Dr. Rob Dillon, Coordinator

Friday, April 19, 2019

FWGNA Volumes 1 - 4 Now Available!

Extra, extra [1]!  Read all about it!  We are delighted to announce that the first formal publications of the Freshwater Gastropods of North America Project are now available for purchase from all the usual online outlets, as well from the publisher at a substantial discount.
Buy all four from the author's profile page
Volume 1, by Dillon, Ashton, Reeves, Smith, Stewart and Watson, reports the results of the largest-scale inventory of freshwater snails ever conducted in the United States. We have reviewed and synthesized macrobenthic collections taken by ten natural resource agencies, malacological holdings at eight museums, and our own original collections from hundreds of sites, covering all freshwater gastropod habitat in Atlantic drainage systems from Georgia to the New York line. For each of the 70 species and subspecies we provide:
  • A dichotomous key for identification.
  • Full-color figures.
  • Range maps at county scale.
  • Notes on habitat, ecology, life history, and reproductive biology.
  • Systematic and taxonomic updates to modern standards.
We propose a new, objective system of conservation status ranking [2], and a new species of pleurocerid snail is described in the appendix [3].

Volumes 2, 3, and 4 are collections of essays, originally appearing in the present blog 2003 – 2019, now edited and rearranged thematically.  Volume 2 collects 29 essays on the systematics and evolution of the freshwater pulmonates of North America, Volume 3 comprises 37 essays on the systematics and evolution of the prosobranchs, and Volume 4 collects 38 essays reviewing ecological and biogeographical themes.  These volumes are intended to support and augment the scientific results reported in Volume 1.

The retail prices for the four individual volumes are $39.95, $34.95, $35.45, and $35.45, respectively.  Although not unreasonable for 250-page glossy color paperbacks in this day and age, I don’t mind telling you that the method by which those retail prices were determined irritated me considerably.  The figures were essentially dictated by amazon.com as the lowest possible sticker-price that would yield $1.00 for the FWGNA [4].  All the rest of the sale either goes to my publisher (Bookbaby) or to Amazon for its marketing services.

So then immediately after the volumes hit the market a couple days ago, Amazon began advertising cut-rate prices.  For Volume 1 today, the amazon.com site is listing “10 used from $34.66” and “21 new from $32.61.”  I do not understand this phenomenon at all.

Here’s the bottom line.  I would encourage you all to cut out the online retailing giants, and purchase FWGNA Volumes 1 – 4 directly from the publisher’s website.  The FWGNA Project [5] will receive a substantially larger fraction of the sale. 

And as an inducement, I have arranged with Bookbaby to sell the entire four-volume set for the discount price of $99.95.  That’s a savings of $45.85 to you, and everybody wins, except Jeff Bezos.

Follow this simple three-step process:
  • Go to my Author Profile Page [here].
  • Add all four titles as listed at the bottom of that page to your shopping cart.
  • Apply the coupon code FWGNA4 to each volume.
The system should discount your package price from $145.80 to $99.95.

Perhaps unsurprisingly, each of the four volumes features a fairly extensive acknowledgement section.  But in addition to those lengthy lists of explicit appreciations, I do want to thank the entire readership of the FWGNA Blog for your support and help over the 20-year gestation of this project.  I have received quite a few helpful comments and suggestions from you all over the history of this forum, sometimes by direct email, other times anonymously commented.  I prefer the former, but appreciate have always appreciated all input, regardless of provenance.


[1] No, this is not an “Extra extra.”  As I understand it, an “Extra” was a second run of a daily newspaper, printed to update the readership on some breaking news.  And an “Extra extra” was a third printing.  So, blog posts aren’t printed.  And even if the present text should ultimately appear in print, which is, after all, one of the primary messages being conveyed in the blog post above, it cannot possibly be extra in any sense.  This is the first run of the blog for April of 2019.  I’m sorry, I just like the sound of “Extra, extra.”

[2] For more on the objective system of incidence ranking piloted by the FWGNA project, see:
[3] For more about my newly-described species of pleurocerid snail, see last month’s post:
  • Pleurocera shenandoa n.sp. [11Mar19]
[4] And no, this is not $1.00 profit.  The set-up costs for these books were a couple thousand dollars each.  There is no way that the FWGNA will ever make any actual profit [5], but profit has never been the point.

[5] And perhaps you remember, the FWGNA Project is a sole proprietorship of Rob Dillon.  So I admit that the distinction between the FWGNA and Rob Dillon is a fine one.  But important.  For more, see:

Monday, March 11, 2019

Pleurocera shenandoa n.sp.

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera shenandoa n.sp.  Pp 101 - 108 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

One of the recurring subthemes we have developed in this blog, over its 20 year history, has been the roll of serendipity in science.  The study of Pleurocera clavaeformis described in our posts of February 2007 and March 2011 [1] and the study of P. semicarinata reported in our post of July 2014 [2] were both designed to confirm cryptic phenotypic plasticity in populations of pleurocerid snails using genetic variance at allozyme-encoding loci.  The sets of populations involved in both of those studies were geographically widespread, which meant that I needed to sample some additional sets of well-characterized pleurocerid populations as controls to calibrate the expected levels of genetic divergence among my sets of experimental populations.

