction bearing larger body whorls for their apex height (A < 0.7B) bear plication (only) on their body whorl. With just those two open circles misclassified above the dashed line above, the Fisher’s exact probability is p = 0.002 [13].

Shell shape and shell sculpture in Spring Ck.

The most likely explanation for this phenomenon, which we have labeled “character phase disequilibrium” [4Jan22] is nonrandom mating.  The data graphed in the figure above strongly suggest some sort of reproductive isolation between the slender-shelled striate pleurocerid population of Spring Creek and the fat-shelled plicate population.  But the data also suggest that reproductive isolation is incomplete.  The largest fraction of the sample, 13/30 = 43%, seem to be hybrids, bearing both plication and striation on their body whorls.

Pleurocera laqueata and Pleurocera troostiana are distinct, reproductively isolated, biological species that hybridize extensively in rivers and streams throughout Middle Tennessee, southern Kentucky, and North Alabama.  Pleurocera troostiana populations are more common in the small creeks, and P. laqueata in the larger rivers, and the mixed populations in streams of intermediate size may comprise more hybrids than purebreds.

In keeping with taxonomic tradition, let us reserve the name P. laqueata for populations bearing shells entirely without striation, and P. troostiana troostiana for populations entirely without plication.  Then the subspecific nomina perstriata, edgariana, and lyonii will apply to the hybrids, according to their degree of shell sculpture.

OK, fine.  What might such widespread hybridization suggest about the evolution of the Pleuroceridae in North America?  Tune in next time.

Notes:

[1] Say, T. (1829) Descriptions of some new terrestrial and fluviatile shells of North America.  New Harmony Disseminator of Useful Knowledge 2(18): 275 – 277.

[2] Lea, Isaac (1838-39) Description of New Freshwater and Land Shells.  Transactions of the American Philosophical Society (New Series) 6: 1 – 154.

[3] Dillon, R.T., Jr.  (2020) The four subspecies of Pleurocera troostiana (Lea 1838), with synonymy.  FWGNA Circular 2: 1 - 5. [pdf]  This is a summary document for the observations, arguments, and hypotheses I advanced in a series of six blog posts to the FWGNA Blog:

• On The Trail of Professor Troost [6Dec19]
• CPP Diary: The Many Faces of Professor Troost [7Jan20]
• Huntsville Hunt [15Apr20]
• A House Divided [10May20]
• What is Melania edgariana? [5June20]
• The Return of Captain Lyon [6July20]

[4] Brief Latinate descriptions:

• Lea, Isaac (1841) Proceedings of the American Philosophical Society 2: 11 – 15.

More complete English descriptions with figures:

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

[5] Anthony, J.G. (1854) Descriptions of new fluviatile shells of the genus Melania Lam., from the western states of North America.  Annals of the Lyceum of Natural History of New York 6: 80 -132.

[6] In fact, I have designated an entire topic entitled “phenotypic plasticity” in the list of “labels” at the right margin of the present blog.  If you click that link you will find 24 essays (as of October 2024) touching upon the component of shell phenotype that is not heritably genetic.  Among the most prominent:

• New clothes for The Emperor [7Feb23]
• Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
• Pleurocera acuta is Pleurocera canaliculata [3June13]
• The Lymnaeidae 2012: A clue [9July12]
• Shell morphology, current, and substrate [18Feb05]

[7] Dillon, R. T., Jr. & S. J. Jacquemin (2015)  The heritability of shell morphometrics in the freshwater pulmonate gastropod Physa.  PLoS ONE 10(4): e0121962. [html] [pdf]  For a review, see:

• The heritability of shell morphology in Physa h^2 = 0.819! [15Apr15]

[8] Wethington, A.R., J. Wise, and R. T. Dillon (2009) Genetic and morphological characterization of the Physidae of South Carolina (Pulmonata: Basommatophora), with description of a new species.  The Nautilus 123: 282-292.  [pdf]

[9] 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]  For a review, see:

• The fat simplex of Maryville matches type [14Oct16]

[10] Dillon, R. T. (2016)  Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825).  Ellipsaria 18(3): 10 - 12.  [pdf]  For a review, see:

• One Goodrich Missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]

[11] I would have preferred to do these measurements on a sample of troostiana from Lea’s type locality at Mossy Creek, about  50 miles NE of Steekee Creek, but my sample size is insufficient.

[12] The gallon jug containing this (whole, unsorted) bulk sample was released to me by TNDEC-DWR personnel in Nashville on 14Jan21.

[13]  Here I count cases above the line and striate = 9, above and plicate = 2, below and plicate = 6, below and striate = 0.  The Fisher’s exact probability of that relationship between shell shape and shell sculpture would be p = 0.002.

The Type Locality of Melania laqueata

Nashville, home of the Grand Old Opry and the Brand-New Parthenon, was founded on a low hill overlooking the Cumberland River in 1779.  Well situated on a deepwater port with an easy float to the Mississippi River, at the northern terminus of the Natchez Trace to walk home again, the town grew quickly.  Nashville was chartered as a city shortly after Tennessee was granted statehood in 1796, and selected as state capitol in 1843, thanks in large part to her favorite son, Andrew Jackson.

Pleurocera laqueata [1]

My faithful readership might remember the thumbnail portrait we sketched back on [6Dec19] of a colorful character named Prof. Gerard Troost (1776 – 1850).  Troost was a pioneering Dutch American geologist, the founding president of the Academy of Natural Sciences in Philadelphia, who in 1825 sailed down the Ohio River with Thomas Say to the utopian community of New Harmony, Indiana.  A scant two years later, however, Troost accepted a call to the University of Nashville, becoming state geologist in 1831.  From that date until his death, he travelled widely across the Volunteer State, becoming (according to the Tennessee Encyclopedia online) “the state’s best-known antebellum scientist.”

Meanwhile, back in New Harmony, his buddy Thomas Say kept the printing presses cranking.  And in 1829 Say described a pleurocerid snail named Melania laqueata, as follows [1]:

“Shell oblong: spire longer than aperture, elevated, conic, acute: volutions moderately convex, with about seventeen regular, elevated, equal, equidistant costae on the superior half of each volution, extending from suture to suture, and but little lower, and becoming obsolete on the body whirl; suture moderately impressed; sinus obsolete.  This species was found by Dr. Troost in Cumberland River.  Aside from a difference in form, it may be distinguished from cancellata, nob., and catenaria, nob., by being altogether destitute of elevated revolving lines.  The young shell is carinated.”

