Dr. Rob Dillon, Coordinator


Friday, July 11, 2014

Elimia livescens and Lithasia obovata are Pleurocera semicarinata

Faithful readers of this blog will by now be familiar with cryptic phenotypic plasticity in the shell morphology of pleurocerid snails – interpopulation variance so extreme as to prompt an (erroneous) hypothesis of speciation.  We first documented the phenomenon in our (2011) survey of Pleurocera clavaeformis in East Tennessee [1] under the term “Goodrichian taxon shift,” and generalized the concept to “cryptic phenotypic plasticity” in our (2013) study of Pleurocera canaliculata ranging from New York through the Midwest to North Alabama [2].

Now in a third installment of the series, published in this month’s issue of Zoological Studies [3], we document what may be the most surprising example of the phenomenon yet brought to light.  And again, it might be easiest to understand the significance of the discovery if we back up to the 19th century and take a running start at it.

Thomas Say described “Melania semicarinata” from “Kentucky” in 1829, one year before C. T. Menke described his “Melania livescens” from “Lake Erie, New York.”  Classically, livescens populations bear short, stubby shells while the shells of semicarinata are more slender.  But the populations inhabiting most of the Midwest seem to vary tremendously in shell morphology, such that the distinction between semicarinata and livescens simply disappears.  Calvin Goodrich (1940) divided the species ranges roughly at the glacial maximum, giving the range of semicarinata as “tributaries of Ohio River, Scioto River, Ohio, to Big Blue River, Indiana; Licking River to Salt River in Kentucky; two creeks of Green River of Kentucky” and that of livescens as “tributaries of Ohio River, east of Scioto River in Ohio; Wabash River and branches, west to Illinois River.  Especially common in the St. Lawrence basin.” [4]

But one gets the impression that this division was rather arbitrary on Goodrich’s part.  Our colleagues Steve Jacquemin and Mark Pyron published a nice morphometrics paper back in 2012 [5] showing that Indiana populations bearing broader shells tend to be found in more lentic environments at higher latitudes, and that populations bearing slender shells tend to be found in lotic waters at lower latitudes.  Our Indiana friends identified all 39 of their study populations as “Elimia livescens,” but Goodrich would have identified the 16 populations they sampled from the southern half of the Hoosier State as Goniobasis semicarinata.

I began thinking about a test of the hypothesis that semicarinata and livescens populations might be conspecific in the spring of 2009.  And it was obvious to me that if I were to use the genetic tools that have stood me in such good stead for 30 years, I would need some sort of external calibration.  The combined range of nominal livescens and semicarinata together would extend across 12 states and two Canadian provinces, much larger than any area I had heretofore surveyed.  How much genetic divergence might one expect among conspecific pleurocerid populations spread across so large a region?

So (again, this was back in 2009) it occurred to me that populations of (what I called at the time) Pleurocera acuta also range from New York through the Midwest to Kentucky, and that nobody has ever questioned their conspecific status.  So along with each study population of either semicarinata or livescens, I resolved to sample a control population of Pleurocera acuta.  And if the genetic divergence among my semicarinata/livescens study populations proved no greater than that of my acuta controls, the evidence would suggest that livescens (Menke 1830) is a junior synonym of semicarinata (Say 1829).  I actually made several field trips through Kentucky, Ohio, and Indiana in the summer of 2009, sampling pleurocerid populations under this original (much simpler) study design.

At this point in my narrative, any of my readership who might be pathologically curious about the process of scientific discovery in your particularly feeble-minded correspondent might insert the essay I wrote last year regarding Pleurocera acuta, P. canaliculata, and P. pyrenellum [6].  It was only in the summer of 2010, after I had already begun work on my semicarinata/livescens project, planning to use acuta as a control, that it dawned on me that the situation with canaliculata/acuta might be as big a mess as semicarinata/livescens [7].  And since I couldn’t interpret my semicarinata/livescens data without a canaliculata/acuta calibration, the canaliculata/acuta problem had to be solved and published first.

And that’s how I stumbled upon the headline news – the relationship of semicarinata/livescens to Lithasia obovata.  Sampling the main Ohio River at the type locality for Pleurocera canaliculata in August of 2011, I encountered the first population of L. obovata I had ever personally examined on the hoof.
The hypothesis that populations which Thomas Say described as “Melania obovata” from the Kentucky River back in 1829 might be a startlingly robust, big-river morph of semicarinata/livescens did not immediately dawn on me, however.  True, juvenile obovata do look a bit like Goniobasis or Elimia, which are the genera to which semicarinata and livescens have historically been assigned. 

And indeed, in 1934 Calvin Goodrich documented the same upstream-downstream relationship between the slender, lightly-shelled “sordida form” of L. obovata and more typical, heavily-shelled “forms” in the Green River of Kentucky that he documented between the slender, lightly-shelled pinguis form of Lithasia geniculata and more typical, heavily-shelled forms in the Duck River of Tennessee [8].  It was Goodrich’s 1934 analysis of shell morphological intergradation in the L. geniculata populations of the Duck River that prompted me to coin the term “Goodrichian Taxon Shift” in his honor in 2007 [1].  And sordida (Lea 1841) was initially assigned to the genus Goniobasis by Tryon, just as pinguis (Lea 1852) was initially assigned to Anculosa.

Was Goodrich’s (1934) Plate 1 riding around in the back of my mind in 2011?  [Click the thumbnail below for a larger version.]  Figures 1 – 9 and 12 depict what Goodrich considered “forms” of Lithasia obovata, including figures 2, 6, and 12, which are all obviously Pleurocera semicarinata.  Figures 10, 11, and 14 depict his “forms” of Lithasia geniculata (Figure 11 is pinguis), which I first extracted and figured way back in 2007 as an illustration of Goodrichian taxon shift.

In the end, I’m not sure what led me to include a sample of 32 Lithasia obovata in a two-phase study already groaning with five populations of nominal acuta, four populations of canaliculata, three populations of semicarinata, three populations of nominal livescens, and a population of nominal pyrenellum from North Alabama thrown in for good measure.  But I’m glad that I did.

I have always considered myself philosophically conservative, in the sense that my initial reaction to change of any sort is negative.  And I am very sensitive to the unhappy side-effects of the taxonomic changes that I have proposed for the North American Pleuroceridae in recent years, especially with regard to the indexing function.  The best general work on the biology of the creatures to which I have dedicated my career is the 1965 paper by B. C. Dazo, “The morphology and natural history of Pleurocera acuta and Goniobasis livescens [9].”  With the publication of my most recent paper, Dazo’s study populations have now become Pleurocera canaliculata and Pleurocera semicarinata, nearly impossible to retrieve by a simple electronic search.

So let’s use subspecies, shall we?  The FWGNA Project has adopted the standard definition of the term “subspecies” as it has been understood since the birth of the Modern Synthesis: “populations of the same species in different geographic locations, with one or more distinguishing traits” [10].  Clearly some populations of Pleurocera semicarinata inhabiting big rivers bear shells so robust as to warrant distinction as P. semicarinata obovata, and other populations in more northern latitudes bear broader shells distinguishable as P. semicarinata livescens.  Such a system will preserve the indexing function of the old names, without compromising our growing storehouse of new evolutionary insights to the ever-surprising freshwater gastropods of North America.

