> Habitat & Distribution

Baker (1945:134) suggested that planorbids of the subgenus Seminolina, to which he assigned H. scalare and H. duryi with all their subspecies, were restricted to peninsular Florida, "north to Bradford County."  Populations of these snails are indeed quite widespread and locally common in a great variety of habitats throughout southern Florida into the Everglades: weedy ditches, marshes, ponds and open lakes, primarily grazing on macrophytic vegetation.

More recently, however, we have discovered that the range of H. scalare duryi extends north through coastal regions of Georgia and South Carolina at least as far as the Myrtle Beach area.  (See my essay of 5Jan21 from the link below for more.)  Here populations usually inhabit ponds in disturbed environments, especially in residential areas, although we are aware of one population in the (relatively pristine) Black River of South Carolina, sympatric with H. trivolvis, and one population in the Atlanta area, obviously introduced.

Populations of North American H. scalare duryi have also been introduced sporadically around Asia, Africa, Europe, and South America (Brown 1994: 408 – 409, Pointier et al. 2005, Fernandez et al. 2010).  Some of these introductions have been intentional, in efforts to control the planorbid hosts of schistosomiasis (Frandsen & Madsen 1979).  But most introduced populations are doubtless the result of unintentional release by aquarium hobbyists.  Helisoma scalare duryi, especially in its red (albino) form, has been domesticated in recent years and widely marketed by the aquarium trade.  The FWGNA incidence rank of H. scalare duryi, lumped together with typical H. scalare scalare, is I-3p, peripheral in our 17-state study area.

> Ecology & Life History

Although little is known about the biology of wild H. scalare duryi populations in their natural range, expatriate populations and aquarium cultures have been quite well-studied.  Some of the best population dynamic data available for any freshwater gastropod anywhere were published for laboratory stocks of H. scalare duryi by deKock & Joubert (1991).  Dillon (2000: 226) speculated that wild H. scalare duryi populations may be R-adapted.

Reared in isolation, H. scalare duryi are able to reproduce successfully by self-fertilization, although demonstrating the reproductive delay and reduced fecundity typical of pulmonates generally (DeWitt & Sloan 1959, Paraense & Correa 1988).  Madsen et al. (1983) estimated the maximum sperm storage duration to be approximately 150 days; strong first-sperm precedence being documented by Norton & Wright (2019).  Fecundity components in H. scalare duryi (reidentified from H. trivolvis) have been estimated by Norton & Bronson (2006), Norton & Newman 2016, and Norton et al. (2018).  See Norton et al. (2008) for a description of mating behavior.

Reared together in culture, Frandsen & Madsen (1979) reported that H. scalare duryi populations have a strikingly negative impact on African populations of the medically-important planorbids Biomphalaria and Bulinus.  These experiments were subsequently expanded and refined by Madsen (1979a,b, 1982, 1984, 1987), tending to rule out allelopathic chemicals and competition for food.  The primary factors may be egg predation (Meyer-Lassen and Madsen 1989) or possibly even direct antagonism (Madsen 1986).

Results such as these led researchers to experiment with introducing H. scalare duryi populations into the wild as a biological control agent for the medically-important planorbids (Frandsen & Madsen 1979).  Returns have not been promising, however (Madsen 1983, Perera et al. 1986, Joubert et al. 1992).

As early as 1945, F. C. Baker observed that “the Helisoma duryi complex includes several races more or less heavily infested with parasitic worms,” going on to report that “Many specimens were so badly infested that most of the organs, especially the genitalia and liver, were completely obliterated.”  Laboratory data on trematode infectivity have been offered by Christensen et al. (1980).  See Dorn (2013) for experimental results with crayfish predation in mesocosms.

> 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.

The shell of Jay’s H. scalaris was higher than wide in aperture view, retaining the obvious sinistral coiling of the juvenile, in an overall morphology described as “physoid” or “scalariform.”  Snails bearing shells of this morphology do not seem to enfold an air pocket, and are typically found grazing on benthic substrates, often in current.

Pilsbry (1934) documented the occurrence of many Helisoma populations in Florida demonstrating smooth transitions in shell morphology between the scalariform and the planispiral.  He suggested that such populations be subdivided into five subspecific nomina: seminole, normale, intercalare, preglabratum and eudiscus in addition to the typical duryi form, often sympatric.

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.

Pilsbry (1934), Baker (1945) and Hubendick (1955) all placed duryi 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 duryi 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 

• See my 29Nov04 post to the FWGNA blog on Gigantic Pulmonates for several additional photos of Charleston-area H. scalare duryi, which I misidentified at the time as H. trivolvis.
• My FWGNA post of 18Feb05 on Shell Morphology, Current, and Substrate also featured a Charleston-area H. scalare populaion, 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."
• Helisoma scalare duryi in a variety of color forms are widely retailed through the online aquarium trade.  See my post of 24Jan18, Snails by Mail for more.
• See my adventure story of 5Oct20, The Flat-topped Helisoma of The Everglades, for a reproduction of Wetherby's original duryi figure of 1879, 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 aquarium forms, 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 duryi from Helisoma trivolvis.  All three of these essays are lavishly illustrated with with figures of anatomy, shell morphology, and habitat.
• 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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