Today, of the (roughly 1,000) names for species of pleurocerid snails described from the waters of North America, Thomas Say’s “Melania laqueata” is twelfth oldest [2].  And populations matching the snails that Gerard Troost sent to Thomas Say from the “Cumberland River,” reidentified as “Goniobasis” laqueata between 1862 and 1980, re-reidentified as “Elimia” laqueata 1980 – 2011, re-re-reidentified as Pleurocera laqueata in the modern day [3], have turned out to be common and widespread in rivers and streams throughout the greater Cumberland and Green River drainages, the upper Kentucky River, and Tennessee River drainages west of Chattanooga.

So, “The Cumberland River” is a big place.  Who could honor the Volunteer Spirit of Tennessee better than a malacologist stepping forward to narrow down (or “restrict’) Thomas Say’s type locality for Melania laqueata to some more precise spot?  And one’s natural first thought – correct me if I am wrong – would be to assume that Gerard Troost collected that first specimen of M. laqueata from the Cumberland River as it runs by his adopted home of Nashville.  But alas.

Modern Nashville

Efforts by the U.S. Army Corps of Engineers to blast the Cumberland River clear of obstacles to navigation began as early as the 1830s.  The first lock and dam on the Cumberland River was constructed at Nashville in 1887, and by the 1920s a system of 15 locks and dams regulated the Cumberland River to a minimum depth of 6 feet through the entire state of Tennessee.  Attention then turned to the generation of hydroelectric power, the COE constructing a series of gigantic dams in the 1940s through the 1970s, including Old Hickory Dam just 20 river miles upstream from Nashville in 1956.

A visit to the Nashville waterfront today betrays no hint of gastropod habitat, nor indeed, home for macrobenthic life of any sort or description.  Downstream the Cumberland River is armored with rip rap boulders.  Upstream the flow is increasingly controlled by the generation schedule at the Old Hickory Dam, daily cycles at the Edenwold Gage often reaching amplitudes of 6 feet.  Slackwater extends 100 miles above the dam, essentially to the base of Lake Cordell Hull, which extends another 70 miles, essentially to Kentucky.  If a viable population of pleurocerid snails of any description survives in the Cumberland River of Tennessee today, I am not aware of it.

It seems to me that we are left with no alternative but to select a tributary of the Cumberland River as the type locality for Thomas Say’s Melania laqueata.  And the tributary closest to Gerard Troost’s base of operations currently inhabited by a viable population of pleurocerid snails matching Thomas Say’s 1829 description would be Browns Creek, a small stream running north through the state fairgrounds to empty into the Cumberland entirely within the modern city limits of Nashville.

I visited Browns Creek at the state fairgrounds on a sunny Saturday morning this April just past, as crowds were beginning to gather for the INEX Spring Nationals at the Fairgrounds Speedway [4].  If you click the image below for an enlargement, you can see a supertruck practicing on the track below, at far left.  Browns Creek runs under that bridge I’ve marked with an arrow.

Tennessee State Fairgrounds

The stream itself doesn’t stink anywhere near as bad as you might expect from its entirely urban catchment.  Sure, there was garbage and litter of all sorts everywhere down in the rather narrowly incised ditch through which Browns Creek runs.  But the water was clear, and coolish for April, and running over riffles, and you could flip rocks and find mayfly larvae.  I’ve waded into much worse.

The pleurocerids were not abundant, but with an hours’ effort I was able to collect N = 29 topotypic Pleurocera laqueata laqueata (Say 1829) from Browns Creek at the state fairgrounds, in Nashville, TN (36.1282, -86.7628).  At this point I propose to restrict the type locality of Melania laqueata Say 1829.

My sample demonstrated the range of shell morphological variation typical of pleurocerid populations everywhere.  But before we follow that thread any further, we need to clarify some terminology.

In his original description, Thomas Say focused on the “regular, elevated, equal, equidistant costae” on the whorls of the shell.  Such scallop-shaped ridges on the whorls have also been called, by other authors at other times, “costations,” “plicae,” or “plications.”  Generally, in previous posts on this blog, I have tended to prefer plications (adj. plicate) to describe that particular shell feature, so let’s try to be consistent.

And Thomas Say also went on to stipulate that the shell of his Melania laqueata was “altogether destitute of elevated revolving lines.”  Such shell features have also been called, by other authors at other times, “spiral lines” or “spiral cords” or “striae” or “striations.”  I have generally preferred striation (adj. striate) in past essays on this blog, so again, let’s stick with that.  Thomas Say’s holotype shell figured way up above demonstrates very strong plications but no striation whatsoever.

So a small sample of the shells born by the newly designated topotypic population of P. laqueata is figured below.  And it should come as no surprise to see significant intrapopulation variation in shell plication.  All are plicate around the apex, but the body whorl of shell on the left is essentially smooth, that of the shell on the right strongly plicate, and the shell in the middle approximately half-plicate, around the top of the body whorl only.

Topotypic P. laqueata

The subject of shell plication in pleurocerid snails has come up at least three times previously in the columns of this blog, maybe more [5], most recently in an essay I published on P. troostiana back in [15Apr20].  My loyal and attentive readership may recall that Calvin Goodrich devoted #3 in his “Studies on the Pleuroceridae” series to plication way back in 1934 [6].  The laboratory rearing experiments of Misako Urabe [7] returned evidence that at least some variation in the strength of shell plication may be an ecophenotypic response to substrate.

And we shouldn’t let this opportunity pass to tip our caps to Thomas Say, the Father of American Malacology, as well.  In a quaint nineteenth-century fashion, I think he may be trying to telegraph that he noticed intrapopulation variance in the plication of Melania laqueata, like a Charles Darwin on the American frontier.  His figured holotype clearly shows strong plication (“costae”) across the entire body whorl “from suture to suture,” much like topotypic shell C above.  But in his description, he specified:

“seventeen regular, elevated, equal, equidistant costae on the superior half of each volution, extending from suture to suture, and but little lower, and becoming obsolete on the body whirl.”

The wording of Say’s written description about plication on “the superior half of each volution … and but little lower” implies to me a morphology more like topotypic shell B.  And that final clause about plication “becoming obsolete on the body whirl” suggests more the morphology demonstrated by topotypic shell A.

Darwin’s theory depended on three hypotheses: that populations vary, that such variation yields fitness differences, and that fitness differences drive evolution.  The first hypothesis is the easiest to test, but historically, was the most difficult to accept.  It is humbling to see a pre-Darwinian systematic biologist such as Thomas Say entertaining an hypothesis that so many 21st-century systematic biologists refuse to consider.