Notes

[1] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]  For more, see:
  • Goodrichian Taxon Shift [20Feb07]
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[2] Dillon, R. T., Jr., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127. [html] [pdf]  For more, see:
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
[3] Dillon, R. T., Jr.  (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gasgtropod, Pleurocera semicarinata.  Zoological Studies 53:31. [html] [pdf]

[4] Goodrich, C. (1940) The Pleuroceridae of the Ohio River system.  Occas. Pprs. Mus. Zool. Univ. Mich. 417: 1-21.

[5] Dunithan, A., S. Jacquemin & M. Pyron (2012)  Morphology of Elimia livescens (Mollusca: Pleuroceridae) in Indiana, USA, covaries with environmental variation.  Am. Malac. Bull. 30: 127 – 133.

[6] Pleurocera canaliculata and the process of scientific discovery [18June13]

[7] And now you know the rest of the story.  In my essay of 18June13 (link above) I wrote, "Standing knee-deep in Savannah Creek (maybe 20 km N of Chattanooga) on that memorable day, it suddenly struck me that the snails crawling around my feet might be Pleurocera acuta."  This revelation was only possible because I had spent several weeks that previous summer travelling around the Midwest hunting Pleurocera acuta.  In retrospect, I think the only way I was able to make the acuta/canaliculata/pyrenellum connection in 2010 was that by that time I had personal field experience across the entire 12-state range of the species, from Yankeeland to Dixie.  This is something that Thomas Say, or George Tryon, or even Calvin Goodrich, could never have contemplated.  Good day.

[8] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae – I.  Occas. Pprs. Mus. Zool. Univ. Mich. 286: 1-17.

[9] Dazo, B.C. (1965) The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae). Malacologia 3:1-80.

[10] For a complete review of FWGNA policies on the taxonomic level of the subspecies, see:
  • What is a subspecies? [4Feb14]
  • What subspecies are not [5Mar14]

Thursday, June 12, 2014

To Identify a Physa, 1978

Word has reached us of the death of Dr. George Ang Te, who passed away late last year in New Hamburg, NY at the age of 67.  Although I never met him, George Te’s 1970s-era research on the worldwide Physidae was a significant influence on my professional career.  In this final installment of our three-part series, we examine Te’s 1978 dissertation, and his continuing legacy.

In the 1970s everybody was in love with computers.  There was just the one computer on the Virginia Tech campus, and it was a huge IBM 360/370, with walls of blinking lights and massive tape drives.  And if you wanted to use it, you pretty much had to write the FORTRAN code yourself.  There were a few “canned” statistical packages available in those days, but even to use canned programs, you had to write your own job control language.  And you had to punch your own cards, in a big room full of keypunch machines – one line of code or data per card.  And you needed an “account” to use the machine.  (Tech undergraduates were allocated $25 per quarter.)  And you left your deck of cards at the input desk, and came back the next day, hoping to find a thick stack of tractor-feed paper with meaningful results in your cubby.  I usually found two-page bombs, with unintelligible error codes, and hefty charges to my account for the service.  I loved computers in the 1970s.

In the 1970s the world of systematic biology was completely consumed by warfare between the forces of phenetics and the forces of cladistics.  Phenetic analysis was seamlessly able to bring the power of electronic computing to bear on problems of classification.  Pheneticists told us to count, measure or score 100 characters on 100 “operational taxonomic units,” punch our cards, and let the room full of blinking machinery objectively classify our OTUs by their overall similarity.

No, cladists argued – classification can only be based on shared-derived characters, and shared-primitive characters must be discounted.  So rather than calculate some gross measure of overall similarity among our OTUs, cladists insisted that character state polarity must be taken into account.  For everything measured, counted or scored, somebody had to figure out what state was primitive, and what state was derived, somehow.   Such subjectivity did not immediately lend itself to electronic computing.

But whatever else the world of systematic biology was focused upon in the 1970s, organisms were not it.  The process of classification was important, and the OTUs being classified, not so much.  And it was into this world that George Te stepped when he entered the PhD program at the University of Michigan in the fall of 1971.

So in addition to J. B. Burch, serving on George Te’s PhD committee were two much-decorated officers in the cladist army, Warren (“Herb”) Wagner and George Estabrook.  Wagner was an exellent botanist, carried to stardom by an algorithm to estimate phylogenetic relationships that became known as “Wagner parsimony.”  Estabrook was a computer scientist, entirely unsullied by any taint of organismal biology, with a head full of very original ideas for estimating cladistic classifications based on character compatibility.  And I think it was primarily Estabrook’s influence that ultimately turned George Te’s dissertation focus, and indeed George Te himself, away from biology.

But he made a promising start at it.  Last month [1] we reviewed Te’s (1975) survey of the Physidae of Michigan, which I characterized as “the zenith of classical malacology in North America.”  So broadening his scope toward a dissertation, Te expanded his regional survey to include 12,163 museum lots of worldwide physids from seven major museums, among which he initially recognized 85 “basic taxa.”  For these 85 basic taxa he scored 71 characters: 37 of shell and 34 of anatomy, each character taking some number of discrete “states” [2].

Estabrook gave George Te two sets of computer programs, one phenetic and the other cladistic.  The former was called “Simgra,” a quirky method of clustering OTUs by a matrix of similarity coefficients.  The latter was a two-part affair, Estabrook’s test for character compatibilities followed by a J. S. Farris routine to extract “cliques.”

The meat of Te’s dissertation was what can only be characterized as methodological gear-grinding, the details of which simply do not matter today.  But what ultimately popped out of the roomful of blinking machinery was a reduced set of 48 “species units,” in five groups with two outliers.   See George Te’s Figure 42, scanned above.

Throughout this retrospective I have tried to refrain, as much as humanly possible, from judging George Te by 21st century standards.  But I ordered a copy of his dissertation from University Microfilms in 1982, and my contemporary impressions of the work were profoundly mixed.

Te’s dissertation did include at least one significant advance – the description of a fifth type of penial morphology, which he called “type-bc, indicating its intermediate nature.”   This was the basis of his physid “Group V – the cubensis group.”  He misclassified most of the species in the group, including cubensis.  But his characterization of this intermediate-but-nonetheless-distinct morphology provided a critical clue to Amy Wethington and me as we began our work on the specific relationships among southeastern physids 20 years later [3].

