Dr. Rob Dillon, Coordinator

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.


[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.


[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!


[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.

Monday, January 6, 2014

Why is Rarity?

We should begin this essay by reminding our readership of the very precise (and perhaps rather peculiar) definition of the adjective “rare” adopted by the FWGNA project, as developed in our essay of last month [1].  We have defined a species as “rare” if the number of samples in which it has been recorded ranks it in the lowest 25% of all species in the fauna under study.  We call this “incidence rarity,” to distinguish it from “abundance rarity,” which is the other way in which rareness is commonly defined [2].

We should also remind our readership that in the present discussion of rarity we have been led by a prominent authority of the subject, Prof. Kevin J. Gaston.  And point out that immediately after Prof. Gaston concluded sections 1.1 and 1.2 of his 1994 book [4] on the definition of rarity, he set his pen to several sections on scale-dependence, and the methods by which a biota might be circumscribed for study.  Because the first explanation for the phenomenon of rarity that occurred to Prof. Gaston was, in his own words, that:
“at local and regional scales, many (species) are not permanent members of the assemblage, do not breed, or do not have self-sustaining populations in the area of interest.  Such species have been variously termed accidentals, casuals, immigrants, incidentals, strays, tourists, transients and vagrants; I shall refer to them as vagrants.”
Prof. Gaston then went on to describe what he termed “the problem of vagrancy.”  Vagrants are typically over-represented in the fraction of the biota that workers in the field identify as “rare.”  Most of the bird species regarded as rare in Britain are vagrants, for example.  But such vagrants are (quite often) not rare elsewhere.  They are what Gaston calls (later, in Chapter 2) “pseudo-rare.”

The data we have gathered for our “Synthesis” of freshwater gastropod incidence in the Atlantic drainages of nine US states [5] do indeed demonstrate a phenomenon rather closely analogous to Gaston’s “problem of vagrancy.”  Our database contains only 4 records of Pleurocera floridensis, for example, ranking P. floridensis well down in the bottom quartile of the 67 freshwater gastropods listed in Table 1 [6].  But there is strong (albeit indirect) evidence suggesting that P. floridensis populations are not rare elsewhere.  The distribution map of S. M. Chambers at left above [7] shows almost 100 populations of P. floridensis (as black circles) in the springs and rivers of western Georgia and Florida.

Pleurocera floridensis is not a “vagrant” as Gaston has defined that term, however.  Populations of pleurocerid gastropods do not blow in and out of our study area like birds [8].  And there is no reason to think that the 4 populations of P. floridensis we have documented (shown as green circles above) are not permanent or “self-sustaining.”  Rather, the reason that that P. floridensis appears rare in Table 1 but almost certainly is not actually rare is that our study area (green) has (by chance) only touched the northern edge of the P. floridensis range.  Pleurocera floridensis is not (literally) a “vagrant,” it is “marginal.”

Here is the definition of the word “marginal” we have adopted for use in the FWGNA project.  A species will be described as marginal if it demonstrates less than median incidence within our area of study, but greater than median incidence outside our area of study.

So our fauna of 67 freshwater gastropod species includes 34 demonstrating incidences below the median of 23 records in our database.  To determine which of these 34 are marginal in our study area will admittedly require some speculation on our part, because no data are available from outside our study area comparable in quality to those available inside.  But based on our reading of the literature [9] it would appear that exactly half – 17 – are marginal.  Although Table 1 shows that these 17 species demonstrate below-median incidence in our study area, the published literature contains evidence suggesting that they may demonstrate above-median incidence elsewhere.

That subset of 7 marginal species appearing in the bottom quartile of Table 1 are, to borrow Gaston’s term, “pseudo-rare.” And here is another remarkable insight brought to us by the good Professor Gaston.  There must also exist a condition he calls “non-apparent rarity,” which is the situation where species are indeed rare, but their rareness is not revealed in the data available.  Non-apparent rarity is the opposite of pseudo-rarity.  And:
“under some definitions of rarity one state cannot occur independently of the other.  For example, under a proportion of species definition, the misclassification of a species as rare when it is not must necessarily entail the misclassification of a species as not being rare when in fact it is.”
To summarize.  All 10 of the species marked in Table 1 as I-1 and I-2 (simply, without a suffix of “m”) are most certainly rare.  The 7 species marked I-1m and I-2m are pseudo-rare.  Then moving up Table 1, the 7 species marked simply I-3 (without suffix) demonstrate non-apparent rarity, deserving to have been ranked in the bottom quartile, but displaced by the 7 marginal species occupying their spots.

