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:

Thursday, November 19, 2015

FWGNA Version 11/15

We are pleased to announce that a new version Nov15 of the FWGNA website is now on line and open for business.  This is not a new operating system by any means - just an incremental point-one upgrade.  But certainly worth a notice, nonetheless.  Click the FWGNA logo at right to check it out.

The refreshed site is based on 12,211 records collected from the Atlantic drainages of our nine-state coverage area [1], up 6.5% from the 11,471 records upon which our previous Oct13 version was based.  Most of the 740 new records come from South Carolina, for which we owe a debt of gratitude to Jim Glover, David Eargle, and Justin Lewandowski of the SC Department of Health and Environmental Control.  The Atlantic drainage fauna has now increased from 67 species to 69, with the addition of (apparently invasive) populations of Melanoides tuberculata and Pyrgophorus parvulus [2].  More about that next month.

All the statewide tabulations (GA, SC, NC, VA, and Mid-Atlantic) have been at least slightly updated, as has the overall FWGNA Synthesis for the Atlantic drainages together [3].  There are a few changes in the FWGNA incidence ranks for the rarer (and hence less stable) species, but nothing drastic.


[1] We have not updated our FWGTN survey of the Tennessee River drainages since 2011.  That chore is long overdue, we must confess.

[2] The total number of species covered by the entire FWGNA site is up from 87 to 89.

[3] The fit of the 69-species distribution of commonness and rarity to a lognormal model continues to worsen, now down to a Shapiro-Wilk W = 0.957 (p = 0.019).

Monday, November 9, 2015

To Only Know Invasives in My General Vicinity ...

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d) To Only Know Invasives in My General Vicinity.  Pp 73 - 79 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

A couple years ago I received a cordial email from a young lady working with a natural resources agency in a big Midwestern state.  She was looking for help developing a key to identify invasive freshwater gastropods in her general vicinity – not the native species, just the important ones.  Here’s what I suggested:

1a) Operculum present . . . (2)
1b) Operculum absent . . . never mind.

2a) Really big! . . Potentially devastating economic impact.
2b) Little . . . (3)

3a) Only a few of ‘em . . . screw it.
3b) Bazillions of ‘em! . . . Potentially devastating economic impact.

So last month we reviewed the USGS Nonindigenous Aquatic Species database [1], an open and earnest federal effort to monitor the distribution and spread of invasive species in the nation’s lakes and rivers, including (at this writing) 42 freshwater gastropods.  This month we’ll broaden our field of view, but sharpen our focus, looking toward state-level invasive species monitoring programs.

State interest in invasive species is typically directed rather narrowly toward perceived threats to agriculture or commerce or human health.  At the very least, the doggone thing from China or wherever it says it came from has got to make itself a nuisance.  So for example, my home state of South Carolina doesn’t really care about the huge populations of invasive viviparids in our reservoirs, unless (I suppose) there’s a significant die-off, and somebody complains about the stink [2].  Which sounds like a problem solving itself.  The South Carolina DNR does, on the other hand, care about our invasive populations of Pomacea, primarily because of its potential to host the rat lungworm [3].  More about that in a future post.

Mud Snail Poster - CaliforniaThe states of California, Oregon and Washington all care about the current range and future spread of Potamopyrgus antipodarium, the “New Zealand Mud Snail” because, to quote the Oregon Department of Fish & Wildlife’s modest web page [4], “there is concern [about] devastating effects to recreational fishing.”  So for more information, Oregon directs its citizens to the New Zealand Mud Snail page at the USDA National Invasive Species Information Center, from whence they might surf over to the USGS-NAS database and report their observation. 

California has developed a larger page on Potamopyrgus, featuring the snail on one nice, colorful poster and one blunt, scary poster (left), both available for download [5].  In addition to mentioning the USGS-NAS database, citizens of California who have observed Potamopyrgus are asked to call or email their Department of Fish & Wildlife directly.