Ah, but.  Thomas Say was very, very certain that the shell of his new Melania laqueata was “altogether destitute of elevated revolving lines.”  What is the situation with striation?  Tune in next time.

Notes:

[1] Say, T. (1829) Descriptions of some new terrestrial and fluviatile shells of North America.  New Harmony Disseminator of Useful Knowledge 2(18): 275 – 277.

[2] Melania laqueata is in a five-way tie for twelfth oldest, to be precise, with the four other pleurocerids described by Thomas Say in 1829: semicarinata, obovata, canaliculata, and trilineata.

[3] The history of the genus of pleurocerid snails to which Say’s Melania laqueata has been assigned is long and tortured.  For a brief review, see:

• Goodbye Goniobasis, Farewell Elimia [23Mar11]

[4] The Nashville Fairgrounds Speedway is the second oldest continually operating motorsports track in the United States.  It hosted Grand National / Winston Cup NASCAR races 1958 – 1984, and NASCAR Busch Series races 1984 – 2000, before being replaced on the schedule by the 1.33 mile Nashville Superspeedway in 2001.  Here’s a quote from the sportscaster Dave Moody (interviewing Sterling Marlin): “If they announced that five old ladies would push baby buggies around that track, 4,000 people would show up.”

[5] Previous essays touching on shell plication in the Pleuroceridae:

• Semisulcospira research: A message from The East [6Jan08]
• Semisulcospira research: A second message from The East [1Feb08]
• What is a subspecies [4Feb14]
• What subspecies are Not [5Mar14]
• Huntsville Hunt [15Apr20]

[6] Goodrich, C. (1934)  Studies of the gastropod family Pleuroceridae – III.  Occasional Papers of the Museum of Zoology, University of Michigan 300: 1 – 11.

[7] Urabe, M. 2000. Phenotypic modulation by the substratum of shell sculpture in Semisulcospira reiniana (Prosobranchia: Pleuroceridae). J. Moll. Stud. 66: 53-59.

Cytoplasmic Male Sterility in the Snake River Physa

Editor’s Note – This is the fourth installment of my three part series on the Snake River Physa controversy.   So if you’re interested in all the public policy craziness surrounding the nominally endangered Physa natricina, I would recommend that you back up to my posts of [14May24], [11June24], and [2July24] before proceeding.  If you’re just interested in the science, on the other hand, you might want to refresh your memory with  my [9June22] essay on cytoplasmic male sterility.

Last month we reviewed a mtDNA sequence dataset collected by Mike Young and colleagues [1] from “The Twelve Phascinating Physa of Bliss,” a small sample taken by IPC/FWS biologists from the tailwaters of the Bliss Dam in the Snake River of Idaho in 2019.  That sample included N = 4 snails bearing a CO1 sequence so strange and divergent that our buddy Mike hesitated even to assign it to the genus Physa.  He called the four snails bearing that sequence “Snake River Candidate Species 3,” or SRCS3 for short.

Blasting Mike’s SRCS3 sequences against the entire worldwide NCBI GenBank, we confirmed that OK510774 (taken as typical for the set of four) was more similar to a planorbid from Bangladesh than to any other physid sequence ever reported, “with five phascinating exceptions.”  SRCS3 was approximately 93% similar to two Physa sequenced in Singapore [2], and three Physa sequenced in South Africa [3].

With those little scraps of data in front of us last month, I speculated that SRCS3 might be a strain of Physa acuta demonstrating cytoplasmic male sterility (CMS).  And at that point I abruptly shifted focus to a remarkable paper published in 2022 by my good friend Patrice David and colleagues, reporting very similar levels of mtDNA sequence divergence in a French strain of Physa acuta unable to mate in the male role, but nevertheless fully fertile as female [4].  Could the Snake River – Singapore – South Africa sequence signal the existence of a second case of CMS, a mitotype strikingly different both from the normal, simultaneously hermaphroditic (N) strain and the divergent, male-sterile (D) discovered by Patrice and his team?

Yes.  Two days after I posted last month’s essay I sent an email to Patrice, subject line “Shout-out on the FWGNA blog,” the entire body of which simply read, “I thought you might be interested to see how your 2022 paper on cytoplasmic male sterility could have an impact on conservation biology here in the USA.”  And Patrice replied with a pdf reprint that knocked my socks off [5].

The paper was published in Evolution by Patrice’s student Fanny Laugier, working with a team of nine coauthors from Montpellier and Villeurbanne [6].  Although available in preprint since March, it just appeared in the physical journal last month.  But let’s back up two steps before we leap forward three.

Cytoplasmic male sterility is well studied in plants.  And given the accelerated rates of mutation demonstrated by mitochondria hosting CMS genes, I do not suppose it is surprising that natural populations of gynodioecious plants often host multiple, independently evolving CMS lines.

And as I mentioned in my essay of [9June22] natural populations of gynodioecious plants hosting CMS mitochondria have also often evolved nuclear genes that restore male fertility.  This results from the selective advantage that a fully functional hermaphrodite has over other members of its population reproducing strictly as a single sex [7].

So, reasoning from numerous well-documented cases in plants, Fanny and her team returned to the Physa populations around Lyon originally studied by Patrice and his colleagues to look for additional CMS strains.  And sure enough, prospecting around with a clever PCR test, they found seven individual snails in two populations bearing a “mitotype K,” startlingly different from both normal hermaphroditic N and the CMS mitotype D they had reported in 2022.

My readership might also remember from [9June22] that CMS mitotype D was, around the entire mitochondrial genome, approximately 44% different from the normal Physa acuta mitotype N.  The 20% CO1 sequence divergence between those two mitotypes was actually less than average.  So the newly discovered mitotype K ultimately proved to be 35% - 57% divergent from N and 35% - 57% divergent from D (ten genes), with CO1 sequence divergences 23% and 28%, respectively.  This is the phenomenon I have termed “mitochondrial superheterogeneity” in many previous posts on this blog [8].

And can you smell what our French chefs are cooking?  Focusing now on the CO1 sequence, the French CMS strain bearing the newly discovered mitotype K matched Ting Hui Ng’s [2] Singapore sequence and Molaba’s [3] South Africa sequence 100%, almost identically.  And the match with Mike Young’s [1] SRCS3 sequence from the Bliss Rapids was 93%, just as I reported last month.  Fanny Languier, Patrice David, and their colleagues had discovered a strain of Physa acuta bearing a wildly divergent mitochondrion worldwide in its distribution.

Laugier [6] Figs. 1B and 1C, modified.

