Dr. Rob Dillon, Coordinator

Monday, December 9, 2013

What is Rarity?

Editor's Note - While the text of the post below remains relevant, in November 2015 the FWGNA database was expanded to include 6.5% additional records and 2 new species.  See the current FWGNA Synthesis page for our most recent analysis.

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 (11,471 records) inhabiting the Atlantic drainages of the United States, from Georgia to the New York line [1].
My initial motivation was simply to expand (and formalize) the analysis I first proposed on this blog in January 2012 [2], 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 [3]. 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 [4].  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 armigera ("jenksii") the least abundant (just N=1 individual), everybody will concur that Planorbula is rare in Oneida Lake [5].  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 [6].  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 [7] 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] The 2013 data and analysis are available for download here - [PDF]

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

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

[4] 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.

[5] 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]

[6]  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.

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

Tuesday, November 19, 2013

Potamopyrgus in US Atlantic Drainages

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d)  Potamopyrgus in US Atlantic drainages.  Pp 41 - 45 in The Freshwater Gastropods of North America Volume 4, essays on Ecology and Biogeography.  FWGNA Press, Charleston.

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]

Monday, September 9, 2013

Plagiarism, Paul Johnson, and The American Fisheries Society

Editor's Notes.  About halfway through this essay I am going to assume that you have previously read last month’s post, regarding Delaware and the nonprofit organization NatureServe.  If you have not already done so, please take a minute to back up to [19Aug13], and then read forward.

This essay was subsequently published as: Dillon, R.T., Jr. (2019d) Plagiarism, Paul Johnson, and The American Fisheries Society.  Pp 243 - 249 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

My first impression of Dr. Paul D. Johnson was that of a “hard-charger.”  It was November of 1998, and I had been invited to Chattanooga to join a committee primarily composed of natural resource managers, fresh from organizing two successful meetings on unionid mussel conservation in the Midwest, interested in expanding their portfolio to include freshwater gastropods and going national.  That weekend we drafted a constitution for The Freshwater Mollusk Conservation Society and made plans for a first general meeting, to be hosted by Dr. Johnson in Chattanooga four months later.  I was pleased to accept the chairmanship of the FMCS Gastropod Committee that Saturday afternoon in November.  And I was honored to nominate Dr. Johnson to the office of President-elect of the entire society in March, from which he ascended to the presidency in 2000.

Even as early as 1998, Dr. Johnson was advocating a “national strategy” for the conservation of freshwater gastropods, to be modeled after a mussel strategy then nearing completion by the group.  This project would involve the development of a list of North American freshwater gastropods prioritized for conservation purposes.  And of course, Dr. Johnson envisioned that such a list would arise from a collaborative effort, presumably coordinated by the FMCS Gastropod Committee. 

Although I was not opposed to the idea (15 years ago), it was my strong opinion that our committee’s first order of business ought to be a comprehensive survey of the continental freshwater gastropod fauna, only after which conservation priorities might be assigned.  I have also developed moral scruples regarding the admixture of science, politics, and public policy, which have deepened in recent years, but no point in going down that road here [1].

In any case, I declined to become involved with Dr. Johnson’s “national strategy,” passing the chairmanship of the FMCS Gastropod Committee to him in 2002.   The effort seems to have subsequently shifted home, from the FMCS to the American Fisheries Society Endangered Species Committee, which has in recent years become a center for such work on the aquatic biota in general.  I have remained on the sidelines, hoping for the best while fearing the worst.  And in June, alas, my worst fears were realized.

In June Dr. Johnson and 13 of our friends and colleagues published a feature article in Fisheries, the peer-reviewed journal of the American Fisheries Society.  It is entitled, “Conservation status of freshwater gastropods of Canada and the United States” [2].

Although the paper extends to 36 journal pages, details regarding the development of the data upon which the Johnson/AFS recommendations of “conservation status” were based are extremely vague.  Here is the single relevant sentence from the methods section, quoted in its entirety:  “Species occurrences within provincial and state boundaries were generated using primary literature, including provincial and state checklists where available, as well as personal communications with professional who are knowledgeable about certain groups or regions.”

Now I have some very, very bad news to report.  If you open a new window in your browser today (9Sept13), go to the USGS website hosting the Johnson/AFS database, and execute a map query for Delaware, you will find almost exactly the same list of 8 species you received from your identical query of the NatureServe Explorer database last month.

This is a peculiar list.  Missing from it are the four most common gastropod species actually inhabiting the freshwaters of Delaware: Physa acuta (aka P. heterostropha), Menetus dilatatus, Ferrissia fragilis, and Lymnaea (Pseudosuccinea) columella.  All four of these species are very nearly cosmopolitan in their distribution throughout eastern North America, and simple reference to the collections of either the DMNH or the ANSP would have returned numerous Delaware records for most of them.

The Johnson/AFS report also includes one species that our extensive field surveys of Delaware and attendant reviews of systematic collections have failed to uncover, Physa gyrina.  Populations of Physa gyrina do inhabit Ridley Creek in Delaware County, southeastern Pennsylvania, so on first reading it certainly seems possible that the Johnson/AFS record might be bona fide.  Or might this record represent a misidentification of Physa acuta?  The DMNH collection does hold a single undated lot of P. acuta (locality just “Wilmington”) misidentified as P. gyrina.