On the other hand, however, it was as clear to me 30 years ago as it is today that the distinctions between most of George Te’s 48 “species units” were largely illusory and entirely undocumented.  Te recognized 27 species units in his “Group IV – acuta” group, for example.  For OTU-66 acuta itself, he wrote “appears to be part of the heterostropha continuum…but is geographically disjunct … and is therefore treated here as separate [4].”   For OTU-50 concolor, he wrote “forms part of the heterostropha continuum, and is the predominant taxon with the type-c penial complex in the northwest.”  He observed that OTU-60 virgata “is the principal southwestern North American component of the heterostropha continuum,” and that OTU-52 osculans “is the predominant taxon of the heterostropha continuum in Mexico,” and that OTU-68 pomilia “is part of the heterostropha continuum and is a southeastern geographic variant of OTU-70 heterostropha.”  And on and on he went.  Surprisingly, he wrote “OTU-71 integra is a predominant Group IV taxon in the Midwest, and is not part of the heterostropha continuum.”

It was extremely frustrating to me in 1982 that, although George Te explicitly stated that his observations quoted above were based on 33 nominal acuta specimens, 13 concolor, 24 virgata, 19 osculans, 29 pomilia, 28 integra, and so forth, he did not tell us where any of his samples came from, and so we who followed him could not verify his observations for ourselves.  With the benefit of 30 years’ hindsight, we now understand that acuta, heterostropha, virgata [7] and integra are conspecific.  We do not know about osculans or concolor [8].   But we do know that pomilia is a distinct species [9], correctly classified in Group V, not Group IV, and the inescapable conclusion must be that none of the 29 individual snails Te examined came from a bona fide pomilia population.

But returning to contemporary times, my bottom line impression of George Te’s 1978 dissertation was one of disappointment.  It could not be used to identify a Physa.  Which is all I ever really wanted it to do.

And can 48 meaningless OTUs be classified in some meaningful way?  Te proposed a system of two subfamilies, four genera, three subgenera, and two “sections,” which yielded the seven baskets he needed for his 48 “species units.”

But by the date of his defense, in December of 1978, it seems clear to me that George Te’s interests had strayed afield from systematic biology, and toward computer science.  Of the 324 pages comprising his dissertation, over 35% were devoted to analytical arcana.  His section on phenetic relationships, for example, was 54 pages of “Simgra” results, showing dozens of “linkage diagrams” at various “similarity levels.”  The roomful of blinking, spinning machinery was beginning to carry George Te away.

His dissertation [10] was never published.  All that ultimately emerged was a six page paper in Archiv fur Molluskenkunde, entitled “New classification system for the family Physidae” [11].  By the 1979 date of that publication, George Te was giving his address as University of Michigan Department of Computer Science.

But to his tiny, obscure 1979 paper must (in all fairness) be added the entire treatment of the Physidae in J. B. Burch’s immensely influential “North American Freshwater Snails,” which first appeared as an EPA publication in 1982.  George Te’s dissertation results became the de facto standard for the classification of the Physidae in the United States, right up in the pantheon beside Baker’s Lymnaeidae, Basch’s Ancylidae, and Goodrich’s Pleuroceridae.  And that quirky physid classification, for all its faults and failings, was the only system adopted by Burch that was, in any sense, objective.

I never met George Te.  But he was, by all accounts, a very nice man.  And he was extremely helpful to me at an early stage of my career.  In a span of just eight years, from 1971 – 1978, he was able to bring order out of taxonomic chaos in one of the most important elements of the North American freshwater benthic fauna.  He advanced our science.  Rest in peace.

Notes

[1] To Identify a Physa, 1975 [6May14]

[2] If this essay were really about George Te’s classification, we’d stop right here.  Only 14 of his 71 characters were legitimately discrete, and his character scoring system was a nightmare, and his entire analysis a house of cards.  But in 2014 it just doesn’t matter.

[3] 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]
  • True Confessions - I described a new species [7Apr10]
 [4] George Te was very, very close to making a breakthrough here.  He wrote:
“Although OTU-66 acuta is widely introduced to many parts of the world, its occurrence in North America has rarely been reported.  Considering the amount of transaction between Europe and North America, the absence of OTU-66 acuta in North America is puzzling.  The logical conclusion is that where OUT-66 acuta has been introduced in North America, it probably interbred and merged with indigenous North American “Physa” (i.e., those taxa in the heterostropha continuum).”
It was not until 2002 that we finally confirmed heterostropha to be conspecific with acuta [5].  Te was a good, conscientious worker, and his observations have substantial scientific value, in direct contrast to some of his contemporaries, who just made stuff up [6].

[5] Dillon, R. T., A. R. Wethington, J. M. Rhett and T. P. Smith.  (2002)  Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra.  Invertebrate Biology 121: 226-234.  [PDF]

[6] See footnote #10 in this essay:
  • Red Flags, Water Resources, and Physa natricina [12Mar08]
[7] Dillon, R. T., J. D. Robinson, T. P. Smith, and A. R. Wethington  (2005)  No reproductive isolation between freshwater pulmonate snails Physa virgata and P. acuta.  The Southwestern Naturalist 50: 415 - 422.  [PDF]

[8] The nomen “concolor” is my lead candidate for the “Snake River acuta-like Physa” or “SRALP,” about which I posted a big series last year:
  • 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]
  • The mystery of the SRALP - “No Physa acuta were found” [2May13]
[9] Dillon, R. T., J. D. Robinson, and A. R. Wethington (2007)  Empirical estimates of reproductive isolation among the freshwater pulmonates Physa acuta, P. pomilia, and P. hendersoni.  Malacologia 49: 283 - 292.  [PDF]

[10]  Te, G. A. (1978)  A systematic study of the Physidae.  Ph.D. Dissertation, University of Michigan, Ann Arbor.  324 pp.

[11] Te, G. A. (1979) New classification system for the family Physidae.  Arch. Moll. 110: 179 -184.

Tuesday, May 6, 2014

To Identify a Physa, 1975


Word has reached us of the death of Dr. George Ang Te, who passed away late last year in New Hamburg, NY at the age of 67 [1].  This is the second installment of a three-part series written to place his 1970s-era contributions to our understanding of the North American Physidae into perspective.

Those of you who bore with me through my (admittedly rather personal) essay last month [2] will not be surprised to learn that between the fall of 1973 and the spring of 1977, I spent most of my spare time loitering about the periodicals-currently-received section of the Virginia Tech library, waiting for the new issues of Nautilus, Malacologia, and Malacological Review to hit the news stands.  And when my eager eyes first drank in that (really rather beautiful) 1975 paper by a graduate student up in Ann Arbor named George A. Te [3], I knew I had found the man who could, at long last, scratch my Physa itch.

Looking back with the benefit of 40 years’ perspective, George Te’s “Michigan Physidae” may have represented the zenith of classical malacology in North America – impressively rigorous in its systematics, pristinely innocent of genetics.  His input data were 3,888 museum shell lots and 252 alcohol lots, cataloged under 34 specific or subspecific nomina.  He analyzed each shell lot subjectively but critically, developing an elaborate scoring system of about 30 characters, for example shell shape = elongate-slender, elliptical-subcylindrical, ovate-globose, and so forth.  A single individual from each alcohol lot was dissected to reveal the penial complex [4], which was scored by four additional characters.  His output data were a (much shorter!) list of 6 species and 8 subspecies confirmed from the waters of the state of Michigan, 7 nomina lowered into synonymy, and 13 nomina subtracted by correction of range or identification.