But that is not the headline news.  Here is the headline.  Mitotype K Physa acuta were not male-sterile!  Our French colleagues cultured up a big batch of Mitotype K Physa and were easily able to cross them with their albino N laboratory line [9], both lines copulating readily in both the male and the female role.

So again, reasoning from many years of accumulated research on cytoplasmic male sterility in plants, Fanny and her team suspected that nuclear genes might have evolved in the mitotype K line to restore male function.  And they launched a 17 – generation introgression experiment to insert mitochondrial lineages derived from their wild-caught (pigmented) K line into the “naïve” nuclear background of their (albino) laboratory line N.

And sure enough!  After just 5 generations of introgression, 69% of the naïve snails bred to bear K-type mitochondria had lost their ability to mate as males, with no such loss whatsoever in control lines bearing the N-type mitochondria.  After 11 generations, the proportion of naïve male-sterile K snails stabilized at around 60%.  No loss in female function was ever detectable in any line.  In my 5July24 email back to Patrice, I wrote:

“Wow, nuclear restorers!  It's a shame we work with crap-brown little trash snails.  If snails were maize, wheat, or rice [10], we'd be rich.”

What wonderful science!  Deductive reasoning, tested by rigorous experimentation carefully designed to proceed from the known to the unknown.  Conducted for the joy of it, for the exploration of evolutionary mechanisms, for the pushing back of the darkness.  The construction of testable hypotheses about the natural world, period, full stop, nothing more and nothing less.  Pure and unsullied science, utterly useless and wonderful!

Laugier [6] Fig. S1, modified.

And yet, it turns out, completely by accident, useful.  Ting Hui’s study in Singapore was directed toward invasive species, the Molaba study in South Africa was parasitological, and Mike Smith’s study on the Snake River directed toward conservation.  Fanny’s results contribute toward an understanding of all those results, and more.

And here is Lesson Number One.  Gene sequences are not species.  A species is a population or group of populations reproductively isolated from all others [11].  Sequence divergence is typically correlated with reproductive isolation [12], of course, no different from morphological divergence.  But gene trees are not species trees, and genes are not species.

So, as I pointed out in last month’s essay [2July24], the Snake River line of Physa acuta that Mike Young and colleagues called SRCS3 is not the same as the French mitotype K.  The two lines are 7% different, as though they arose from a single mutation in some mitochondrial DNA repair gene sometime in the past and have subsequently diverged.  Could such a mutation have occurred more than once in the evolutionary history of the Physidae, here in North America, where Physa seem to have first evolved?  You betcha.

There is no reason that the CO1 gene sequence that Gates, Kerans and colleagues [13] recovered from the stunted Physa below the Minidoka Dam, roughly 15% different from the N line at Bliss (OK510580), couldn’t be evidence of yet another CMS strain in Physa acuta.  Gates and Kerans identified that sequence as “Physa natricina.”  But gene sequences are not species.  Species are defined by reproductive isolation.  And we have no data on reproductive relationships between the Minidoka population and the dirt-common Physa acuta downstream whatsoever.  And plenty of evidence otherwise [14].

Nor is there any reason in the world that the peculiar mitotype shared by Mike Young’s SRF14, the Owyhee Wet Rock Physa of eastern Oregon [15] and scattered in odd lot Physa populations from California to British Columbia couldn’t be evidence of a CMS strain in Physa gyrina.  MtDNA sequence data, and the gene trees we make with them, are (at best) weak null models of population relationships, correlated with speciation, nothing more [16].

Do not misunderstand me.  I am not demanding controlled breeding studies between every pair of the 2.2 x 10^6 species described from Planet Earth.  But for nominally endangered species, such as Physa natricina, before we enact pages of Federal regulations and spend millions of dollars on conservation, we could at least run a couple of $12 allozyme gels and test for evidence of assortative mating [17]  with a nuclear polymorphism or two, am I right?

Notes

[1] Young, M.K., R. Smith K.L. Pilgrim, and M.K. Schwartz (2021)  Molecular species delimitation refines the taxonomy of native and nonnative physinine snails in North America.  Scientific Reports 11: 21739. https://doi.org/10.1038/s41598-021-01197-3

[2] Ng, T.H., Tan, S.K. & Yeo, D.C. (2015) Clarifying the identity of the long-established, globally-invasive Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in Singapore. BioInvasions Rec. 4, 189–194.

[3] Molaba, G.G. et al. (2023) Molecular detection of Fasciola, Schistosoma and Paramphistomum species from freshwater snails occurring in Gauteng and Free State provinces, South Africa.  Veterinary Parasitology 320: 109978.  https://doi.org/10.1016/j.vetpar.2023.109978

[4] David, Patrice, Cyril Degletagne, Nathanaëlle Saclier, Aurel Jennan, Philippe Jarne, Sandrine Plénet, Lara Konecny, Clémentine François, Laurent Guéguen, Noéline Garcia, Tristan Lefébure, Emilien Luquet (2022) Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination.  Current Biology 32: 2325 - 2333.  https://doi.org/10.1016/j.cub.2022.04.014.  For a review, see:

• Cytoplasmic Male Sterility in Physa! [9June22]

[5] Patrice went on to apologize “I told my PhD student Fanny a dozen times to send this to you, that you would probably make good use of it because it was solving a controversy, apparently she didn’t, so I do it myself.” Yes, please send me your reprints!  I lost my library privileges when I was banned from campus in 2016.  I really haven’t had access to most of the scientific literature published since.

[6] Laugier, Fanny, Nathanaëlle Saclier, Kévin Béthune, Axelle Braun, Lara Konecny, Tristan Lefébure, Emilien Luquet, Sandrine Plénet, Jonathan Romiguier, and Patrice David (2024) Both nuclear and cytoplasmic polymorphisms are involved in genetic conflicts over male fertility in the gynodioecious snail, Physa acuta.  Evolution 78 (7): 1227–1236. https://doi.org/10.1093/evolut/qpae053

[7] Yes, mitochondria bearing CMS genes are “selfish” in the sense of the Richard Dawkins (1976) classic.   Don’t get me started on Richard Dawkins.