Like California, the State of Washington also has a nice web page devoted to Potamopyrgus, with a downloadable fact sheet, although no keen posters [6].  There is, however, a “Report online” link from the Washington Potamopyrgus page where a citizen might open a (vanilla) online reporting form to send a potential sighting to the Washington DNR.  Or one might elect to download a “WA Invasives” app for one’s smartphone.  Impressive.

The point I’m making here, however, is that none of these three states apparently gives a rip about the populations of European Lymnaea (Radix ) auricularia which have spread all over the American West in the last 100 years, but apparently have not risen to the level of a threat, or a pest, or even a “nuisance.”  You’re just a fat little brown snail.  No smartphone app for you.

There are exceptions to this general rule.  Back in 2011 we featured a “Citizen’s Watch” initiative in the state of Wisconsin [7], broadly charged with monitoring invasive species of all sorts, as well as general aspects of the physical environment, water quality, and so forth, for specific lakes.  The Maine Volunteer Lake Monitoring Program [8] also features an invasive species component which, although focused on aquatic weeds, includes a mechanism to report “suspicious” aquatic organisms of all phyla.  Click the header below for the entire form:

 Maine Suspicious Organisms Report

The high end of invasive species monitoring programs seems to be occupied by a set of nine states (and one Canadian Province) who have adopted a NatureServe system called “ImapInvasives.”  This is an online system that rivals (and in many respects, surpasses) the USGS-NAS database in its scope and versatility.  Open the link below in a new window and check it out, if you’re not already familiar with the program.

If you click the “Login/Maps” button at the top of the iMapInvasives home page you will find a list of the ten states and provinces currently subscribing, from whence you might be tempted to follow links to try the system out.  ImapInvasives is designed as a monitoring system for exotic biota of all sorts – weeds, bugs, slugs, stoats, goats, giraffes, Giselles, and especially rabbits.  Only four states list freshwater gastropods on their public maps at present: Maine (Bellamya), Pennsylvania (Bellamya, Potamopyrgus), Oregon (Bellamya, Potamopyrgus [9], Melanoides), and Arizona (Bellamya, Melanoides, L. auricularia).

The “public maps” viewable from the several states listed above are disappointingly low in their resolution – returning just county or watershed (8-digit HUC) of record, not the precise locality.  They also show only a subset of the species actually tracked, in some cases.  To see a detailed map, or access locality data, or contribute fresh data to these systems, one must be a registered user.

Over the last couple years I have developed a cordial relationship with Ms. Amy Jewitt, the invasive species information assistant working for the Pennsylvania Natural Heritage Program.  She offered to register me for the Pennsylvania site, and I can report that it is as easy to use and informative on the inside as the USGS-NAS system is on the outside. 
Bellamya chinensis on iMapInvasives
Pennsylvania actually tracks the entire rogue’s gallery of invasive freshwater gastropods in its iMapInvasives system as implemented for registered users, including Bithynia and Viviparus as well as Bellamya and Potamopyrgus.  The Bellamya are sorted correctly (I have reason to think) into 33 records of chinensis and 18 records of japonica, with just one single record of “species uncertain.”  The reason I have reason to think the Pennsylvania identifications are correct is that most of their records are ones I forwarded to Ms. Jewitt from the FWGNA database back in 2013.  And she continues to send me jpegs for identification, as new sightings are reported.

So how well-coordinated are the state and federal efforts?  One of my prompts to launch the present series on invasive species came in late September when I happened to receive emails from both Ms. Jewitt and Mr. Matt Cannister, a contractor working with the USGS office in Gainesville, both with attached Bellamya jpegs.  Mr. Cannister’s response to my question was quite positive: “there is usually a high level of information sharing between state and federal biologists and ourselves here at NAS.”  Ms Jewitt was more guarded, albeit hopeful:
“As of right now, we do not have any of the USGS data in PA iMap and we have not sent any of our data to them, though we hope to change this in the very near future.  There is one big task that needs to happen on our end first before we can begin data sharing with them; however, once that happens, the PA iMap database will certainly have much more useful information to provide.”
I should conclude this month’s essay by re-assuring everybody that I thumbs-up-like online governmental programs to monitor invasive species.  They are a mixed blessing for the FWGNA project, which is, after all, an effort to survey of an entire fauna, and subject to the phenomenon I identified last month as invasion-biased oversampling.  But nothing I have written in my last couple posts is intended to reflect negatively on the hardworking public servants at any level, state or federal, who are doing their best to deal with the rising tide of exotic species inundating our shores.

