Dr. Rob Dillon, Coordinator

Wednesday, December 16, 2015

The Many Invasions of Hilton Head

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d) The Many Invasions of Hilton Head.  Pp 81 - 93 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

The history of the oblong, 109 kmpatch of South Carolina coastland now identified as “Hilton  in Head Island” is brief, but eventful.  During the glacial cycles of the last million years, the pricey patch of beachfront real estate between Beaufort and Savannah has been alternately inundated and dewatered well inland, only upon rare and fleeting occasions, evolutionarily speaking, presenting itself as an island.  But by 1663, at initial entry into the logbook of Capt. William Hilton [1], it had acquired the topography typical of a Carolina-Georgia “sea island,” separated from the mainland by winding estuaries and extensive Spartina marshes.

The front of such a sea island is typically decorated with a broad beach of white sand, backed by parallel dunes between which water collects, yielding pools of varying persistence and freshness.  Grasses and palmettos on the front dunes yield to live oaks decked with Spanish moss in the island interior, which together collect every photon of light, leaving very little understory vegetation.  The gastropod fauna, both terrestrial and freshwater, can be surprisingly diverse.

Bombardment of Ft Walker
But between 1700 and 1860 Hilton Head Island was entirely deforested and converted to intensive row-crop agriculture [2].  The first successful crop of long-staple “sea island cotton,” legendary for its silky texture, was harvested from Hilton Head in 1790.  At the outbreak of the Recent Unpleasantness, there were over 20 working plantations on Hilton Head Island, and all surface water on the island ditched, diked, dammed, and carefully controlled.

The Union seized Confederate Fort Walker, located at the northern end of Hilton Head Island, in the Battle of Port Royal on November 7, 1861.  The massive amphibious invasion was co-commanded by Gen. Thomas W. (Tim) Sherman and Admiral Samuel F. Du Pont.  During four years of military occupation, the island population swelled to exceed 40,000 troops [3], camp followers, and freed slaves.  After the war the economy of the entire region was shattered, and the island gradually returned to forest.  By the 1950s the population of Hilton Head had dipped as low as 300 residents, essentially all the descendants of freedmen [4].

If the first invasion of Hilton Head was agricultural, and the second military, the invasion that stepped off in the summer of 1956 was motivated by the allure of cheap real estate.  For on May 19, 1956 the James F. Byrnes Bridge was dedicated to admit automobile traffic from the mainland.  And in 1957, real estate developer Charles E. Fraser formed “Sea Pines Company” and began subdividing residential properties on the island for sale.

It is difficult to overstate the importance of Sea Pines Plantation as a model for commercial
"Harbour Town" at Sea Pines
development, both elsewhere on Hilton Head and all along the Carolina-Georgia coast.  In the next thirty years the island was partitioned into a dozen gated communities with private beaches, golf courses, yacht harbors and tennis clubs.  Elaborate networks of roads, thousands of lavish houses, and the services to support them cannot be constructed without extensive recourse to the bulldozer.  But to the extent possible, Charles Fraser and the developers who followed him endeavored to maintain at least the appearance of nature, as that noun had come to be locally understood, given invasions #1 and #2.  They left some trees, anyway [5].

So last month I made passing reference to the interest of my colleagues at the Department of Natural Resources in the recent invasion of South Carolina by Pomacea apple snails [6].  I was first contacted by Ms. Elizabeth Gooding, a Wildlife Biologist I working on the SCDNR Pomacea project back in February of 2015.  And I was most gratified to discover, even at that early date, that Ms. Gooding and her colleagues were interested in our entire freshwater gastropod fauna, not just the Pomacea.  The study design called for a stratified random sample of 100 ponds across the five coastal counties of South Carolina [7].  Given that only four populations of Pomacea have (to this date) been documented in the Palmetto State, Ms. Gooding and her colleagues realized that they were setting themselves up to shoot a lot of blanks.  Thus the study plan that matured as a solution to the evils of invasion-biased oversampling may have been born as an antidote for boredom.

