Dr. Rob Dillon, Coordinator

Monday, November 29, 2004

Gigantic Pulmonates

Editor's Note #1.  This essay was published as: Dillon, R.T., Jr. (2019b)  Gigantic pulmonates.  Pp 113-120 in The Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston.
Editor's Note #2. I subsequently re-identified the planorbid population inhabiting this "fancy landscape pond" as Helisoma scalare duryi.  See my posts of [5Jan21] and [7Feb23].

The fancy landscape at the office park near my home in Charleston features a narrow pond, perhaps three meters deep and over 100 meters long, with concrete lining and a natural earth base. It is inhabited by our three local pulmonate weeds (Physa acuta, Helisoma trivolvis, and Lymnaea columella) as well as by mosquito fish, tadpoles, and a variety of macrophytes both submerged and emergent. This pond has long served as the water source for my laboratory cultures of pulmonates, and as such I have visited it approximately every two weeks for 6 - 8 years.

About a month ago the pond was drained for cleaning. A crew subsequently spent quite a few days shoveling leaves, mud, vegetation and organic debris from the basin, leaving a few shallow pools scattered on a clean mud bottom. The photo at left above shows the drying pond at its lowest end, with the concrete standpipe that ordinarily regulates water depth in the background. A high-resolution close up of one of the drying pools with Helisoma trivolvis (both the living and the dead) much in evidence, can be downloaded [here]. As the figure below illustrates, the individual Helisoma were exceptionally large.
A similar phenomenon also seems to occur occasionally in the Physa acuta population inhabiting the main pond at Charles Towne Landing State Park. I have many years of casual observations on this population, which has served as my source of control snails for experiments on the reproductive biology of Physa since 1988. In culture, P. acuta from this population reach maturity at about 6 mm shell length (seven weeks post hatch), and have never grown larger than about 12 mm when reaching the end of their life at 12 - 13 months of age (Wethington & Dillon 1993, 1997). For years I never saw an individual larger than about 12 mm in the wild. But on a casual visit in March, 2001, I discovered that the population was comprised almost entirely of gigantic individuals, 15 - 20 mm in shell length (below).

What might cause such sporadic cases of gigantism? Growth is indeterminate in all the basomatophoran pulmonates of which I am aware. But since growth rate slows dramatically at onset of reproduction, maximum size effectively becomes a function of size at maturity. Following Charnov and similarly minded evolutionary biologists from the 1980s and 1990s, I have suggested that size at maturation in pulmonates may be a function of survivorship schedule (Dillon 2000: 140 - 149).

In both my local Helisoma population and my local Physa population, gigantism appeared in the fall or winter and was not associated with parasitism. Both populations seemed stressed by low water, and probably severe temperatures. But neither population appeared to be starving, at least in the long term. And perhaps most strikingly, in neither population were eggs or juveniles in evidence. It is my hypothesis that gigantism in pulmonates may be prompted by some environmental perturbation postponing, perhaps even canceling, reproductive maturity.

Such a perturbation, although perhaps ongoing as gigantic individuals are observed, must have begun earlier in the season, when maturation would in an ordinary year have taken place. Its effect would be to improve survivorship above expected levels, perhaps through an unusual reduction in predation, parasitism, or disease. Did something kill the predators that ordinarily live in the ponds with my gigantic pulmonate populations without adversely affecting the snails themselves, triggering a growth spurt?

And do systematic malacologists have a history of being fooled by gigantism? About 300 km up the coast from where I write this essay is the home of Helisoma ("Planorbella") magnifica, a pulmonate of truly gigantic proportions (Figure below from plate 96 of Baker). Long feared extinct, a couple scattered populations of H. magnifica were re-discovered in 1988 near Wilmington, North Carolina, by Bill Adams and Andy Gerberich. The subsequent history of this species, tragic and comic by turns, is a tale best left for another day, and another teller. I know that a sample of H. magnifica was held in culture for a number of years, and that at least occasionally, magnifica adults bore offspring subsequently identified as H. trivolvis, with which magnifica co-occurs. The morphology of H. magnifica does not suggest simple gigantism of H. trivolvis, however, the magnifica shell being much more boxy and broad than the trivolvis shells shown above. The extent to which the shape of the pulmonate shell may be influenced by environmental cues is another topic upon which we might chew in the future.

