Dr. Rob Dillon, Coordinator

Tuesday, January 5, 2021

Collected in Turn One

If you’re just joining us.  This is the fifth essay in a long-running series on planorbids of the genus Helisoma in Florida. You really should be familiar with last month’s essay [3Dec20] before going forward, and it would help if you backed all the way up to 9Sept20 and read forward through 5Oct20 and 9Nov20 as well.

Is it possible for anyone alive today to visualize the lush and tangled jungle that must have greeted Mr. Charles Dury as he explored “places along the coast of Volusia County” in 1874?  Exiting I-95 at the US 1 interchange toward Ormond Beach in the late summer of 2020, I myself most certainly could not.

On the clipboard riding in the passenger seat beside me was a copy of Pilsbry’s (1934) review of Wetherby’s (1879) description of Helisoma duryi [1], “given to me by Mr. Charles Dury.”  And here is the Pilsbry quote I had circled in red:

“I am informed by Mr. Ralph Dury that in the trip of 1874 his father [Charles] 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 [2].”

You, my readership, are now informed by Dr. Robert Dillon that the entire eastern border of Volusia County is, today, one enormous, congested sprawl.

My plan was to focus on the historic drainage of the Tomoka River, which like most of the Atlantic side of Florida, has been diked and filled by intensive development activities spanning many, many years.  The water was a bit brackish at my first stop, near the Ormand Beach airport (point X), so I drove a couple miles inland to the borrow-pit lakes at Ormond Beach’s Central Park (O).

Eastern Volusia Co, FL
And I must say that I was pleasantly surprised by the freshwater gastropod diversity [3] that greeted my eyes in the shallow, weedy margins of Ursa Minor Lake (29.2728, -81.0721).  The Helisoma population was sparse, however, and I only found a couple adults, and so I moved another 10 km south to a network of ditches draining toward the Tomoka River at Daytona (Point D).

Notice the checkered-flag motif on the wall above the ditch that I here offer as the H. duryi type locality [4], figured below.  Squatting down and dipping through the weeds, the adult Helisoma that met my eye would most certainly have been characterized by Henry Pilsbry as “large planorbes,” diameter ranging up to 2.54 cm that morning in August.  Standing up, I could see the Turn 1 grandstands of Daytona International Speedway.

I’m a NASCAR fan [5].  The relationship between snail collecting and stock car racing is exactly the same as the relationship between science and public policy.  Not compatible, but not incompatible either [6].

Essentially all the adult Helisoma I found alive in the eastern Volusia County region that Charles Dury apparently visited in 1874 seemed to bear slender or narrow shells with tight coils – significantly more slender than the figures of the type lot published by Pilsbry, see [3Dec20].  This was true both at Ormond Beach and at Daytona.  I think this may be the weedy, ditchy shell morphology.  But on the bank of the ditch at Daytona I found one relict shell that seems to match Pilsbry’s figures very nicely.  Might this be a memorial to what the eastern Volusia county environment looked like, 146 years ago?

So when I got home to Charleston, I dumped my fresh samples of bona fide Helisoma duryi out on the lab bench, got out the scope and looked at them real hard.  And I also pulled a nice batch of Helisoma trivolvis out of my collection from all over North America, including a topotypic sample I collected from an impoundment of French Creek way up in NW Pennsylvania in 2008 [7].  And the distinction, to be precise, is not shell form.

Ditch at Daytona [4]

In overall appearance, Helisoma duryi shells are short, tall, fat, skinny, and all over the place.  If the figure below does not convince you of that observation, look back at the figure I posted on [5Oct20], of shell morphology in H. duryi collected at the 40-mile bend, deep in The Everglades.  And compare those shells to the shell figure I posted on [9Nov20], depicting the shell morphology developed by that same population in culture.

For a while, I thought that I might be able to detect a difference in the tightness of the coil.  Some planispiral populations of H. duryi seem to demonstrate significantly more whorls to reach a given shell diameter than one ever observes in H. trivolvis (O and D below).  But again, look at the relic H. duryi shell, which matches Wetherby’s type.  The tightness of that shell coiling is not detectably different from H. trivolvis.

But every authority I have ever read has always mentioned, somewhere early in his description of H. duryi, something about shell shininess.  Wetherby [1] wrote, “Shell thick, shining, straw color, of medium size.”  For his new subgenus Seminolina, Pilsbry [2] wrote “The smooth or malleate surface is not thread-striate, usually glossy.”  Baker [8] agreed, saying “Surface smooth, usually glossy, without the threadlike striae of Pierosoma.”  The first character Thompson [9] offered us in his couplet #86 of dichotomous key was “shell dull” vs. “shell glossy.”  The former leads us to H. trivolvis, the latter to H. duryi.