I hate to draw a line here, but (once again) I sense I may be nearing the maximum length this peculiar format can comfortably contain.

I will close by noting, however, that we have barely “touched the margin” of the question we have undertaken to address, the “why” of rarity.  The second explanation that occurred to Prof. Gaston, actually better described as a class of explanations, would be sampling error.  Species may appear rare because they are small or cryptic, clumped or aggregated, undersampled by the gear, and so forth.  The related sins of "conservation-biased oversampling" [10] and trash-snail undersampling occurred to me as I read this section of Gaston's book.

Professor Gaston does not begin to explore the causation of bona fide rarity until Chapter 6, which he divides into two subsections:  “environmental factors” and “colonization ability.”  And it is not until page 126 (in 163 pages of text) that it occurs to Prof. Gaston that human activities might have some effect on rarity, or that rarity might be anything other than a perfectly ordinary phenomenon of nature.

Yes, rare species are more likely to become extinct than common species.  And it is toward the subject of conservation that Prof. Gaston ultimately turns, in his Chapter 7.  But as he does so, this is his observation:  “Considerations of rarity lead almost ineluctably to the topic of conservation.  Indeed, it seems a popular belief that the two issues are inseparable.  Previous chapters should have established beyond any doubt that this is not so.”  Amen.


[1] What is Rarity?  [9Dec13]

[2] Actually there is a third way in which rarity can be understood – high habitat specificity.  This sets up a 2x2x2 classification by geographic range, local population size, and habitat specificity, with eight cells in the box.  Only species in one of these cells (demonstrating a wide range, large population sizes and low habitat specificity) will be considered common by everybody.  The other seven cells contain species that are rare, in some sense [3].  See Rabinowitz, A. B. (1981) Seven forms of rarity.  Pp 205 – 217 in “The Biological Aspects of Rare Plant Conservation” (Ed. H. Synge) Wiley, NY.

[3] Honestly, it is very easy to explain why species are rare.  The more difficult question would be “Why is anything common?”

[4] Gaston, K. J. (1994) Rarity.  Chapman & Hall Population and Community Biology Series 13. 205 pp.

[5] See the FWGNA Synthesis page for additional details.

[6] FWGNA Synthesis Table 1.  The 67 freshwater gastropod species inhabiting US Atlantic drainages, ranked by total incidence. [pdf]

[7] Chambers, S. M. (1990)  The genus Elimia (Goniobasis) in Florida (Prosobranchia: Pleuroceridae)  Walkerana 4: 237 - 270.

[8] Which is not to imply that aerial dispersal of freshwater gastropods is unimportant!  See [17Nov05

[9] The most important references here were:
Jokinen, E. H. 1992.  The Freshwater Snails (Mollusca: Gastropoda) of New York State.  Albany: New York State Museum.  112 pp. 
Thompson, F. G. 1999.  An identification manual for the freshwater snails of Florida.  Walkerana 10 (23): 1 – 96.

[10] Toward the Scientific Ranking of Conservation Status - Part III [19Mar12]

Monday, December 9, 2013

What is Rarity?

In recent weeks I have received some very generous compliments regarding the brand new “Synthesis” of freshwater gastropod abundance posted on the FWGNA site.  Essentially what my colleagues and I have done is delete the old “Recommendations” pages that used to be available separately from the FWGGA, FWGSC, FWGNC and FWGVA sites and replaced that little corner of cyberspace with a combined analysis of the “distribution of commonness and rarity” across all 67 species of freshwater gastropods inhabiting the Atlantic drainages of the United States, from Georgia to the New York line.

 FWGNA Synthesis
My initial motivation was simply to expand (and formalize) the analysis I first proposed on this blog in January 2012 [1], adding data from five Mid-Atlantic states to data already in hand for the four states further south.  I confess that I was a bit disappointed, however, when the enlarged dataset did not fit a lognormal model this second time around, because I had already written a nice discussion section explaining why it should, and exploring all manner of elaborate implications about broad-scale ecological relationships in this diverse assemblage of freshwater gastropods over evolutionary time.  But I was teetering on the ragged edge of rejecting the lognormal in January 2012, with my Shapiro-Wilk W barely nonsignficant at p < 0.065, and an additional 2,597 records (and 10 species) pushed me off to p < 0.032.  So the new FWGNA “Synthesis” is entirely nonparametric.  Such is science.