Budgets are small and personnel are few, and I certainly understand why natural resource agencies must pick (at most!) a couple invasive freshwater gastropods that seem the most intimidating, and (on the flip side) a couple nominally “endangered” freshwater gastropods that seem the most pathetic, and let the other 95% of our North American freshwater gastropod fauna fend for itself.  They’re just little brown snails.


[1] To Only Know Invasives [16Oct15]

[2] Just Before the Bust [5Aug14]

[3] Pomacea Spreads to South Carolina [15May08]
     Two Dispatches from the Pomacea Front [14Aug08]
     Pomacea News [25July13]

[4] Oregon Department of Fish & Wildlife [New ZealandMud Snails]

[5] California Dept of Fish & Wildlife [New Zealand Mud Snails]

[6] Washington Invasive Species Council [New Zealand MudSnails]

[7] Dispatches from the Viviparid Front [12Sept11]

[8] Maine Volunteer Lake Monitoring Program [html]

[9] Alert readers will notice that I just reviewed the Oregon Potamopyrgus page about six paragraphs earlier in this essay, and said nothing about iMapInvasives.  Yep, strange but true.  I cannot find a link from any agency of the State of Oregon to iMapInvasives.  Maybe this is just an oversight?  But it is consistent (I am afraid) with the general inhospitality of the iMapInvasives system.

Friday, October 16, 2015

To Only Know Invasives

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019d) To Only Know Invasives.  Pp 63 - 71 in The Freshwater Gastropods of North America Volume 4, Essays on Ecology and Biogeography.  FWGNA Press, Charleston.

I’ve never compiled any statistics, but it is my impression that the most common category of inquiry I find delivered to my email inbox on any long-term basis probably bears the modifier “invasive” on the subject line.  Such messages are usually requests for identification of putatively exotic freshwater gastropods, with jpeg images attached.  Sometimes these inquiries come from professional biologists working with agencies, and other times just from ordinary concerned citizens.  I always try to help.

So a couple years ago I receive a variant of the typical email described above, from a NOAA biologist in the Great Lakes area.  The attached jpeg image depicted a couple beach worn shells of the common pleurocerid Pleurocera semicarinata livescens.  And in the body of his message the biologist confessed, amidst other routine background matter, “I really only know the invasives.”  He was quite certain that these freshwater gastropods were not Bithynia, or Bellamya, or Potamopyrgus.  But he didn’t know what they were.

And this struck me as a sad way of looking at the world.  One is reminded of a clinical practitioner who never leaves the hospital – only knowing the sick, never meeting the well. 

But in a larger sense, the existence of a biologist, or an agency, or indeed multiple governmental agencies, who “only know invasives” has become an article of public policy in recent years.  The Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 established a body called the “Aquatic Nuisance Species Task Force,” with representation from 13 federal agencies, which in turn established a National Nonindigenous Aquatic Species Information Center “to collect, analyze, and disseminate information about the presence and distribution of nonindigenous aquatic species and their effects.”  That Center is currently located at the US Geological Survey’s Southeast Ecological Science Center in Gainesville, Florida [1].

