Dr. Rob Dillon, Coordinator

Thursday, December 3, 2020

The Emperor Speaks

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  The Emperor Speaks.  Pp 253 – 260 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

I will begin my essay this month confessing an error that I committed in the late summer of 2018, as relayed to this group two months ago.  Here is a direct quote from my 5Oct20 essay on the “Flat-topped Helisoma of the Everglades”

“So, reading Wetherby’s description in the calm of my office here one sunny morning in Charleston a couple years ago, I was stricken with the impression that the type locality of Helisoma duryi might could use a bit of narrowing-down.  And I swiveled my chair and pulled my well-thumbed copy of F. C. Baker’s (1945) “The Molluscan Family Planorbidae” off the bookshelf.”

That was lazy of me.  I should have consulted Baker's mentor, the Elderly Emperor [1] Dr. Henry A. Pilsbry (1862 - 1957).

His regal ghost still flickered, dimly, through the hollow corridors of the mollusk collection at the Academy of Natural Sciences during my years as a graduate student in Philadelphia.  When I arrived at that venerable institution in the summer of 1977, the first stop on my first tour was the “Pilsbry Chaos,” a pile of boxes, papers, and shells through which curatorial assistants were still laboring, 20 years after the great man’s death. 

From H. B. Baker [2]
He was born on a small farm near Iowa City and seems to have developed his interest in land and freshwater shells at the nearby University, from whence he was awarded his B.Sc. in 1882.  Pilsbry then became a newspaper man, briefly, as was his contemporary Calvin Goodrich [3], moving to New York City as a proofreader in 1887.

He rocketed to malacological stardom almost immediately thereafter, at the age of 25.  On Thanksgiving Day of 1887, Pilsbry was invited to Philadelphia by George W. Tryon [4], who offered him a job as his assistant.  When Tryon died suddenly in February of 1888, Pilsbry inherited Tryon’s position as Conservator the Conchological Section, and Editor of the Manual of Conchology.  Pilsbry sat behind that high desk at the ANSP for 70 years, until they carried him out on a plank [5].

In 1889 he founded “The Nautilus,” the Volume 70 galleys of which were on the cluttered desk around which his plank was wedged.  Frank Collins Baker also came to Philadelphia to work with Pilsbry in 1889, and left the next year profoundly affected [6].  In 1890 Pilsbry organized the American Association of Conchologists, the first of several precursors to the American Malacological Union, of which he was elected first president.  In 1899 he was awarded a doctorate of science by his alma mater, the University of Iowa, the first of three doctorates he was ultimately to receive [7].

Pilsbry’s primary interest was in the North American land snails.  H.B. Baker characterized his (1895) “Guide to the Study of Helices” as “the most brilliantly original, iconoclastic book that ever has been written about the subject.”  Of land snails.  His four-volume “Land Mollusca of North America” (1939 – 1948) is the alpha of the American terrestrial gastropod fauna even unto the present day and may ultimately (I fear) prove to be the omega as well.

But the Elderly Emperor was widely published in freshwater, marine, and fossil malacology as well, from all over the world.  No one can count the sum of his works.  His biographers wrote, “an estimate between 3,000 and 4,000 possibly might cover the number of published articles that flowed from his facile pen.” Paging through the Burch canon [8], I count 57 species or subspecies of North American freshwater gastropods described by Pilsbry surviving even unto 1980, plus eight Pilsbry genera and three Pilsbry subgenera, in eight families.  Accepting Burch’s estimate of approximately 500 species, Pilsbry may be credited with describing over 10% of our freshwater gastropod fauna.  Not bad for a secondary interest.

So early in his career Pilsbry began taking regular vacations to Florida [10].  And in 1934 he published a large and wide-ranging paper in the Proceedings of the ANSP entitled “Review of the Planorbidae of Florida, with notes on other members of the family [11].”  The first 17 pages of that work were subtitled “I. The Large Planorbes of Florida,” which since not followed by a second section subtitled “The Small Planorbes of Florida,” turns out to have been what he meant by “The Planorbidae of Florida” in his main title, screw all those little ones [12].  The second 20 pages of Pilsbry’s 1934 paper were subtitled “Notes on Other Planorbidae,” which turned out to be an ambitious review of planorbid systematics across The Americas, with descriptions of a bunch of new species from three continents.  He described his new genus Australorbis about halfway through that second section, assigning Say’s (1818) glabratus to it, not helping [13].

Paratype lot of H. duryi in the UMMZ [9]

But it is the first half of Pilsbry’s 1934 paper that has brought him to our attention this month.  He began with Helisoma trivolvis, which (of course) is widespread throughout North America, which he allocated to the Dall subgenus Pierosoma.  He then undertook to describe a new subgenus, Seminolina, with Helisoma scalare (Jay 1839) as the type [14].  He also assigned to his new Seminolina two fossil species of Dall (conanti and disstoni) and “the Helisoma duryi complex.”  In the duryi complex he recognized, in addition to Wetherby’s typical subspecies of 1879, intercalare (Pilsbry 1887), preglabratum (Marshall 1926), and three new subspecies: seminole, normale, and eudiscus.  We touched briefly upon all this taxonomic churn back in October.  Sorry to bring it up again.

And regarding the type locality of Helisoma duryi, Pilsbry wrote: “Wetherby’s locality “Everglades of Florida” was vague and doubtless inexact.  I am informed by Mr. Ralph Dury [15] that in the trip of 1874 his father visited places along the coast of Volusia County – Tomoka River, Port Orange, Daytona, Halifax River […] It seems likely therefore that H. duryi was found somewhere along the eastern border of Volusia County.

D’oh!  Back in 2018, with F. C. Baker’s 1945 monograph open in my lap [16], I had convinced myself that a good typical (if not necessarily type) locality to sample H. duryi might be located on the Tamiami Trail at the 40-mile bend.  That is what sent me dodging airboats way down in The Everglades in October of 2018 [17], and that is why I had such high hopes for Cindy Norton’s 2019 breeding experiments [18].  In retrospect, I should have consulted The Elderly Emperor first.

In my own defense, here is the verbatim quote from Wetherby: “This shell was given me several years ago, by Mr. Charles Dury, who brought it from the Everglades of Florida.  It was also among the shells received from the Miami country.”  Volusia County is not in The Everglades, even under the most expansive modern definition of that term.  And Volusia is 250 miles north of Miami, and always has been.

Excuse logged.  Now go back to Florida, Dillon, and do your job right.

Digging into the Pilsbry paper further, it materializes that The Elderly Emperor examined Wetherby’s actual type lot, which Bryant Walker got hold of somehow, which sits in the UMMZ collection to this day.  That set of shells comprises a holotype (UMMZ 83501) and nine paratypes (UMMZ 83502) as figured above.  Pilsbry measured and figured four of the ten, including the holotype, which is where I got “19.5 mm” for footnote [3] of my October post.  And if you can believe it, Pilsbry split one of the shells out of Wetherby’s type lot of Helisoma duryi duryi into his own newly-described Helisoma duryi seminole.  See figure #4 in the Pilsbry montage below.