For what its worth, our colleague Art Bogan and his colleagues (2002) reported mtDNA sequence divergence in the 4 - 9% range between a sample of four H. magnifica and sample of 9 H. trivolvis collected in Union County, a couple hundred kilometers west of Wilmington. Given Art's observation that sequence divergence within conspecific Helisoma populations ranged over 3%, however, a comparison of magnifica to a trivolvis population with which it co-occurs would have been much more cogent. But regardless of the status and fate of H. magnifica, the existence of great phenotypic plasticity in shell morphology should remain foremost in the minds of all of us struggling with the taxonomy and classification of freshwater pulmonates.

  • Adams, W.F. & A. G. Gerbeich (1988) Rediscovery of Planorbella magnifica in southeastern North Carolina. Nautilus 102: 125-126.
  • Baker. F. C. (1945) The Molluscan Family Planorbidae. University of Illinois Press, Urbana.
  • Bogan, A.E., M. Raley, & J. Levine (2002) Conservation status of the magnificent ramshorn, Planorbella magnifica (Pilsbry, 1903), endemic to the lower Cape Fear River Basin, North Carolina. Abstract, American Malacological Society, Charleston.
  • Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs. Cambridge University Press.
  • Wethington, A. R. & R. T. Dillon (1993) Reproductive development in the hermaphroditic freshwater snail, Physa, monitored with complementing albino lines. Proc. R. Soc. Lond. B 252: 109 - 114. [pdf]
  • Wethington, A. R. & R. T. Dillon (1997) Selfing, outcrossing, and mixed mating in the freshwater snail Physa heterostropha: lifetime fitness and inbreeding depression. Invert. Biol. 116: 192-199. [pdf]

Wednesday, October 27, 2004

Proceedings of the Charleston Symposium

As you may recall, the 2002 meeting of the American Malacological Society here in Charleston featured a symposium entitled, "The Biology and Conservation of Freshwater Gastropods." Our colleague Amy Wethington organized a related special session at that same meeting, "Pulmonates in the Laboratory."

Now (at long last!) I'm pleased to report that 12 papers from those two sessions have reached publication in the American Malacological Bulletin, Volume 19: 31 - 144. A table of contents is provided below.Single issues of the AMB are available from the office of the Editor-in-Chief: Dr. Janice Voltzow, Department of Biology, University of Scranton, Scranton, PA 18510-4625. The cost: $40 for members, $48 for non-members. Email Janice for more details: voltzowj2@Scranton.edu

Thanks to all of you on this list who were involved in bringing this symposium to successful fruition.

----------- AMB 19:31-144 (14Oct04) ---------------
  • Dillon, R. T., Jr. The biology and conservation of freshwater gastropods: Introduction to the symposium.
  • Richards, David C. & Dianne C. Shinn Intraspecific competition and development of size structure in the invasive snail, Potamopyrgus antipodarum.
  • Mower, Christina B. & Andrew M. Turner Behavior, morphology, and the coexistence of two pulmonate snails with molluscivorous fish: A comparative approach.
  • McCarthy, Thomas M. Effects of pair-type and isolation time on mating interactions of a freshwater snail, Physa gyrina (Say, 1821).
  • Brown, Kenneth M., and Paul D. Johnson Comparative conservation ecology of pleurocerid and pulmonate gastropods of the United States.
  • Dillon, Robert T., Jr., Charles E. Earnhardt, and Thomas P. Smith Reproductive isolation between Physa acuta and Physa gyrina in joint culture.
  • Dillon, Robert T., Jr., & Robert C. Frankis High levels of mitochondrial DNA sequence divergence in isolated populations of the freshwater snail, Goniobasis.
  • Stewart, Timothy W. & Robert T. Dillon, Jr. Species composition and geographic distribution of Virginia's freshwater gastropod fauna: A review using historical records.
  • Britton, David K., & Robert F. McMahon Environmentally and genetically induced shell shape variation in the freshwater pond snail Physa (Physella) virgata.
  • McMahon, Robert F. A 15-year study of intrapopulation, interannual shell-shape variation in a freshwater, pulmonate limpet population (Pulmonata: Basommatophora: Ancylidae).
  • Glaubrecht, Matthias Leopold von Buch's legacy: Treating species as dynamic natural entities, or Why geography matters.
  • Wethington, Amy R. & Robert Guralnick Are populations of physids from different hot springs distinctive lineages?
And forthcoming in the next AMB:
  • Jokinen, Eileen H. Pond mollusks of Indiana Dunes National Lakeshore: Then and now.