The most visible product of the USGS Information Center has been the development of the Nonindigenous Aquatic Species database, which can, under some circumstances, be a useful resource, if you understand its limitations, which are extensive.  Do open this site in a new window and check it out, if you’re not familiar with it already:

The USGS-NAS tracks a staggering 44 species (or categories of species) of freshwater gastropods: 11 ampullariids, 8 viviparids, 3 thiarids, 7 miscellaneous prosobranchs, 7 planorbids, 5 physids, and 3 lymnaeids.  Of this extensive list only 32 are exotic; 12 species or categories are native to North America, but have expanded their ranges so recently or so dramatically, possibly by human agency, as to attract the attention of The Feds.  Current distributions of these 44 species are displayed as good quality dot maps which are zoomable and clickable, to see the underlying records.  Public users are allowed to add records to the database through a process that is not terribly difficult or onerous, which certainly has a downside, but which (on the whole) I think is good.

The site is, however, a taxonomic mess.  It reminds me of my son’s bedroom, when he was nine years old.  Stuff is thrown around everywhere, and it would be very difficult to find anything you might want in there, if (hypothetically) you wanted to find it, and most of the stuff you can see when you pass by the open doorway is just junk, and should be thrown out.

USGS-NAS chinensis
For example, the USGS-NAS collects data on four separate categories of the Asian Mystery Snail: Cipangopaludina japonicaCipangopaludina chinensisCipangopaludina chinensis malleata, and "Cipangopaludina species.”   The most current science suggests, however, that just two species of mystery snail have been introduced to North America, best identified as Bellamya japonica and Bellamya chinensis.  It is very difficult for laymen, or indeed most field biologists, to distinguish japonica from chinensis.  In fact, both of the thumbnail photos of shells adorning the clickboxes for chinensis and japonica on the USGS-NAS website depict chinensis [2].

I really don’t trust the species-level identifications upon which any of the four “Cipangopaludina” maps are based, and would advocate combining all those data into a single map that simply shows “Bellamya species.”   If the mission of the USGS-NAS database is indeed to disseminate information about the “distribution of nonindigenous species and their effects,” B. chinensis and B. japonica are ecologically equivalent [3], and combining them would make sense.

USGS-NAS "japonica"
(not, actually)
The situation is much more complicated in the Ampullariidae, I fear.  It is not entirely clear how many exotic species of Pomacea have been introduced into US waters, but the best hypothesis at this writing is probably four: Pomacea maculata (aka insularum) which is the most common, Pomacea canaliculata (bona fide) which is very similar but more rare, Pomacea diffusa (aka bridgesii) which is smaller and not as voracious, and Pomacea haustorum, which is much larger and rare.  I myself am not sure I could distinguish maculata from canaliculata (bona fide) in the field, although their egg masses are distinctive [4].

So users of the USGS-NAS database will find these four differently-sized pies cut into seven differently-sized pieces: maculata, canaliculata, diffusa, bridesii, haustorum, cumingi, and “Pomacea species.”  Again, I really don’t trust any of the identifications upon which any of the resulting maps are based, and would advocate collapsing the categories ecologically, into “Exotics that do eat macrophytes” (maculata, canaliculata, haustorum, and probably “species”) and “Exotics that do not” (diffusa, bridgesii, and cumingi).

I have similar misgivings about almost all the pulmonate groups.  Lymnaea auricularia can be identified unambiguously, but the distinction between “Planorbella duryi” and “Helisoma species” gives me pause, and just the thought of clicking on some of those physid links scares me to death.  What the heck is exotic “Physella species?”  Some of the things in my 9-year-old son’s bedroom did not warrant close examination.

But to be clear, I am not criticizing the hard-working public servants who are doing their best to administer the USGS Nonindigenous Aquatic Species database.  They are trying to play baseball in a bluegrass jam session, bless their hearts [5].

And the most important feature of the USGS-NAS database is not the maps, but rather the ready
availability of the data upon which the maps are based.  Some of the USGS records are ancient extractions from vague literature reports, while others are rigorously documented and vouchered at major museums.  As long as we users can right-click and judge the quality of the records ourselves, I can’t see how any damage can be done.  And some benefit might certainly be had.

So what fraction of the USGS-NAS data might be useful for the FWGNA project?  And should we move to incorporate it?