[8] Mitochondrial superheterogeneity (mtSH), where two or more of the members of a single population demonstrate greater than 10% divergence in any single-copy mtDNA gene, not sex linked, seems to be remarkably common in freshwater gastropods.  In pulmonate populations, I wouldn’t be surprised if most or all mtSH is ultimately traceable to CMS.  In prosobranch populations, however, I think mtSH is a signature of great age, plus low-frequency long distance dispersal, the “Jetlagged Wildebison Model.”  Here is a sample of my previous posts on mtSH:

• The Snails the Dinosaurs Saw [16Mar09]
• Mitochondrial superheterogeneity: What we know [15Mar16]
• Mitochondrial superheterogeneity: What it means [6Apr16]
• Mitochondrial superheterogeneity and speciation [3May16]
• Mitochondrial heterogeneity in Marstonia lustrica [3Aug20]

[9] Dillon, R.T. and A.R. Wethington (1992) The inheritance of albinism in a freshwater snail, Physa heterostropha. Journal of Heredity 83:208-210. [pdf]  For nice review, see:

• Albinism and sex allocation in Physa [5Nov18]

[10] The clever manipulation of cytoplasmic male sterility, together with nuclear restorers, has been one of the more important methods by which plant breeders have achieved the outcrossing of normally self-pollinating crop plants.  I am quite sure that a lot of money has been made with that genetic technology.  With Physa acuta, however, the prospects are not as lucrative.

[11] This is the biological species concept, most closely associated with the work of Ernst Mayr.  It should need no restatement, much less a defense.  But the best paper Jerry Coyne ever wrote was a defense of both Mayr and his species concept, here:

• Coyne, J. A. (1994) Ernst Mayr and the origin of species.  Evolution 48: 19 – 30.

[12] 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. [html] [pdf].  For a review, see:

• What is a species tree? [12July11]

[13] Gates, K. K., B. L. Kerans, J. L. Keebaugh, S. K. Kalinowski & N. Vu (2013) Taxonomic identity of the endangered Snake River physa, Physa natricina (Pulmonata: Physidae) combining traditional and molecular techniques.  Conserv. Genet. 14: 159-169.  For a review, see:

• The Mystery of the SRALP: No Physa acuta were found [2May13]

[14] Rogers, D.C. & A.R. Wethington (2007) Physa natricina Taylor 1988, junior synonym of Physa acuta Draparnaud, 1805 (Pulmonata: Physidae).  Zootaxa 1662: 45 - 51.  For a review, see:

• Red flags, water resources, and Physa natricina [12Mar08]

[15] Moore, A.C., J.B. Burch, and T.F. Duda Jr. (2015) Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations.  Conservation Genetics 16: 113 – 123.

[16] I have made this argument as many times as the number of essays listed under the label “Gene trees” above.  For an overview, see:

• Gene trees and species trees [13July08]

[17] For an explanation of gametic phase disequilibrium, its power to distinguish species, and its extension to character phase disequilibrium, see:

• What is character phase disequilibrium? [4Jan22]
• Character phase disequilibrium in the Gyraulus of Europe [4Feb22]
• Just 125 Species of Pyrgulopsis in the American West [7Sept22]

The Twelve Phascinating Physa of Bliss

Editor’s Note – This is the third episode in a three part series on the Snake River Physa controversy, prompted by the recent paper of M. K. Young and colleagues [1].  If you’re new to this blog, and seriously interested in the science, I would recommend that you back up two months and read my essays of [14May24] and [11June24] before continuing onward.  I’ve also written six older posts on the subject 2008 – 2013, but no point in going back that far, screw it.

In August of 1979 the Idaho Power Company (IPC) received a permit to study the construction of a proposed A. J. Wiley Hydroelectric Project on the Snake River at RM 565, about 1.5 miles west of the little town of Bliss [2].  The proposed dam would have impounded a narrow reservoir as deep as 85 feet nearly eight miles upstream, to the foot of the Lower Salmon Falls Dam.  Inundated would have been the childhood home of a lichen biologist named Dr. Peter Bowler.

By 1992, Bowler had developed a second career as a malacologist, thick as thieves with Dwight Taylor, Terry Frest, and Ed Johannes.  A new species of Physa had been described, Physa natricina, which although found as a fossil throughout northern Utah and southern Idaho, was known alive only from RM525 to RM571 of the Snake River [3].  The “Snake River Physa” had been fast-tracked onto the Federal Endangered species list [4], and the A.J. Wiley Project had been permanently shelved.

The Snake River at RM 570

So last month we reviewed the exciting story of heroic efforts by IPC biologists working in the tailwaters of the Swan Falls Dam, RM 420 – 449, ultimately yielding hundreds of small physids matching the Physa natricina phenotype in at least some respects [11June24].  Because the IPC sample was taken 76 miles downstream from the type range, however, we referred to this population as “Snake River natricina-like Physa,” or SRNLP for short.  Ultimately, the SRNLP from RM 420 – 449 proved a genetic match with Physa acuta, a trash snail invasive on five continents [1].

This is not the situation at RM 675, however.  Previously we have reviewed very similar efforts by biologists engaged by the U.S. Bureau of Reclamation in the tailwaters of the Minidoka Dam 100 miles upstream of the type range of Physa natricina [5].  There, the SRNLP bear a unique mtDNA haplotype, not matching Physa acuta.

Between the Swan Falls Dam and the Minidoka Dam, in the actual type range of Physa natricina, previous research results have been equivocal.  Rogers and Wethington examined hundreds of physids sampled from this stretch of river by IPC biologists 1995 -2007, ultimately unable to distinguish the entire lot from Physa acuta [6].  John Keebaugh disagreed, selecting a small subsample as bona fide Physa natricina.  But the genetic results from the Keebaugh subsample were dubiously reported and never vouchered [7].

Are there no reliable genetic data for Snake River physids sampled between RM 525 and RM 571?  Well, yes, there are.  Twelve very interesting and extraordinarily diverse mtDNA sequences from the tailwaters of the Bliss Dam at RM 559, to be precise.

The Snake River in Idaho, modified from Gates et al [7]

It materializes that the Idaho Power Company has interests that extend beyond the conservation biology of freshwater gastropods.  Sport fishing is an important part of the local economy of the small communities along the Snake River, for example, and mercury levels in the game fish a matter of some concern to the IPC [8].  IPC biologists also have an ongoing project to survey mercury levels in the macrobenthos, as a component of the diet of the white sturgeon.