On 3Aug13 I sent an email inquiry to Dr. Johnson, asking if he could provide a reference to the primary literature or any other source available to him supporting his report of P. gyrina in Delaware, with 12 of his coauthors on the CC line.  Dr. Johnson has not favored me with the courtesy of a reply [3].

The match between the Johnson/AFS database and the NatureServe database is simply too close to be coincidental.  But at no point in his paper does Dr. Johnson acknowledge NatureServe as the origin of his primary data – not in methods, results, or acknowledgments.  The NatureServe organization is mentioned only on pages 250, 252 and 263 with regard to its system of conservation ranking, and cited only with respect to conservation ranking in the reference section.  The Johnson paper does not include a citation to the NatureServe Explorer as explicitly required by NatureServe for the fair use of its data.

The match is not perfect.  The exotic Bellamya (“Cipangopaludina”) chinensis was deleted from the Johnson/AFS Delaware list, and Helisoma (Planorbella) trivolvis added, indeed #11 on the confirmed list soon to appear on the FWGNA website.  The generic nomina of Physa gyrina and Fossaria obrussa have been emended to Physella and Galba.

But the evidence of plagiarism is pervasive.  My thirty years of experience grading the genetics lab reports of lazy college sophomores have (alas!) given me way too much practice identifying the phenomenon [4].  I have footnoted analyses of the situations in West Virginia [5] and New Jersey [6] below.  If these examples do not constitute sufficient evidence to convince my readership that the extensive data table reproduced in the appendix of the paper by Johnson and his colleagues did not originate from the NatureServe Explorer, tell me how many more such examples are necessary, and I will supply them.

Dr. Paul D. Johnson and his 13 colleagues stole a crappy, spurious dataset off the internet, tweaked it to the point they thought nobody would catch them, put their names on it, and transferred it into the peer-reviewed literature without attribution.  Shame on everybody involved: Arthur E. Bogan, Kenneth M. Brown, Noel M. Burkhead, James R. Cordeiro, Jeffrey T. Garner, Paul D. Hartfield, Dwayne A. W. Lepitzki, Gerry L. Mackie, Eva Pip, Thomas A. Tarpley, Jeremy S. Tiemann, Nathan V. Whelan and Ellen E. Strong.  Your mothers taught you all better.

And both the American Fisheries Society and the USGS Southeast Ecological Science Center are now accessories to egregious plagiarism.  The Johnson paper must be retracted, with apologies to NatureServe and to the scientific community at large.

Because the damage extends beyond that done to the professional reputations of Paul Johnson and his 13 collaborators.  The greatest damage is that done to science.  For what was merely the conventional ignorance of the worldwide web has now been transformed, by its publication in what appears to be a reputable journal, into ignorance of a high and aggravated nature, disgorged by 14 professionals whose credentials would lead one to expect some minimum level of scientific rigor, wrongly.  And perhaps a bit of integrity, for a change.

Postscript – On the date this essay was posted I mailed a formal letter to Dr. John Boreman, the President of the AFS, accusing Dr. Johnson and his 13 coauthors of plagiarism.  Dr. Boreman then tasked Mr. Jeff Schaeffer, the chief science editor of Fisheries, to conduct an individual review.  And 11 days later, I received a letter from Mr. Schaeffer finding that my “letter of complaint and supporting information from the blog do not meet” the standard of plagiarism.  I do not know what testimony the 14 authors of the Johnson paper might have offered on their own behalf.  I myself was not interviewed.


[1]  The language, culture, and values of science are not incompatible with those of law, politics and public policy, but they are not compatible either.  And over the years it has become clearer to me that much damage is done by workers with either worldview when we try to force a fit with the other, directly analogous to the damage done when a false compatibility is forced between public policy and religion, or science and religion, for that matter.  See any of my essays labeled “Worldview Collision” at right for more.

[2] 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.

[3] The complete correspondence record is as follows.  On 2July13 I sent an email to Dr. Johnson inquiring if he might be willing to share his database of occurrences with me, and asking for additional detail on the method by which these data were converted to conservation status recommendations.  Twelve (of his 13) coauthors were on the CC line.  (I have been unable to find an email address for Tarpley.)  I received no reply from any of the 13 recipients.  On 3Aug13 I sent a second email to Dr. Johnson, again with 12 coauthors on the CC line, simply requesting information regarding the occurrence of Physa gyrina in Delaware.  I received one fragmentary reply from Mr. Jay Cordeiro, who abruptly broke off our correspondence when I asked for clarification.  And I have heard nothing since.

[4] The sad science of plagiarism detection focuses on the “shared bonehead error,” or SBE.  If student #2 copies the errorless lab report of student #1, he will not be caught.  If student #2 copies a lab report, finds errors and fixes them, he will not be caught.  In fact, if student #2 copies a lab report containing reasonable errors, for example “three squared equals six,” he will not be caught.  The key to detecting plagiarism is the situation where student #2 copies a bonehead error, for example “three squared equals seven.”  Such “shared bonehead errors” are like fingerprints.

[5] NatureServe’s West Virginia list includes 28 species.  To this list Johnson/AFS added nine nomina – five valid species and four junior synonyms.  But there are two SBE omissions on both lists: Lyogyrus granum and Physa (“Physella”) gyrina.  This despite the fact that reference to the ANSP collection online would return 1 West Virginia lot of the former and 6 lots of the latter.