An element of serendipity was introduced into both studies when the genetics of the “well-characterized” control populations turned out to be as surprising as the experimental populations.  The additional research this necessitated, for P. simplex (reported in Sept – Nov 2016) [3] and P. canaliculata (reported in June 2013) [4] yielded additional insights into the evolutionary biology of an enigmatic group of organisms that becomes more fascinating to me every time I step in the creek.

So, there were actually two sorts of controls for the studies referenced above, calibration standards and mobility standards.  In the very first study of allozyme polymorphism I ever published [5], way back in 1980, I set Pleurocera simplex population WYTH as a mobility standard, defining the mobility of the most common allozyme band in that population as 100 SDEMM (standard Dillon electrophoretic mobility millimeters.)  And in every study of allozyme polymorphism in pleurocerid populations I have published since that date, a total of 13 in all, I have included a population linked somehow to my 1980 population WYTH, calibrating every allele at every locus by its mobility relative to that standard.
Figure 4 of Dillon [2]
So in both my 2013 study of cryptic phenotypic plasticity in Pleurocera canaliculata [4], and in my related 2014 study of P. semicarinata [2], my mobility standard was population SV.  This was the same population of P. semicarinata I called “PINE” in my 1980 paper, which I ran beside population WYTH thirty-five years previous, hence calibrating the mobilities of all allozyme bands in the 2013 and 2014 papers in units of SDEMM.

Now look at Figure 4 of my 2014 study, reproduced above.  “Standard” Virginia semicarinata population SV is more genetically similar to P. canaliculata than to any member of the (nominally conspecific) semicarinata/livescens/obovata cluster.  In fact, my P. semicarinata “standard” seems to be the most genetically divergent population in the entire study!

What is this semicarinata population from Virginia I called SV in 2014, which I called PINE in 1980?  And what made me think that it was Pleurocera semicarinata in the first place?

Let’s flip the calendar back another five years to 1975, when the young Rob Dillon was a sophomore at Virginia Tech, blissfully bumping around on the backroads of the upper New River drainage with boots, nets and buckets thrown in the back of a state pickup truck.  You may recall, from my essay of May 2014 [6], that my undergraduate research thesis was entitled “Factors in the distributional ecology of upper New River mollusks (Va/NC).”

Goodrich’s papers [7,8] were available in the university library in 1975, but this was before the publication of Burch’s EPA key [9], so I had no pictures.  Nor (of course) any access to reference collections.  So, I confess that I simply guessed.  Goodrich listed just two species of Goniobasis from the area, Goniobasis proxima as a trans-Appalachian inhabitant of the highlands of North Carolina and G. simplex in the Bluestone River of West Virginia, apparently captured over from the Tennessee drainage.  That suggested to me that my softwater species in the New River must be G. proxima, and my hardwater species must be G. simplex.

Even a college sophomore knew crappy science when he saw it, and he was not happy about that, at all.  I do remember thinking, 45 years ago, that there seemed to be at least TWO hardwater species of Goniobasis in the upper New River drainage in addition to the single softwater species but had no idea how to start solving the problem.

A method to start solving the problem presented itself in grad school at the University of Pennsylvania.  My advisor at the ANSP, Dr. George Davis, had a big grant to work out the systematics of unionid mussels with the brand-new technique of allozyme electrophoresis, and was generous enough to allow me to bring my samples of pleurocerids into his lab.  And absorbed onto the little paper wicks applied onto the butt ends of the first gels I ever ran was the proteinaceous goo of ground-up Goniobasis from the upper New River drainage.

Our gels clearly showed one species in the softwater and two species in the hard [5].  And by now I did have a research collection at my fingertips.  The softwater species was (indeed) Goniobasis proxima, upon which I focused my dissertation, and most of the rest of my career, and one of the two hardwater species was (indeed) Goniobasis simplex.  That other hardwater species seemed to match Goniobasis semicarinata.
Figure 2 of Dillon & Davis [5]
Of course, this was just shell-matching.  That is all that pleurocerid taxonomy had ever been, as of 1980.  And Goodrich [7] gave the range of Goniobasis semicarinata as “Tributaries of Ohio River, Scioto River to Big Blue River, Indiana; Licking River to Salt River in Kentucky,” saying nothing about Virginia at all.  I assumed at the time, and continued to assume for many years, that the range of G. semicarinata must extend from Ohio through West Virginia up some 200 km of Kanawha River drainage into the upper New River basin, unbeknownst to Goodrich.