And so it came to pass that on 17Sept19 a team of IPC and USFWS biologists gathered to sample macrobenthos from the tailwaters of the Bliss dam at RM 559 with a suction dredge.  Their sample came from wadable depths – not the middle of the river – and Physa were not the target [9].  But the 12 individual Physa recovered as bycatch were sent to Mike Young and colleagues at the National Genomics Center for Wildlife and Fish Conservation in Missoula for analysis, along with that big sample of P. acuta from the Swan Falls tailwaters we obsessed over last month [1].  And the results were fascinating:

P. gyrina, Snake R. [10]

N = 1 Physa gyrina.  This is the most widespread of the Snake River physids, inhabiting the main stem throughout the state of Idaho, upstream beyond the Minidoka Dam and into many of the tributaries.  Populations of P. gyrina are not as invasive as P. acuta, however, demonstrating slower growth rates, larger sizes at first reproduction, and lower fecundities per unit body mass.  The CO1 sequence similarity between the singleton P. gyrina recovered by IPC biologists at Snake River mile 559 (SRP199 = OK510769) and the type P. gyrina sequence from Council Bluffs, Iowa (AY651187) is 99.0%.  Good.  No surprises there.

N = 5 Physa acuta.  Arbitrarily selecting the first-listed of the five sequences in this most common category of physids recovered from the Bliss tailwaters, the divergence of SRP001 (= OK510580) from the P. acuta type sequence (River Garonne, AY282589) is 4.7%, and the divergence with our typical P. acuta sequence from the Swan Falls tailwaters (SRP051 = OK510624) is 3.4%.  Apparently, a population of very ordinary, typical [11] Physa acuta was found, again.  This is additional confirmation, if any was needed, that the range of a trash snail invasive on five continents does in fact extend 76 miles up the Snake River from Swan Falls to the type locality of Physa natricina.  Fine.  No surprises there, either.

N = 4 Snake River Candidate Species 3 (“SRCS3”).  Four individual physids sampled from the Snake River at RM 559 bore a haplotype so divergent from all other sequences in the worldwide database that Mike Young, bless his heart, considered the evidence “consistent with their assignment to a separate genus.”  Ultimately, however, Mike decided simply to nominate this subset of four as #3 in his slate of 18 new “candidate species” for the physinine malacofauna of North America.

P. acuta, Snake R. [13]

Picking the first SRCS3 sequence from my buddy Mike’s Supplementary Table 2 as typical (SRP206 = OK510774), we can immediately confirm nearly negligible CO1 sequence similarities of 76.1% with Bliss acuta (OK510580) and 75.8% with Bliss gyrina (OK510769).  Indeed, blasting OK510774 against the entire NCBI GenBank database, we discover that Mike’s SRCS3 is more similar to Indoplanorbis exustus from Bangladesh than to (essentially) any other physid sampled from anywhere else in the world, with five phascinating exceptions.

Mike Young’s SRCS3 sequences are 93.2% similar to a pair of CO1 sequences uploaded by Ting Hui Ng and colleagues from their famous survey of freshwater gastropods in the ornamental pet trade of Singapore (KP182981 and KP182982).  I reviewed Ting Hui’s paper shortly after its 2016 publication [14], adding a footnote to her earlier paper focusing on Singapore Physa in particular [15], and a footnote connecting the Singapore Physa to the situation in the Snake River of Idaho, which now looks genuinely prophetic [16].

Mike Young’s SRCS3 sequences are also 92.1 – 93.9% similar to a set of three sequences very recently uploaded to GenBank by Gantshe Molaba and colleagues [17] from a 2023 parasitological study in South Africa (ON953193, ON953197, ON953200).  The South African sequences and the Singapore sequences are virtually identical.

What is going on here?  In Singapore and in South Africa and now in the Snake River, physids bearing an extravagantly divergent haplotype have been recovered together with Physa acuta bearing a typical haplotype, and in none of these cases did the authors of the papers report any corresponding morphological distinction between divergent and typical snails.

This situation looks precisely like the situation in France, and indeed right here in Charleston, South Carolina, elegantly demonstrated by our good friend Patrice David and his colleagues [18] to be a signature of cytoplasmic male sterility.  Patrice has hypothesized that mutation in some component of the mitochondrial DNA repair mechanism has simultaneously increased mitochondrial mutation rates in such lines and robbed their bearers of fertility in the male function.  But because they retain female function and mate freely with typical P. acuta (serving as male), they have not speciated.  They are still Physa acuta.

From David et al. [18]

Male-sterile variants of P. acuta are also at least as invasive as typical P. acuta, possibly even more so.  It is impossible to resist pointing out that the middle Snake River Valley is home to scores of fish hatcheries and fish farms, jostling each other for every cubic foot of the spring water that gushes forth from its scenic canyon walls, yielding truckloads of rainbow trout, cutthroat trout, salmon and steelhead.  It was almost certainly these hatcheries, and the fisheries they support, that introduced Potamopyrgus into the American West back in 1987.  Surely, they could bring exotic Physa into this country as well, yes?

Sure, maybe.  But we should also immediately note that Physa acuta is a North American native, and it is just as likely that we exported a male sterile variant to Singapore as Singapore exported one to us.  And the Snake River strain is not the Singapore strain.  They are 6.8% different, as though they sprang from a single mitochondrial mutation, and have subsequently diverged.

And the hypothesis that SRCS3 really is a valid biological species cannot be ruled out.  I’ve been footnoting the possibility of a second acuta-like Physa in the Pacific Northwest for years now – even suggesting Haldeman’s (1843) concolor as a possible name for the critter [19].  Wouldn’t it be great to do some old-fashioned breeding studies here?  Informed by old-fashioned nuclear markers – allozymes, microsatellites, anything?  I have a dream.

N = 2 Snake River Form 14 (“SRF14”).  Mike Young, bless his heart, declined to nominate the two individual snails (SRP200 and SRP201) recovered from the Bliss tailwaters bearing CO1 sequence OK510770 as a “candidate species” unto themselves, instead referring to them as “Form 14” of the 34 “forms” of physinine snails he ultimately distinguished in his stupendous gene tree.  His OK510770 sequence is 92.6% similar to Bliss gyrina (OK510769) and just 83.6% similar to Bliss acuta (OK510580).

Blasting the CO1 sequence Mike Young obtained from his two SRF14 individuals against the entire NCBI GenBank database, we discover that OK510770 is 95 - 96% similar to a set of 13 sequences obtained by University of Michigan (UMMZ) researcher A. C. Moore, with Jack Burch and Tom Duda, from a “highly restricted freshwater snail lineage” they introduced to the world in 2014 as the “Owyhee Wet-Rock Physa,” or OWRP for short [20].

The Owyhee River enters the Snake River in eastern Oregon at RM 395, downstream 165 miles from the Bliss Dam.  (See map below.)  It would be quite the adventure, but if one could float those 165 miles down the Snake to the Oregon border then pioneer back up the Owyhee approximately 130 miles through deep, arid canyons one would arrive at a geothermal spring complex inhabited by a population of Physa that apparently reminds everybody, both in their morphology and in their life habit, of the federally endangered Physa zionis of Utah.