Wetherby’s type lot. #2 = holotype, #4 = H. d. seminole

Now would be an opportune time, I suppose, to make explicit what has, to this point in my essay, been implicit.  Henry Pilsbry was innocent of the Modern Synthesis.  The only species concept of which he was aware was the nineteenth-century “organism or group of organisms recognized as distinct by a competent taxonomist.”  Which Pilsbry, without question, was.  So, if His Imperial Majesty recognized a species, then it was a species, by definition.

And exactly the same for subspecies.  Under today’s modern synthesis of evolutionary thought, we define subspecies as “populations of the same species in different geographic locations, with one or more distinguishing traits [19].”  Pilsbry never considered that “different geographic locations” thing.  Subspecies were what he recognized as subspecies, just the same as species were what he recognized as species, only with less of whatever that species juice might be.

So although Pilsbry examined the type lot in Bryant Walker’s collection, it materializes that he never had any fresh Helisoma duryi duryi from anywhere in Volusia County in front of him.  Nor did his protégé Baker.  The type locality remained only slightly less mysterious to The Elderly Emperor than to me, reading his words in the calm of my office a couple months ago.

Volusia County is today home to approximately a half-million residents, 122 motels, 5 Walmart Supercenters, and the World Center of Racing.  Next month, we race off to Daytona!


[1] R. Tucker Abbott (1958) coined that sobriquet on page 103 of his contribution to the Pilsbry festschrift: "From the Pilsbry Chair of Malacology."  Nautilus 71: 100 – 103.

[2] I have gleaned most of the biographical details relayed here from Baker, H.B. (1958) Henry Augustus Pilsbry 1862 – 1957.  Nautilus 71: 73 – 83.

[3] Calvin Goodrich (1874 - 1954) was an early-modern malacologist, Pilsbry the paragon of the late pre-modern.  For more, see:

  • The Legacy of Calvin Goodrich [23Jan07]

[4]  We explored the relationship between George Tryon (1838 - 1888) and his immediate predecessor at the ANSP in:

  • Isaac Lea Drives Me Nuts [5Nov19]

[5] Not really, but darn close.  He suffered a heart attack at his desk in September of 1957 and died in his sleep in October.

[6] For a bit of background on my malacological hero, see:

  • The Legacy of Frank Collins Baker [20Nov06]

We will hear much more about the relationship between Baker and Pilsbry in coming months.

[7]  Pilsbry was ultimately awarded doctorates of science by the University of Iowa (1899), the University of Pennsylvania (1940), and Temple University (1941).

[8] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[9] We thank Taehwan Lee of the University of Michigan Museum of Zoology for braving the perils of the worldwide Coronavirus panic to assemble and photograph for us the lovely montage of H. duryi paratype lot 83502 reproduced above.

[10] Here’s a quote from T. L. McGinty (Nautilus 71: 97 – 100):  “Early in 1937, Dr. Pilsbry secured a cottage in Lantana, Florida, and each succeeding winter visit to his Florida home brought the Doctor new friends.”

[11] Pilsbry, H. A. (1934)  Review of the Planorbidae of Florida, with notes on other members of the family.  Proceedings of the Academy of Natural Sciences of Philadelphia 86: 29 – 66.

[12] Helisoma duryi, you may recall from my essay of October, was originally described by Wetherby (1879) as neither large nor small, but rather “medium-sized.”  Pilsbry (1934) folded the medium-sized planorbes in with the large.  I suppose we, the students who follow in the great man’s footsteps, should be grateful.

[13] Here’s a direct quote from H. B. Baker’s Pilsbry obituary [2]:

“Very rarely, when in a Puckish mood, did he (Pilsbry) wield his prestige to establish dubious cognomens; thus he argued against the use of Mesomphix instead of Haplotrema, but contrarily replace Planorbina guadaloupensis by (Biomphalaria) Australorbis glabrata (1934).”

I don’t know what that means, but it sounds important, so feel obligated to pass it along.

[14] John Clarkson Jay (1839) spelled the species name “scalaris” to agree in gender with the feminine Paludina.  Pilsbry re-spelled it scalare when he moved the specific nomen under the neuter noun-construct Helisoma in 1934.  Thanks to Harry Lee for the insight.

[15] We tipped our hat to Mr. Charles Dury in October footnote [4].  His son Ralph E. Dury (1899 – 1984) was Director of the Cincinnati Museum of Natural History for almost 60 years.

[16] Baker, F.C. (1945) The Molluscan Family Planorbidae. Urbana: University of Illinois Press. 530 pp.

[17] If you haven’t read it already, you might be entertained by:

  • The Flat-topped Helisoma of The Everglades [5Oct20]

[18] And as long as you’re reviewing my previous posts, you might as well bring yourself up to date:

  • Foolish Things With Helisoma duryi [9Nov20]

[19] To refresh your memory on the definition of the word “subspecies” as adopted by the FWGNA Project, see:

  • What Is A Subspecies? [4Feb14]
  • What Subspecies Are Not [5Mar14]

Monday, November 9, 2020

Foolish Things with Helisoma duryi

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Foolish Things with Helisoma duryi.  Pp 245 – 251 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

I first met Dr. Cynthia G. Norton in 2005, at the AMS meeting in Asilomar, California.  And I was immediately impressed by her warmth, by her outgoing personality, and by the rigor of her scientific inquiry, which in my long experience, is a rare combination.  She was interested in the same sorts of research questions in Helisoma that Amy Wethington and I were working upon at that time in Physa: mating behavior, sex allocation, and the reproductive biology of simultaneous hermaphrodites generally.  And we have corresponded regularly ever since, cataloguing the similarities in our research findings we have uncovered over the years, which are many, as you might expect.  The most important differences are that Helisoma mate reciprocally, face-to-face, while Physa mate unilaterally, male on top of female, after which swapping may occur.  And quite unusually for basommatophoran pulmonates, Helisoma rarely seem to self-fertilize [1].

In 2009 I lured Cindy into a collaboration with me involving the European Planorbarius corneus, an unpublishable disaster that yielded the sum total of one post on the FWGNA Blog [2].  But I love the quote that Cindy has always used to close her emails, even unto the present day: "You will do foolish things, but do them with enthusiasm - Colette [3]."

So in May of 2018 the mailman came knocking on my door with Volume 36(1) of the American Malacological Bulletin.  I was aware that Cindy had been working with an albino strain of Helisoma trivolvis for several years at that point.  So, I was not surprised to find her note confirming that albinism is inherited simple Mendelian recessive [4].  I think that has pretty much turned out to be true throughout the animal kingdom.  Amy and I had documented two non-complimenting albino loci in Physa back in 1992, which we used as a tool for all manner of interesting studies of reproductive biology [5].