The 1980 paper I published with George Davis ultimately involved two populations of G. simplex, six populations of G. proxima, and the four populations we identified as “Goniobasis semicarinata,” shown on the bottom row of our Figure 2 as reproduced above.  Image (k) depicts population “PINE” from Little Pine Run, a tributary of the New River in Pulaski County, which (renamed “SV”) was ultimately to serve as a mobility standard for my 2013 & 2014 studies of cryptic phenotypic plasticity in canaliculata [4] and semicarinata [2].  In addition to two other “semicarinata” populations from the New River drainage, we also included population “ROA” from Mill Creek in Montgomery County (image J), a tributary of the North Fork Roanoke River.

Yes, the Roanoke River is an Atlantic drainage.  And in subsequent years, as the FWGNA survey extended throughout all the Atlantic drainages from Georgia to the New York line, I discovered populations of what I called Goniobasis or Pleurocera semicarinata north down the length of the Great Valley of Virginia, in tributaries of the Roanoke, James, and Shenandoah Rivers as far north as Waynesboro, my home town [10].

This went on for many years.  And at no time, from my initial judgement call in 1980 to maybe perhaps 2010, had I ever studied at any length a bona fide, Midwestern population of semicarinata on the hoof.

In 2010 the FWGNA project began to devote an increasing amount of our attention to the freshwater gastropod fauna of Ohio River drainages.  This led to the population genetic surveys that ultimately disentangled the evolutionary relationships between what had historically been called Goniobasis semicarinata, G. livescens, Pleurocera canaliculata, P. acuta, and Lithasia obovata.  Perhaps the most striking result of those studies was the tremendous ecological adaptability of what we now call Pleurocera semicarinata – populations now known to inhabit the entire range of waterbodies in the Midwest, from the smallest creeks to the grandest rivers, including lake shores of both sand and rock.  In addition to this, or perhaps because of this, populations of bona fide P. semicarinata demonstrate tremendous plasticity of shell morphology.

That plasticity certainly extends to cover the shell morphology demonstrated by what I had been calling semicarinata in the Great Valley of Virginia.  But 30 years of field experience had impressed upon me that Virginia populations seem to be entirely restricted to small creeks.  They do not extend into larger rivers or lentic bodies of water of any sort, nor do they demonstrate anywhere near the range of shell phenotypic plasticity.  There also seemed to be body color differences – the Midwestern populations typically demonstrating a brighter orange coloration and the Virginia populations darker.  I’m not entirely convinced of this, but it is worth mentioning.

And in 2014 we extended the FWGNA survey into the Ohio drainages of West Virginia.  There really are no Pleurocera populations of any species inhabiting any tributaries of the Kanawha river through most of the state.  The Virginia populations I had been calling semicarinata appear to be isolated by over 200 km of uninhabited waters from the nearest bona fide semicarinata population in Ohio.

So all that, together with the genetic results shown in 2014 Figure 4 reproduced above, combined to force me into describing a new species, in the appendix of the first hardcopy volume of the FWGNA project, coming soon [11].  This new species, Pleurocera shenandoa, is locally quite common and widely distributed throughout a long-settled part of the world, where one might reasonably expect the biota to be well-known and well-characterized.

It is not.  Perhaps the primary theme of my 20 years of blog posts, and the entire set of four FWGNA volumes to be published soon, is that even here in the 21st century, in the home of the best science the world has ever known, we remain stunningly ignorant of even the most commonplace.  We spend billions of dollars shooting space probes to the moons of Jupiter, and not a nickel to understand the little brown snails in the creeks behind our own houses.  Shame on us all.


[1] Dillon, R. T. Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53: 265-277. [PDF] See:
  • Goodrichian taxon shift [20Feb07]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[2] Dillon, R. T., Jr.  (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata.  Zoological Studies 53:31. [PDF] See:
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
[3] Dillon, R. T., Jr (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]
  • The cryptic Pleurocera of Maryville [13Sept16]
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]
  • The fat simplex of Maryville matches type [14Oct16]
Dillon, R. T. (2016)  Match of Pleurocera gabbiana (Lea, 1862) to populations cryptic under P. simplex (Say, 1825).  Ellipsaria 18(3): 10 - 12.  [PDF]
  • One Goodrich missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]
[4] 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]  See:
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Pleurocera canaliculata and the process of scientific discovery [18June13]
[5] Dillon, R.T. and G.M. Davis (1980) The Goniobasis of southern Virginia and northwestern North Carolina: Genetic and shell morphometric relationships. Malacologia 20: 83-98. [PDF]

[6] Sweet, gauzy memories of my college days:
  • To identify a Physa, 1975 [6May14]
[7] Goodrich, C. (1940) The Pleuroceridae of the Ohio River system.  Occas. Pprs. Mus. Zool. Univ. Mich. 417: 1-21.

[8] Goodrich, C. (1942) The Pleuroceridae of the Atlantic Coastal Plain.  Occas. Pprs. Mus. Zool. Univ. Mich. 456: 1-6.

[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] Sweet, gauzy memories of Waynesboro, VA.  Well, not so much:
  • The Clean Water Act at 40 [7Jan13]
[11] Dillon, R. T., Jr. (2019) Description of a new species of freshwater snail (Caenogastropoda: Pleuroceridae) from the Great Valley of Virginia.  Appendix 1 in Dillon, Ashton, Reeves, Smith, Stewart & Watson, The Freshwater Gastropods of North America, Volume I.  Atlantic Drainages, Georgia through Pennsylvania.  FWGNA Publishing, Charleston, SC.