[6] NatureServe’s New Jersey list includes 22 species, three of which are exotic or introduced, reducing the list to just 19.  To these 19 Johnson/AFS added ten nomina – nine of which are specifically valid.  But again there is one SBE omission [7] on both lists: Helisoma (Planorbella) campanulatum, of which the ANSP collection online holds 15 New Jersey lots.

[7] There is also at least one glaring “shared reasonable error” on the two New Jersey lists.  Neither includes Ferrissia fragilis, which is #8 most common of the 30 species (no not 22, no not 28) soon to be documented on our Freshwater Gastropods of the Mid-Atlantic website.  This omission does not qualify as a “shared bonehead error,” however, because no national collection (to my knowledge) actually holds a single New Jersey record of F. fragilis.  The ANSP does hold 8 lots of F. fragilis from New Jersey, but five are curated as “Ferrissia sp.” in their online database, and three are misidentified as Ferrissia rivularis.  Thus the absence of F. fragilis from the New Jersey tabulation of Johnson/AFS does not constitute evidence of plagiarism.  It is a glaring example of that simple, conventional ignorance which we all ought to be working to fix, together.

Monday, August 19, 2013

Delaware, and what we think we know, that we do not

Editor’s Note. This essay was subsequently published as: Dillon, R.T., Jr. (2019d) Delaware, And What We Think We Know, That We Do Not.  Pp 235 - 241 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

Delaware, our first state, is surprisingly diverse.  Its Piedmont north is primarily industrial and urban, but there are some lovely (and surprisingly rich) rocky streams tumbling across the low fall line.  Large, prosperous farms dominate the sandy coastal plain of the south, where I was personally startled to find blackwater swamps, dominated by bald cypress.  And with the Delaware Museum of Natural History in Wilmington, the Academy of Natural Sciences 20 miles north in Philadelphia and the National Museum of Natural History 70 miles east in Washington, the state of Delaware is cradled in the greatest concentration of malacological talent in the USA, if not the world.

Should we be surprised to learn that Delaware has never seen a survey of its freshwater gastropod fauna?  Or that the systematic collections of the DMNH, the ANSP and the USNM hold but 151 lots of freshwater gastropods collected in The First State combined, under 31 nomina, 11 of which are misidentifications or junior synonyms?  It seems likely to me that this situation arises from some sort of “neglected backyard effect,” our North American imaginations so transfixed by the Brazilian Amazon and the Black Smokers of the Abyss that we tend to overlook the little creeks running through our own city parks back home.

I am now pleased to report that a remedy is at hand.  In coming months the FWGNA project will roll out a new website entitled, “The Freshwater Gastropods of the Mid-Atlantic States” which will include surveys of Delaware, Maryland, New Jersey, eastern Pennsylvania, and the West Virginia panhandle.  Our extensive original collections, sampled from approximately 144 sites spread down the 100 mile length and across the 30 mile breadth of The First State, will confirm that a fairly reliable estimate for the number of gastropod species inhabiting the freshwaters of Delaware is indeed 20, as might have been predicted from the meager museum records available.

We will be sharing much more detail about the freshwater gastropods of all the Mid-Atlantic states in the coming months.  My focus for the remainder of this essay is not on what we now know, but on what we thought we knew, that we did not.

I am not a fan of the nonprofit environmental group called, “NatureServe,” or its online database “Explorer.”  But if you were to open a new window in your web browser today (19Aug13), go to www.natureserve.org, hit that “Explorer” button on the right panel, and execute a query for freshwater snails AND Delaware, the search engine will return a list of just eight species.

This is a peculiar list.  Missing from it are the four most common gastropod species actually inhabiting the freshwaters of Delaware: Physa acuta (aka P. heterostropha), Menetus dilatatus, Ferrissia fragilis, and Lymnaea (Pseudosuccinea)columella.  All four of these species are very nearly cosmopolitan in their distribution throughout eastern North America, and simple reference to the collections of either the DMNH or the ANSP would have returned numerous Delaware records for most of them.

The NatureServe report will also include one species that our extensive field surveys of Delaware and attendant reviews of systematic collections have failed to uncover, Physa gyrina.  Populations of Physa gyrina do inhabit Ridley Creek in Delaware County, southeastern Pennsylvania, so on first reading it certainly seems possible that the NatureServe record might be bona fide.  Or might this record represent a misidentification of Physa acuta?  The DMNH collection does hold a single undated lot of P. acuta (locality just “Wilmington”) misidentified as P. gyrina.

On 28July13 I sent an email inquiry to Dr. Bruce Young, Director of “Species Science” for NatureServe, asking if he could provide a reference supporting his report of P. gyrina in Delaware.  Dr. Young replied the next day, apologizing that “for common species we do not tie specific records to individual references or museum collections.”  Ms. Margaret Ormes of NatureServe followed up with “I just looked at our database and my best guess is that it was added (as Physella gyrina) based on Burch (1989).” [1]

I do appreciate the courtesy of the NatureServe staff, but the facts are very plain and very ugly.  The mission of NatureServe is advocacy, not science, and Physa gyrina is a trash snail, and they do not care whether arbitrarily-chosen trash snails actually inhabit arbitrarily-chosen states.  The NatureServe Explorer database is a smokescreen and a ruse, designed to lend false scientific credibility to a political agenda.