Albino planorbids are much more spectacular than albino physids, however.  Planorbids have re-evolved hemoglobin as a respiratory molecule, and when albinism blocks their normal production of melanin, their bodies are rendered startlingly red [6].  Red-colored planorbids, universally marketed as “ramshorn” snails, may be the most commonly encountered aquarium snail in the world, that somebody actually wants to be in there [7].

The peculiar thing is this.  Nobody seems to know what those commercially-available “ramshorn” snails actually are, or where they might have come from [8].  The most common speculation is that they are Floridian Helisoma duryi, although H. trivolvis is sometimes mentioned in this regard, and Asian Indoplanorbis exustus, and even European Planorbarius corneus.  And indeed, even the relationship between the two American species, H. duryi and H. trivolvis, has simply never been clear.

So the seed of an idea began to germinate.  Might Cindy Norton be lured into another foolish collaboration with yours truly?  In June I wrote her to propose a set of breeding experiments with the two sister Helisoma species, and in light of her motto, I should not have been surprised that she replied affirmatively, with enthusiasm.  Cindy and I roughed out an experimental design involving three lines: her albino H. trivolvis, a stock of Helisoma duryi that I would supply from Florida, and a control (pigmented) stock of H. trivolvis from Charleston [9].  And now you know what, exactly, I was doing dodging airboats in The Everglades in late October of 2018 [10].

Cindy's culture technique
Like most animals with internal fertilization, pulmonate snails can store sperm for a long time – probably their entire lives [11].  Most of the Helisoma I sent to Cindy from Florida in October of 2018 were juveniles and might not be inseminated, I suppose.  But the pigmented wild stock Helisoma I sent her from South Carolina immediately thereafter were adults, and most of Cindy’s albino line were adults as well.  So before experiments could begin, Cindy would need virgins from all three lines.  She put our Florida snails (F) in one tank and our Carolina snails (C) in another and waited for eggs.

And indeed, both the Florida experimental-snails and the Carolina control-snails survived their harrowing late-autumn flights to St Paul, and indeed, both lines began to lay eggs.  And Cindy began to collect and isolate hatchlings from those two lines, and from her already-established albino (A) line as well.  Helisoma reach sexual maturity about 70 – 80 days post-hatch in Cindy’s lab [1], so our experiment entered a second waiting phase.

Alas, Cindy wrote me in January that the Florida hatchlings did not survive.  And in fact, most of the wild-born Florida snails also gave up the ghost in their first couple months under Cindy’s experimental conditions, as well.  The Carolina and Albino lines thrived, as did their lab-born hatchlings, so she felt as though the problem was not temperature, water quality, or any other aspect of her experimental technique.  The Florida snails just did not seem to culture well.  Some snails don’t.

But let’s back up a step.  You will recall that most of the snails I sent Cindy from Florida were juveniles.  On a whim, Cindy had paired 8 of those Florida-born snails with lab albinos, and 8 of the Carolina-born snails with lab albinos, and began collecting eggs from the albinos, looking for pigmented F1.  And here is what she found:

  • From the Carolina x Albino crosses, 5/8 of the albino mothers produced at least some pigmented offspring.  A count of 34 egg masses laid by these five mothers yielded on average 54% pigmented.  The other 3 albino mothers produced only albino progeny.
  • From the Florida x Albino crosses, 4/8 of the albino mothers produced at least some pigmented offspring.  A count of 24 egg masses laid by these four mothers yielded an average of 17% pigmented.  Three of the other mothers laid only albino offspring, and one mother was unproductive.

Yow.  When I read those results back in January of 2019, the first thing that jumped off my computer screen and landed in my lap was the similarity between the 5/8 outcross figure in the CxA control and the 4/8 figure in the FxA experiment.  Pigmented offspring from almost exactly half of eight matings, in both CxA and FxA, really?  And almost as amazing was Cindy’s discovery of mixed-phenotype egg masses from outcrossed mothers of both crosses.

Let me back up two steps and get a running start at this entire experiment.  As I mentioned at the top of this essay, Cindy’s previous research results seem to suggest that Helisoma rarely self-fertilize.  So exactly five of the eight albino snails she paired with Florida snails had previously mated with albino fathers, but nevertheless copulated with Florida snails, and bore some FxA hybrid F1.  And exactly four of the eight albino snails she paired with Carolina snails had previously mated with albino father, but nevertheless copulated with Charleston snails, and bore some CxA hybrid F1.  In mixed clutches [12].  The implied mating compatibility between a Helisoma trivolvis population from South Carolina and a Helisoma duryi population from the Everglades of Florida, bordering indeed on reproductive uniformity, is striking.

So were those hybrid F1 fertile?  Alas, we will never know.  In March Cindy regretted to inform me that all her hybrids expired – both the FxA and the CxA hybrids – which she attributed to problems with temperature regulation in the lab.

Helisoma egg mass, showing mixed phenotypes

Ultimately, the 2018 collaboration that I talked Cindy into with Helisoma duryi ended up being only slightly less foolish than the 2009 collaboration I talked her into with Planorbarius corneus.  One, single oral presentation was the sum total yield [13].  I was not in the audience when she presented our results at the University of Salford in April of 2019.  But I feel sure that she did so enthusiastically.

But wait, there’s more.  About 7 – 8 paragraphs ago I mentioned that “most” of the adult H. duryi I sent Cindy back in October 2018 died in January.  But some did survive, to maturity.  And in fact, Cindy was able to carry a pure culture of H. duryi in her St. Paul laboratory for a couple generations, alongside her pure controls from Carolina.  Photographs of which have been assembled into the montage that opened this essay.

Most of Cindy’s lab-born snails, including the first lab-born generation of H. duryi from Florida, developed typical, planispiral shells.  And a few developed shells with a low apex.  Now look back at last month’s essay.  Their parents, collected from submerged macrophytes in The Everglades, bore that peculiar, elongated (“scalariform” or “physoid”) shell morphology typically associated with H. scalaris.  Let’s explore that phenomenon further, shall we?


[1] Norton C.G. & B.R. Newman (2016)  Growth, reproduction and longevity in the hermaphroditic freshwater snail Helisoma trivolvis.  J. Moll. Stud. 82:178 – 186.

[2] I wrote about my travails bringing Planorbarius corneus into the USA for Cindy’s 2009 experiments in:

  • Non-plants, non-pests, and non-sense at the USDA [17Dec08]

[3] I confess I had to google her.  Sidonie-Gabrielle Colette (1873 – 1954) was the French author best known for her 1944 novella Gigi.

[4] Norton, C.G., A.F. Johnson, and B.M. Nelson (2018)  The genetic basis of albinism in the hermaphroditic freshwater snail Planorbella trivolvis.  Amer. Malac. Bull. 36: 153 – 157.

[5] Dillon, R.T., Jr and A.R. Wethington (1992)  The inheritance of albinism in a freshwater snail, Physa heterostropha.  Journal of Heredity 83: 203 – 210. [pdf]  For more see:

  • Albinism and sex allocation in Physa [5Nov18]

[6] Terwilliger, R. C. (1980)  The structures of invertebrate hemoglobins.  American Zoologist 20: 53 – 67.