Friday, February 15, 2019

Dates of Publication

Editor’s Note – This brief post is offered as an appendix to my essay of 6Feb19, “Ferrissia fragilis (Tryon 1863).”

Several of you have pointed out, in separate correspondence with me over the last week, that taxonomic priority is established not by date of oral presentation, but by date of publication.  This does not change the order of the three freshwater limpet descriptions I reviewed in my essay of 6Feb19, nor indeed anything in my essay at all.  But for the record, the precise, formal publication dates of the three limpet taxa I discussed in that essay were: 
  • Gundlachia californica Rowell, (31)May1863
  • Ancylus fragilis Tryon, 13July1863
  • Gundlachia meekiana Stimpson, (31)Dec1863
Our good buddy Dr. Harry Lee, from Jacksonville (FL), is quite the gifted scholar.  Yesterday morning he sent me a thoroughly-researched letter on the subject, together with supporting documentation, which I have made available from the links below: 
  • Letter from H. G. Lee, regarding the nomen Ferrissia fragilis [pdf]
  • Extract from Levinton et al. (2010) [pdf
And to be clear.  Just because I myself am not a priest, nor a scribe, nor a Pharisee does not mean that I don’t value the services they can provide.  Everybody appreciates a scribe now and then.  Thanks, Harry!

Wednesday, February 6, 2019

Ferrissia fragilis (Tryon, 1863)

At 9:30 on the starry but moonless night of April 16, 1863, seven armored gunboats under the command of Admiral David Porter, accompanied by three army transports and a steam ram, began a stealthy voyage down the Mississippi under the guns of Vicksburg.  Water-soaked bales of hay were stacked around their boilers and pilot houses, and coal barges lashed to their starboard flanks.  At 11:10 PM, all hell broke loose [1].

Wikimedia Commons
On the evening of April 20, four days later, the regular biweekly meeting of the California Academy of Natural Sciences convened at 622 Clay Street, San Francisco, seven members present, Dr. Trask in the chair.  Dr. Cooper communicated the description of a new mollusk, recently discovered by Rev. Joseph Rowell in the waters of the Feather River, Gundlachia californica [2].  Images of the little limpet, “length about sixteen one-hundredths of an English inch,” showed a shell apex distinctly different from the embryonic shell origin.  Philip Lutley Sclater, Esq., of London was elected a corresponding member, three species of reptiles from San Mateo added to the cabinet, and the Academy adjourned.

Shortly after 5:00 on the evening of May 2, 1863, the troops on the right flank of General Joseph Hooker’s army at Chancellorsville, VA, stacked their rifles and began to prepare their suppers.  They were amused to see large numbers of deer and rabbits break out of thickets to the west and come bounding toward them.  The men cheered and waved their caps at the startled forest creatures, until the next thing they saw froze the laughter in their throats.  Total casualties at the end of the battle were 3,500 killed and 19,000 wounded.

May 2, Just before dawn.  Wikimedia commons
A bit more than two weeks later, on the afternoon of May 26, 1863, The Academy of Natural Sciences of Philadelphia convened for its regular weekly meeting, 19 members present, Mr. Lea in the chair.  The agenda was lengthy: 9 papers presented and ordered to be published, including an ambitious contribution by T. B. Wilson & J. Cassin proposing a third kingdom of life, the Primalia.  Mr. George W. Tryon read a paper describing seven new species of freshwater gastropods, finishing with Ancylus fragilis [3].  The 4 millimeter limpet, apex elevated, acute and “curved backwards,” had been sent to him from California by Rev. J. Rowell.

On June 3, 1863, General Robert E. Lee began to concentrate his army of 75,000 at Culpeper, in preparation for an invasion northward.  And on the morning of June 8, Union General Alfred Pleasonton probed south across the Rappahannock with six brigades of cavalry, approximately 10,000 horsemen, to gauge Lee’s disposition.  Around noon Pleasonton encountered a roughly equal force of confederate cavalry under Gen. Jeb Stuart at Brandy Station.  Sabers, pistols, and carbines flashing in the sun, the largest cavalry engagement ever fought on American soil was underway.

Cavalry Charge Near Brandy Station, by Edwin Forbes.
Two weeks later, on the afternoon of June 17, the Boston Society of Natural History convened at Tremont Street, Prof. Wyman in the chair.  Mr. Stimpson read a paper on the genus Gundlachia, in which he counted five species, including G. californica, described just two months prior [4].  He went on to describe a sixth species, G. meekiana, collected from the vicinity of Washington DC, similar in all respects to G. californica, but with a less ovate aperture.  One additional paper was read, two communications received, six donations to the museum logged, and the Society adjourned.

