Dr. Rob Dillon, Coordinator

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.