Ms. Gooding and I kept in touch as the 2015 field season progressed.  I also enjoyed getting to know Ms. Tiffany Brown, an undergraduate who worked on the project during the summer.  It was upon a 7/27 email from Ms. Brown that our story now turns: 
Good afternoon Dr. Dillon, I request your assistance one more time in identifying these freshwater snails. The snail labeled K (in the attached jpeg) is interesting because referring back to your guide and dichotomous key, this doesn't seem to be a South Carolina snail and was found in a Beaufort County pond.
Tiffany’s “Snail K” is the old world thiarid Melanoides tuberculata, of course, invasions of which are well-documented in Florida, Texas, and scattered about the American West, but heretofore unknown in South Carolina [8].  I wondered immediately whether her collection might represent an established population or a singleton aquarium refugee.  So I replied requesting more complete locality data, and inquiring about her sample size.  Ms. Brown answered that her sample size was N = 1.  And her sample pond was on Hilton Head Island.

M. tuberculata, Hilton Head
Most of the private developments on Hilton Head today garrison both an outer checkpoint to protect the well-to-do from the unwashed masses, and inner checkpoints, to protect the genuinely wealthy from the merely well-to-do.  Although the coordinates sent to me by Ms. Brown showed the SCDNR sample sited deep within the second line defenses of a jealously-guarded enclave called “Palmetto Dunes,” satellite imagery suggested a connection through a series of ditches and moats to a sample point that might be more lightly defended.  I resolved to attempt an invasion of my own.

Saturday, 22Aug15, was S-day.  I was unwittingly waved through the first line of defenses by young men wearing orange vests, apparently assuming I was attending some public event of which I knew nothing.  I then parked in the complex of members-only tennis courts and club houses, donned my camo, and proceeded by footpath to a moat running between the parking-for-guests-only Marriott and a machine gun nest guarding the Palmetto Dunes keep.

As my eyes adjusted to the dim light in the ditch under the jungle of bayberry and greenbrier, I was able to distinguish three things: water, sand, and Melanoides tuberculata.  The snails were grazing at densities around 100-200/m2 through a fine layer of organic sediment all over the coarse sand bottom.  All ages and size classes represented – clearly an old and well established introduction.  The water was brackish to the taste, although I had no equipment to measure salinity with me that particular afternoon [9].  I also found hydrobiids very common on the dead leaves and woody debris, which I took to be Littoridinops.  I cast about for perhaps 10 – 15 minutes looking or other freshwater gastropods, but given the salinity, was not surprised by the absence of any additional species.  I was in and out unscathed in 30 minutes.

But any hope I might have harbored that a single pinpoint strike could adequately sample so complex a biota as that of Hilton Head was doomed to disappointment.  The hydrobiids I collected from the organic debris in that brackish ditch were not Littoridinops.  The little sample that spilled into the dish under my dissecting scope Monday morning demonstrated a Promethean diversity of shell morphology – short & fat, tall & skinny, dark & pale.  Some shells even bore crenulations or short spines on the whorl shoulders, reminiscent of (even surpassing!) the shell polymorphism one sometimes sees in Potamopyrgus.  The females bore embryos in a brood pouch, again as in Potamopyrgus.  But males were well-represented, bearing a cochliopine penial morphology, like Littoridinops.  I had stumbled upon a population of the hydrobiid genus Pyrgophorus.
Pyrgophorus parvulus, Hilton Head

The natural range of Pyrgophorus is usually given as the Caribbean rim: Cuba, the Lesser Antilles, Venezuela, Mexico, Texas and Florida [10].  As one might expect for a population of snails bearing such diverse shell morphology, the literature includes over 40 specific nomina assigned to Pyrgophorus, Hershler & Thompson [11] “uncertain if more than just a few of these are valid or if only one should be recognized.”  The oldest name available on the list is Pyrgophorus parvulus, described by Guilding from the Caribbean Island of St. Vincent in 1828.

My SCDNR colleagues were most interested to hear the news of not just the one, but two exotic freshwater gastropod populations on Hilton Head.  And together we began to plan additional expeditions, more heavily-reinforced than my commando raid of 22Aug15.  In an SCDNR vehicle, one can (generally) obtain access to even the holiest sanctums of Hilton Head with a simple declaration of intent.

On S2-day, 5Nov15, Elizabeth Gooding and I established that the Melanoides and Pyrgophorus populations extended throughout most of the brackish ditches and ponds of the Palmetto Dunes development.  We also discovered that the ponds and ditches of the Shipyard Plantation and Long Cove developments neighboring to the immediate South and West were fresh – apparently isolated from the Palmetto Dunes system by low dikes of some older vintage.