Science is the construction of testable models about the natural world.  Such models need not necessarily be accurate, nor even based on reliable data.  I am not fussing here about the discrepancy between the NatureServe estimate of 8 freshwater gastropod species in Delaware, and the FWGNA estimate of 20.  Nor am I fussing about the basis of the NatureServe estimate, which is spurious.

Science will fix these things, eventually.  There are scientists who will drive 16,139 miles in a 1992 Nissan pickup, camp in the rain, and eat Dinty Moore beef stew for a week to not find Physa gyrina in Delaware.  There are scientists who will dump three years of unsorted DNREC-DWR macrobenthic samples into little Petrie dishes and squint through microscopes for hours to not find Physa gyrina in Delaware.  There are scientists who will sleep five nights on an air mattress in Alexandria to ride the orange line downtown to pull every drawer of physids out of the US National Museum to not find Physa gyrina in Delaware.  Science ultimately fixes carelessness, sloppiness, and neglect.

No, the worst thing about the Explorer database is not that it is wrong, but that it reinforces conventional ignorance, and in so doing works against the cause we all seek to advance.  Fresh young students, casting about for research ideas, will tend to assume that the freshwater gastropod fauna of North America is in some sense “known,” and divert their energies elsewhere.  A proposal to the NSF Biotic Surveys and Inventories Program will be reviewed more favorably if it is directed toward the Ubangi River than to the Nanticoke, even though the freshwater gastropod fauna of the Ubangi is better documented [2].  Reinforcement of conventional ignorance is not the main problem I have with the NatureServe organization [3], but it is certainly in the top ten.

How big is your budget, Dr. Young, and how large is your staff?  The FWGNA project has now surveyed all or part of ten states.  This we have done with no more than $5K in grant support total, over 15 years.  Those 16,139 miles we drove in 2012-13 were unreimbursed.  We have paid $49/night out of our own pockets to sleep on air mattresses in Alexandria.  We bought our own Dinty Moore.

I will end this essay, however, on a note of hope.  Despite the layers of pseudoscientific hoo-hah in which NatureServe cloaks its Explorer website, it remains just a website, run by a political advocacy group.  And (how many times must we warn our students?) the internet is a wild and woolly marketplace of information, both the good and the bad, and the buyer must beware.  Each report returned by the NatureServe Explorer comes with the following disclaimer: “All documents provided by this server are provided “as is” without warranty as to the currentness, completeness, or accuracy of any specific data.”  In the final analysis, the Explorer database is not peer-reviewed science, and it does not pretend to be.

Thank heaven no legitimate researcher would ever confuse spurious data made available from the website of an advocacy group with reliable science.  Thank heaven.


[1] Yes, it does seem plausible to me that the initial state-by-state freshwater gastropod distributions uploaded to the NatureServe Explorer may have been the work of some intern hired to extrapolate the broad ranges found in J. B. Burch’s North American Freshwater Snails.  Burch lists 15 subspecies of P. gyrina, the ranges of at least two of which certainly include Delaware.  The range of Physella gyrina aurea is given as “New Jersey to Kansas, south to Arkansas and Florida,” and that of Physella gyrina cylindrica is given as “Ontario and New York south to Virginia.”  Then did our young intern simply miss Delaware when extrapolating “New England to Ohio, Tennessee and the Virginias” for P. heterostropha?  And did that intern miss Delaware when extrapolating “Eastern United States, from Maine west to Iowa, south to Texas and Florida” for Menetus (Micromenetus) dilatatus?  And so forth?  The entire enterprise is a house of cards.

[2] Brown, D. S. (1994)  Freshwater Snails of Africa and their Medical Importance (Second Edition).  Taylor & Francis, London.  608 pp.

[3] My main problem with NatureServe is the pseudoscientific method by which they gin up “conservation status ranks.”  See:
  • Toward the Scientific Ranking of Conservation Status [12Dec11]

Thursday, July 25, 2013

Pomacea News

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d)  Seven dispatches from the Pomacea front.  Pp 19 - 28 in The Freshwater Gastropods of North America Volume 4, essays on Ecology and Biogeography.  FWGNA Press, Charleston.

The rate of scientific advance on the invasive species front always seems to far outstrip the (rather languid) research progress we seem to log on any of our native freshwater gastropod fauna.  Thus it has become our occasional custom to publish batches of news to catch us up on particular invasive species, bundled for convenience.  So here, for the general edification of the readership, are four news items featuring Pomacea.

Clarifying the Identities
A laurel, and hearty thank-you are due to our friend Ken Hayes and his colleagues at the University of Hawaii (and elsewhere) for their contribution published late last year in the Zoological Journal of the Linnean Society, “clarifying the identities of two highly invasive Neotropical Ampullariidae” [1].  This is research from the old school, emphasizing comparative anatomy, histology, biogeography, and traditional taxonomic scholarship to resolve a problem that has bedeviled the Pomacea community for 200 years, both North American and South.  I am persuaded that the most common invasive species here in the southeastern United States is best identified as Pomacea maculata (Perry 1810), rather than P. insularum (d’Orbigny 1835) as most workers have previously supposed, and have updated the FWGNA page accordingly [2].

Ken’s primary focus is the distinction between P. maculata (previously insularum) sampled from Florida and the other common invasive species of global import, Pomacea canaliculata, sampled from Hawaii.  If I have any quibble with his study [3], it would be that no mention is made of P. canaliculata in Florida, which leaves me wondering about the reliability of reports by Florida natural resource agencies [4] that both species co-occur widely in The Sunshine State.