During the literature review for this most recent analysis I came across a little jewel of a book which I think deserves a wider audience: “Rarity” by a (then) Fellow at the British Museum, Kevin J. Gaston [2]. The first chapter of Gaston’s book offers an especially insightful dissection of the meaning of the adjective, “rare,” beginning with all the dictionary definitions and proceeding through (what seems to be, almost!) every imaginable scientific usage in modern history.

The author takes as a given that the adjective “rare” must be a comparative term.  Things can only be rare relative to other things, which are called common.   At first blush, it might seem odd that Gaston does not explicitly consider such “absolute rarity” measures as seem to be popular in state natural heritage agencies here in the USA.  For example, North Carolina botanists define “significantly rare” as “generally with 1 – 100 populations in the state,” apparently unscaled by anything [3].  But the (unstated) implications of such an approach must be that plants with greater than 100 populations are insignificantly rare in North Carolina, and that the significants are rare relative to the insignficants.  If my wife passes me a trapezoidally-shaped bowl with 100 red candies in it, I would not consider red candy to be rare.  But if the bowl contained 100 red candies and 1000 browns, I might.

Now if there are more than two colors of candy in the bowl, say 100 red, 500 green, and 1000 brown, an additional complication may arise.  Perhaps the rareness of red remains unchanged, as 100/1000 or 100/1100?  Or perhaps red is now even more rare, as 100/1600?  Or perhaps rareness is best expressed as 1/3, unweighted by frequency?  The most interesting analysis in Gaston’s Chapter 1 is his demonstration that the most robust definition of a rare species is “the x% with the lowest abundances or smallest range sizes in the assemblage.”  Gaston would prefer the rank statistic for his bowl of candy, the rareness of red being 1/3.

Back in my essay of January 2012, I expressed a concern that most of the literature (with which I was familiar at the time) focused on the rarity of individuals within communities.  So if F. C. Baker’s samples of Oneida Lake contained 32 species, with Planorbula jenksii the least abundant (just N=1 individual), everybody will concur that Planorbula is rare in Oneida Lake [4].  But FWGNA data are incidences across a wide geographic area, not individuals within a single community.  So if the fauna of US Atlantic drainages from Georgia through Pennsylvania comprises 67 species, and Aplexa hypnorum is found in the lowest number of sample sites (just N=1 pond), is Aplexa rare in the same sense that Planorbula is rare?

Yes, Gaston reassures us that any technique we might apply to analyze the abundance-rarity of snail individuals within a single lake will generalize to the incidence-rarity of snail populations across nine states.  Note that his definition of rarity above makes no distinction between “lowest abundances or smallest range sizes.”  The ecological causes for these two types of rarity most certainly do differ, but the analytical consequences are the same.

Ultimately, although some statistics to measure rarity are more robust than others, the dividing line between rare and common must need be a subjective decision.  So what is a reasonable value for “x” in Gaston’s definition three paragraphs above?  Gaston’s remarkably insightful first chapter also includes a Table 1.4 reviewing 20 studies (mostly of plants and birds) in which the authors have both defined the word “rare” and provided data sufficient to calculate rarity, in its “proportion of species” sense [5].  And Gaston’s judgment suggested to him that a fair consensus might be 25%.

So ultimately, Gaston settled on what he called “the quartile definition,” defining rare species as the 25% with the lowest abundance or lowest incidence.  And this is the definition we have adopted for the FWGNA project as well, expanding Gaston’s concepts across the entire nine-state study area to elaborate a five-tiered system.  We elect to set aside the rarest 5% of the freshwater gastropod species in a special incidence category I-5, leaving the next 20% as I-4, and successive (increasingly widespread) quartiles I-3, I-2, and I-1.

So now that we have defined rarity, what are its causes?  That might be suitable grist for a future post.  But in closing I probably ought to re-emphasize the main point of the present essay, which at one time I should have imagined might be obvious to all my colleagues in this field, but which (I now have some reason to fear) may be lost on some.

Among the more colorful comments I received regarding my post of 9Sept13 [6] was the following: 
“You also go on about how this or that COMMON species of snail has been negligently overlooked in the list for Delaware or New Jersey.  Planorbella campanulata, Physella gyrina, P. acuta, P. heterostropha – who gives a S**T.   It’s f***ing Delaware and New Jersey and it’s not the FOCUS OF THE PAPER – which if you need reminding is about conservation of rare species.”
A species can be rare only by comparison with other species.  Unless we know what is common, we cannot recognize what is rare.  The comment above reminds me of the frat brother who said he’d rather have two slices of pizza than half a pizza, because he “didn't give a S**T about denominators.”