Tuesday, September 28, 2004

Goniobasis and Elimia

The freshwater ceritheacean family Pleuroceridae is generally reckoned to encompass seven North American genera. Although the names for six of these genera are widely agreed upon, the name of the largest genus has become controversial in the last 20 years. Here I review the history of the genus name Goniobasis and its recently resurrected synonym, Elimia. I conclude that Elimia should have been suppressed, but that at this juncture the loss of either name would result in more confusion than retaining both.
  1. The genus Elimia was proposed by H & A Adams (1854) without type designation. Since the 16 species contained in the genus were extremely diverse, including what today would be recognized as three different genera (Goniobasis, Pleurocera, and Lithasia), the genus was not accepted by contemporary workers. It was specifically rejected by Haldeman (1863).
  2. The genus Goniobasis was proposed by Lea in 1862 with a description sufficient to distinguish the group.
  3. The first monograph of the American Pleuroceridae ("Strepomatidae") was that of Tryon (1873). He recognized Goniobasis as a natural group. Regarding Elimia, he wrote: "We quote the full lists of species given by Messrs. Adams, in order that the insufficiency of their genera may become more apparent from the incongruous assemblage of shells of which they have composed them." (pg xii)
  4. Pilsbry & Rhoads (1896) revived Elimia, designating acutocarinata as type by virtue of its first listing in the alphabetical arrangement of H & A Adams. But Pilsbry used Goniobasis for the three species of that group covered in his 1896 paper, and in many subsequent papers (e.g., Pilsbry 1916). He himself never used the genus Elimia.
  5. Hannibal (1912) designated osculata (Lea 1862) as the type species of Goniobasis.
  6. The North American mollusks were reviewed by Walker (1918). Walker used Goniobasis, writing (Pg 149): "Dr. Pilsbry has more recently decided that Goniobasis should be restored to its former position as a generic term, on the ground that Elimia was a composite group."
  7. The family Pleuroceridae was next monographed by Goodrich, in a series of papers published from 1922 - 1944 (e.g., 1936, 1940, 1942). Goodrich used Goniobasis. Although the literature continued to contain occasional references to Oxytrema (Raf.) and rare uses of Elimia, almost all workers followed Goodrich through the next 50 years.
  8. The literature contains, I believe, about four or five instances of the genus Elimia between 1918 and 1978 in total. I would estimate the usage of Goniobasis during this 60-year period to be one or two per year. A review of the Zoological Record 1970 - 1979 shows 13 instances of Goniobasis, 3 of Oxytrema, and none of Elimia.
  9. Burch (1979) resurrected Elimia for his (1980, 1982) monograph "North American Freshwater Snails." He wrote (1982:271), "Elimia H & A Adams 1854 is used in place of its better known synonym Goniobasis Lea 1862." Burch (2001) subsequently added, "Since Elimia H. & A. Adams 1854 has clear priority over Goniobasis Lea 1862, an appeal could have been made by me (or someone else) to the International Commission on Zoological Nomenclature in an attempt to conserve the name Goniobasis. But having knowledge of - and in fact participating in - the long battle to get the genus name Pleurocera conserved to fit its common usage convinced me that such an endeavor to save the use of the junior synonym Goniobasis would be futile, and in any event would take an inordinate amount of time, and certainly try the patience of malacologists."
  10. No consensus has followed. The Zoological Record for the period 1980 - 1989 reported 29 uses of Goniobasis and 9 uses of Elimia, and for the period 1990 - 2000 reported 10 uses of Goniobasis and 37 uses of Elimia. The classification of Vaught (1989) used Goniobasis, and that of Turgeon et al. (1998) used Elimia. The U.S. Endangered species list uses Elimia.
As the clear choice of Haldeman, Tryon, Pilsbry, Walker, Goodrich, and almost every other professional malacologist for over 100 years, the nomen Goniobasis has been attached to a great and valuable literature. The nomen Elimia, which should have been suppressed 25 years ago, has nevertheless also become attached to a significant literature. The loss of either name at this point would be unconscionable. Thus it seems to me that both names ought to remain in currency, and that authors preferring Goniobasis should refer to "Elimia" in their text, while authors preferring Elimia also refer to "Goniobasis."

Those of us who are familiar with the English language have come to accept, in fact even expect, synonyms. If I ask for or request a soda or a pop with my hot dog or frank I will get or receive the same thing. Synonyms are a stable component of daily communication, and do not necessarily lead to confusion. So regarding Goniobasis or Elimia, as my daughter used to say, "Whatever!"