And do P. maculata and P. canaliculata hybridize?  In the final paragraph of their discussion Hayes and colleagues mention “possible hybridization events,” although we are offered no details.  Then what is the basis of the reproductive isolation between them?  There have been several nice descriptions of mating behavior and sex pheromones in Pomacea [5] – mate choice tests would appear to be low-hanging fruit.  Any students out there looking for a good thesis research topic?

Modeling the Invasion
Kudos are also due to Jeb Byers and his colleagues at the University of Georgia for their paper published in PloS ONE this past February, “Climate and pH predict the potential range of the invasive apple snail (P. insularum) in the southeastern USA” [6]

Jeb based his model on Pomacea records from the USGS Nonindiginous Aquatic Species Database (accessed in late 2009) cast onto a presence/absence grid of 1x1 km plots.  The (N=68) such plots where invasive Pomacea was present were primarily Floridian, but extended into six other states: TX, LA, MS, AL, GA and SC.  Jeb and colleagues also pre-screened 19 climate variables from BioClim to pick two of temperature and three of rainfall, and downloaded pH data from the USEPA Storet database.

The weakness of inferences based on any such model are fairly obvious – if the vast majority of the 1x1 km plots are suitable, but still have no invasive Pomacea, how strong can be the inference on absence?  But I do think the author’s prediction that the pH may prove too low for Pomacea in the Okefenokee Swamp should be robust, which is good news.

Dispatches from the Front
And indeed, our good friend Maj. Alan Covich has just returned from a scouting expedition to the Okefenokee Swamp at the head of a large detachment from the Society for Freshwater Science.  No enemy activity is reported in the area.

Meanwhile, reports from the Department of South Carolina are mixed.  On the positive side, the northernmost population of Pomacea does not seem to have advanced beyond its beachhead at Socastee [7].  A colleague and I canoed the Intracoastal Waterway downstream from the site of introduction last summer, finding no evidence of Pomacea at any point in our trip.  It was our impression that the habitat may prove unsuitable. 

The tidal fluctuation in the Intracoastal Waterway at Socastee is at least two feet (as witnessed from the photo below) to which one must add over a foot of slosh for the wake of passing motorboats.  The water is quite darkly stained and the cypress canopy generally dense throughout the shallows, both of which tend to keep aquatic macrophytes at negligible densities.  We simply did not see much food or habitat for Pomacea in that particular part of the world.

On the negative side, however, the Socastee population itself still appears quite healthy in the weedy ponds and ditches where it was first reported in 2008.  This despite the fact that the winter of 2009-10 was one of the coldest recently experienced in South Carolina [8], with a low of 18 degrees F recorded on January 11 at Conway, 10 miles to the north.  From the standpoint of climate, I am afraid that Jeb Byers’ map may prove a robust predictor, as well.
And also on the negative side, in August of 2010 word reached us of a second introduction in South Carolina, this at Lake Marion just south of the I-95 bridge.  Our friend Larry McCord of Santee Cooper (the utility that administers the lake) reported that “the population has been treated with a low level of copper in hopes to avoid spread to other areas.” 

With large populations of invasive Viviparus georgianus, V. subpurpureus [9], and Bellamya japonica jostling each other for grazing room on its dense beds of Corbicula, Lake Marion was already wearing the yellow jersey in this year’s “Tour de Malacological Infestation.”  Now Pomacea is threatening to eat all the invasive waterweed as well.  Bummer.

Poisonous eggs
I long ago ceased trying to understand why particular odd little snippets of scientific trivia find their way into the popular press.  But in early June a PLoS ONE paper by M. S. Dreon and an international team on toxic Pomacea eggs [10] was selected for feature by the AAAS online organ “Science Now" [11] and then propagated around the world by e!Science News, Reddit, Facebook, and what have you.

The “hook” used by Science Now was that the composition of Pomacea egg neurotoxin is “unusual for animals” and that “the apple snail creates it in an unprecedented way.”  But were I the editor of some popular science outlet, or indeed one of the authors trolling for popular attention and looking for bait, I would have emphasized the weirdness that any toxin should be found in any egg at all.

Embryos are edible.  Somewhat counter to the statement of Dreon and colleagues that “many invertebrates defend their eggs by endowing them with deterrent chemicals,” across all of evolutionary biology, plants and animals alike, the phenomenon is stunningly rare [12].  It is easy to observe that a mother-ampullariid must value her (relatively large) eggs much more highly than a mother-physid or a mother-pleurocerid, but when we expand the comparison to a mother-hen, a simple adaptationist explanation becomes more difficult to sustain.  Although Orians & Janzen punted their original 1974 article with “it is possible to be toxic if one is an embryo, but under most circumstances it isn’t worth the price,” they ultimately concluded, “We think that refining this answer is worth the price.”  Are we any closer to refining this answer if, now 40 years later, we have forgotten the question?


[1] Hayes, K. A., R. H. Cowie, S. C. Thiengo, and E. E. Strong (2012)  Comparing apples with apples: clarifying the identities of two highly invasive Neotropical Ampullariidae (Caenogastropoda).  Zool. J. Linn. Soc. 166: 723-753.

