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:
Thanks for any feedback that you might provide!
[Baffled in Missouri]
Hello Dr. Dillon,
[Bothered in Indiana]
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 , “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!
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 , 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 . 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 :
|Helisoma trivolvis population of Wakendaw Lakes|
That’s quite a vivid demonstration of ecophenotypic plasticity , 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 ) 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.
 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.
 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]
 More about the Helisoma trivolvis population of Wakendaw Lakes:
- Shell morphology, current, and substrate [18Feb05]
 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]
 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]
 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]