Adams, H. and A. Adams. 1853 - 58. The genera of recent Mollusca; arranged according to their organization ( 3 Vols.). John Van Voorst, London.
Burch, J. B. 1979. Genera and subgenera of Recent freshwater gastropods of North America (North of Mexico). Malacological Review 12:97-100.
Burch, J. B. 1982. North American freshwater snails: identification keys, generic synonymy, supplemental notes, glossary, references, index. Walkerana 4:1-365.
Burch, J. B. 2001. On the genus name Goniobasis (Elimia - Gastropoda: Pleuroceridae) and other recent nomenclatural inconsistencies. Walkerana 12:97-105.
Burch, J. B. and J. Tottenham. 1980. North American freshwater snails: species list, ranges, and illustrations. Walkerana 3:1-215.
Goodrich, C. 1936. Goniobasis of the Coosa River, Alabama. Misc. Publ. Mus. Zool. Univ. Mich. 31:1-60.
Goodrich, C. 1940. The Pleuroceridae of the Ohio River system. Occas. Pprs. Mus. Zool. Univ. Mich. 417:1-21.
Goodrich, C. 1942. The Pleuroceridae of the Atlantic coastal plain. Occas. Pprs. Mus. Zool. Univ. Mich. 456:1-6.
Haldeman, S. S. 1863. On Strepomatidae as a name for a family of fluviatile Mollusca, usually confounded with Melania. Proc. Acad. Nat. Sci. Phila. 15: 273 - 274.
Hannibal, H. 1912. A synopsis of the Recent and Tertiary freshwater Mollusca of the California Province, based upon an ontogenetic classification. Proc. Malacol. Soc. Lond. 10:112-211.
Lea, I. 1862. Description of a new genus (Goniobasis) of the family Melanidae and eighty-two new species. Proc. Acad. Nat. Sci. Phila. 14:262-272.
Pilsbry, H. 1916. Goniobasis in western Pennsylvania. Nautilus 30:4-5.
Pilsbry, H. and S. Rhoads. 1896. Contributions to the Zoology of Tennessee, Number 4, Mollusca. Proc. Acad. Nat. Sci. Phila. 1896:487-506.
Tryon, G. W., Jr. 1873. Land and Freshwater Shells of North America. Part IV, Strepomatidae. Smithsonian Miscellaneous Collections, vol. 253. Washington, DC.
Turgeon, D., J. et al. 1998. Common and Scientific Names of Aquatic Invertebrates from the United States and Canada: Mollusks. Special Publications, vol. 26. American Fisheries Society.
Vaught, K. 1989. A Classification of the Living Mollusca. American Malacologists, Melbourne, FL.
Walker, B. 1918. A Synopsis of the Classification of the Freshwater Mollusca of North America, North of Mexico. Misc. Pubs., vol. 6. University of Michigan Press, Ann Arbor.

Thursday, August 26, 2004

Lioplax in South Carolina

Every year about this time I deliver a lecture to my Biology 111 students entitled, "The Scientific Method." I always emphasize that this may be the most important lecture of their careers as scientists. And I begin by defining "science" as the construction of testable models about the natural world, accenting the word testable. A model need not be correct - in fact, it is commonly argued that no model of the natural world can ever be proven correct. But I always try to drive home to the freshmen that any hypothesis, theory or model of nature that is testable, verifiable, or falsifiable can make a contribution to science. Anything else is screwing around with a lab coat on.

Judging from subsequent examinations, however, I've been forced to conclude that the minds of college freshmen are generally focused elsewhere on the first day of class. And to judge from the quality of the typical paper published in the literature of evolutionary biology today, there are precious few opportunities to make up the missed material between the freshman year and the Ph.D. I suspect there's something about DNA sequence data in particular that promotes clueless data-dredging, but I digress.

Earlier this summer my faith in the scientific method was refreshed by an email from Ms. Laura Kirk, Park Interpreter at Lee State Natural Area near Bishopville, SC. She reported collecting a snail from the nearby Lynches River unidentifiable through the dichotomous key I published on my brand new "Freshwater Gastropods of South Carolina" web site, not matching any of the photos or descriptions I had posted. She attached a photo of the beast, which after some correspondence I came to realize was Lioplax subcarinata (above). Hers was the first modern record of the species in this state.

The March 9 Version of my FWGSC web site, which was still up (as of August 24, 2004, but badly in need of revision), constituted a testable model of the natural world. I stated that there are 24 freshwater gastropod species in South Carolina. And in a matter of two months, a scientist with whom I had no prior contact disproved that hypothesis, bringing the total to 25. (There may now be a 26th species as well, but that's another story.)