[2] The (revised) Pomacea maculata page on the FWGNA site [html]

[3] Actually, it would appear that I have listed more than one quibble above.  And add this nasty clinker from page 744, “Molecular data confirm that P. maculata and P. canaliculata are two distinct species.”  Hayes and colleagues are referring to mtDNA sequence divergence here!  Good grief.

[4] Florida Fish & Wildlife Commission.  “Non-native Apple Snails in Florida.” [pdf]

[5] Takeichi, M., Y. Hirai & Y. Yusa (2007)  A water-borne sex pheromone and trail following in the apple snail, Pomacea canaliculata.  J. Moll. Stud. 73: 275-278.
Burela, S. & P. R. Martin (2011)  Evolutionary and functional significance of lengthy copulations in a promiscuous apple snail, Pomacea canaliculata (Caenogastropoda: Ampullariidae).  J. Moll. Stud. 77: 54-64.

[6] Byers, J., W. McDowell, S. Dodd, R. Haynie, L. Pintor, and S. Wilde. 2013. Climate and pH predict the potential range of the invasive apple snail (Pomacea insularum) in the southeastern United States. PLoS ONE 8: e56812. [html]

[7] Previous posts:
  • Pomacea spreads to South Carolina [15May08]
  • Two dispatches from the Pomacea front [14Aug08]

[8] See Figure 1 in this (weirdly related!) paper:
Dorcas, M. E., J. D. Willson & J. W. Gibbons (2011) Can Burmese pythons inhabit temperate regions of the southeastern United States?  Biol. Invasions 13: 793-802.

[9] Invasive viviparids in South Carolina [29Oct03]

[10] Dreon, M., M. Frassa, M. Ceolin, S. Ituarte, J-W. Qiu, J. Sun, P. Fernandez, and H. Heras. 2013. Novel animal defenses against predation: a snail egg neurotoxin combining lectin and pore-forming chains that resembles plant defense and bacteria attack toxins. PLoS ONE 8: e63782.  [html]

[11] ScienceShot: Invasive snails protect their young with odd poison. [html]

[12] Orians, G. H. & D. Janzen (1974)  Why are embryos so tasty?  American Naturalist 108: 581-592.

Tuesday, June 18, 2013

Pleurocera canaliculata and the Process of Scientific Discovery

Editor's Note - This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera canaliculata and the process of scientific discovery.  Pp 71 - 76 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

Looking back on my post of two weeks ago [1], I fear I may have left the impression that Rob Dillon is smarter than Calvin Goodrich, and indeed smarter than any other malacologist who has ever waded knee-deep into the rivers of the greater Midwest before or since.  That's probably not an uncommon failing of mine, but nothing could be farther from the truth.  In fact, the revelation that the gastropod populations we have been calling Pleurocera acuta for 189 years are actually the same as what we've been calling Pleurocera canaliculata for 192 only dawned on me after several years of being dope-slapped by the glaringly obvious.  And even then, I was only able to figure out the "Goodrichian" relationship between P. acuta and P. canaliculata backwards, through P. pyrenellum.

So to set the situation aright, I hereby offer an amendment to my essay of 3June13.  And in this retelling I propose to lay bare for my readership the agonizingly slow process by which this particular snail-guy's mind actually works.

The initial revelation that Goodrichian taxon shift might be even more dramatic than Calvin Goodrich himself realized first dope-slapped me in the back of my head eight years ago, as I scored a set of allozyme gels comparing pleurocerid populations from the upper Powell drainage on the Virginia/Tennessee border.  I wrote this the bottom of my data sheet late in the afternoon of October 4, 2005: "Good grief!  Pleurocera unciale and Goniobasis clavaeformis at the mouth of Indian Creek are a single random-breeding population!"  John Robinson [2] and I wrote this initial evidence of Goodrichian taxon shift into a gray literature report in 2007 [3].  I then sampled additional populations of nominal clavaeformis, unciale, acutocarinata, curta, carinifera and vestita across East Tennessee and North Georgia in 2008-09, ran a big mess of additional gels, and published the formal paper subsuming the genera Goniobasis and Elimia under Pleurocera in 2011 [4].

Meanwhile, my general survey of the freshwater gastropod fauna of East Tennessee was moving forward.  I spent at least a week or two working in the rivers and streams between Knoxville and Chattanooga each field season between 2007 and 2010, collecting (what I recorded as) P. clavaeformis (in its various phenotypic forms) in essentially every stream I visited for four years.  And incidentally, I also observed that the big river populations (historically referred to P. unciale or curta) sort-of petered out around Knoxville, to be replaced in the main Tennessee River by Pleurocera canaliculata.

I took a second dope-slap to the back of my noggin at approximately 4:30 PM on August 14, 2010.  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.  At that point in my career, I had collected Pleurocera acuta up in Kentucky, Ohio and Michigan, but would not have expected a P. acuta population anywhere within 500 km of the stream where I was standing.  When I got back to my truck, I wrote this in my field notes: "(star)Wow(star)  Is this Pleurocera acuta??? My eyes are opened!"

So back in Charleston, I pulled four years of old samples off the shelves, dug the old literature out of the file cabinets, and (over the course of several weeks) pieced together the following line of reasoning.

Inference.  The snails I collected in Savannah Creek on 14Aug10 must be what Goodrich would have called "Pleurocera pyrenellum," not Pleurocera acuta.  Goodrich (1940) gave the range of P. pyrenellum as "tributaries of the Tennessee River in Morgan and Lancaster Counties, Alabama, and Walker County, Georgia" [5].  Walker County wasn’t much more than 20-30 km downstream from Savannah Creek.  So "pyrenellum" my snails would seem to be.