Admittedly, the little brown pulmonate limpet that we today call Ferrissia fragilis is not the most striking element of the North American malacofauna.  It is, however, the fourth most common freshwater gastropod in the Eastern United States, behind Physa acuta, Campeloma decisum, and Menetus dilatatus [5].  Populations of Ferrissia fragilis are ubiquitous on aquatic vegetation and organic debris in every lake, pond, and riverine backwater nationwide, Canada to Mexico, sea to shining sea.  Including at the mouth of Pennypack Creek, in north Philadelphia.

Why do you suppose that these exceptionally abundant and wide-ranging little gastropods were completely overlooked by every American biologist working in every puddle of fresh water for half a century, and then simultaneously discovered by three completely separate societies of learned men, meeting in San Francisco, Philadelphia, and Boston, during a single eight-week period of 1863?  What might account for the sudden, passionate interest among young well-born gentlemen of the urban North in freshwater limpets?  I will leave that question to the speculation of my readership.

The Bartow County Yankee Killers [6]
I will, however, take a paragraph to remind you all of several previous essays touching upon Ferrissia fragilis [7].  You may recall, from my essays of 10June09 and 9Nov12, that the freshwater limpets were a particular research interest of Bryant Walker’s (1856 – 1936), and that the definitive monograph was contributed by Paul Basch in 1963 [8].  Both Walker and Basch recognized Ferrissia fragilis as a widespread and important element of the North American malacofauna, and listed californica (Rowell 1863) and meekiana (Stimpson 1863) as junior synonyms of fragilis (Tryon 1863).  And you may also remember my essay of 8Dec10 reviewing the excellent work of Andrea Walther and colleagues [9] synonymizing several additional well-known names under fragilis, including walkeri (Pilsbry & Ferris 1906) and mcneilli (Walker 1925).  So that today, the FWGNA Project recognizes just two species of Ferrissia: rivularis and fragilis.

Up until recently it has been quite easy to ignore the extraordinarily trivial and obscure detail that the meeting of the California Academy which heard the description of G. californica preceded the meeting of the ANSP which heard the description of A. fragilis by five weeks.  For some reason I cannot fathom, however, here in 2019 it has become less easy.

The issue of the American Malacological Bulletin freshly arrived on my desk last month included a research note announcing the “discovery of the freshwater limpet, Ferrissia californica (Rowell, 1863)” on the Island of Montserrat [10].  Tryon’s nomen “fragilis” does not appear in title, abstract, key words, or the first five paragraphs of its introduction.

Do systematic biologists of the 21st century feel some heightened sense of obligation to the Rev. Rowell, now asleep in Christ for 100 years?  Have our oaths to uphold the International Code of Zoological Nomenclature suddenly become more solemn?  Is the iron fist of the ICZN Commission grown more fearsome?

I do not know.  I am neither priest nor scribe nor Pharisee, I am a scientist.  The names I assign to populations of freshwater gastropods are hypotheses of evolutionary relationship – my best hypothesis, without compromise.  If I find that more than one name has been assigned to a population or group of populations, each of which conveys the same evolutionary hypothesis, I will select the name that, in my judgement, conveys my hypothesis to the broadest audience. 

That name, in the case of the 4 mm freshwater limpets with the eccentric shell apex, is Ferrissia fragilis (Tryon, 1863).  The letter of some legalistic code about which I was not consulted, administered by some commission I do not recall electing, does not enter into the calculation.

But let me hasten to make another point, and to make it as forcefully as I have made the previous one.  I would not presume to impose my selection of any scientific name on anyone else.  In fact, I earnestly hope that other scientists will develop other hypotheses about the evolutionary relationships of the populations I refer to Ferrissia fragilis.  Such a situation would be the mark of an active science.  And if it is the judgement of some other worker that Rowell’s nomen californica transmits information more effectively than Tryon’s fragilis, far be it from me to second-guess.

I have no problem with synonyms.  I do not think that taxonomic synonyms necessarily lead to scientific confusion, any more than I expect the college dean to become confused if I tell him to kiss my peachy-pink posterior or my rosy-red ass, on the way out the door.  Synonyms are pervasive in the English language, and we are richer for it.

Indeed, I think it will be a service to preserve both names.  So just this morning I have added Rowell’s “Ferrissia californica” directly under the header “Ferrissia fragilis” at the top of my FWGNA page.  And entered the nomen into the list of synonyms available from the website pull-down.  And written the present essay, wherein both names are connected.  In this fashion, the future generation of graduate students, perhaps na├»ve about the fragilis/californica situation, will be able to google-up and connect their disparate literatures.

And finally.  Difficult though it may be to understand [11], some non-negligible fraction of my colleagues have, from time to time, associated into committees to develop formal lists of accepted or approved names that we, “the community,” will be sanctioned to apply to the diverse biota of this, our great country.  I would suggest that all members of such committees re-read the first six paragraphs of the present essay.  And get a life, every one of you.