It was in a shallow pond by the main road through Shipyard Plantation that Ms. Gooding and I discovered a large and dense population of yet another freshwater gastropod invader, Biomphalaria havanensis.  The natural range of Biomphalaria (listed as either obstructa or as havanensis, see note 12 below) was given by Malek [13] as Texas, Louisiana, Florida, Mexico, Puerto Rico, and Cuba.  The FWGNA database contained but two previous records of Biomphalaria in southern Atlantic drainages, a Charleston population I documented [14] in 1992 (now perhaps extinct?) and a 1960 record from McIntosh County, Georgia, that I have been unable to confirm.  Hilton Head Island may today be home to the only viable Biomphalaria population north of Florida.

Ms. Gooding samples Biomphalaria
The Biomphalaria were crawling at about 50 - 100/m2 on leaves, detritus, and sparse aquatic macrophytes in shallow water uniformly across the bottom of our shallow pond.  Also present was a large population of Physa acuta and lesser densities of Helisoma trivolvis, Hebetancylus excentricus and Laevapex fuscus.  Ms. Gooding and I determined that the Biomphalaria population extended through canals and roadside ditches at least 1 – 2 km beyond Shipyard Plantation into the community.

And in the Long Cove Subdivision, just across William Hilton Parkway from Palmetto Dunes and Shipyard Plantation, we discovered a dense and apparently healthy population of Bellamya japonica.  Good grief!  Bellamya introductions are not uncommon in the larger impoundments and reservoirs of the Carolina mainland, but this is the first record of a sea island population, to my knowledge.  The serpentine system of ponds we sampled in a residential section of Long Cove was also inhabited by large populations of Physa acuta, Helisoma trivolvis, and Hebetancylus, and we picked up a couple Lymnaea columella as well.

I don’t think it has appeared on anybody’s radar screen as yet, but it is my impression that the range of Hebetancylus has been expanding up from the south significantly in recent years [15].  And if you asked anybody with any knowledge of freshwater gastropods in Europe, Asia, Africa or South America, he’d tell you that our North American Physa acuta, Helisoma trivolvis, and Lymnaea columella can be spectacularly invasive everywhere else in the rest of the world.

On S3-day, 14Dec15, Elizabeth, Amy Fowler and I expanded our survey south to include Sea Pines Plantation and the Wexford subdivision.  Although we did not confirm any of our (now four!) nonindigenous freshwater gastropod populations in the quarter of the island occupied by Sea Pines, we were most impressed by the locally heavy infestations of the dreissenid mussel Mytilopsis leucophaeata, in ponds of salinity as low as 1.2 ppt.

Riding back to Charleston on the evening of 14Dec15, it occurred to me that I had spent three full field days sampling a freshwater benthic community comprised entirely of invasive species.  At some time scale, this insight is trivial.  Hilton Head didn’t even exist at the last interglacial period, so its entire freshwater and terrestrial biota must be invasive at a scale of 105 years [16].  But the gastropod community my SCDNR colleagues and I have been sampling this fall looks 102 invasive to me, and might even be 101 invasive.  If Capt. William Hilton, Gen. Tim Sherman, or Mr. Charles Fraser had left us any freshwater gastropod data, we’d have a better estimate.

It never hurts to remind ourselves occasionally that all biotas are dynamicMelanoides tuberculata and Pyrgophorus parvus turned out to be species #68 and #69 on the list of freshwater gastropods documented from the nine-state Atlantic drainage region that has been the focus of FWGNA activities thus far.  Which means that the old 67-species “Synthesis” of the distribution of commonness and rarity we published back in 2013 [17] is already obsolete, just two years later.

So if I must re-run the entire overall synthesis, it occurred to me that I might as well add the 740 fresh records that have accumulated in the FWGNA database over the last two years.  And so the bottom line for the present essay is that in the last couple months I have uploaded an almost entirely fresh “v11/15” of the FWGNA website, with new state and regional totals, new line maps, a new synthesis, and new incidence ranks [18].  I’d like to blame Hilton, Sherman, or Fraser for all this additional data churn.  But I suppose it’s just inevitable.


[1] “The Lands are laden with large tall Oaks, Walnut and Bayes, except facing on the Sea, it is most Pines tall and good.”  Read more at the Heritage Library of Hilton Head Island, here: [html]

[2] The best historical chronology I’ve found on the web is available from the Town of Hilton Head Island, here: [html]

[3] The 54th Massachusetts Infantry Regiment, made famous by the Academy-award-winning film “Glory,” was posted on Hilton Head for several months in 1863, prior to their ill-fated attack on Battery Wagner.