Therefore Pleurocera pyrenellum populations inhabit Tennessee tributaries further upstream than Goodrich realized.  Going back through my old collections, I recognized P. pyrenellum at 10-12 additional sites, extending up the Tennessee Valley all the way to the environs of Knoxville [6].  They were usually mixed with populations of P. clavaeformis, and (dazzled as I had been by the phenotypic variety of clavaeformis) I had simply missed them.

Revelation.  Travelling down the main Tennessee valley from Knoxville to Chattanooga, Pleurocera canaliculata begins to appear in the main river (red above) as P. pyrenellum begins to appear in the tributaries (pink).  This was the third major dope-slap I took during my long bewilderment with the East Tennessee Pleuroceridae, although I didn't record the date and time of the whacking.  The relationship between pyrenellum and canaliculata looks exactly like the Goodrichian relationship between unciale/curta and clavaeformis (blue above).

Hypothesis.  But the range of P. canaliculata extends throughout the greater Midwest, up the Ohio River all the way to Pittsburgh, while nominal pyrenellum populations are restricted to drainages of the Tennessee.  So might the small-stream form upstream from all those populations of canaliculata in Ohio, Kentucky, Indiana and Illinois be what everybody calls P. acuta in Yankeeland?

The point of this confession is that my colleagues and I were no more able to see the Goodrichian relationship between acuta and canaliculata standing on the shores of the Wabash River than Calvin Goodrich [7].  I flipped the whole story around when I told it two weeks ago, so that it "made sense," following the conventional practices of science.  I made the process of discovery look logical, even inevitable, when it most certainly was not.  I'm really a bonehead.  Ask my wife.

Allow me to close with another of my philosophical peregrinations.  My regular day-job is teaching majors-level genetics at a very ordinary liberal arts college of regional reputation.  It’s a big chore – there are three or four other members of my department doing the same thing.  All younger than I, of course.

So I organize the subject matter of genetics historically, the way I myself learned it, starting with Mendel, then Morgan, then Beadle then Avery and McLeod and McCarty and Watson and Crick and so forth.  And just when it begins to look like we might get anywhere near the present day, I go back to Hardy and Weinberg and finish with the "Modern Synthesis" of the 1930s and 40s.

Some of my colleagues tell me that undergraduate students "don’t get this."  They prefer to start their majors-level genetics sections with our modern understanding of the molecular basis of inheritance, because given the underlying mechanism, Mendel's 1866 results become a lot easier to explain.  And the kids prefer to focus on today's science, in any case.  They don't care about peas.

That line of argument is so self-evidently horrific to me that in my first draft of this essay, I left it dangling in the wind, without comment.  Like Stede Bonnet on the Charleston battery.

But I shall close with the simple observation that the K-12 teaching of science and the profession of science in higher education are two entirely different things.  In the former we are building palaces, and in the latter we are laying roads.  I trust that my readership has enjoyed this month’s brief foray into road building.  Next month, it’s back to the masonry.


[1] Pleurocera acuta is Pleurocera canaliculata [3June13]

[2] John D. Robinson was working on his MS in Marine Biology with me at the time.  He went on to earn his PhD at the University of Georgia and has recently moved to the Cornell area.

[3] Goodrichian Taxon Shift [20Feb07]

[4] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[5] Goodrich, C. (1940)  The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich. 417: 1 -21.

[6] By the 8/2011 debut of the FWGTN website, I had documented 8 populations of P. canaliculata in the big rivers below Knoxville and 24 populations of nominal "P. pyrenellum" in the tributaries.  Since I've decided to save the nomen "pyrenellum" as a subspecies (more about that in a future post) the distribution of both shell forms is still apparent on the pdf  map available from the P. canaliculata page on the FWGNA site.

[7] In fact, the Indiana survey of Pyron et al. (2008) retained both P. canaliculata and P. acuta as quite distinct species.  For a pdf of Pyron’s survey, see:
  • The Freshwater Gastropods of Indiana [23Jan09]

Monday, June 3, 2013

Pleurocera acuta is Pleurocera canaliculata

Editor's Note - This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera acuta is Pleurocera canaliculata.  Pp 65 - 70 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

I am pleased to report that the second paper in what I expect will be a continuing series on Goodrichian taxon shift in the North American Pleuroceridae has recently reached formal publication [1].  In this most recent installment, the team of Dillon, Jacquemin and Pyron uses a combination of genetic and shell morphometric techniques to demonstrate that the specific nomina acuta (Rafinesque 1824) and pyrenellum (Conrad 1839) are junior synonyms of Pleurocera canaliculata (Say 1821).  As “Pleurocerus acutus” was designated the type of the genus in ICZN Opinion #1195 of 1981 [2], this finding will be of more than the usual taxonomic interest.

Our analytical approach was patterned after that used by Dillon (2011) on the Pleurocera clavaeformis populations of East Tennessee [3].  We used allele frequencies at nine polymorphic allozyme loci to show that two populations of nominal P. acuta (from Indiana and Kentucky) and one population of nominal P. pyrenellum (from north Alabama) were each more genetically similar to the P. canaliculata population immediately downstream than any of the six populations was to any nominal conspecific.  We then used landmark-based morphometics to explore one of these “Goodrichian” shifts in greater detail, the historically important acuta-to-canaliculata transition found in the Wabash River of Indiana.