This Saturday just past I traveled to Lee State Natural Area and was able to collect a large sample of Lioplax from the Lynches River. The river is perhaps 10 meters across in this region of the state, rather steeply incised into its banks. Fallen trees and woody debris constitute the only solid substrate. The water is generally turbid but has fairly good flow, over a bottom of sand and mud (below.)

The Lioplax were burrowing in extremely flocculent mud in backwater eddies. "Burrowing" is a poor word for it, because the mud was very loosely aggregated in these pools, and would seem to require almost no extra expenditure of energy to penetrate. The snails seemed more "suspended in" than "burrowing through" the substrate. I made my collection by wading knee-deep into 4 cm of water and feeling with my hands.

The animals looked much like Campeloma, with very pale body coloration. They were unusually small for viviparids, none larger than 16 mm in shell length. But my examination of four individuals ranging from 12 mm to 15 mm showed all to be mature, one male and three females bearing early-stage embryos. Their shells bore spiral cords of varying strength, some quite fine.

The relevant paragraph on the "Recommendations" page of the present (9Mar04) version of my FWGSC site reads as follows: "There may be cause for some conservation concern regarding Lioplax subcarinata. This fairly large and conspicuous species is restricted to Atlantic drainages, nominally ranging from New York to South Carolina (Clench 1962). But Lioplax seems to have been extirpated from New York (Jokinen 1992), and we have been unable to confirm its presence here at the southern terminus of its range as well. Lioplax does seem at least locally common in North Carolina and Virginia, however, so the situation may not as yet be critical."

Although the penultimate sentence of that paragraph will obviously require some modification, I think the other sentences remain okay as they currently read, at least for the present. I still suspect that there may be some cause for conservation concern regarding Lioplax, depending on its abundance through the meat of its range to the north. Stay tuned!

Thursday, July 29, 2004

Documenting the Downward Spiral

We were pleased to receive email notification late last week that the proceedings of the Vienna Symposium on Molluscan Biodiversity and Conservation have found their way to press. The table of contents is available as a PDF download from the FWGNA web site:
  • Journal of Conchology Special Publication No. 3 [PDF]
Longtime members of this list may remember that I offered a report on the Vienna Symposium upon my return in September 2001. Although the original symposium did not include any talk specifically dealing with freshwater gastropods, the recently published volume features a paper on the Lake Tanganyika gastropod fauna by Todd and colleagues. It is available as a special publication of the Journal of Conchology, contact Dr. Mary Seddon for the details: Mary.Seddon@nmgw.ac.uk

I do think we've seen increased awareness of molluscan conservation issues in recent years. In fact, I personally have a hard time keeping track of all the books, articles, and other resources documenting the downward spiral. Herewith is a brief bibliography:
  • Lydeard, C. et al. (2004) The global decline of nonmarine mollusks. BioScience 54: 321 - 330. Chuck Lydeard is joined by a gang of 15 coauthors in this general review featuring three "highlighted faunas:" Pacific land snails, unionoid mussels, and spring snails of the Australian outback, as well as conservation strategies.
  • Black, S. H., M. Shepard & M. M. Allen (2001) Endangered invertebrates: the case for greater attention to invertebrate conservation. Endangered Species Update 18: 42 - 50. Scott Hoffman Black is the executive director of the Xerces Society, an advocacy group for invertebrate conservation. One could accuse Xerces of being biased toward insects, but so was God. Scott's article does include references to freshwater mollusks, and may be available in PDF format from the Xerces web site.
  • Neves, R.J., A E. Bogan, J. D. Williams, S. A. Ahlstedt, and P. W. Hartfield (1997) Status of aquatic mollusks in the southeastern United States: A downward spiral of diversity. Chapter 3 in Aquatic Fauna in Peril: the Southeastern Perspective (Benz & Collins, eds.) Southeast Aquatic Research Institute Publication 1. This 42-page work features the most complete review of the conservation status of any regional freshwater gastropod fauna.
  • Lydeard, C. & R. L. Mayden (1995) A diverse and endangered aquatic ecosystem of the southeast United States. Conservation Biology 9: 800-805. This work includes a comprehensive review of the Mobile Basin gastropod fauna, past and present.
If anybody would like to recommend additional resources generally relevant to the conservation of freshwater gastropod faunas, by all means bring them to my attention.