[1] Toward the Scientific Ranking of Consevation Status – Part II.  [9Jan12]

[2] Gaston, K. J. (1994)  Rarity.  Chapman & Hall Population and Community Biology Series 13.  205 pp.  Prof. Gaston is currently at the University of Exeter.

[3] Two big reports, the North Carolina “Rare Plants List,” and the “Rare Animals List,” are downloadable here:
Despite being entitled “Natural Heritage Program List of Rare Plant Species of North Carolina 2012” and “Natural Heritage Program List of Rare Animal Species of North Carolina 2012,” neither of these reports has much to do with rarity.  Both seem to be focused almost entirely upon such unscientific categories as “endangered,” “threatened,” “imperiled,” “vulnerable,” and “secure.”  A definition for “significantly rare” is, however, to be found at the bottom of page 6 in the plants list.

[4] Dillon, R. T. (1981)  Patterns in the morphology and distribution of gastropods in Oneida Lake, New York, detected using compuer-generated null hypothses.  American Naturalist 118: 83-101.  [PDF]

[5]  None of Gaston’s 20 examples included any references to the voluminous gray literature generated by natural heritage agencies here in the USA - state, federal, or NGO.  I cannot pretend to more than passing familiarity with any of it, myself.  But in the few cases of which I am aware, such as the NC “Rare Plants List” cited above, rarity definitions are incomplete.  Gaston might have summed up the “significantly rare” species of plants in North Carolina for his numerator, but we are not given the total species count for our denominator.

[6] Plagiarism, Paul Johnson, and The American Fisheries Society.  [9Sept13]

Tuesday, November 19, 2013

Potamopyrgus in US Atlantic Drainages

Hidden deep inside last month’s big release of the new “Freshwater Gastropods of Mid-Atlantic States” web resource was at least one item of unwelcome news.  A population of  the New Zealand Mud Snail, Potamopyrgus antipodarum, has been discovered in Spring Creek, a small tributary of the Susquehanna River in Centre County, PA.  This is the first record of Potamopyrgus from a US Atlantic drainage.

The circumstances surrounding the discovery are interesting, possibly even edifying.  The Pennsylvania Department of Environmental Protection has long maintained a water quality monitoring site #WQN415 on Spring Creek at the County 3001 bridge, at which they collect regular macrobenthic samples.  And I paid a call on the PADEP lab in Harrisburg this past spring, as an important stop on my regional tour of similar facilities around the East, reviewing the freshwater gastropod components of (what typically turn out to be) large collections of vials stuffed with large collections of mostly bugs.  And I actually held a vial of macrobenthos collected on 7Oct10 from WQN415 in my hand this past March 20, and saw (what I remember as) a very large number of very minute hydrobiids washing around at the bottom.  And misidentified them as Fontigens.

I wrote “F. nickliniana teensy and weird” on line #415 of the spreadsheet I carried that day on my clipboard.  In retrospect, I’m surprised that I remember anything at all about any particular vial among the thousands I have examined over the last 18 months.  But I don’t think any of those (hundreds?) of tiny little gastropods contained in vial #415 was much over 1 mm standard shell length.  I don’t remember any individuals that looked like legitimate adults.  But surely a sample containing hundreds of individuals couldn't be comprised entirely of juveniles, could it?  In retrospect, my eyes were not prepared to see what they were looking at.

So six months later, our good friend Steve Means of the PADEP sent me an email inquiry with “New Zealand Mud Snail in Spring Creek, Centre County” on the subject line.  And the jpegs attached to his email (one of which is inserted above) clearly depicted adult Potamopyrgus antipodarum in the 4-5 mm range, collected this most recent summer at WQN415.  Oops!  A bit red-faced, I added P. antipodarum as species #41 to the Mid-Atlantic photo gallery, and composed species page #87 for the FWGNA site [1].

Potamopyrgus is a notorious invader, making its first North American appearance in Idaho’s Snake River back in 1987.  Populations spread to Montana in 1995, Oregon and California in 1997, and as far as Arizona in 2002 [2]. Most of the western populations seem to be associated with trout fishing, and it has been speculated that their spread has been facilitated either by untidy anglers, or by gut passage in the fish themselves.

Meanwhile back East, populations of Potamopyrgus were first reported in Lake Ontario in 1991, Lake Superior in 2001, and Lake Erie in 2005 [3].  The eastern populations seem more associated with commercial shipping, the implication being that they might represent a separate introduction via bilge water.  And recent research has indeed confirmed that the eastern and western populations represent genetically distinct clones [4].

