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.

No comments:

Post a Comment