[7] I feel sure that Physa acuta is found in more aquaria worldwide than Helisoma, but almost always as a pest.  I also see an awful lot of Melanoides tuberculata populations hiding in aquarium gravels … whether by accident or design is an interesting question.  See:

[8] I reviewed the gastropod fauna of local and online aquarium dealerships, including their planorbid component, in:

[9] No, not Wakendaw Lakes, which we have now featured twice in this blog, originally in [18Feb05] and again in [9Sept20].  The “Wakendaw Lakes” subdivision is in Mt Pleasant, which is a Charleston suburb East of the Cooper River (32.8289, -79.8569).  The control Helisoma trivolvis that I sent Cindy in 2018 (and indeed sent her for earlier experiments as well) was from Charles Town Landing, a state park very near my house West of the Ashley, on the other side of Charleston (32.8073, -79.9897).

[10] If you didn’t read it last month, you might enjoy:

  • The flat-topped Helisoma of The Everglades [5Oct20]

[11] Dillon, R. T., T. E. McCullough, and C. E. Earnhardt. (2005)  Estimates of natural allosperm storage capacity and self-fertilization rate in the hermaphroditic freshwater pulmonate snail, Physa acuta.  Invertebrate Reproduction and Development 47: 111-115.  [pdf]

[12] Norton, C.G. & M.K. Wright (2019)  Strong first sperm precedence in the freshwater hermaphroditic snail Planorbella trivolvis.  Invertebrate Reproduction and Development DOI: 10.1080/07924259.2019.1630019.

[13] Norton, C.G. R.T. Dillon, Jr., K. Tweeten & N. Ezenagu (2019) What is a Species? Biological and Phylogenetic Data in the Genus Helisoma (Planorbella).  Simultaneous and Sequential Hermaphroditic Organisms Workshop, Salford, England.

Monday, October 5, 2020

The flat-topped Helisoma of The Everglades

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  The flat-topped Helisoma of The Everglades.  Pp 235 – 243 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

Spoiler Alert.  This is the beginning of a series on Floridean Helisoma that will span eight additional episodes, continuing on into 2023.  It will materialize that the 40-Mile Bend is not the type locality of Helisoma duryi [3Dec20], nor is "Helisoma duryi" the best identification for the snails I collected there in 2018 [7Feb23].  But the observations I made in The Everglades turned out to be important.  And it's a good story.  Read on.

Albert G. Wetherby (1833 – 1902) was for six years a professor of geology and zoology at The University of Cincinnati.  Then he got burned out and quit [1].  But in 1879 this little-known scientist published a little-known paper in a little-known journal entitled, “Notes on some new or little known North American Limnaeidae [2].”  And it was there that Planorbis (Helisoma) duryi was first described.

Wetherby described the shell as “thick, shining, straw color, of medium size, slightly waved by indistinct transverse ridges… spire very regular, flat or very slightly concave.”  It was not immediately clear what the author meant by “medium size,” as no measurements were offered, and his figure was without scale [3]. Weatherby noted that the shell [in the singular, 3] before him “was given me several years ago, by Mr. Charles Dury [4], who brought it from the Everglades of Florida.  It was also [5] among the shells received from the Miami country.”

Planorbis (Helisoma) duryi [2]

“The Everglades of Florida” is a big place.  The national park of the modern era is a 2,300 square mile wilderness extending over three South Florida counties.  More broadly, the USGS/FWS defines the Everglades Ecoregion as The Kissimmee River, Lake Okeechobee, and drainage fields south, extending over all or part of 18 South Florida counties, for a total of 7,800 square miles [6].  Even a large-sized snail would be difficult to track in such a place.  Let alone medium.

So, reading Weatherby’s description in the calm of my office here one sunny morning in Charleston a couple years ago, I was stricken with the impression that the type locality of Helisoma duryi might could use a bit of narrowing-down.  And I swiveled my chair and pulled my well-thumbed copy [7] of F. C. Baker’s (1945) “The Molluscan Family Planorbidae” off the bookshelf [8].

As my faithful readership will recall,  Baker’s overly-ambitious monograph was published posthumously, Part I (“Classification and General Morphology”) being left incomplete.  But under the Subfamily Helisomatinae (F. C. Baker 1928), genus Helisoma (Swainson 1840), Subgenus Seminolina (Pilsbry 1934) Baker was able to catalog, prior to his departure for the cloud of witnesses malacological, seven valid taxa: scalare (Jay), preglabratum (Marshall), and five subspecies of duryi.  For the geographical distribution of all seven Seminolina taxa together, he wrote, “As far as known, this group is found only in the peninsula of Florida north to Bradford County [9].”  

Everglades [10], Tamiami Trail in red

The five subspecies of duryi included Wetherby’s typical form and four added by Pilsbry [11]: normale, seminole, intercalare, and eudiscus.  Baker did not distinguish the typical form of duryi from Pilsbry’s subspecies normale, and in fact, does not appear to have examined any typical specimens at all.  But the first locality that Baker listed for Helisoma duryi normale was “Tamiami Trail, 40 miles west of Miami.”  OK, I thought to myself, that’s it.  As a typical locality for Helisoma duryi, if not necessarily the type locality of the species, that should be good enough.

The Tamiami (Tampa-to-Miami) Trail has a longer and more interesting history than one might think.  Work on the initial sections of an automobile highway across the Florida peninsula began in 1915, if you can believe it, just 7 years after Henry Ford debuted his Model T.  The most spectacular section, running east-west across The Everglades, was built between 1923 and 1928, construction teams blasting a canal through the marsh bedrock and raising a roadbed with the fill.  A significant engineering discrepancy that developed between the eastbound and westbound teams was corrected with features now known as the 40-Mile Bend and the 50-Mile Bend, initial plans for a 45-Mile Zigzag ultimately falling out of favor.

Today the 40-Mile Bend area is home to the Miccosukee Tribe of Indians, who operate a museum, a restaurant, a gift shop and a general store.  And airboat rides.  Mom, Dad, and The Kids will find ample opportunity along this entire stretch of the Tamiami Trail to tour the Everglades on airboats of all design and description.

 So Sunday afternoon 21Oct18 I gave my wife a peck on the cheek and pointed my trusty [12] Mazda pickup south toward The Everglades.  And Monday afternoon found me turning off the Tamiami Trail into the 40-Mile Bend boat ramp parking lot.  I emerged blinking and stretching.  And has long been my habit, stuck my hands into my pockets and walked over to the water’s edge to conduct a preliminary assessment.

And what I saw was utterly unlike anything I have ever seen at the end of any other boat ramp any other place in my entire life.  The water was crystal clear to the bottom.  How deep is it in there, I wondered, five feet?  More?  It was impossible to judge at such a sparkling clarity.  And hundreds of bass, bream, and carp, all with their fishy little hands stuck in their fishy little pockets, looked back up at me, hungrily.  With a bamboo pole and a dozen crickets, I could have fed myself for a week.