The Spring Creek population obviously inhabits an eastern longitude, but is quite intimately associated with trout fishing.  PADEP water quality monitoring station WQN415 is located approximately 1-2 km downstream from the Bellefonte State Fish Hatchery, on a stretch of river marked “Fisherman’s Paradise” in my topographic map book [5].  I understand from Steve that the hatchery does not import trout into its facility, from the West or anywhere else, and that recent benthic surveys show lower densities of Potamopyrgus at the hatchery, increasing downstream toward WQN415.  This strongly suggests that the source of the introduction has not been the fish, but the fishermen.

Might some transcontinental angler have carried a sticky creel or muddy set of boots all the way from Montana to Pennsylvania?  Or is the Spring Creek population a fresh introduction of the eastern clone, which has heretofore seemed primarily associated with commercial shipping [6]?  I understand that our colleagues Ed Levri of Penn State Altoona and Mark Dybdahl of Washington State University are working on this question as we speak.

Meanwhile, the Pennsylvania Fish and Boat Commission has issued the press release available from the link below:
  • PFBC Issues Alert to Contain Invasive Species in Centre County [pdf]
Further spread throughout the Susquehanna drainage would seem inevitable, however, especially to nearby tributaries popular with the anglers.  Reference to the PFBC website [7] reveals a smorgasbord of Class – A streams in central Pennsylvania, including Logan Branch a few km to the east of Spring Creek and Buffalo Run a few km to the west.  Logan Branch even hosts a state trout hatchery.  Were I a researcher in the central Pennsylvania area interested in freshwater community ecology or energy flow, I’d begin planning my baseline studies in Logan Branch right now.


[1] For a broad review of the general biology, life history, ecology and systematics of the New Zealand Mud Snail, see the FWGNA species page...
[2]  ...or the USGS Nonindigenous species database:
[3]  See “Invaders Great and Small”  [19Sept08]

[4]  From the Aquatic Nuisance Species Taskforce:
  • National Management and Control Plan for the New Zealand Mudsnail [pdf]
[5] The image of the “Fishermans Paradise” sign comes from the PFBC website:
[6]  Although populations of the Great Lakes clone typically seem to reach maximum abundances at depths of 4 meters and below, recent surveys have uncovered populations in two small streams in western New York state, draining into Lake Ontario perhaps 300 km north of Spring Creek.  See Levri, Colledge, Bilka & Smith (2012) The distribution of the invasive New Zealand mud snail in streams in the Lake Ontario and Lake Erie watersheds.  BioInvasions Records 1: 215-219.

[7] Here’s a very nice interactive map from the PFBC:

Wednesday, October 30, 2013

Freshwater Gastropods of Mid-Atlantic States

We are pleased to announce that a new web-based resource, the Freshwater Gastropods of Mid-Atlantic States by R. T. Dillon, M. A. Ashton, and T. P. Smith, is now available from the FWGNAwebsite.  Hit the big FWGNA logo at right, and click the Mid-Atlantic region for a tour!

This is the sixth region to be added to the FWGNA site since its debut in 2003, extending our coverage from Georgia to the New York line, raising the total species reviewed from 79 to 87.
The new Mid-Atlantic site covers Delaware, Maryland, New Jersey, eastern Pennsylvania and the West Virginia panhandle.  Our database of 2,893 freshwater gastropod records was developed from the collections of the US National Museum, the Academy of Natural Sciences of Drexel University, the Carnegie Museum of Natural History, and the Delaware Museum of Natural History, the macrobenthic surveys of the Maryland DNR (Annapolis), the Pennsylvania DEP (Harrisburg), and the Delaware DNREC (Dover), and our own original fieldwork.  The new website features a dichotomous key and a photo gallery for all 41 species recovered from the five-state area, as well as range maps and notes regarding their ecology, life history, taxonomy and systematics.

Also new for 2013 is a page entitled “Synthesis,” which can be visited directly here:
In this analysis we combine our 2,893 Mid-Atlantic records with 8,568 data from Virginia, North Carolina, South Carolina and Georgia to generate a distribution of commonness and rarity over the entire 67-species Atlantic drainage freshwater gastropod fauna.   We suggest a new (nonparametric) system of “incidence ranks” as a supplement to, if not necessarily a replacement for, the subjective system of “conservation status ranks” currently in vogue with natural resource agencies [1].

All reference to conservation as a motivation or intent has now been removed from the FWGNA website, insofar as possible.  Henceforth the Freshwater Gastropods of North America project will restrict itself to science.


[1]  This is an expansion and refinement of the approach I first suggested for the 57 species of freshwater gastropods in:
  • Toward the Scientific Ranking of Conservation Status - Part II [9Jan12]