Fig 1 of Moore et al [20] modified

Both the Owhyee Wet-rock Physa and Physa zionis are tiny, stunted little things, with grotesquely expanded body whorls, even more stunted and even more grotesque than Physa natricina.  What is the chance that Mike Young’s SRF14 might be Dwight Taylor’s long-sought and much-endangered species?

The evidence does not point in that direction.  As most of my (highly specialized) readership will be aware, pulmonate snails of the family Physidae bear several distinct types of penial morphology.  The most common type was called “Type C” by George Te in 1975 [21] – bearing a one part, muscular penial sheath.  Type C physids include Physa acuta, Physa natricina [22], and Physa zionis.  Also quite common in North America are physids bearing “Type B” penial morphology – a two part penial sheath – such as demonstrated by Physa gyrina.  If that distinction is unfamiliar to you, and you have more than a casual interest in the Snake River Physa controversy, I would assign the 2007 paper of Wethington & Lydeard [23] for homework.

Wethington & Lydeard showed that penial morphology correlates with molecular phylogeny.  So, although neither the IPC/USFWS team that fished them from the Bliss rapids nor Mike Young, bless his heart, dissected any of the 12 fascinating physids under discussion here, nor (indeed!) did the UMMZ group dissect any of their Owyhee Wet Rock Physa, it turns out that both the SRF14 sequence and the 13 OWRP sequences all clustered together with Type B Physa gyrina, not with Type C Physa acuta/natricina/zionis.

So, the next four paragraphs will be a digression, but indulge me.  The UMMZ group sequenced 13 Owhyee Wet-rock Physa for three genes, ultimately obtaining five slightly different CO1 sequences (KF305393 – 405).  While a match to P. zionis was not confirmed, those sequences did demonstrate a 95% match to a set of 11 CO1 sequences (KF305406 – 416) obtained from a population identified as Physa gyrina inhabiting Aqua Fria Creek, about 20 miles west of Yosemite National Park in central California.

And in 2018, four years after the publication of the OWRP paper, our hardworking friends at the Centre for Biodiversity Genomics (CBG) in Guelph uploaded to GenBank a “Physidae sp.” sequence collected from a beaver pond by the Similkameen River in southern British Columbia (MG421809) that also demonstrated a 95% match to the OWRP.  The Similkameen River flows south into the Okanogan River, joining the Columbia River in central Washington.

Now for the punch line.  It materializes that Mike Young’s SRF14 sequence, collected by IPC/USFWS biologists from the Bliss Rapids of the Snake River, is a stunning 99.7% similar to that “Physidae” sequence collected by the CBG from the beaver pond by the Similkameen 500 miles to the northwest.

Do the four populations mapped above, inhabiting wet rocks on the eastern edge of Oregon, a creek in central California, a beaver pond in southern British Columbia, and now discovered in the Bliss rapids of the Snake River of Idaho, represent a single previously unrecognized species of Type B Physa, extraordinarily variable, widespread across western North America?  Or might they have evolved from a male-sterile variant of Physa gyrina, as SRCS3 seems to have evolved from Physa acuta?

Again, as was the case with SRCS3 and P. acuta, Mike Young’s SRF14 population is sympatric with a population of Physa gyrina of the typical phenotype, bearing mitochondria of the typical haplotype.  Dare I dream that some worthy and industrious graduate student might someday survey these populations for variation at some sort of nuclear marker or markers and look for reproductive isolation, or lack thereof?  Followed by some breeding studies, perhaps?

There was a time in my career when I could have answered every question posed in this entire essay with a day in the field, four days in the lab, and $100 for reagents and expendable supplies.  Get a nice, big sample of Physa from the Bliss Dam tailwaters, paying attention to microhabitat.  Examine their anatomy and shell morphology critically, sort them into Type B and Type C subsets.  Within those subsets, are nuclear markers segregating in Hardy-Weinberg equilibrium?  How many biological species of Physa are we dealing with here?  Give me another six months and a student or two, and the breeding experiments done would have been done.  How many million dollars and years of wandering in a wilderness of ignorance and superstition might timely reference to a scientist, who actually knows something about the biology of the living, breathing animals he is studying, have saved?

N = 0 Minidoka SRNLP.  Although otherwise bearing a stunningly diverse array of mitochondria, our little sample of 12 physids from the tailwaters of the Bliss Dam did not include any bearing a CO1 sequence matching that of the peculiar little misshapen Physa recovered from the Minidoka Dam tailwaters 116 miles upstream, ascribed by Gates, Kerans and Keebaugh to Physa natricina [7].  True, the IPC/USFWS biologists did not sample midstream, where Physa bearing such haplotypes were recovered at Minidoka.  This is a weak test, indeed.

But these are all the relevant data we have, for 18 years of effort by multiple teams of hardworking, well-meaning biologists and millions of dollars flushed down the toilet.  So, since Physa acuta do indeed appear to be common in the shallows of the Snake River in the type range of Physa natricina, and since populations identified as Physa natricina in the Swan Falls tailwaters, both in the shallows and at midstream, are synonymous with P. acuta, the strongest hypothesis at present remains that the population Dwight Taylor described as Physa natricina in its RM 525 – 571 type range is a junior synonym of Physa acuta, as suggested by Rogers and Wethington in 2007.  And the SRNLP population in the roiling tailwaters of the Minidoka Dam is Something-Else-God-Knows-What.

To summarize.  On [12Mar08] I posted the following overly optimistic observation:

“Science is a self-correcting process. It is gratifying to see two of our own, Rogers and Wethington, designing the research program and publishing the paper that has turned us back from our 20-year blunder. But at such a cost! Literally millions of dollars have been wasted monitoring, managing, and protecting a snail that anyone on six continents could find in the ditch behind his local McDonalds, licking special sauce off the hamburger wrappers.”

Now 16 more years have passed, the Rogers and Wethington paper ignored, and millions of additional dollars wasted.  In 2008 I was laboring under the assumption that the world of science and the world of public policy could find some common frame in which to communicate.  Although I went on to write, “Science and politics do not mix,” in retrospect, I should have written that science and public policy cannot mix.

I am playing bluegrass music.  The natural resource agencies are playing baseball [24].  And the Idaho Power Company, just trying to run a business heaven help them, is standing in the batter’s box with a fiddle.