[1] This account of the action at Vicksburg, together with those of Chancellorsville and Brandy Station following, are extracted from Shelby Foote’s (1963) classic The Civil War, A Narrative.  Volume II, Fredricksburg to Meridian. Vintage Books, 988 pp.

[2] Rowell, J (1863) Description of a new Californian Mollusc.  Proceedings of the California Academy of Sciences Series 1, 3: 21 – 22.

[3] Tryon, G. W. (1863) Descriptions of new species of fresh water Mollusca, belonging to the families Amnicolidae, Valvatidea, and Limnaeidae; inhabiting California.  Proc. Acad. Natl. Sci. Phila. 15: 147 – 150.

[4] Stimpson, W. (1863) Malacozoological Notices No. 1, On the genus Gundlachia.  Proc. Boston Nat. Hist. Soc. 9: 249 -252.

[5] This result is from 18,974 records of 99 species in four regions: the Atlantic, the Ohio, East Tennessee, and (very preliminarily) The Cumberland.  Download the presentation here:
  • The freshwater gastropods of The Ohio: An interim report [27June17]
[6] From left, Daniel, John, and Pleasant Chitwood of Company A, 23rd Georgia Infantry.  Daniel and John were captured at Chancellorsville on May 2, 1863.  This image scanned from Miller, W J. & B. C. Pohanka (2006).   An Illustrated History of the Civil War.  Barnes & Noble.

[7] My previous essays on Ferrissia:
  • Just One Species of Ferrissia [10June09]
  • Two Species of Ferrissia [8Dec10]
  • Bryant Walker’s Sense of Fairness [9Nov12]
[8] Basch, P.F. (1963) A review of the recent freshwater limpet snails of North America (Mollusca: Pulmonata). Bull. Mus. Comp. Zool. Harvard Univ. 129: 399–461.

[9] Walther, A. C., J. B. Burch and D. O’Foighil (2010) Molecular phylogenetic revision of the freshwater limpet genus Ferrissia (Planorbidae:Ancylinae) in North America yields two species: Ferrissia (Ferrissia) rivularis and Ferrissia (Kincaidilla) fragilis. Malacologia 53: 25-45.

[10] Coote, T, K. A. Schmidt, R. E. Schmidt, & E. R. McMullin (2018) Discovery of the freshwater limpet, Ferrissia californica (Rowell, 1863) (Gastropoda: Planorbidae), from streams of Montserrat, West Indies, a new addition to the Caribbean fauna.  American Malacological Bulletin 36: 291 – 295.

[11] I myself probably do understand it, however.  I think committees form to standardize the names of the diverse elements of the American biota to facilitate governmental regulation.  And with governmental regulation comes governmental funding.  I don’t want to be cynical – I’m pretty sure my colleagues on such committees think that they are furthering the cause of conservation, and that whatever taxpayer’s dollars might be expended on their salaries are well-justified.  I used to think that, too.

Monday, January 14, 2019

The best estimate of the effective size of a gastropod population, of any sort, anywhere, ever

First, a quick refresher on week #4 of your population genetics class.  One of the assumptions of Hardy-Weinberg equilibrium is that population size is effectively infinite.  If that assumption is met (together with all the other ones) gene frequencies will not change.  But if the population is small, gene frequencies will change by sampling error, for the same reason that if I flip a fair coin twice, there a 50% chance of that either the head will not be represented, or the tail.  That phenomenon is called “genetic drift.”

So, suppose we were standing on the edge of a huge field of peas, say 100,000 plants, polymorphic at the seed color locus – green and yellow – meeting all the Hardy-Weinberg assumptions (no selection, no migration, random mating and all that, Note 1).  And suppose we sampled 100 plants, and shucked a bunch of pods, and estimated the frequencies [2] of the green and yellow genes in the remaining 99,900.  And then we waited a year, let all 99,900 plants reproduce, cover the field with a new generation, and sampled another 100.  And we didn’t obtain exactly the same gene frequencies in year 2 that we did in year 1.

Much of the reason that the year-2 gene frequencies didn’t match the year-1 frequencies would certainly be our own sampling error – that we sampled a finite number from year 1 and a finite number from year 2.  But part of that allele frequency variance (new term) might also be due to the sampling error of the peas themselves – only a finite number of peas reproduced to yield the year-2 population.  Might we have been wrong about that 99,900 estimate?  Maybe, effectively, there weren’t 100,000 peas in the field at all?

The Duck Pond at Quarterman Park
So effective population size (Ne) is the size of an idealized population that demonstrates the same allelic frequency variance as the population under study.  There are many possible reasons why Ne is always less than or equal to N, the headcount (or stem count) population size.  Often much less.

Well, I hate to be pedantic, but there’s another (equivalent, but somewhat more difficult) definition of Ne.

Suppose we were to sample 100 peas from our population at a single date but analyzed gene frequencies at two polymorphic loci, not just one.  So, say seed color (green/yellow) and seed shape (wrinkled/smooth).  There is no reason to expect any relationship between seed color and seed shape – knowing green shouldn’t allow you to predict wrinkled.  That is true regardless of whether the seed color locus and the seed shape locus are on the same chromosome or not, because (in a large, randomly-breeding population) crossing-over would ultimately erase any initial relationship between the alleles.