Among the strengths of Te’s 1975 work were his excellent historical review of the taxonomy of the Physidae [5], his extensive tabulations of the shell and penial character states for the 14 taxa he considered valid elements of the Michigan fauna, a plate of small (but serviceable) shell photos, and a (remarkably ambitious) dichotomous key to the shell morphology.  Perhaps his most significant contribution was a formal taxonomy for physid penial morphology, which still remains useful today [below, note 6].

Those same 40 years of hindsight also make it easy to find fault with George Te’s paper on a number of grounds, however.  He studied “museum lots,” not populations.  There is (almost) no evidence that Te recorded any observations on the living creatures themselves, or indeed that he conducted any fieldwork whatsoever [7].  And Te’s species concept was rigidly typological, betraying no hint that he understood the concept of variance, either within populations or between them.  So (for example) the shell length for 623 museum lots of Physa integra was given as 12.7 mm.

The worst consequence of Te’s typological species concept – which would ultimately render his 1978 dissertation virtually useless – seems to have been that he did not feel it necessary to provide locality data (or equivalently, catalog number) for any specimen he ever examined [8].  I understand that if all 623 of his lots of Physa integra did indeed bear elongate-ovate shells 12.7 mm in standard length, it will not matter where any particular individual specimen of P. integra was collected.  But they didn't, and it does.  A lot.

Nevertheless, against the background of the medieval ignorance that prevailed regarding the North American Physidae prior to 1975, George Te’s Michigan paper was a tremendous advance.  Physids are the most widespread freshwater gastropod in the waters of eastern North America.  And all across this great country of ours, the environmental movement was dawning.  The National Environmental Policy Act was passed in 1970, the Clean Water Act in 1972 [9], and the Endangered Species Act in 1973.  Agencies and industries were suddenly being held accountable for environmental “impacts” of all sorts, but a secondary trade had not as yet arisen to meet the demand for environmental support and consultation.  So contracts for environmental work flooded into the universities, which were the only place where any technical expertise in addressing such questions resided.  Expertise that included, I suppose, that of a 20-year-old undergraduate biology major at Virginia Tech.

Meanwhile, back in southern Virginia, ten years of controversy had already swirled around Appalachian Power Company’s “Blue Ridge Project,” a double-dam, pumped-storage facility on the New River straddling the North Carolina line [10].  So in 1975, the Virginia Tech Center for Environmental Studies issued me a pair of boots, a pair of buckets, a kick-net and a pickup truck [11].  And off I went up the New River Valley, sampling 89 sites and ultimately identifying 6 unionid species, 4 pisidiid species, and (approximately) 10 species of freshwater snails.

By this stage of my young career I was familiar with Baker’s (1911) lymnaeid monograph [12] and his (1945) planorbid plates [13], as well as Basch’s excellent (1963) work on the ancylid limpets [14].  And now, just when I needed him the most, a promising young graduate student in Ann Arbor threw me a lifeline on the Physidae.

I first wrote George Te in March of 1976, inquiring whether he might be willing to examine my samples of physids from southern Virginia.  His reply was prompt, courteous, and affirmative.  And over the next five months, we exchanged a pleasant correspondence focusing primarily on the New River Physa, which he found most interesting.  He recognized two species – Physa hendersoni widespread throughout the upper (soft water) mainstem and tributaries, and Physa pomilia in scattered hard water sites downstream.  George Te considered the occurrence of P. hendersoni in the upper New River especially interesting, as “little is known about (its) range beyond its type locality in South Carolina.”  I accepted his identifications as gospel, of course, transferring both to my (1977) undergraduate honors thesis [15], and to the (1982) paper I published in Freshwater Biology with my adviser, E. F. Benfield [16].

In retrospect, I cannot imagine how George Te’s identifications of my New River Physa samples could have been more wrong.  The physid populations widespread in soft waters upstream are P. acuta, and the scattered hard water populations downstream are P. gyrinaPhysa hendersoni is a junior synonym of P. pomilia [17], and is entirely absent from the New River drainage, under all aliases, as far as is known today.

But science is not about being correct, it is about being testable.  And George Te’s 1975 Michigan paper, even without genetics, field observations, or indeed locality data, was the first scientific treatment of the North American Physidae since the dawn of the modern synthesis.  The photos, dichotomous key, and tabulations of character states it contained were sufficient to allow Amy Wethington and myself to return to Michigan in 2003 and re-identify multiple biological populations of the typological species Te recognized in 1975.  These we used to calibrate the genetic tools and breeding studies we were developing to conduct a fresh review of the family.   No, there are not six valid biological species of physids in Michigan [18].  But that is not important.

And George Te’s research contributions were by no means complete with his 1975 paper.  His higher-level taxonomy was still very traditional in 1975, all Michigan species referred to the large, inclusive genus Physa favored by Bryant Walker [19] and earlier workers, except the oddball Aplexa hypnorum.  Where did that unfortunate generic nomen “Physella” come from?  Tune in next time!

Notes

[1] George Te (1946 – 2013) Poughkeepsie Journal, 12Nov13.  [html]

[2] To Identify a Physa, 1971 [8Apr14]

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

[4] Te’s (1975) paper did not include data on any aspect of the anatomy other than penial complex.  But he added (promisingly) that the types of penial sheaths “correlate with observations on differences in the renal complex, mantle digitations, mantle border, radula, and other anatomical structures (Te, unpubl).”

[5] Te’s thorough review of the decades of abject confusion surrounding the nomen “Physella” should be required reading for any taxonomist who has ever considered subdividing the Physidae.

[6]  The image with this month's post depicts Te’s 1975 diagrams of the four penial complex types: Aplexa-type, Type-a, Type-b, and Type-c.  The differences that Te perceived among the various "subtypes" are not significant.

[7] Well, that’s a bit unfair.  Te mentioned that he made “direct field collections” (locality unspecified) in his discussion of penial morphology, and thanked P. T. Clampitt “for helping me collect P. parkeri and P. integra at Douglas Lake” in his acknowledgements.  But any field observations he may have made apparently had zero influence on his 1975 paper.  In Table 4, for example, we read that the habitat preference of 171 “museum lots” of Aplexa hypnorum was “woods pool,” and the habitat preference of 343 museum lots of P. gyrina was “pools and ditches,” and so forth.

[8]  We will return to the subject of George Te’s (1978) dissertation next month.

[9]  The Clean Water Act at 40 [7Jan13]

[10] APCO’s Blue Ridge Project was ultimately stopped in late 1976, when a 26.5 mile section of the New River in North Carolina was added to the National Wild and Scenic Rivers system.  Here’s an excellent thesis reviewing the controversy:
  • Woodard, R. S., Jr. (2006)  The Appalachian Power Company along the New River: The defeat of the Blue Ridge Project in historical perspective.  M.A. Thesis, Virginia Tech, Blacksburg.  139 pp. [pdf]
[11] Those were the glory days!  The peg board behind the desk at John Cairns’ Center For Environmental Studies must have held keys to at least 20 vehicles.  And even undergraduates could check one out, with a state gas card in the glove compartment.  But my adviser  Fred Benfield, said this: “If you wreck that truck, Boy, just keep going.  Never come back.”

