> Habitat & Distribution

Pilsbry (1934) gave the range of H. scalare as “southern Florida,” where populations are indeed quite common, but our recent research suggests that the type locality for the species was in the Florida panhandle, much further north.  We are also aware of one extralimital population inhabiting riprap rocks below a small dam in a suburb of Charleston, South Carolina.

Populations of Helisoma scalare bearing the typical shell morphology are usually discovered grazing over submerged macrophytic vegetation, rock, or other benthic substrates in cooler, clearwater lakes, spring-fed rivers, and in the Everglades proper.  Helisoma scalare scalare populations are not typically found in emergent or floating vegetation, nor are they characteristic of weedy ditches, swamps, or warm-water ponds.  If transferred to aquarium culture, their second generation generally develops the planispiral duryi shell morphology.
trade.  The FWGNA incidence rank of H. scalare scalare, lumped together with its planispiral subspecies H. scalare duryi, is I-3p, peripheral in our 17-state study area.

> Ecology & Life History

The typical “scalariform” or “physoid” shell morphology of Helisoma scalare, higher than wide in aperture view, is thought to develop in populations that are entirely benthic, not aspirating an air bubble within their mantles.  Such populations must then be restricted to waters with some minimum dissolved oxygen – perhaps flowing, perhaps deeper, presumably cooler.  The scalariform shell morphology also seems to be associated with clearer waters, where the risk from fish and other crushing predators might be greater.

We are not aware of any good ecological studies of scalariform H. scalare scalare populations in their natural environment.  Populations of H. scalare bearing planispiral shell morphology are, however, among the best-studied freshwater gastropods worldwide.  Such populations, usually identified as “Helisoma duryi” are easily adaptable to aquarium culture and invasive on four continents.  Much of the research reviewed on our Helisoma scalare duryi page will be applicable to H. scalare scalare.

> Taxonomy & Systematics

John Clarkson Jay described Paludina scalaris in 1839 from the “Everglades of Florida,” but our recent research suggests that the type locality was Wakulla Springs, in the Florida panhandle near Tallahassee.  See our essays of 6Dec22 and 5Jan23, available from the links below, for more.  Albert G. Wetherby followed with a description of Planorbis (Helisoma) duryi in 1879, also from the “Everglades of Florida.”  Our research has also suggested a more northerly type locality for Wetherby's population as well, somewhere in the vicinity of Daytona, 400 km east of Tallahassee.  See our essays of 3Dec20 and 5Jan21 for more.  Neither of these putative type localities is in the Everglades Ecoregion, as formally defined today. 

The shell of Wetherby’s H. duryi was wider than high in aperture view, developing the planispiral morphology typical of the Planorbidae.  In life, snails bearing shells of this morphology aspirate an air pocket, which they enfold under their mantle, lifting the weight of the shell from their backs.  The volume of entrapped air can be so adjusted that the snails are neutrally or even positively-buoyant, allowing them to graze in floating macrophytic vegetation.

Pilsbry (1934) described a series of four subspecies of Helisoma duryi (normale, seminole, intercalare and eudiscus) which, together with the typical subspecies duryi and preglabratum of Marshall (1926), demonstrated a seamless transition in shell morphology between H. duryi and H. scalare.  Pilsbry considered that multiple subspecies were often present in the same population, sympatrically.  He set aside the entire group, H. scalare and H. duryi with its six subspecies, in the subgenus Seminolina.  Baker (1945) concurred.

Our recent re-evaluation of both the anatomical and the shell morphological data has returned no evidence of a distinction between Helisoma duryi and the earlier-described H. scalare.  See our essay of 7Feb23 from the link below for more.  Indeed, the modern synthesis of evolutionary thought precludes sympatric subspecies such as hypothesized by Pilsbry.  Thus we have advocated the recognition of just two (typically allopatric) subspecies, the scalariform (higher than wide in aperture view) and the planispiral (wider than high).  And since Jay’s nomen scalaris/scalare is senior over Wetherby’s duryi by 40 years, the scalariform shell must be considered typical, with duryi the subspecific form.