40-Mile boat ramp

My curiosity thus piqued, I launched my kayak and paddled off into an utterly foreign world.  Heaven knows I have boated hundreds of marshes, swamps, and wetlands of all sorts over the entire eastern USA in my long life.  Without exception, all have been soft-bottomed, muddy, and filled with decaying vegetation – in a word, swampy.  But here I found myself floating over a bottom of limestone rock, through a non-swampy wetland.

True, I did launch into that canal alongside the roadway, which must have been excavated for fill rock.  But even as I paddled out into the marsh, I could look down through crystal-clear water and see solid substrate.  In fact, at one place where the bedrock approached the surface, I got out of my kayak and walked.  On rock, in a wetland!  Little bluegills and bream nibbling the hairs on my leg.  Otherworldly.

And a third remarkable aspect of the Everglades environment, beyond the water clarity and the substrate, dawned upon me as I paddled.  I could find no floating aquatic vegetation whatsoever – no water hyacinth or duckweed or Elodea or Hydrilla or anything that looked like Elodea or anything that looked like Hydrilla.  Floating macrophytic vegetation, so common everywhere else in my many years of kayaking experience, is what I have always thought of as typical Helisoma habitat.  But the aquatic jungle through which I was paddling that afternoon was rooted-emergent and rooted-submerged only.

I found my Helisoma hiding deep in the rooted, submerged macrophytes.  Putting to work the net I mount tied to the stern of my kayak [13], I dipped in the crystal clear water at arm’s length, ran through the grass and weed beds, and with a bit of effort, was able to collect a decent sample of Helisoma.  The snails were almost entirely juveniles, invisible to me at the surface and (more evolutionarily important) to the schools of hungry bream, which I feel sure would have immediately picked off any stray pulmonate foolish enough to raise a tentacle.  They were quite pale in their body coloration – light grayish or even whitish [14].  Perhaps they emerge to graze at night, I thought to myself.

Suddenly my reverie was interrupted by a distant, but fast-approaching roar.  And my thoughts were jerked rudely back sixty years from the warm, sunny October day I was then enjoying in my exotic little patch of paradise.  And cast to the family room of my modest home in Waynesboro, Virginia, and to a favorite television show of my youth.  Lincoln Vail of “The Everglades.”  Airboat coming!

My kayak and I were at that juncture floating invisibly in tall, emergent grasses.  And the thunder was immediately upon me, as quick as I could raise an orange paddle.  Lincoln Vail and his family of passengers veered abruptly to my port side and missed me clean (literally clean) by maybe ten feet.  But the prop wash – or whatever you call the spray those gigantic airboat motors kick up behind them, threw a 40-gallon rainstorm over me, horizontally, in the blink of an eye.

Back at the boat ramp, toweling off, I found opportunity to reflect upon the biological observations of the afternoon.  Every Helisoma I had collected during the previous three hours had been netted from macrophytes submerged in water at depths no less than two feet, with no approach to the surface ever in evidence.  Nor would there seem to be any rationale for an individual Helisoma to approach the surface, given the absence of floating macrophytic habitat in the environment I had just been bathed in.  Nor (indeed) would it be safe even to expose oneself to the surface, if one were a snail of that predilection, given the predation risk from the ravenous schools of bream.  This population of Helisoma must be entirely benthic.  I did not gather any experimental confirmation, but I’ll bet dollars to donuts that the mantle cavities of every individual planorbid I collected that afternoon were 100% filled with water.  No air pockets.

Helisoma from the 40-mile Bend

Now a second observation followed from the first.  I had found perhaps 40 – 50 individual Helisoma, only about 10% of which seemed to be adults.  But those adults did not demonstrate the planispiral shell morphology typically associated with Helisoma duryi, as depicted in Wetherby’s original figure way up above.  Rather, they retained the elongated, obviously-sinistral "scalariform" or "physoid" shell morphology that has come to be associated with the other medium-sized planorbid of The Everglades, Helisoma scalare.  Might the relationship between Helisoma duryi and Helisoma scalaris find analogy in the Helisoma population of Charleston’s Wakendaw Lakes [15]?

By 3:30 I had loaded my kayak back into my truck, exited the 40-Mile parking lot, and turned east toward the rush hour traffic of Miami.  And by 4:45 I was walking in the door of the FedEx shipping center at 21st Street with a box of Helisoma cradled under my arm [16].  The cost of overnight delivery to St. Paul, Minnesota, turned out to be $99.35.  Ouch.

Next month, our story follows that box.


[1] Harper, G. H. (1902) Albert G. Wetherby.  Nautilus 16: 10 – 12.

[2] Wetherby, A.G. (1879)  Notes on some new or little known North American Limnaeidae.   The Journal of the Cincinnati Society of Natural History 2: 93 – 100.

[3] It materializes that there were nine shells in Wetherby’s type lot, and that the diameter of the holotype shell was 19.5 mm.  We’ll follow up in a later post.

[4] Charles Dury (1847 – 1931) was a Cincinnati-area naturalist of the Old School, primarily interested in insects.  His obituary was published in the Ohio Journal of Science 31: 512 – 514.

[5] Interestingly, Prof. Wetherby seems to have been drawn into the subject of planorbids, generally, by the chronic, centuries-old confusion over what exactly is (or was) the Planorbis glabratus that Thomas Say had in his hand when he wrote his description in 1818.  Weatherby thought that most of the planorbids in his “large collection of shells from the Miami country of Florida” might be P. glabratus.  Which is why he wrote "also" here in his description.  This subset is NOT Planorbis glabratus.  Although I didn't pursue this topic any further in 2020, I did come back to it on [7Mar23].

[6] Bailey, R. G. (1980)  Description of the Ecoregions of the United States.  USDA Forest Service Misc. Publication No. 1391, 83 pp

[7] My copy is autographed “Charlotte Dawley Sept. 1950.”  It was then stamped “Rowland M. Shelley.”  It then passed to W. F. (Bill) Adams, who gave it to me in 2007, when he retired from the Wilmington office of the Corps of Engineers.  Thank you, Bill, wherever you are.

[8] Baker, F.C. (1945) The Molluscan Family Planorbidae. Urbana: University of Illinois Press. 530 pp.  For more about Baker and his remarkable work, see:

  • The Legacy of Frank Collins Baker [20Nov06]
  • The Classification of the Planorbidae [11Apr08]

[9] Bradford county is situated further north than the area covered by the map of the Everglades I have reproduced above.  But Baker’s restriction of Seminolina to areas south of Bradford County will become important in essays to follow.

[10] By Kmusser - Own work. City and Federal lands data source: National Atlas. County and urbanized areas data source: U.S. Census Bureau. Hydrology data source: National Hydrography Dataset. WCAs, EAA, and Management District boundary source: South Florida Water Management District. National Marine Sanctuary data source: NOAA, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9034125

[11] Pilsbry, H. A. (1934)  Review of the Planorbidae of Florida, with notes on other members of the family.  Proceedings of the Academy of Natural Sciences of Philadelphia 86: 29 – 66.