[12] Baker, F.C. (1911) The Lymnaeidae of North and Middle America. Recent and fossil. Chicago Academy of Sciences Special Publication No. 3. 539 pp.
  • See, The Legacy of Frank Collins Baker [20Nov06]
[13] Baker, F.C. (1945) The molluscan family Planorbidae. Collation, Revision, and Additions by H.J. Van Cleave. University of Illinois Press. Urbana, Illinois.  530 pp.
  • See, The Classification of the Planorbidae [11Apr08]
[14] 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.

[15] Dillon, R. T., Jr. (1977) Factors in the distributional ecology of upper New River mollusks (Va/NC).  Unergraduate Honors Thesis, Virginia Tech, Blacksburg.  [pdf]

[16] Dillon, R.T. and E. F. Benfield (1982) Distribution of pulmonate snails in the New River of Virginia and North Carolina, U.S.A.: Interaction between alkalinity and stream drainage area. Freshwater Biology 12: 179-186. [pdf]

[17] Dillon, R. T., J. D. Robinson, and A. R. Wethington (2007)  Empirical estimates of reproductive isolation among the freshwater pulmonates Physa acuta, P. pomilia, and P. hendersoni.  Malacologia 49: 283 - 292. [pdf]

[18]  Dillon, R. T., and A. R. Wethington (2006)  The Michigan Physidae revisited: A population genetic study.  Malacologia 48: 133 - 142. [pdf]

[19]  Walker, B. (1918) A Synopsis of the Classification of the Freshwater Mollusca of North America, North of Mexico. Misc. Pubs., no. 6. Ann Arbor: University of Michigan Press.
  • See, Bryant Walker’s Sense of Fairness [9Nov12]

Tuesday, April 8, 2014

To Identify a Physa, 1971


Word has reached us of the death of Dr. George A. Te, who passed away late last year in New Hamburg, NY, at the age of 67.  This month we begin a short series of posts aimed at placing his 1970s-era contributions to American malacology in their historical context.

I grew up a collector.  By the age of five I had filled the basement of my home in Waynesboro, Virginia, with a huge collection of rocks, shells, bones, fossils, feathers, butterflies and wildflowers.  I was especially fond of snakes, keeping as many as ten or twelve (in various states of health) through most of my elementary years, and planted a cactus garden out back.

By the age of 12 I had begun to specialize on the shells, most of which were self-collected on our annual family vacations to various beaches, but some of which were purchased at shell shops, or traded with other hobbyists worldwide.  I also especially loved the Devonian fossil exposures in Highland County to our immediate west, and the diverse Miocene/Pliocene exposures around the Chesapeake Bay, and did not neglect the lands snails in my own back yard.

I wanted to know everything about everything I collected, starting with, of course, its name.  Shell books were relatively easy to get, and by the age of 15 I had a large library with coverage around the world.  I also had a well-thumbed copy of Burch’s (1962) How to Know the Eastern Land Snails, and a 1963 reprint of the Maryland Geological Survey’s Miocene Plates.  In fact, I occasionally prevailed upon my mother to drive me 25 miles over the mountain to Charlottesville, where the University of Virginia library contained a respectable collection of paleontological references, including good coverage of the local Paleozoic.  The only element of the molluscan fauna that I could not identify, from Cambrian to the present day, on land or sea from here to Timbuktu, were the freshwater snails.

I have already reminisced, at some length, about my languid childhood days on the South River behind my house [1].  The rocks were (and still are) covered with little black snails we called "periwinkles" [2].  I didn't collect them for the same reason I didn't collect ants.  They were dirt-common, and (in retrospect) I think my collecting button only got pushed when I saw something that seemed unusual, which periwinkles were not.  And I knew for a fact that, just as was the case with ants, I could not identify those periwinkles, even if I dug through every "Mollusca" card in the University of Virginia Library, which I had.

Interestingly, I was familiar with popular guidebooks on "pond life."  And in such guidebooks one often found pages devoted to common pulmonate snails, such as the "ramshorn snails" (Helisoma) and "tadpole snails" (Physa) that inhabited the South River backwaters and marginal pools.   So by the summer of 1970, as I was finishing my first year at Waynesboro High School, I was familiar with my local generic Physa population, specifically anonymous though they continued to be.

It probably need not be said that I was very much involved in science fairs through my entire K-12 career.  And from some general reference, now lost to memory, it was pointed out to me that the shells of juvenile Physa are so transparent that one might observe (and count) their heart beat.  From this germ of an idea was born my entry in the 1971 Virginia Junior Academy of Sciences competition, "Responses to Stimuli in the Freshwater Snail, Physa" for which I set up an aquarium and began my first Physa culture.  My good buddy Dave and I spent quite a few long hours doing strange stuff to hatchling Physa in Petri dishes, counting heart beats under a dissecting scope borrowed from the school [3].

So in any case, by the 1970-71 school year, there was at least one young man in the Shenandoah Valley of Virginia who was very, very interested in learning everything he could about Physa [4].  And quite frustrated by the reference materials available at the University of Virginia, Madison College (in Harrisonburg), and even at the Virginia Tech library, where he spent a few hours during the Junior Academy of Sciences meeting itself. 

I do (vividly) remember my bus trip to Champaign-Urbana in August of 1971, for a National Order of the Arrow convention at the University of Illinois [5].  I spent half a day in the Illinois Library, where I first laid eyes on Baker’s (1911) monograph on the Lymnaeidae, which may have been the best reference work I had ever seen, to that point in my young life [6].  But the highlight of my summer was the discovery of a 1930s-era MS Thesis by a woman named Thelma Fish Brown, entitled "The Biology of Physa anatina, a Snail Living in a Sewage Treatment Plant" [7].  For years thereafter I referred to my South River Physa as "Physa-anatina-question-mark," on the basis of the few hours of enlightenment I was able to gather from its pages [8].  It was the first time I had ever seen a specific nomen applied to a snail population of the genus Physa.

Just the tiny victory of finding one, lone Physa reference in the summer of 1971 was apparently such a life-altering experience that I remember it clearly, over 40 years later.  Why should 50-million-year-old Turritella lifted from the Eocene clay near Fredricksburg be so much easier to identify than dirt-common little brown snails in my own back yard?  I would like to say that I rose from my library carrel, shook my fist at the heavens, and swore a mighty oath that one day I would right this grievous injustice to American Malacology.  I didn't.  But four years later, when I stepped forward as a junior at Virginia Tech to survey the freshwater mollusks of the 2,748 mi2 upper New River basin, I did not undertake the task naively.

And meanwhile, at the World Center of Malacology three states north, a bright young student from The Philippines named George A. Te was enrolling in the Ph.D. program at the University of Michigan.  How might our destinies intertwine?  Tune in next time.