There has also been considerable confusion regarding the distinction between H. scalare (especially in its planispiral, duryi form) and the much more widespread Helisoma trivolvis, common throughout North America.  The shells of juvenile H. trivolvis are marked with threadlike spiral striations and (typically) an acute apical keel, while those of juvenile H. scalare are much smoother and glossier, lacking both features. See my essays of 3Dec21, 5Jan21, and 9Feb21 from the links below for more about that particular issue.

Pilsbry (1934), Baker (1945) and Hubendick (1955) all placed scalare in the genus Helisoma, subgenus Seminolina.  Taylor (1966) raised Baker’s subgenus Planorbella to the genus level on the basis of the apparent axis of shell coiling, and moved scalare underneath it.  Burch (1989) followed Taylor.  But the apparent axis of planorbid shell coiling is a plastic trait, varying even within the Helisoma scalare/duryi complex, and hence we prefer the system of Pilsbry, Baker, and Hubendick.

> Maps and Supplementary Resources

• Helisoma scalaris distribution in Atlantic drainages (2023)
• To learn more about the aquarium culture of Helisoma scalare duryi, just google “Ramshorn snails.”  There are quite a few websites run by aquarium hobbyists and the businesses that serve them, with tips and pointers about keeping your pets healthy and happy.  You will also find many lovely photos and videos of living H. scalare duryi available online, especially of the red albino variant.

> Essays 

• My FWGNA post of 18Feb05 on Shell Morphology, Current, and Substrate featured a Charleston-area H. scalare population, misidentified as Helisoma trivolvis.  That essay featured photographs of the shells of both subspecies, the typical form and the duryi form, together with ecological notes.
• See my post to the FWGNA blog of 11Apr08 for a review of the Classification of the Planorbidae.
• Or view the (Hubendick 1955) classification of North American planorbids in a tabular format [here].
• See my post of 26Sept14 for good, comparative figures illustrating "The egg masses of freshwater pulmonate snails."
• See my adventure story of 5Oct20, The Flat-topped Helisoma of The Everglades, for a growth-series figure of H. scalare scalare, together with notes and observations about its south Florida habitat.
• My essay of 9Nov20, Foolish Things with Helisoma duryi, reported the results of a breeding experiment undertaken by our colleague Cynthia Norton in 2019.  That post features photos of several duryi forms from the aquarium, as well as a nice close-up of a developing egg mass.
• See my essays of 3Dec20 (The Emperor Speaks), 5Jan21 (Collected in Turn One) and 9Feb21 (The Emperor, the Non-child, and the Not-short Duct) for a detailed account of my personal journey toward distinguishing Helisoma scalare duryi from Helisoma trivolvis.  All three of these essays are lavishly illustrated with with figures of anatomy, shell morphology, and habitat, almost all of which is equally applicable to H. scalare of the typical form as well.
• See my essays of 6Dec22 (In the Footsteps of the Comte de Castelnau) and 5Jan23 (The Helisoma from the Black Lagoon!) for another adventure story, this one in search of the type locality of Helisoma scalare scalare, finding mostly H. scalare duryi.  Lots of additional figures of habitats and shell morphology for both subspecies are available here as well.
• On the basis of all evidence - anatomical, shell morphological, ecological and historic - I synonymized Wetherby's (1879) duryi under Jay's (1839) scalaris/scalare in my essay of 7Feb23, New Clothes for The Emperor, retaining duryi at the subspecific level to distinguish planispiral populations.
• In my essay of 7Mar23 I offered two answers to the rhetorical question, "What was Planorbis glabratus?"  The first answer was that glabratus Say 1818 is a senior synonym of both scalaris/scalare Jay 1839 and duryi Wetherby 1879.  And the second answer was to forget the first answer and act like the question was never asked.  But there's a lot of interesting biology and some important lessons to be learned about glabratus, scalaris and duryi between one and two, nonetheless!

> References

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