FWGNA > Species Accounts > Planorbidae > Helisoma scalaris scalaris
Helisoma scalaris scalaris (Jay 1839)
“Planorbella (Seminolina) scalare” 
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> Habitat & Distribution

Pilsbry (1934) gave the range of H. scalaris as “southern Florida,” but modern scholarship suggests that the type locality for the species lies in the Florida panhandle, somewhere in the vicinity of Tallahassee.  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 scalaris 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 scalaris scalaris 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.

> Ecology & Life History

The typical “scalariform” or “physoid” shell morphology of Helisoma scalaris, 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 physoid 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 physoid H. scalaris scalaris populations in their natural environment.  Populations of H. scalaris 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 scalaris duryi page will be applicable to H. scalaris scalaris.

> Taxonomy & Systematics

 John Clarkson Jay described Paludina (Helisoma) scalaris in 1839 from the “Everglades of Florida,” which modern scholarship suggests to have been somewhere in the vicinity of Tallahassee.  Albert G. Wetherby followed with a description of Planorbis (Helisoma) duryi in 1879, also from the “Everglades of Florida.”  Modern scholarship suggests that Wetherby’s type locality was somewhere in the vicinity of Daytona, 400 km east of Tallahassee.  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 preglabratum of Marshall (1926), demonstrated a seamless transition in shell morphology between H. duryi and H. scalaris.  Pilsbry considered that multiple subspecies were often present in the same population, sympatrically.  He set aside the entire group, H. scalaris and H. duryi with its five subspecies, in the subgenus Seminolina.  Baker (1945) concurred.

The modern synthesis of evolutionary thought, however, precludes sympatric subspecies.  Thus we have advocated the recognition of just two (typically allopatric) subspecies, the scalariform (higher than wide) and the planispiral (wider than high).  And since Jay’s nomen scalaris 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. scalaris (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. scalaris are much smoother and glossier, lacking both features.

Pilsbry (1934), Baker (1945) and Hubendick (1955) all placed scalaris 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 scalaris underneath it.  Burch (1989) followed Taylor.  But the apparent axis of planorbid shell coiling is a plastic trait, varying even within the Helisoma scalaris/duryi complex, and hence we prefer the system of Pilsbry, Baker, and Hubendick.

Launch into phylogenetics paragraph.  Cite Morgan et al. (2002).

> Supplementary Resources

  • Helisoma distribution in Atlantic drainages (2013)
  • To learn more about the aquarium culture of Helisoma scalaris 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. scalaris 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. scalaris 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. scalaris scalaris, 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.

> References

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