The history of the oblong, 109 km2 patch 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.
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| 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
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| "Harbour Town" at Sea Pines |
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.
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| 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.
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| 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.
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| 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 dynamic. Melanoides 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.
Notes
[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]
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:
- To Only Know Invasives [16Oct15]
[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.
[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:
[18] See last month’s formal announcement for additional
details:
- FWGNA Version 11/15 [19Nov15]