[12] Well, five of those letters are correct, anyway.

[13]  Here’s how to do it:

  • Collecting freshwater snails by kayak [13June11]

[14] I think it is probably a general rule, throughout the Animal Kingdom, that melanin production is induced by light.  I think many of those little Everglades Helisoma may have spent their entire lives in darkness – grazing on macrophytic tissues and detritus deep in the weeds, terrified by the bream.

[15]  The present essay fits somewhere in the middle of an extended saga chronicling my own personal struggle for biological understanding of the large Helisoma.  For background on the Wakendaw Lakes population, see:

  • Shell morphology, current, and substrate [18Feb05]
  • Juvenile Helisoma [9Sept20]

[16] Somewhat amazingly, to me in any case, I was forced to detour by some random drainage pond in Miami to net up water weed to pack my Helisoma in.  I couldn't find any water weed suitable for packing anywhere in The Everglades.

Wednesday, September 9, 2020

Juvenile Helisoma

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Juvenile Helisoma.  Pp 227 – 233 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other Essays FWGNA Project, Charleston, SC.

Before launching into the subject of this month’s essay, I do want to emphasize that I always enjoy hearing from you.  I’m retired, and bored, and I (honestly!) do not have a whole lot better to do than sit at my computer and correspond with colleagues from around the world, about a subject that has fascinated me since childhood.  So let’s open up the Ol’ Mailbag and see what the postman brings.  Click the captions under the thumbnail jpegs to see the larger, original images:

Greetings Dr. Dillon:

Click caption
We have collected an aquatic snail that has puzzled me for years (photo attached).  At first, it seemed unique enough that we could record it without paying much attention to biogeography.  We used the genus Planorbella (based on Burch’s EPA publication) to document the counts when we collected it during bioassessment work.  Since I began working on aquatic snail distribution, I have tried to get better at snail IDs and to possibly learn a little.

Thanks for any feedback that you might provide!
[Baffled in Missouri]

Hello Dr. Dillon,

Click caption
I hope this email finds you well.  I'm afraid I'm cold-emailing you (if such a thing exists) to ask for advice re: some gastropods I'm keying out as a small cog of a [federally-funded] monitoring program […] Finally, my other big headache is my stubborn inability to feel comfortable IDing to genus the small Planorbidae down to genera.  I had a weird Helisoma this year, with bizarre whorling too. It happens, I guess! […] So I'd like to ask if you'd be kind enough to peek at a few pictures (in .zip form) I've attached of stubborn-to-ID snails.

Much obliged,
[Bothered in Indiana]

Dear Robert,

Click caption
You may remember me. I contacted you about [another question] in 2011.  I have another riddle to submit to you.  We sampled those gastropods in a small river in Quebec.  We’ve never seen that before. It looks like some Amerianna or Planorbella that are not present in our region.  Can you tell me the identification? They can come from an aquarium?

Thanks for your help,
[Bewildered in Quebec]

Dear Baffled, Bothered, and Bewildered,

The snails depicted in all your jpegs are juvenile Helisoma trivolvis.  They don’t look much like their parents, do they?

Their most striking feature is that flat apex, when viewed sinistrally, which is the way you have all depicted them, which is correct.  As the shell grows the later whorls cover up that flat apex, so it it disappears.  The adult shell doesn't seem to have any apex at all, which is another way of saying that the adult shell is "planispiral."  Bottom line is that, several weeks after hatch, juvenile Helisoma trivolvis seem to switch rather dramatically and mysteriously from obviously-sinistral to just-plain-flat.

Helisoma anceps, by contrast, has an indented apex when viewed sinistrally, even as a juvenile.  Indeed, the apex of H. anceps is indented no matter how you look at it, which makes anceps shells pretty darn near perfectly-planispiral.  Just-plain-flat from birth.

Both H. trivolvis and H. anceps can be found in a wide variety of habitats, but H. trivolvis is a better floater, and is more common in lentic environments, especially in macrophytic vegetation.  Helisoma anceps is more common in lotic environments – especially in calm backwaters – generally grazing on solid substrates.  See the figure below for a comparison of juvenile H. trivolvis, H. anceps, and H. campanulatum, from up lakes north, included here for completeness.

Have you ever heard the old saw [1], “Ontogeny recapitulates phylogeny?”  Planorbids evolved from a left-handed ancestor that probably looked something like a modern physid.  Through subsequent selection they have evolved planispiral shells – possibly so that the air bubble enfolded by their shells forms a more stable float – and in the adults of many species it is now difficult to see which way their shells are coiling, left or right.  Usually.  But there’s a big asterisk to that generalization.

More to follow!

The shells of juvenile Helisoma
At this point in my essay, allow me to speak directly to you, the readership of this blog, rather than as a correspondent to a third party.  I did a bit of a disservice to our colleague Baffled-in-Missouri at the top of this essay.  His email continued with a lot of excellent insights and additional questions, from which I have extracted the following:
“If you have the time and interest to respond, I would like your opinion about the following speculations: Following your modified classification of Hubendick [2], I would lean toward calling the snails in my photo Helisoma scalare, or possibly Helisoma duryi.  Since I now realize that these species are from Florida, I would have to suspect an introduction to Missouri.  There seems to be several scientific journal articles to support the idea of a possible aquarium introduction of these species worldwide. […] The other possibility is one you have written about many times.  Do you think phenotypic plasticity is a possibility?  Many times, we also have co-occurring records of Helisoma sp. that have the more typical form.  This form always seems to be the more mature individuals.  Even if not fully grown, the typical form is always much larger that the Helisoma scalare form.”
Here Baffled-in-Missouri has broadened the subject in an interesting direction – adult shell morphology.  He is referring to an essay I contributed to this blog way back in 2005, sharing my observations on a single Helisoma population inhabiting two strikingly-different environments in the “Wakendaw Lakes” subdivision on the other side of the Cooper River from Charleston [3].  It might help to open that essay in another window and keep my photo of that study area handy [15Feb05].

While the Helisoma inhabiting the little retention pond upstream demonstrate typical shell morphology, those that have colonized rip-rap rocks in the flowing-water environment below the pond retain their flat-topped, obviously-sinistral juvenile morphology into adulthood.  Here’s an improved version of the figure I originally published in 2005, which I fixed up for my 2019 book [4]:

Helisoma population of Wakendaw Lakes
That’s quite a vivid demonstration of ecophenotypic plasticity [5], isn’t it?  Both shells are from adults, photographed at the same scale.  Snails in the pond above the dam are grazing in the macrophytic vegetation, using their shells as buoyant floats, like normal.  Snails below the dam are grazing on rocks, holding their shells low on their backs against the current.