Notes

[1]  The Clean Water Act at 40 [7Jan13]

[2] These were Leptoxis carinata which, according to the 15-member AFS Committee on the Scientific and Vernacular Names of Mollusks, are correctly referred to as the "crested mudalia."  But I was not interested in common names, even when I was five years old.  I was never common.

[3] No, I didn't win.  In fact through my entire career of scientific competition, K-12, college and graduate school, I won a total of one yellow third-place ribbon.  Glory has never been my motivation.

[4] And not catching the eye of any of the ladies, anytime soon.  Goodness gracious, what a nerd!

[5] Have you seen the arrow?

[6]  See, The Legacy of Frank Collins Baker [20Nov06]

[7] Brown, T. F. (1937) The biology of Physa anatina Lea, a snail living in a sewage treatment plant.  Amer. Midl. Natur. 18: 251-259.

[8] The population of Physa in the South River behind my house was common trash Physa acuta, of course.  But I’ll bet dollars to donuts that Thelma Fish Brown’s sewage-treatment-plant population was also Physa acuta as well.  So it seems likely to me that my 1971 identification may have been (perversely) correct.

Wednesday, March 5, 2014

What Subspecies Are Not


Last month [1] we belabored the definition of the word “subspecies,” using as our illustration Pleurocera catenaria dislocata, populations of which inhabit scattered small streams of the lower piedmont and coastal plain from Virginia to Georgia.  And we warned, somewhat cryptically, about the “mischief” that might be precipitated should the concept of the subspecies be misunderstood, promising to develop that theme more fully this month.  So this month let us return to our example of P. catenaria from the lower piedmont and plumb the taxonomic history of the nomen “dislocata” to substantially greater depth.

Edmond Ravenel and L. A. Reeve probably ought to share joint credit for describing “Melania dislocata,” but Ravenel (1834) is usually accorded the honor, by virtue of his earlier publication date [2].  In either case, the nomen “dislocata” was uniformly transmitted at the specific level by Tryon and other 19th century authorities.  Based upon his keen appreciation for the plasticity of pleurocerid shell morphology [3], however, Calvin Goodrich lowered the nomen to subspecific rank under Goniobasis catenaria in 1942 [4].  Burch followed Goodrich, listing “Elimia catenaria dislocata” on page 132 of his 1989 “North American Freshwater Snails.”

I have never cared for the American Fisheries Society’s “Common and Scientific Names of Mollusks” book [5], the second edition of which was published in 1998.  But the policy of Donna Turgeon and her committee of 14 regarding the rank of subspecies was reasonable, and clearly stated on page 15: 
“Most subspecies are not suitable subjects for common names, but those forms that are so different in appearance as to be distinguished readily by lay people or for which a common name constitutes a significant aid in communication may merit separate names.  There is a wide divergence of opinion concerning the criteria for recognition of subspecies.  We have usually not named subspecies.”
So for example, Burch (following Goodrich) recognized 50 distinct subspecific nomina in the North American Pleuroceridae.  The great majority of these (44, to be exact) were not transferred by the Turgeon/AFS committee to their 1998 book, and I am not complaining.  Subspecies distinctions are subjective, and often (as we have seen) without evolutionary basis, and if a committee of secondary authorities simply resolves to lump all subspecific nomina under their parent species as a matter of policy, fine.  Turgeon and colleagues did, however, save one subspecific nomen in the Pleuroceridae, Leptoxis crassa anthonyi, together with its cognate L. crassa crassa, presumably because in the judgment of the committee, anthonyi is “distinguishable readily by lay people.”  Okay, I won’t second-guess [6].

My problem lies with the other five subspecific pleurocerid nomina, all in Burch’s concept of the genus “Elimia.”  In these five cases, the Turgeon/AFS committee surreptitiously raised the subspecific nomina of Goodrich/Burch to the species level: albanyensis, inclinans, vanhyningiana, viennaensis, and (you guessed it) dislocataSubspecies are not species!  Here in the modern era, specific nomina are not somebody’s subjective opinion – they are testable hypothesis about the reproductive relationships among sets of populations.  What misanthrope thought he could arbitrarily, high-handedly, without any explanation or indeed any notice, promote five subspecific nomina [7], carrying with them no assumption of heritability much less reproductive relationship, to the level of species [8]?

This sort of shenanigans was not the worst thing about the 1998 AFS publication by Turgeon and her committee, but it was certainly in the top five.  How many other such spurious taxonomic judgment-calls might be hidden within its 526 pages?  How many natural resource agencies and conservation NGOs may have uncritically accepted this shoddy excuse for scholarship and propagated it about the country as some sort of “standard?”

I would confess that some personal irritation with the treatment given the southeastern Pleuroceridae by the Turgeon/AFS committee at least partly motivated several population genetic surveys I undertook subsequent to 1998.  Last month we reviewed the 2002 paper I published with A. J. Reed, reporting evidence for multiple independent origins of the dislocata phenotype, strongly refuting the Turgeon/AFS surreptitious elevation of dislocata to the species level [9].  And in 2011 I published a paper in AMB with J.D. Robinson, synonymizing viennaensis and albanyensis back under Pleurocera catenaria, again, and inclinans under Pleurocera floridensis, again [10].  One might hope that hard data published in the peer-reviewed literature should trump the subterfuge of colleagues mired in nineteenth-century typology.  But no.

The 2013 AFS publication by Paul Johnson and committee [11] is 36 pages long, but all one ever need examine is the single mysterious paragraph in which the authors purport to describe their methodology, not saying more than they say.  In our post of 9Sept13 [12] we trained the scientific equivalent of exegesis on the single sentence by which the authors described their biogeographic methods, discovering something most foul hidden underneath.  Here we turn our attention to the two sentences by which they described their taxonomic methods, as follows:
“This list was derived from Turgeon et al. (1998) and updated with subsequently described species and systematic revisions.  Subspecies are not recognized.”
 I initially interpreted the four-word rubric, “Subspecies are not recognized” to mean that the Johnson/AFS Committee collapsed subspecific nomina under their specific cognates.   But upon closer study, it would appear that Johnson and colleagues actually meant “Subspecies are raised to the full species level.”

Only one (obvious) set of North American freshwater gastropod subspecies actually survived the 1998 transfer from Burch/Goodrich to Turgeon/AFS: the Leptoxis (“Athearnia”) crassa crassa and L. crassa anthonyi pair.  Johnson and colleagues raised both of these nomina to the species level, on the strength of no research of which I am aware.  Let us call this pattern “down-down-up.”  The nomen anthonyi was a subspecies prior to 1998, continued as a subspecies 1998-2013, and then elevated to the full species level.