But what if you did find a relationship between seed color and seed shape in the pea field?  This is often called “gametic phase disequilibrium” to distinguish it from the (hard) linkage disequilibrium you might discover between loci on the same chromosome using a controlled cross.  That would mean that the population wasn’t infinitely large and randomly-breeding.

So finally.  Effective population size is the size of an idealized population that demonstrates the same allelic frequency variance or the same gametic phase disequilibrium as the population under study.  Geeze, that turned out to take longer to explain than I imagined when I started this essay eight paragraphs ago.  I appreciate your forbearance.

Effective population size is a parameter in every model of theoretical population genetics ever published, neutral or otherwise.  That number is really, REALLY important.  And also, really difficult to obtain.

I only know of ten estimates of Ne in gastropod populations ever published, in total, for all environments: 2 marine, 4 terrestrial, and 4 freshwater [3].  And frankly, many of those ten are pretty darn spurious.

OK, I’m going to change subjects entirely here.  But don’t forget all the boring population genetics stuff you had to slog through above, because you’re going to need it again, shortly.

I taught Genetics Laboratory 305L at The College of Charleston for 33 years.  And Investigation #9 in my Genetics 305L lab manual was “The analysis of genetic polymorphism in a natural population using allozyme electrophoresis.”  I needed a sample of 31 individual somethings from some polymorphic population for each section, which toward the latter half of my career [4], became three sections per semester, two semesters per year, that’s N = 186 somethings.  And after years of messing around with pleurocerid snails and Mercenaria hard clams, those somethings became Physa acuta.

Now my second-favorite population of Physa in the world [5] inhabits the Duck Pond at Quarterman Park in North Charleston.  Amy Wethington and I first sampled that population (“NPK”) in 1990 in conjunction with our study of sea island biogeography [6], and I knew it was polymorphic at three allozyme-encoding loci: Isocitrate dehydrogenase (Isdh, 3 alleles), 6-phosophogluconate dehydrogenase (6pgd, two alleles) and Esterase-3 (Est-3, two alleles, Note 7).

So, in May of 2009 I collected my first big sample of Duck Pond Physa for the students working on Genetics Lab Investigation #9.  In many respects the one-hectare pond at Quarterman Park is perfect Physa habitat – shallow, quiet, and very rich, fed by runoff from the neighborhood.  Every day the kids bring bags of stale bread to feed the ducks, which they empty and throw into the pond with all the other picnic garbage.  Occasional whiffs of sewage.  Physa paradise.

Physa paradise
Only three factors keep that pond from becoming a block of Physa solid enough to walk across.  One is the flocks of ducks and geese, which probably prefer the Physa over the bread.

A second is the general lack of habitat.  Some 15 – 20 years ago the City of North Charleston undertook a complete renovation of the Duck Pond, draining it and shoring up the walls with bulkheads.  So, the modern pond has no vegetation of any sort, nor indeed any littoral zone.  Physa are common only on the floating allochthonous debris and garbage that accumulates at the eastern end, by the drain, and rather rare elsewhere.

And a third is the summer maximum temperatures, which can be brutal.  The City of North Charleston also installed a couple fountain pumps during those renovations a few years ago, but during the hot months, I feel certain that all aerobic life must be confined to the top centimeter or two of the pond.  And concentrated at the eastern end.

All those stipulations registered, I had no problem collecting several hundred Physa at the Quarterman Park Duck Pond in the spring of 2009.  I knelt on the walkway at the eastern end of the pond and hand-plucked Physa off the floating debris.  Or sometimes I found it easier to wash higher concentrations of snails off large sticks and bread bags into my bucket.  The entire sample didn’t take more than 30 minutes to collect.

And in the fall of 2009, the students enrolled in my three sections of Genetics Lab 305L estimated gene frequencies at three loci in 93 of them.  And ditto for another N = 93 in the spring of 2010.  And I went back to the Duck Pond to fetch more.

This went on for seven years, from 2009 to 2015.  The actual date of the sampling varied a bit from year to year, depending on the density of the snail population, which in turn, seemed to vary with the weather.  I could always find at least a few Physa in the Duck Pond – any day, 12 months per year.  But to collect the hundreds I needed annually, I needed a bloom.

After a few years of experience, I began to notice a relationship between Physa blooms and the blooming of the azaleas.  In the Charleston area, as I am sure elsewhere, azaleas bloom in response to a pulse of warmth and sunshine – the stronger the pulse, the more brilliant the display.  The bloom typically takes place in March here and lasts for several weeks.  My annual observations suggested that the Physa bloom at the Quarterman Park Duck Pond typically commenced around the week the azaleas dropped their flowers – in early to mid-April.  The population typically expanded through May, contracted in June, and died back almost entirely in the summer.