And here is the most interesting thing about this phenomenon, to me, anyway.  The snails on the rocks, retaining their juvenile shell morphology into adulthood as they do, look sinistral, as planorbids actually are.  But the Helisoma inhabiting the pond, have (as is typical for the species) flipped their shells so far across their backs that they seem to have gone beyond planispiral to dextral.  Typical pond-dwelling Helisoma "look correct” the way I have depicted them above.  I’m not sure why this is so, but turn your computer monitor upside down and look at the  pond-dwelling snails again if you don’t believe me.  Or just look back at the original figure in my 2005 post, where the typical Helisoma shell looks like it's upside down.

That, by the way, is why “Planorbella” is (at best) a subgenus under Helisoma.  The distinction (originally drawn by Baker [6]) has to do with whether the adult is apparently right-handed or apparently left-handed, a trait which can vary within a single population, ecophenotypically.

So Baffled-in-Missouri also brought up the question of Helisoma scalaris/scalare and Helisoma duryi, which I myself also touched upon in 2005.  Those are Florida species, primarily distinguished by their flat-topped, sinistral-looking shell morphology carried into adulthood.  What, exactly, are Helisoma scalare and Helisoma duryi?  Tune in next time.


[1] That phrase, originally coined by Ernst Haeckel, has pre-Darwinian roots.  It was appropriated by twentieth-century philosophers and charlatans (most notably Stephen Jay Gould) and twisted every way it could possibly be twisted, to the point that nobody knows what it means, much less whether Haeckel’s theory has any validity or not.  Wikipedia has a pretty standard review if you want to google it.

[2] Hubendick, B. (1955) Phylogeny in the Planorbidae. Trans. Zool. Soc. London 28: 453-542.  For a modern elaboration, see:
  • The Classification of The Planorbidae [11Apr08]
[3] More about the Helisoma population of Wakendaw Lakes:
  • Shell morphology, current, and substrate [18Feb05]
[4] My 18Feb05 essay was subsequently published as:
Dillon, R.T., Jr. (2019b)  Shell morphology, current, and substrate.  Pp 121-126 in Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston. [FWGNA Publications]

[5] This present essay is the 28th I have published on the ever-fascinating subject of ecophenotypic plasticity in freshwater gastropod shell morphology.  Hit the “Phenotypic Plasticity” label in the margin at right if you don’t believe me.  My series on the stagnicoline lymnaeids is probably the most relevant.  Start at the end, here:
  • The Lost Thesis of Samantha Flowers [3Sept15]
[6] Baker, F. C. (1945) The Molluscan Family Planorbidae. University of Illinois Press, Urbana. 530 pp.  For more about my hero, see:
  • The Legacy of Frank Collins Baker [20Nov06]

Monday, August 3, 2020

Mitochondrial heterogeneity in Marstonia lustrica

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b) Mitochondrial heterogeneity in Marstonia lustrica.  Pp 193 – 201 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

A raised-letter certificate of appreciation, with wax seal and red ribbon, is hereby awarded our good friend Tom Coote for his important contribution to the general understanding of mtDNA sequence divergence in the freshwater gastropod fauna of North America.  The paper he based on his 2011 dissertation [1], published late last year in Northeastern Naturalist [2], offers us the best survey of intraspecific mtDNA sequence variation among hydrobioid populations as yet available, anywhere [3].  His results will surprise many and enlighten us all.

Populations of the (relatively large-bodied) Marstonia lustrica can reach locally high densities on rocks and macrophytes in big lakes and other cold, clear bodies of water above the glacial maximum.  In recent years they have disappeared from many habitats where they were once common [eg Oneida Lake, 6] and hence have been the object of some conservation concern.  Our buddy Tom obtained COI and NDI sequences from 20 M. lustrica populations scattered across six northern states and one Canadian province.  He recorded at least one COI sequence from 18 of those 20 populations, for a total of 54 sequences.  And he recorded at least one NDI sequence in 14 of the 20, including the two populations that he missed with the COI, for a total of 41 sequences.

Let us first look at Tom’s COI results.  That big black triangle at the top of his maximum likelihood COI tree below (labelled “modal”) represents the most common sequence, and variants of no greater than 0.5%, which he recorded in 39 of his 54 individuals.  Tom documented at least one copy of the modal sequence in 15 of the 18 populations for which he had any COI data.

Sample sites, from Coote [2]

Now look immediately below the black triangle.  The three sequences marked “HLMN,” from Harriet Lake, Minnesota, differed from the modal sequence by 1.4%.  And the sequences marked KBMI (B and C, from Keeweenaw Bay, in Lake Superior) differed from the mode by 0.5%.  And the sequences marked LLMN, from Limestone Lake, MN, differed from the mode by 2.6%, and so forth.  The maximum divergence from the mode was 4.5%, recorded by two sequences collected at Grand Island, New York (GINY-A and GINY-D) and the one collected in Indiana’s Tippicanoe River (TRIN).  The maximum COI divergence anywhere in Tom’s dataset was 5.7%, between GINY-A and IRMI-A, from Isle Royal in Lake Superior.

 Now perhaps the most striking features of Tom’s COI tree are the three branch tips circled in red.  These are not Marstonia lustrica.  These tips are occupied by sequences fished from GenBank for Marstonia pachyta (1.4% from modal M. lustrica), M. comalensis (3.0% from modal M. lustrica) and M. hershleri (2.7% from modal M. lustrica).  These three sequences, recorded from Marstonia populations in Alabama, Texas, and Alabama respectively, are nested inside Tom’s larger Marstonia lustrica COI tree.

Does this mean that Tom Coote has discovered a big swarm of new Marstonia species?  Does his TRIN/GINY sequence for example, 4.5% different from modal M. lustrica, sitting on its own branch of the tree, way outside three other good Marstonia species from Alabama and Texas, reveal to us the existence of an undescribed species of hydrobiid lurking in the Niagara River of upstate New York, cryptic under Marstonia lustrica?  No, of course not.

Because Tom obtained four COI sequences from the GINY population, and two of them (GINY-B and GINY-C) were modal.  The Marstonia lustrica population on the Grand Island shores of the Niagara River is heterogeneous for (at least) two COI sequences – the common one that is found everywhere across six northern states and a rare one, 4.5% different, that matches – if you can believe it – a population in Indiana.

COI maximum likelihood tree, from Coote [2]

Well, then.  How about that sequence Tom found at Moira Lake way up in Ontario (MLCA-B), the one that seems to match a Marstonia hershleri haplotype collected down in the Heart of Dixie?  Maybe Moira Lake is inhabited by an Alabama species?  No, of course not.  Because again, Tom obtained three COI sequences at Moira Lake, and two of them (MLCA-A and MLCA-C) were modal.  The Moira Lake population is heterogeneous for two COI haplotypes differing by 2.4%.

What could that mean?  Maybe Tom’s NDI results will cast some light on his peculiar COI data.  He did not publish his NDI results separately, preferring to offer us a concatenated COI+NDI network in his Northeastern Naturalist paper.  But the figure down below shows the NDI (only) neighbor-joining tree from his dissertation [1].