The Johnson/AFS Committee also preserved the five subspecific nomina raised surreptitiously to the species level by the Turgeon/AFS committee as species, directly counter to research published between 1998 and 2013 in most cases.  It is difficult to see how all fourteen of my esteemed colleagues could have missed the 2002 paper I published in Malacologia on the catenaria/dislocata pair or the 2011 paper that John Robinson and I published in AMB on albanyensis, viennaensis and inclinans.  So the implication must be that the Johnson/AFS committee was aware that these nomina are only subspecifically distinct (at best!) but elected to re-raise them back to the species level (again), under the four-word rubric, “Subspecies are not recognized.”  This is the down-up-down-up pattern I illustrated at the top of this essay: subspecies prior to 1998, raised to species level, demoted between 1998 and 2013, then raised a second time.

There is one other pattern in the interplay of the two AFS Committees: up-down-up.  The Turgeon/AFS committee followed Goodrich/Burch in recognizing several species that research since 1998 has shown do not warrant recognition at the species level.  The 2011 population genetic study of Dillon & Robinson [10], for example, suggested that the “timida” phenotype is independently derived in populations of Pleurocera floridensis, and in that same year Dillon (solo) demonstrated an identical phenomenon for the “unciale” phenotype of P. clavaeformis [13].  These taxa were recognized at the specific level prior to 1998, lowered to the subspecies level between 1998-2013, then raised to the species level again by the Johnson/AFS committee.

The bottom line is that at least 12-15 pleurocerid nomina listed at the specific level by the Johnson/AFS committee are (at best) subspecies.  But why does this matter? Isn’t everything I have written in my essays over the last couple months just one big prima donna sniping at a little clique of other prima donnas over angels on the head of a pin?  Well actually, it matters a lot.  And I will tell you why.

Subspecies are not valid units of management!  I cannot claim any more than a layman’s understanding of the politics that motivated federal endangered species legislation in 1973, or the state-level laws and policies that have followed it in the last 40 years.  But it seems to me that advocates of such policies have, at least in recent years, primarily rationalized their efforts as desirable for the protection of “biodiversity.” And (again, I think!) the implication is that such “biodiversity” is supposed to be heritable.  Which means if (as research suggests) the traits that distinguish pleurocerid populations bearing nomina valid only at the subspecific level, such as dislocata, viennaensis, timida and unciale are ecophenotypic responses to the environment, with no demonstrably heritable basis, efforts directed toward their conservation are unwarranted.

So at last we arrive at our summary paragraph.  As I mentioned in my February essay, the FWGNA currently recognizes four sets of subspecies, all in the family Pleuroceridae.  We have preserved these subspecific distinctions primarily for their indexing function – all the nomina involved are associated with significant (often historic) literature.  But we have combined subspecific nomina for analysis of relative abundance, and for the assignment of FWGNA Incidence ranks [14].  We make no assumptions regarding the evolutionary significance of the morphological distinctions upon which subspecific nomina are based, nor are any (at present) warranted.

Notes

[1] What is a Subspecies? [4Feb14]

[2] The situation is complicated.  Ravenel listed Terebra dislocata Say (a marine gastropod) on page 11 of his (1834) “Catalogue of Recent and Fossil Shells,” with no figure or description.  Reeve provided a formal description of Melania dislocata in his (1861) “Monograph of the Genus Melania,” stating as he did “The shell here figured is Ravenel’s original type of this species, from the collection of Mr. Anthony.”  So all the older references, including Tryon and Goodrich, attribute dislocata to Ravenel (1834).  But Burch, Turgeon/AFS and Johnson/AFS prefer Reeve (1861).

[3] Goodrich’s appreciation for phenotypic plasticity in the Pleuroceridae has been a long-running theme on this blog.  I’ve published a series of essays touching on it, notably:
  • The Legacy of Calvin Goodrich [23Jan07]
  • Goodrichian Taxon Shift [20Feb07]
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
[4] Goodrich, C. (1942) The Pleuroceridae of the Atlantic Coastal Plain.  Occas. Pprs. Mus. Zool. Univ. Mich. 456: 1-6.

[5] Turgeon, D.D., J.F. Quinn, A.E. Bogan, E.V. Coan, F.G. Hochberg, W.G. Lyons, P.M. Mikkelson, R.J. Neves, C.F.E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione, and G.D. Williams (1998) Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks (second edition), American Fisheries Society Special Publication 26, Bethesda, Maryland, 526 pp.

[6] That’s absurd, of course.  The spectacle of 15 Ph.D. Malacologists convening around a long table to determine which crappy little brown creek-snails “lay people” can or cannot distinguish, and inventing “common names” for the former, sounds like a scene from Alice in Wonderland.

[7] Chambers (1990) synonymized inclinans and vanhyngiana under Pleurocera (“Elimia”) floridensis, viennaensis under Pleurocera (“Elimia”) curvicostata, and albanyensis under Pleurocera (“Elimia”) boykiniana.  The sequence data of Mihalcik & Thompson (2002) and the population genetic survey of Dillon & Robinson (2011) both generally confirmed the hypotheses of Chambers, although demonstrating that boykiniana is a junior synonym of Pleurocera catenaria, and suggesting that viennaensis might go under catenaria as well.  In any case, there is no evidence that any of those five pleurocerid taxa is specifically distinct: albanyensis, inclinans, vanhyningiana, viennaensis, or dislocata.  And there are at least three recent papers with good, hard data pointing in the opposite direction.  See Dillon & Robinson from footnote [10] below.

[8] On page 13, the “expert in the field of molluscan systematics who updated the checklist for this volume” for freshwater gastropods is given as Fred G. Thompson.

[9] Dillon, R.T. & A.J. Reed (2002)  A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23-31. [pdf]

[10] Dillon, R.T., Jr & J.D. Robinson (2011) The opposite of speciation: Genetic relationships among the populations of Pleurocera in central Georgia.  Amer. Malac. Bull. 29: 159-168. [pdf]

[11] Johnson, P. D., A. E. Bogan, K. M. Brown, N. M. Burkhead, J. R. Cordeiro, J. T. Garner, P. D. Hartfield, D. A. W. Lepitzki, G. L. Mackie, E. Pip, T. A. Tarpley, J. S. Tiemann, N. V. Whelan & E. E. Strong (2013)  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247 – 282.

[12] Plagiarism, Paul Johnson, and the American Fisheries Society [9Sept13]

[13] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]  I am quite sure that the Johnson/AFS committee saw this paper, since it appeared in their cited references.

[14] What is Rarity? [9Dec13]

Tuesday, February 4, 2014

What Is A Subspecies?


Subspecies are populations of the same species in different geographic locations, with one or more distinguishing traits.  This is the modern textbook definition [1], due primarily to the 1942 work of Ernst Mayr [2].  The concept is so simple and basic to the training that most of us probably received as undergraduates that it is difficult to imagine how anybody could become confused about it.  But they do.

No, not any of you reading this essay!  I’m sure you all get it.  But (hard though it may be to believe) there are some evolutionary biologists active in our field today who imagine that Mayr’s definition of the noun “subspecies” says more than it does.  So indulge me while I make three (obvious) points. 