I’ve read those words many times in the past, and the distinction between dull and shiny/glossy has never been clear to me.  Some of the bona fide H. duryi shells lying in piles on the lab bench before me were certainly shiny or glossy.  But some (like shell D below) most certainly were not.  And some H. trivolvis shells seem sort-of shiny, maybe.  Shininess is not measurable by any equipment conventionally available, and the cut point between duryi and trivolvis in international-shell-shininess-units has never been calibrated by any prior worker, in any case.  I needed something more.  Something objective.

O = Ormond, D = Daytona, Dr = relict

So what about those “thread-like striae?”  In the figure below I have collected all four of the images I published back in September, depicting juvenile Helisoma trivolvis.  And I have compared them to images of juvenile Helisoma duryi, collected at the 40-mile Bend, at Ormond, and at Daytona.  Also Lake Munson, way up in North Florida near Tallahassee – we’ll come back to that locality in a future essay.

If you click the image and examine an enlargement, the distinction is vivid.  The shells of juvenile H. trivolvis demonstrate what Pilsbry called “thread-like spiral striations” and the shells of juvenile H. duryi do not.  Projected out into adulthood, I am sure this yields the “shell dull” vs. “shell glossy” distinction that authorities have always noted.  But in juvenile shells the subjective element of the distinction is removed.

Also striking is that strong carination near the apex of the juvenile trivolvis shells (arrow), which Pilsbry called an “acute keel.”  That feature is not clear in the adult shells, at all, but adult trivolvis do tend to demonstrate “boxier” whorls than the more smoothly-planispiral duryi, with which may be a later manifestation of the juvenile keel.  The whorl-boxy character is not helpful unless you’ve seen a lot of both shells on the bench in front of you.  But once you’ve seen it, whorl-roundedness or boxiness seems to be a fairly reliable method of distinguishing the species as well as the threadlike spiral striations.

So I will conclude this month’s essay with another confession of error, my second in two months.  And this error is a whopper.

Juvenile H. trivolvis (above) and H. duryi (below)

Could I ask you all to look back at an essay I wrote in February of 2005 on shell morphology, current, and substrate in the Helisoma population of Wakendaw Lakes?  Open this link [18Feb05] in a new window.  I actually dredged that 2005 essay up again this past September, as an example of ecophenotypic shell variation in the planorbids generally.  You could look at my post of [9Sept20] too if you want.

For 15 years I have identified those snails from the Wakendaw subdivision east of the Cooper as Helisoma trivolvis.  But I fetched up a sample of juveniles this fall and scoped them out, and their shells are smooth as a baby’s bottom.  The Wakendaw Lakes population is Helisoma duryi.

I suppose I should not have been surprised, since that population came to my attention because it was biphasic, showing strikingly different shell morphologies on pond weeds above the little dam, and on riprap rocks in the current below.  Wakendaw Lakes look like a little patch of Florida, on the other side of Charleston, in retrospect.

But even here in my own neighborhood West of the Ashley, where all the Helisoma are uniformly planispiral.  The gigantic planorbid population in that office park about which I blogged on [29Nov04] is Helisoma duryi.  And most embarrassingly of all, the Charles Town Landing population that I sent to Cindy Norton as a “control” for the breeding experiments I detailed in [9Nov20] were also Helisoma duryi.  No wonder she found such strong evidence of reproductive compatibility between her Carolina population and the Helisoma population I collected at the 40-Mile Bend!  Ultimately, the most foolish thing about Cindy’s 2018 breeding experiments was her collaborator.

In recent months I have re-examined, and in many cases re-sampled, populations I have previously identified as Helisoma trivolvis from a broad swath of the southern Atlantic drainages.  I have discovered one population of H. duryi in coastal Georgia, which I collected on Sapelo Island in 2005, and one way up in the Atlanta area, certainly a recent introduction.  I have also confirmed 15 duryi populations in coastal South Carolina, from way down on Hilton Head Island, where I mentioned “H. trivolvis” in my blog post of [16Dec15], all the way north to the Myrtle Beach area.

I’ve been screwing up my local Helisoma for years.  In my own defense, I might quote Baker [8], who limited the range of H. duryi, and indeed the entire Pilsbry subgenus Seminolina, to “only in the peninsula of Florida north to Bradford County.”  Burch [10] quoted Pilsbry’s “Northern to southern Florida.”  No prior authority ever seems to have imagined that H. duryi might range as far north as the Carolinas.