Good grief!  Sticking way out on a distant limb of Tom’s NDI tree we find the same three sequences that were sticking way out on that distant limb of his COI tree: GINY-A, GINY-D, and TRIN-A.  And again, GINY-B and GINY-C are lumped into the mode.  And there’s MLCA-B sticking way out as well, again with MLCA-A and MLCA-C in the mode.  Could the striking similarity of Tom’s COI and NDI trees be a coincidence?

No, of course not.  The two M. lustrica data sets we are comparing here correspond precisely as the COI and 16S data sets that Nathan Whelan and Ellen Strong developed for pleurocerids in Alabama [7] and published in 2016.  And indeed, Dillon & Frankis documented an identical correspondence between COI and 16S haplotypes in Pleurocera proxima of the Southern Appalachians way back in 2004 [8].

At this point I would invite you to open a new browser window and re-read my blog post of 15Mar16, entitled “Mitochondrial superheterogeneity: What we know.”  In that 2016 essay I reviewed the striking patterns of mtDNA sequence divergence reported by Whelan & Strong in North Alabama pleurocerids and compared them to a set of 18 other such studies published for a variety of other gastropod populations both terrestrial and freshwater.  Of the six bullet-points that Whelan & Strong confirmed for us (or taught us!) about mitochondrial superheterogeneity (mtSH), let me call your attention to #2, “peculiar patterns of interpopulation haplotype sharing” and #3, “tight coupling in the evolution and distribution of separate mitochondrial genes.”  Do those phenomena look familiar?

NDI Neighbor-joining tree, from Coote [1]

The patterns in mtDNA sequence divergence in Tom Coote’s M. lustrica populations correspond strikingly to the patterns that Whelan & Strong documented in the Leptoxis populations of North Alabama.  The only difference is one of scale: Marstonia lustrica haplotypes only vary to a maximum of 4.5% within populations, while intrapopulation sequence heterogeneity in Alabama pleurocerids ranges up to 20%, and elsewhere in the southern Appalachians up to 21.9% [9].

Marstonia populations sampled above the glacial maximum are younger than the pleurocerid populations of the American South.  I feel sure that M. lustrica, the original population of that species, diverged from whatever its ancestor might have been prior to the Pleistocene, probably up north somewhere, and that it spread, and that isolated populations diverged in a fashion not unlike the pleurocerids.  Glacial advance may well have extinguished the species through most of its range, to survive only in isolated refugia.  Populations have then subsequently re-colonized the northern latitudes, but not in a smooth wave rolling from south to north.  Rather, individual colonists have been airlifted north from genetically divergent refugia in a chaotic and unpredictable fashion, some lakes skipped and other lakes colonized twice.

I would now invite you to open another browser window and re-read my essay of 6Apr16, “Mitochondrial superheterogeneity: What it means.”  My “wildebison model” will be easy to recognize in the paragraph above.

And finally, I would invite you to review my essay of 3May16, “Mitochondrial superheterogeneity and speciation.”  Under the wildebison model of mtDNA genome evolution, we now understand that results such as Tom Coote has reported for COI and NDI do not call into question the specific identity of scattered Marstonia lustrica populations.  Just the opposite.  Strikingly divergent haplotypes such as he has labelled GINY-A and TRIN on the distant branches of his gene trees constitute direct, positive evidence that the Marstonia populations inhabiting the Grand Island shores of the Niagara River in upstate New York and the Tippencanoe River of Indiana are conspecific.

One last point, and then a summary, begging your indulgence.  All of my 2016 essays were directed toward the phenomenon of mitochondrial superheterogeneity, which I defined on 15Mar16 as follows: “A population can be said to demonstrate “mitochondrial superheterogeneity” when two or more of its members demonstrate 10% sequence divergence or greater for any single-copy mtDNA gene, not sex-linked.”  Well, Tom Coote’s Marstonia populations do not display mtSH by that definition.  Their maximum intrapopulation divergence (GINY-A and GINY-B) is just 4.5%.  So, let us call intrapopulation sequence divergence greater than 2% but less than 10% “simple mitochondrial heterogeneity” (mtH) nothing super about it, shall we?  I think future research will find mtH to be the rule in freshwater gastropod populations, not the exception.

Gene trees are dependent variables, not independent.  Examined in isolation, they can lead the naïve researcher to terrible misunderstandings about the evolution of the group which he studies.  But bringing a robust understanding of the evolution of the group to his study, a wise researcher can peer through the window of a gene tree to find evolutionary processes of great generality and importance.


[1] Coote, T. W. (2011) The phylogeography of Marstonia lustrica: Understanding the relationship between glaciation and the evolution and distribution of a rare snail.  Ph.D. dissertation, University of Massachusetts, Amherst.  Open Access Dissertations 399. [html]

[2] Coote, T. W. (2019)  A phylogeny of Marstonia lustrica (Pilsbry 1890) (Gastropoda: Hydrobiidae) across its range.  Northeastern Naturalist 26: 672 – 683.

[3] Well, maybe I should back up and qualify my opening paragraph just a little bit.  Tom’s research involved more populations of any single hydrobioid species than have ever been heretofore surveyed, although not more individuals.  Tom Wilke and colleagues did publish, back in 2000, a survey of COI sequence variation across ten populations of the medically-important pomatiopsid Oncomelania hupensis sampled from SE Asia, ten individuals per population [4].  The maximum sequence divergence recorded in the Wilke study was a not-insubstantial 2.2%.  Here in North America, the largest study of intraspecific mtDNA sequence variation for any hydrobioid published to date was that of Hershler and colleagues [5], involving 12 Idaho populations of Taylorconcha serpenticola, 5 individuals per population.

[4] Wilke, T., G.M. Davis, C-E. Chen, X-N. Zhou, X.P. Zhang, Y. Zhang & C.M. Spolsky (2000) Oncomelania hupensis (Gastropoda: Rissooidea) in eastern China: molecular phylogeny, population structure, and ecology. Acta Tropica 77: 215-227.

[5]  Hershler, R., H-P. Liu, T.J. Frest, E.J. Johannes & W.H. Clark (2006) Genetic structure of the Western North American aquatic gastropod genus Taylorconcha and description of a second species. Journal of Molluscan Studies 72: 167-177.

[6] For more about the lost malacofauna of Oneida Lake, see:

  • Harman, W. N. & J. L. Forney (1970) Fifty years of change in the molluscan fauna of Oneida Lake, New York.  Limnology and Oceanography 15: 454 – 460.
  • Dillon, R. T., Jr. (1981)  Patterns in the morphology and distribution of gastropods in Oneida Lake, NY, detected using computer-generated null hypotheses.  American Naturalist 118: 83 – 101.

[7] Whelan, N.V. & E. E. Strong (2016)  Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.  With subscription: [html]

[8] Dillon, R. T., Jr. & R. C. Frankis. (2004)  High levels of DNA sequence divergence in isolated populations of the freshwater snail, Goniobasis.  American Malacological Bulletin 19: 69 - 77.  [PDF].

[9]  Dillon, R. T., Jr. & J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11. [PDF].  For a personal perspective, see:

  • The snails the dinosaurs saw [16Mar09]