Wednesday, May 26, 2004

Somatogyrus in the Southeast - Darkness, Confusion, and a New Hope

I have spent most of my professional career in the company of researchers more knowledgeable of the Hydrobiidae than I. So on the (increasingly frequent) occasions when I collect hydrobiids myself, or have them sent to me by others, or field inquiries from workers around the country regarding this enigmatic family, I have tended to "pass the buck." Or I have done a slapdash job. The purpose of this essay it to record my own personal awakening regarding strikingly similar members of three hydrobiid genera, Amnicola, Gillia, and (especially) Somatogyrus, of considerable conservation interest here in the American southeast. This will be a confession of sorts, addressed to several of my colleagues to whom I have supplied erroneous information in the past, and (I hope) a spur for the reexamination of assumptions that may have been made in similar situations by others elsewhere.

Had I been asked ten years ago for an accounting of the hydrobiid fauna of South Carolina, I would have certainly listed Gillia altilis (the type locality of Gillia being the Santee Canal north of Charleston) and perhaps added Amnicola limosa. The only previously published SC list is that of Mazyck (1913) who includes "Amnicola sp." as well as Gillia. These two vanilla-looking snails are nearly indistinguishable on paper, although Amnicola is smaller, with two ducts on its verge, while the verge of the larger Gillia is single-ducted. But I have always thought of A. limosa as a species of northern lakes, rather than South Carolina swamps, and the NatureServe Explorer database lists Gillia alone.

And in fact, when I first began seriously surveying the freshwater gastropods of this state in connection with the FWGNA project in the mid-1990s, I did begin finding little vanilla-looking hydrobiids with double-ducted verges in slow-moving lowcountry streams which I identified as A. limosa, as well as a smattering of vanilla-looking hydrobiids in the midlands and upstate with single ducts, which I called "Gillia altilis."

The occurrence of Somatogyrus in South Carolina never crossed my mind. The primary reference to Somatogyrus is that of Walker (1904), who described two species from Atlantic drainages further north: S. pennsylvanicus from the Susquehanna River and S. virginicus from the Rapidan River. And the relevant couplet in Burch's key to the Hydrobiidae offers this choice: "Shell generally thick and solid, columella thickened. Mississippi and Gulf of Mexico drainage (except S. pennsylvanicus and S. virginicus) - Genus Somatogyrus" or "Shell rather thin, columella not thickened. Atlantic drainages from NJ to SC - Gillia altilis." This turns out to be a bit misleading, because Thompson described a third Somatogyrus from an Atlantic drainage in 1969 (S. tenax from the Broad River, a tributary of the Savannah) and Krieger added a fourth from the Alcovy and Yellow Rivers (of Georgia's Altamaha System) in 1972. But Fred Thompson himself, in a presentation at the American Malacological Society meeting last June, indicated that he was unaware of any hydrobiids other than Gillia and perhaps Amnicola in South Carolina.

Lower left - Somatogyrus virginicus (Chauga River, SC). Center - Gillia altilis (Charleston Museum specimen, topotypic from the Santee Canal). Upper right - Amnicola grana (Salkehatchie River at Yemassee, SC). Lower right - Amnicola limosa (Salkehatchie R., Bamberg Co., SC).

Looking back, I suppose my awakening began upon hearing Charles Watson's excellent talk at the FMCS meeting in Chattanooga in 1999. Watson reported that he was unable to distinguish among any of the three (nominal) species of Georgia Somatogyrus (tenax, alcoviensis, and rheophilus of the Flint River, a Gulf drainage) "using the characters given in the literature for separating these species." Watson continued, "The supposedly diagnostic characters appear to be subject to more variation than the original authors realized." Watson's paper also highlighted to me how very close the Broad River of Georgia is to the Chauga River of South Carolina, which is inhabited by a population of hydrobiids I'd assumed to be Gillia.

My awakening was completed just last year, when I first critically examined bona fide Gillia in Walker's collection at the University of Michigan. Adult Gillia commonly bear shells in the 7 - 8 mm range, but the snails I'd been calling "Gillia" in SC were rarely much more than 4 - 5 mm. And bona fide Gillia are not heavily shelled, which makes sense for the Carolina lowcountry canal that was the type locality. So the small, very heavily-shelled snails I'd been finding on rocky substrates in the midlands and upstate must be Somatogyrus, not Gillia!

Currently I have records of at least three Somatogyrus populations in SC, inhabiting the Chauga River and Stevens Creek of the Savannah drainage and the Big Cedar Creek of the Broad/Santee drainage near Columbia. And in the course of a larger survey of the southeast currently underway with my colleagues Brian Watson, Tim Stewart, and Will Reeves, we have discovered populations in the Uwharrie, Green, and Eno Rivers of North Carolina. We have unconfirmed reports of Somatogyrus in the James River of Virginia. Just two weeks ago I rediscovered Somatogyrus in the Rapidan River, the type locality of S. virginicus, where it had been feared extinct.