The only exception happened in 2012, when no Physa bloom occurred at all.  The spring of 2012 was exceptionally hot in Charleston – the mean March temperature (65.3 degrees F) the second-highest value in the 80-year record of the National Weather Service.  I was never able to make a collection that year.  I visited the pond every couple weeks from March to July, and could always find a few snails, but never in the quantity that would prompt me to get on my knees and start washing bread bags into buckets.

So I was relieved of my duties at the College of Charleston in February of 2016, and ultimately banned from campus for a Woodrow Wilson quote [8], bringing my study to an end with the 2015 field season.  The paper reporting the results obtained by myself and my team of 540 undergraduates was published early last year in Ecology and Evolution, citation from Note [9] below.

I should thank my good friend and former student Dr. John Robinson for putting me onto a really excellent freeware resource called “NeEstimator,” developed by Chi Do and colleagues [10].  The software calculates effective population size using both allelic frequency variance (which are called “two-sample methods”) and gametic phase disequilibrium (“one-sample methods”).  All the statistics and other gory details are available in my paper.

The bottom line turned out to be that Ne for the Physa population at the Quarterman Park Duck Pond was infinite in 2009 and 2010, dipped in 2011 to somewhere around Ne = 100, dipped again between 2011 and 2013 to around Ne = 50, popped up to around Ne = 200 in 2014, and then rebounded to infinite again in 2015.  These results are remarkably consistent across both my one-sample and the two-sample analyses, which are independent, and really tend to strengthen their mutual credibility.

I suppose the first explanation that might occur to one would be a population bottleneck in the 2012 year.  But bottleneck effects are notoriously long-lasting… once allelic diversity is lost, it takes many, many generations to regain it.

I think the key factor in the volatility of Ne demonstrated in this study may be cryptic population subdivision.  In retrospect, the striking dip in apparent population census size I observed in 2012 may have been localized at the east end of the pond, and its subsequent recovery due to immigration from elsewhere within a Physa population subdivided by distance.

Some of the most influential studies of population subdivision published ever have been conducted using land snail models. Cain and Currey (1963) described small-scale variation in the frequencies
of shell color morphs in the English land snail Cepaea as “area effects,” attributing the phenomenon to genetic drift [11].  Could freshwater gastropod populations perceive their environments much differently than Cepaea?

At minimum, these results should be received as a cautionary tale by those researchers, including yours truly, a sinner, who would represent the evolutionary relationships between populations by single samples, even as large as 200, even with multiple polymorphic loci, collected from single sites at single dates.

And as for the practice of sampling individual genes from individual snails from individual populations to represent an entire biological species?  That’s just plain White-House-stupid.


[1] I know that garden peas are self-pollinating.  Give me this one, for the sake of the example, OK?  Geeze, you must have really irritated your tenth-grade biology teacher.

[2] And I also realize that, because of dominance, you’d have to assume HWE to get gene frequencies at the seed color locus in garden peas.  Doggone it, now you’re beginning to piss me off.

[3] See the introduction section of my paper from note [9] below for the references.

[4] From 1983 into the mid-1990s, I only taught one section per semester – perhaps 15 students.  But the number of lab sections I taught per semester increased from two to three in the latter half of my career, as my lecture sections were assigned to adjunct faculty more sensitive to the self-esteem of the customers.

[5] My first-favorite Physa population inhabits the pond at Charles Towne Landing State Park.  See:
  • To Identify a Physa, 1989 [3Oct18]
  • Albinism and sex allocation in Physa [5Nov18]
[6] Dillon, R.T., and A.R. Wethington (1995) The biogeography of sea islands: Clues from the population genetics of the freshwater snail, Physa heterostropha. Systematic Biology 44:401-409.  [PDF]

[7] Dillon, R.T., and A.R. Wethington (1994) Inheritance at five loci in the freshwater snail, Physa heterostropha. Biochemical Genetics 32:75-82. [PDF]

[8] Who Decides What Must Be on a Syllabus?  Inside Higher Ed, 8Aug16.  [html]

[9] Dillon, R. T. (2018) Volatility in the effective size of a freshwater gastropod population.  Ecology and Evolution 8: 2746 - 2751. [https://doi.org/10.1002/ece3.3912]  [PDF]

[10] Do, C., Waples, R., Peel, D., Macbeth, G., Tillett, B., & Ovenden, J.  (2014) NeEstimator v2: Re-implementation of software for the estimation of contemporary effective population size (Ne) from genetic data. Molecular Ecology Resources, 14, 209–214. https://doi.org/10.1111/1755-0998.12157

[11] Cain, A. J. & Currey, J. D. (1963) Area effects in Cepaea. Phil. Trans. R. Soc. London Series B 246: 1-81.  Cain, A. J. & Currey, J. D. (1968) Studies on Cepaea III: Ecogenetics of a population of Cepaea nemoralis (L) subject to strong area effects.  Phil. Trans. R. Soc. London Series B: 253, 447-482.  Ochman, H., J. S. Jones & R. K. Selander (1983) Molecular area effects in Cepaea.  PNAS 80: 4189 – 4193.