[4] Although set on adjacent Daufuskie Island, Pat Conroy’s (1972) memoir “The Water Is Wide” (and its Hollywood adaptation, “Conrack”) are especially evocative of this era.

[5] I met Charles Fraser in Washington in 1983, when I was a AAAS fellow, working on Section 404 of the Clean Water Act.  I remember his rising, as the first speaker at our first advisory panel meeting, to recite a verse from Sidney Lanier’s “The Marshes of Glynn.”  I don’t remember his doing of much else.

[6] I have three previous posts on our local Pomacea invasion:
  • Pomacea spreads to South Carolina [15May08]
  • Two dispatches from the Pomacea front [14Aug08]
  • Pomacea News [25July13]
[7] Nothing published as yet, but here’s a flavor of the project:
Gooding E., Brown T., Kingsley-Smith P., Knott D., Dillon R., and Fowler A. (abstract) The spread and potential impacts of freshwater invasive island snails (Pomacea maculata) in coastal South Carolina, USA.  Nineteenth International Conference on Aquatic Invasive Species, Winnipeg, CA.  (Upcoming April 10 – 14, 2016) [pdf]
[8] My regular readership will remember, however, that the USGS Nonindigenous Aquatic Species Database does contain a 2001 report of Melanoides tuberculata in coastal North Carolina.  See:
And my regular readership may also begin to perceive, dimly, what brought me to poking around in the especially untidy corner of the internet occupied by the USGS-NAS earlier this fall.  And prompted me to launch this entire series on invasive species, which shows no signs of ending, here three months later.

[9] Measurements that Ms. Gooding and I took in November from the ditch inhabited by the Melanoides and Pyrgophorus populations returned a (remarkably high) salinity of 14.5 ppt.  To the north and east, both populations extend into salinities as high as 17.9 ppt.  Zowie!

[10] Harrison, A. D. (1984)  Redescription of Pyrgophorus parvulus (Gastropoda: Hydrobiidae) from St. Vincent, St. Lucia, and Grenada, West Indies.  Proc. Acad. Natl. Sci. Phila. 136: 145-151.

[11] Hershler, R. & F. G. Thompson (1992)  A review of the aquatic gastropod subfamily Cochliopinae (Prosobranchia: Hydrobiidae).  Malacological Review Supplement 5: 1 - 140.

[12]  For many years there was a great deal of uncertainty regarding the identity of Biomphalaria havanensis at its type locality in Cuba, and hence no consensus on the identity of populations here in the USA.  But see:
  • Yong, M, Pointier J-P. & Perera,  G. (1997)  The type locality of Biomphalaria havanensis (Pfeiffer 1839).  Malacological Review 30: 115-117.
  • Yong, M., Gutierrez, A., Perera G., Durand P. & Pointier J-P. (2001)  The Biomphalaria havanensis complex (Gastropoda: Planorbidae) in Cuba: A morphological and genetic study.  Journal of Molluscan Studies 67: 103 - 111.
[13] Malek, E. (1985)  Snail hosts of schistosomiasis and other snail-transmitted diseases in tropical America: A manual. Washington, D.C., Pan American Health Organization.  325 pp.

[14] Dillon, R. T., Jr. & A. Dutra-Clarke (1992)  Biomphalaria in South Carolina. Malacological Review, 25: 129-130. [PDF]

[15] My old colleague, the late Julian Harrison, reported the first South Carolina population in:
Harrison, J. R. (1989) The freshwater limpet Hebetancylus excentricus (Morelet) in South Carolina (Abstract).  ASB Bulletin 36(2): 110.
[16] Amy Wethington and I demonstrated this phenomenon more rigorously in:
Dillon, R. T., Jr., and A. R. Wethington (1995) The biogeography of sea islands: clues from the population genetics of the freshwater snail, Physa heterostropha. Syst. Biol. 44: 400-408.  [pdf]
[17] I posted two essays describing the 2013 FWGNA “Synthesis” in considerable detail:
The 11/15 version of this analysis (currently online) features a somewhat enlarged data set, but is otherwise identical in approach.

[18] See last month’s formal announcement for additional details:


  1. were there signs of Mammal predation on the Bellamya?

    1. Fred - No, I didn't see any such sign at the particular point we sampled on Hilton Head last month. I have seen obvious mammal predation of Bellamya populations elsewhere in South Carolina, however. Muskrats seem to take a LOT of Bellamya downstream from the Wateree Dam, if you remember my post of 5Aug14, for example.