I have compared the morphology of these populations to topotypic S. tenax from the Broad River of Georgia, as well as to S. alcoviensis from the Apalachee River, and found all populations indistinguishable. There are no recorded anatomical differences - all Somatogyrus have simple single-ducted verges with no accessory lobes or crests. Thompson differentiated his S. tenax from S. virginicus by shell character only - "by its smaller size, sloping unshouldered whorls and perforate shell." But the shells born by all populations I have examined show quite variable adult size, shoulder shape and umbilical perforation. It seems clear that a single species of Somatogyrus is widely distributed in piedmont streams from Virginia to Georgia, and that the nomen S. virginicus has priority. It also seems likely to me that this same species extends (or has extended) up to the Susquehanna River in Pennsylvania and down to the Gulf drainages of Georgia, under the aliases S. pennsylvanicus and S. rheophilus.

In retrospect, two factors contributed to the long darkness I suffered regarding hydrobiids in the southeast. One was an over-reliance on historical distribution records, which must reflect collection effort. I assumed that Somatogyrus did not occur in the Carolinas because I could find no record of it, but now it would appear that there is no record of it because nobody has ever really looked. And the second factor was the reliance I have placed on written descriptions, dichotomous keys and published illustrations, rather than on bona fide specimens. Burch's couplet is correct in its first half - the shell of Somatogyrus is indeed much more "thick and solid," and its columella is indeed "thickened" compared to Gillia, and if I had compared shells of genuine Gillia to my specimens, I wouldn't have made mistakes for so many years.

The South Carolina Hydrobiidae are compared above. To be complete, I have included a fourth hydrobiid species from this state, the much-undercollected Amnicola grana. Amnicola grana is as vanilla as the other three local species in shell morphology, but bears an operculum that blossoms from multispiral to paucispiral as it ages, and never reaches much more than 3 mm. I've only recorded it from a few sites in the South Carolina lowcountry, but a rigorous effort would doubtless uncover more.

Somatogyrus alcoviensis has a "global conservation status" of G1 (critically imperiled) on the NatureServe Explorer database, S. virginicus is ranked G1G2 (imperiled) and S. tenax is G2G3 (vulnerable). There's also mention of a "Somatogyrus sp. 1 from North Carolina" in the NatureServe database, with no suggestion regarding conservation status. So the good news is that the single species, S. virginicus, properly understood to range from Virginia to Georgia, is not is not as endangered as some have feared. But the bad news is that Gillia altilis is probably not as widespread as previously believed. I have not seen any specimens recently collected from this state, and its range (almost strictly Atlantic coastal plain, according to Thompson 1984) has been heavily impacted by urbanization. And so the closing moral of this essay is an old one, which bears repeating. The accuracy with which we target our conservation efforts is critically dependent upon an understanding of the distribution, abundance, and taxonomy of the creatures we intend to protect.

The figure below shows strikingly high densities of Somatogyrus on rocky shoals in the Apalachee River, Georgia, May 2004. Some individuals have been stranded by lowering water levels. Click on the figure and zoom in for a closer look in the cracks.


Krieger, K. A. (1972) Somatogyrus alcoviensis, a new gastropod species from Georgia (Hydrobiidae). Nautilus 85: 120 - 125.
Mazyck, W. (1913) Catalog of Mollusca of South Carolina. Contributions from the Charleston Museum, Vol. II. (P. Rea, ed.). Charleston, SC, Charleston Museum. 39 pp.
NatureServe Explorer: http://www.natureserve.org/explorer/index.htm
Thompson, F. (1969) Some hydrobiid snails from Georgia and Florida. Quart. J. Florida Acad. Sci. 32: 241-65.
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Watson, C. (2000) Results of a survey for selected species of Hydrobiidae (Gastropoda) in Georgia and Florida. In Freshwater Mollusk Symposia Proceedings, Part II, eds. Tankersley, Warmolts, Watters, Armitage, Johnson & Butler, pp. 233 - 244. Columbus: Ohio Biological Survey.

Tuesday, March 23, 2004

Report from Tuscaloosa

I'm pleased to report that registration topped 100 at the FMCS gastropod workshop at the University of Alabama last week. The weather was warm, the facilities adequate, and our hosts most gracious. A good time was had by all.