Notes

[1] Young, M.K., R. Smith K.L. Pilgrim, and M.K. Schwartz (2021)  Molecular species delimitation refines the taxonomy of native and nonnative physinine snails in North America.  Scientific Reports 11: 21739. https://doi.org/10.1038/s41598-021-01197-3

[2] Malde, H. E. (1981)  Geologic factors pertinent to the proposed A. J. Wiley Hydroelectric Project, No. 2845, Bliss, Idaho.  U.S. Department of the Interior, Open File Report 81-569.  75 pp.

[3] Taylor, D. W. (1988) New species of Physa (Gastropoda: Hygrophila) from the western United States. Malacological Review 21: 43-79.

[4] US Fish & Wildlife Service (1992). Endangered and threatened wildlife and plants; Determination of endangered or threatened status for five aquatic snails in south central Idaho. 50 CFR Part 17. Federal Register 57(240)59244-57. (December 14, 1992)

[5] Gates, K. K., and B. L. Kerans (2010) Snake River Physa, Physa (Haitia) natricina, Survey and Study.  Report to the US Bureau of Reclamation under agreement 1425-06FC1S202.  87 pp.  For a review of this report, see:

• The Mystery of the SRALP: A bidding… [5Feb13]
• The Mystery of the SRALP: A twofold quest! [1Mar13]
• The Mystery of the SRALP: Dixie Cup showdown! [2Apr13]

[6] Rogers, D. C. & A. R. Wethington (2007) Physa natricina Taylor 1988, junior synonym of Physa acuta Draparnaud, 1805 (Pulmonata: Physidae). Zootaxa 1662: 45-51.  For a review, see:

• Red Flags, Water Resources, and Physa natricina [12Mar08]

[7] Gates, K. K., B. L. Kerans, J. L. Keebaugh, S. K. Kalinowski & N. Vu (2013) Taxonomic identity of the endangered Snake River physa, Physa natricina (Pulmonata: Physidae) combining traditional and molecular techniques.  Conserv. Genet. 14: 159-169. For reviews, see:

• The Mystery of the SRALP: No Physa acuta were found [2May13]
• The SRALP and the SRNLP: A New Hope [14May24]

[8] Willaker, J.J., C.A. Eagles-Smith, J.A. Chandler, J. Naymik, R. Myers and D.P. Krabbenhoft (2023) Reservoir stratification modulates the influence of impoundments on fish mercury concentrations along an arid land river system.  Environmental Science & Technology. DOI: 10.1021/acs.est.3c04646

[9] This is a personal communication from my friend Jim Chandler at IPC.  No reports or publications are as yet available for the sturgeon/food web/methylmercury research project.

[10] The specimen of P. gyrina figured above was collected by RTD from wetlands below the Minidoka Dam on 19Sept2010.

[11] Actually, I think the Snake River population of P. acuta may represent the “Clade B” form of Ebbs, Loker, and Brant [12].  The sequence similarity of OK510580 to Clade B sequence MF694449 from Montana was 98.2%.

[12] Ebbs, E.T., Loker, E.S. & Brant, S.V. (2018) Phylogeography and genetics of the globally invasive snail Physa acuta Draparnaud 1805, and its potential to serve as an intermediate host to larval digenetic trematodes. BMC Evol Biol 18, 103. https://doi.org/10.1186/s12862-018-1208-z

[13] The specimen of P. acuta figured above was collected by RTD from the Snake River at Bliss, 19Sept2010.

[14] Ng, Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130.  For a review, see:

• What’s Out There? [9Oct17]

[15] Ng, T.H., Tan, S.K. & Yeo, D.C. (2015) Clarifying the identity of the long-established, globally-invasive Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in Singapore. BioInvasions Rec. 4, 189–194.

[16] If you write five crazy things per month for 25 years, eventually at least one will turn out to be a prophecy.  See footnote [6] of my [9Oct17] post for an example.

[17] Molaba, G.G. et al. (2023) Molecular detection of Fasciola, Schistosoma and Paramphistomum species from freshwater snails occurring in Gauteng and Free State provinces, South Africa.  Veterinary Parasitology 320: 109978.  https://doi.org/10.1016/j.vetpar.2023.109978

[18] David, Patrice, Cyril Degletagne, Nathanaëlle Saclier, Aurel Jennan, Philippe Jarne, Sandrine Plénet, Lara Konecny, Clémentine François, Laurent Guéguen, Noéline Garcia, Tristan Lefébure, Emilien Luquet (2022) Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination.  Current Biology 32: 2325-2333.  https://doi.org/10.1016/j.cub.2022.04.014.  For a review, see:

• Cytoplasmic Male Sterility in Physa! [9June22]

[19] The hypothesis “that some physid bearing a type-C penial morphology, but not correctly identified as either P. natricina or as P. acuta, might inhabit rivers of the Pacific Northwest” was Hypothesis #2 (of 3) in my post of [14Sept10].

[20]  Moore, A.C., J.B. Burch, and T.F. Duda Jr. (2015) Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations.  Conservation Genetics 16: 113 – 123.

[21] Te, G. A. (1975) Michigan Physidae, with systematic notes on Physella and Physodon (Basommatophora: Pulmonata).  Malacological Review 8: 7-30.  For a review, see:

• To Identify a Physa, 1975 [6May14]

[22] Figure 6 in Dwight Taylor’s original description [3] clearly shows a type-C penial morphology.  And Jack Burch’s Fig 2.3 confirms that the SRNLP population in the Minidoka tailwaters are also type-C Physa [5].

[23] Wethington, A.R., & C. Lydeard (2007) A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences.  Journal of Molluscan Studies 73: 241 - 257. For a review, see:

• The Classification of the Physidae [12Oct07]

[24] The relationship between the worldview of science and the worldview of public policy is analogous to the relationship between music and sports.  They have different languages, cultures, and values.  They are not “compatible” in any sense, neither are they “incompatible” in any sense, because they are absolutely, utterly different.  Nothing I have written in this essay, nor indeed  any scientific findings of any sort, can ever have any effect on public policy.  I play the banjo, and the USFWS plays baseball.

Most of advocates for science in public policy are harmless naïfs, enamored of big government, Democrats the lot of them.  I worked with a building full of that sort 1982-83, as a AAAS Congressional Fellow on Capitol Hill.  They write reports which nobody reads.  No harm in that.

But should any person step forward, claiming to be a scientist, promising to conduct research to inform some issue of public importance, in exchange for contract money or grant, beware!  This is a charlatan, and pseudoscience is his snake oil.