On at least two occasions Calvin Goodrich himself expressed doubts about the distinction between P. acuta and P. canaliculata, specifically mentioning the Wabash populations in 1937 [4].  So we borrowed 18 lots of historically-collected Wabash Pleurocera from the University of Michigan Museum of Zoology [5] and documented a significant correlation between shell robustness (as measured using relative warp analysis) and river size at point of collection, as estimated by catchment area.  The seven museum lots bearing more slender, high-spired shells (collected above Wabash River kilometer RK500) were identified by Goodrich as acuta, and the six lots bearing broader, heavier shells collected downstream from RK400 were identified as canaliculata.  Goodrich recognized a mixture of the two nominal species in the five lots collected in the 100 km immediately downstream from RK518, where the Tippecanoe River joins the Wabash, essentially doubling the catchment area (sites 8 - 12 in the map below).

Situations such as we document in the RK400-500 region probably explain why Calvin Goodrich, the man for whom Goodrichian taxon shift was named, did not synonymize acuta under canaliculata himself back in 1939.  The two nominal “species” sometimes seem to mix and retain a degree of distinctiveness.  The phenomenon appears even more dramatically in places where very small streams communicate directly with large mainstem rivers, such as that marked “12” in the map below.  It is not unusual to find snails bearing very slender, high-spired shells washed down into much broader, more robustly-shelled populations inhabiting the main river at spots such as this, looking for all the world like a pair of bona fide, reproductively-isolated species.

Way back in 2007 I defined "Goodrichian Taxon Shift," as “intraspecific variation in freshwater gastropod shell morphology along an environmental cline of such magnitude as to prompt the erroneous recognition of multiple nominal taxa” [6].  The phenomenon was conceived as narrowly applicable to freshwater snails, as Calvin Goodrich himself focused his own research.

But after some soul-searching, my colleagues and I have decided to broaden the concept and coin a new term for it.  So in our paper just published, we propose the term “cryptic phenotypic plasticity,” which we define as “intrapopulation morphological variation so extreme as to prompt an (erroneous) hypothesis of speciation.”  We think that the sort of insight Calvin Goodrich brought to the study of pleurocerid snails in the 1930s and 1940s has the potential to make a contribution to the understanding of evolutionary processes beyond our small community of freshwater gastropod cognoscenti, even in the present day.

Goodrich himself was strongly influenced by the research of A. E. Ortmann on unionid mussels.  Ortmann described a correlation between river size and shell robustness way back in 1920, on the basis of which he sank quite a few unionid taxa into synonymy [7].  And Ortmann credited the idea to Wilson & Clark (1914) [8].  So in some sense it might not be fair to continue to call the phenomenon “Goodrichian” anything, as we forward cryptic phenotypic plasticity onward to evolutionary science as a whole.

The extent to which cryptic phenotypic plasticity may occur in the biotas of land and sea remains to be seen, however.  It seems unlikely to me that marine and terrestrial environments manifest themselves as variably to the populations that inhabit them as fresh waters, at least for the molluscan fauna with which I have any professional familiarity.  Were the Nucella (“dog whelk”) populations of the Pacific intertidal an (historic) example of cryptic phenotypic plasticity?  What component of the baroque taxonomy of Cerion populations in the Bahamas might be attributable to ecophenotypic plasticity in their shells [9]?  Our colleagues with expertise in environments such as these are hereby invited to take notice.


[1] 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]

[2] Joe Morrison and the Great Pleurocera Controversy [10Nov10]

[3] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]
  • Mobile Basin III: Pleurocera Puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[4] Goodrich, C. (1937)  Studies of the gastropod family Pleuroceridae – VI.  Occas. Pprs. Mus. Zool. Univ. Mich. 347: 1 – 12.
Goodrich, C. (1939)  Pleuroceridae of the St. Lawrence River basin.  Occas. Pprs. Mus. Zool. Univ. Mich. 404: 1 – 4.

[5] The UMMZ was Goodrich’s home institution from 1924 to 1944.  All the pleurocerid lots curated during this period would have been identified by him, if not collected by him personally.  See “The Legacy of Calvin Goodrich.” [23June07]

[6]  Goodrichian Taxon Shift [20Feb07]

[7] Ortmann actually did this backwards.  In his (1918) “The Nayades of the Upper Tennessee Drainage, with Notes on Synonymy and Distribution” (Proc Am Phil Soc 57: 522) he wrote, “A large number of the ‘species’ described by Lea… and of those listed by Lewis… and subsequently described by various authors…are mostly synonyms.”  And as evidence he cited, “a rule…that one and the same shell assumes different shapes in the large rivers and in small streams and headwaters, a rule the existence of which will be shown elsewhere.”  Then in 1920 Ortmann published his “Correlation of shape and station in freshwater mussels (naiades)” (Proc Am Phil Soc 59: 269-312).

[8]  Wilson, C. B. & H. W. Clark (1914)  The mussels of the Cumberland River and its tributaries.  Bur. of Fisheries 781: 1 – 63.

[9]  I thought Woodruff & Gould pretty much answered this question back in the 1980s (eg, Biol. J. Linn. Soc 14: 389-416).  But Harasewych (Nautilus 126: 119-126) seems not to have been paying attention.