FMCS president Tom Watters opened the water quality portion of the conference Monday and Ken Brown spoke on the status of freshwater snails in the southeast Tuesday, both talks I was sorry to miss. The meat of the gastropod conference was served on Wednesday, with 9 talks of 30 - 45 minutes each, organized systematically. The nice young folks making the presentations were all well-prepared and thorough, compensating for lack of experience with extra measures of enthusiasm. Perhaps the main message of the day was that identifying freshwater gastropods ain't brain surgery. A bit of background, a key reference or two, patience and willingness to learn are all one really needs. Here's a download of the workshop "Primer":

Perez, Clark, & Lydeard (2004) FMCS Freshwater Gastropod Identification Workshop [PDF]

Most of the Wednesday presenters were, to borrow Kathryn Perez's apt turn of phrase, "channeling Dr. Burch." The two exceptions were Amy Wethington, with her Ph.D. dissertation on the Physidae recently defended, and (of course) Jack Burch himself. Jack used the occasion to repeat his long standing quibbles with Hubendick's (1951) monograph on the Lymnaeidae, arguably the greatest work on any family of mollusks ever published. He prefers a modification of the systematic arrangement of F. C. Baker which, innocent of the modern synthesis, was based almost entirely on shell characters. Jack reviewed some micro-Ouchterlony results he obtained years ago which seem to support the Baker classification. I would love to see these data published in the peer-reviewed literature, where they might be objectively evaluated.

Amy Wethington has 15 years of direct experience with the Physidae and co-authorship of about a dozen peer-reviewed papers on various aspects of their biology. Her classification, based on hundreds of DNA sequences, allozymes, anatomy, reproductive biology and ecology, would reduce the number of North American species from 40 to about 10, and genera from four to two. Oddly, the editors of the meeting's Gastropod Identification Workbook preferred* the 23 - genus classification of Taylor (2003), based on features of penial anatomy only Dr. Taylor can see, which if they exist, are demonstrably immaterial to the snails themselves. Amy paid for a 20-page supplement out of her own pocket, fairly and objectively reviewed all competing classifications of the Physidae, and emerged as the hero of the meeting. Here's a download of Amy's Supplement:

Wethington, A. R. (2004) Family Physidae [PDF]

Thursday's presentations dealt with overarching topics, especially taxonomic methods and ecological applications. The highlight of the day for me was a dispatch from the front lines of freshwater gastropod conservation couriered by Steve Ahlstedt and Paul Johnson. Steve reviewed his many years of transplantation experiments with Io, and Paul reported his more recent successes with captive propagation. I used my presentation on freshwater gastropod distribution and ecology to introduce a new web resource, the Freshwater Gastropods of South Carolina.

A nice variety of freshwater gastropods, both the living and the dead, kept mute witness on the side tables during the two day event. The "show-your-shells" social Wednesday evening was a big hit - I myself learned quite a lot from the interesting assortment of specimens carted to Tuscaloosa from the four corners of this great land. Laurels are due to meeting chairman Chuck Lydeard and his hardworking young colleagues David Campbell, Stephanie Clark, Kathryn Perez and Jeffrey Sides. Well done everybody!

*P.S. Chuck Lydeard has asked me to forward to the group an explanation for his choice of the Taylor classification over the Wethington classification for the Physidae chapter of FMCS Workbook. Apparently it was easier to format the poorer classification to fit strict workbook guidelines.

---------[From Chuck Lydeard 25Mar04]-------------

As co-editor of "A Primer to Freshwater Gastropod Identification" (Perez, Clark, and Lydeard), I would like to explain our decision to opt for Taylor's (2003) classification scheme of the Physidae for the workbook. First, we chose to adopt rather strict guidelines for ALL authors to follow including a brief introduction of the family, a small table giving a few species and their conservation rank, and a general description and illustration for each genus in the family. The oral presentations were also supposed to adhere to the guidelines offered for each family account. Taylor (2003) offers the latest published account of the family. Taylor's anatomical renderings for each genus are very useful and provide an opportunity for the reader to see the entire male reproductive tract with labels. Taylor has about 40 years of experience with freshwater mollusks including physids, so he is certainly knowledgeable about the group. Of course, our use of Taylor (2003) should not be construed as an endorsement of his findings, nor should it be construed as a dismissal of Amy Wethington's dissertation findings. I was Amy's dissertation advisor, so I certainly appreciate and value what she accomplished during her time as one of my graduate students. However, I did not think all the intra-generic details she was willing to provide was necessary for the workbook and certainly would have altered our format appreciably. In closing, we are pleased with the workbook and feel it fulfills its purpose as a primer to gastropod identification reasonably well. We hope you will all look to the future for published phylogenetic studies about various freshwater families of gastropods.

Chuck Lydeard