Point number 1 is that no definition of the word “subspecies,” ancient or modern, has ever specified anything about heritability.  Yes certainly, the morphological differences that distinguish subspecies A and B might indeed have a genetic basis.  In fact, subspecies are sometimes referred to as “insipient species.”  But even if the “one or more distinguishing traits” are entirely ecophenotypic in their origin, arising as plastic responses to differences in environment A and environment B, the validity of the subspecific status of populations A and B would not be compromised.

Point number 2 follows from point number 1.  If the trait(s) by which a subspecies is distinguished need have no heritable basis, then populations united under the same subspecific trinomen need share no evolutionary history.  Similar ecophenotypic responses can be elicited multiple times independently.

And point number 3 is that the modern definition of the subspecies also says nothing about the existence of morphological intermediates.  Yes, it is certainly possible that subspecies A and B might be universally distinguishable.  But typically just the opposite is the case.  To quote a recent review [3], “Intergradation at the boundary between two geographic replacement forms is the touchstone of trinominalism.”  So if, for example, a third population C exists, in an intermediate region or an intermediate environment between population A and population B, the demonstration of a perfectly intermediate C phenotype would not compromise the subspecific status of A and B.  Subspecies only need be distinguishable somewhere, not everywhere.

So to take a real example.  Many rivers of the Piedmont and upper Coastal Plain from Virginia to Georgia are inhabited by populations of (typical) Pleurocera catenaria, bearing shells with prominent costae, dissected by spiral cords.  These populations reach maximum abundance in rocky shoals in medium to large rivers.   Smaller and sandier tributaries, entering the larger rivers farther east in the coastal plain, are sometimes inhabited by populations with much plainer shells, lacking spiral cords and often demonstrating reduced costation as well.  Goodrich [4] referred these populations to a subspecies, Pleurocera ("Goniobasis") catenaria dislocata.

I strongly suspect that the shell differences between dislocata populations and typical catenaria populations have little heritable basis [5].  And in fact, a population genetic survey I published in 2002 [6] suggested that the dislocata phenotype seemed to have evolved multiple times independently.  The matrix below shows that the genetic identify between a Savannah dislocata population (“Srp”) and a Savannah catenaria population (“McC”) was 0.86, and that between a Broad/Santee dislocata (“Sant”) and Broad/Santee catenaria (“Cola”) was 0.89, but that the genetic identity between the two dislocata populations was only 0.81.


And although dislocata populations are distinctive in South Carolina, they intergrade with typical catenaria throughout the Tar River drainage of North Carolina.  The figure at the top of this essay shows three shells from Red Bud Creek in Nash County, a dislocata individual on the left without spiral cords, a typical catenaria on the right with strong cords, and an intermediate individual in the middle.

But none of these considerations – the questionable heritability of the distinguishing traits, their multiple independent origins, or the existence of intermediate forms – compromises the validity of Goodrich’s subspecific designation Pleurocera catenaria dislocata.

Many of my colleagues tell me that they “don’t like subspecies.”  And without question, the description of subspecies has fallen out of favor in modern evolutionary biology.  It is an old-school concept, dependent on the taxonomist’s subjective understanding of the adjective, “distinguishing.”  And modern classifications are supposed to reflect the evolutionary history of the populations being classified.  It seems misleading to lump a subset of Pleurocera catenaria populations under the subspecific trinomen “dislocata” that have, for reasons we admittedly do not understand, come to look similar independently.

On the other hand, some of us (on rare occasions, perhaps) find the composition of freshwater gastropod communities provides helpful information about the environment from which they were sampled [7].  So if populations of Pleurocera catenaria dislocata are indeed associated with shallower, sandier creeks than typical P. catenaria catenaria, the trinomial distinction would seem to serve a valuable function.

Moreover, the Linnean system of nomenclature was instituted neither to recapitulate the evolutionary history of the organisms being classified, nor to facilitate ecological generalizations.  Carl von Linne conceived his system of taxonomy for information retrieval, like a Dewey-decimal filing system for critters [8].  And very simply, a trinomial carries more information than a binomial.

So in the final analysis, I find the ecological and “information retrieval” arguments for subspecies more compelling than the evolutionary arguments against them.  There does exist a small (but not negligible) scientific literature associated with the pleurocerid nomen “dislocata,” which would be lost (or become much more difficult to Google, anyway) if that nomen were to disappear today.

And even more the shame for such better-known pleurocerid nomina as acuta, pyrenellum, and unciale/uncialis from the American interior.  The populations described by these names are indeed geographically separate (usually) and morphologically distinctive (usually) to the point that they have been recognized as valid species until quite recently [9].  Why not save the nomina as subspecies?   As long as we are clear that there may be no heritable basis for the distinction between Pleurocera canaliculata acuta and Pleurocera canaliculata canaliculata, and that the acuta phenotype seems to have arisen many times independently, and that intermediate populations exist between the typical, robust canaliculata form and the more slender acuta form [10], it seems to me that the information indexed to the name “acuta” is worth saving.

So at present, the FWGNA project recognizes four sets of subspecies, all in the family Pleuroceridae:  the catenaria/dislocata pair, the clavaeformis/unciale pair, the floridensis/timida pair, and the canaliculata/pyrenellum/acuta triplet [11].  Although none of these subspecific distinctions likely has any evolutionary basis, all these nomina most certainly do serve important indexing functions, and hence warrant preservation.

But everything I have written in my essay this month is predicated upon a firm understanding of the word “subspecies,” as defined in the first sentence of this essay.  What mischief might be visited upon evolutionary science by our colleagues should any of them become confused?  Tune in next month!

Notes

[1] Quoted verbatim from the glossary of Price, P. W. (1996) Biological Evolution. Saunders College Publishing.

[2] Mayr, E. (1942) Systematics and the Origin of Species. Harvard University Press, Cambridge, MA.

[3]  Mallet, J. (2007) Subspecies, semispecies, superspecies.  In Encyclopedia of Biodiversity [pdf]

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

[5] Urabe, M. (2000) Phenotypic modulation by the substratum of shell sculpture in Semisulcospira reiniana (Prosobranchia: Pleuroceridae). J. Moll. Stud. 66: 53-59.  See:
  • Semisulcospira Research: A Message from The East.  [6Jan08]
[6] Dillon, R.T. & A.J. Reed (2002)  A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23-31.

[7] See “Models of Species Distribution” (pp 391 – 407) in my book, The Ecology of Freshwater Mollusks (Cambridge University Press, 2000). [html]

[8] The information-retrieval function of zoological nomenclature is not incompatible with its scientific function, but not compatible, either.  See:
  • When Worlds Collide: Lumpers and splitters.  [4Sept12]
[9] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]  See:
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[10] Dillon, R.T.,Jr., S. J. Jacquemin & M. Pyron (2013)  Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127 [html] [pdf]  See: 
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Pleurocera canaliculata and the Process of Scientific Discovery [18June13]
[11] Although web pages are not (at present) available for the typical Pleurocera floridensis floridensis, nor for Pleurocera canaliculata acuta.  Coming soon, I promise.