Helisoma duryi becomes species #70 on the list of freshwater gastropods documented from the nine-state Atlantic drainage region of North America [11].  And here is the natural follow-up question, I suppose.  Is this species native or introduced to the region?  Pretty much all 17 of the H. duryi populations north of Florida of which I am aware [12] inhabit disturbed environments.  I can offer no better answer than the one that occurred to me on my ride home from Hilton Head Island five years ago.  Quoting my essay of [16Dec15]:

“I had spent three full field days sampling a freshwater benthic community comprised entirely of invasive species.  At some time scale, this insight is trivial.  Hilton Head didn’t even exist at the last interglacial period, so its entire freshwater and terrestrial biota must be invasive at a scale of 10^5 years.  But the gastropod community my SCDNR colleagues and I have been sampling this fall looks 10^2 invasive to me and might even be 10^1 invasive.”

Everything is invasive, and we humans are invasive, and it never hurts to remind ourselves occasionally that all biotas are dynamic.  As is science.


[1] Weatherby, 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.  For more about Weatherby and his Helisoma, see:

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

[2] 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.  For more about Pilsbry and his 1934 contribution to our understanding of the Planorbidae, see:

[3] The freshwater gastropod fauna of Ursa Minor Lake: Helisoma scalaris duryi, Biomphalaria havanensis, Gyraulus parvus, Physa acuta, Physa pomilia, Melanoides tuberculata, Lymnaea columella, Pyrgophorus parvulus, Pomacea paludosa.

[4] The type locality for Helisoma duryi (Wetherby 1879), here designated: Ditch leading to the Tomoka River at the corner of Bayless & Fentress Blvds, 6 km W of Daytona Beach, Volusia County, FL. (29.1891, -81.0786)

[5] I’ve only attended the Daytona 500 once, in February of 2006, a race in which Jimmy Johnson took the checkered flag.  48 can kiss my ass.

[6] I’ve explored the relationship between science and public policy so often in the 20-year history of this blog that I’ve developed a separate label in the right-hand margin way up above, “Worldview Collision.”  The relationship is exactly analogous to science and sports, or music and sports, for that matter.  My daddy was both a baseball-player and a banjo-picker, but he never tried to make the two compatible.

[7] Thomas Say (1819) wrote that the “ingenious naturalist, Mr. C. A. Lesueur” found his sample of Planorbis trivolvis “in French Creek, near Lake Erie.”  My sample of H. trivolvis, which I offer here as topotypic, came from Howard Eaton Reservoir, an impoundment of upper French Creek in Erie County, PA.  (42.1476, -79.7658)

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

[9] Thompson, F.G. (1999)  An identification manual for the freshwater snails of Florida.  Walkerana 10 (23): 1 – 96.

[10] 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).

[11] The 69-species list (FWGNA synthesis V2.1) was the one ultimately published as Table 2 in:

  • Dillon, R.T., Jr., M.J. Ashton, W.K. Reeves, T.P. Smith, T.W. Stewart, & B.T. Watson (2019a) Atlantic drainages, Georgia through Pennsylvania.  Freshwater Gastropods of North America, Volume 1.  FWGNA Press.  199 pp.   [FWGNA Publications]

Version 2.1 has subsequently been supplanted by FWGNA synthesis version 3.0 (with 102 species), currently on the website [synthesis].

[12] The only exception of which I am aware is a record of Helisoma duryi in the rather pristine Black River near Andrews, SC.  That population seems to be sympatric with H. trivolvis.  Significant in a couple respects, I think.

Thursday, December 3, 2020

The Emperor Speaks

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.

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 worldwide, 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.”  I am sure Pilsbry must have had some reason to emend Jay’s scalaris to “scalare,” probably worried about agreement in gender, but such practice only paints another wash of black onto a landscape already Rembrandtian in its murkiness.

[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

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 that 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

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 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 4 – 5 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 scalaris.  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] Weatherby, 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.  I don’t know.  But I do know that the controversy surrounding Planorbis (ultimately Biomphalaria) glabrata, later understood as the host of schistosomiasis in the new world, is the third rail of American freshwater malacology.  And I also know that I have never been brave enough to touch it.  Maybe one day, when I am even older, and even stupider.

[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 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

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 adults, 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.  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.  But trivolvis is unambiguously sinistral.

Both H. trivolvis and H. anceps can be found anywhere, 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 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.

But in Helisoma trivolvis, newly-hatched juveniles do show an apex (flat-topped but still distinct) and (hence) are very obviously sinistral.  As they mature, they flip their shells across their backs and become more planispiral, losing their apparent axis of coiling.  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 trivolvis 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 H. trivolvis 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 trivolvis 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 H. trivolvis 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 H. trivolvis look “right” 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 H. trivolvis 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 scalaris 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 trivolvis 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]