At this point I should register a scruple.  The primary goal of the FWGNA project has always been to survey the distribution and abundance of the North American freshwater gastropod fauna, as objectively as possible.  Early on we identified a phenomenon I called “conservation-biased oversampling" [6] which is the (self-defeating) tendency for agencies to conduct surveys directed toward putatively-endangered species.  And I have consistently tried to avoid incorporating data which might reflect such bias in the FWGNA database, albeit with mixed success.

The USGS-NAS database very clearly demonstrates a related phenomenon, which I hereby dub “invasion-biased oversampling.”  I suppose if the USGS ran a parallel site, where biologists might upload data on the native species of freshwater gastropods they found in addition to the nonindigenous species recorded at each site, such a bias would not be a problem.  But they don’t, so it is.

With that dab of balm applied to my tender conscience, last week I spent a few hours fishing around in the USGS-NAS database [7], looking for good-quality freshwater gastropod records that might reasonably be added to the FWGNA.  I focused on the Atlantic-drainage fauna of the nine-state region from Georgia to the New York line, the region with which I am most familiar.  I disqualified the Bellamya (“Cipangopaludina”) records, the Pomacea records, and any weirdo pulmonate records for the reasons outlined above.  This left me with just the four taxa it is harder to mess up: Viviparus, Bithynia, Potamopyrgus, and Melanoides.

USGS-NAS V. georgianus
Well actually, Viviparus is a bit problematic.  The NAS database contains 235 records in total: 227 of Viviparus georgianus, 7 of Viviparus viviparus and 1 of Viviparus subpurpureus.  The distinction between (the putatively native) V. georgianus and (the European) V. viviparus simply is not clear [8], but lumping those two categories and sorting geographically, I was able to extract 10 records.  Three of those records corresponded to populations already in the FWGNA database.  Four referred to Viviparus populations in the Potomac River near Washington, of which I was already aware, but which I have been unable to confirm.  The other three records were from the 1970s, not vouchered or accompanied by sufficient data [9].

The USGS-NAS database also contains (a very similar) 247 records of Bithynia tentaculata, of which 9 fall in our study area.  All nine of these records refer to the Potomac River Bithynia population already well-documented in the FWGNA database.

Although the USGS-NAS database contains a whopping 1,210 records of the New Zealand Mud Snail, Potamopyrgus antipodarium, only one, single record falls within our nine-state study area.  That would be the population in Spring Creek, Centre County (PA) with which we are already quite familiar [10].

Similarly, the 94 records of Melanoides tuberculata in the USGS-NAS database also include but a single observation in our study area.  But it is a good one.  Record #153847 reports a 2001 collection of M. tuberculata in the small coastal town of Southport, NC.  Zooming the USGS map inward shows the collection site at Bonnetts Creek, near the entrance to the “Olde Southport Subdivision.”  And reference to the online database at the North Carolina State Museum shows a lot of 82 Melanoides tuberculata shells collected in Southport on 27Jan06 under catalog number 40541.  Super – can’t beat that.

Although I have seen secondary reports of Melanoides in North Carolina [11], this is the first well-documented, bona fide record of that species I have ever seen north of Florida, east of Texas.  So the
USGS-NAS Melanoides
USGS-NAS database can, at least occasionally, serve for the effective dissemination of “information about the presence and distribution of nonindigenous aquatic species” as it was designed to do, even to such a specialist as myself.

But let me conclude this month’s essay with another anecdote from the old mail bag.  A couple years ago I got an email from a colleague in Europe asking a simple question.  He wanted an estimate of the number of freshwater gastropod species in the US Great Lakes.  I was gratified to be able to answer promptly, and I did.  I said I didn’t know [12].   So my European correspondent wrote me back incredulous, and asked, “Do you mean to tell me that the US Government charges 13 Federal Agencies to record and monitor the latitude, longitude, date and sex of every pallid 5 mm European gastropod that sets foot onto American shores, but spends not one nickel and gives not one rip about any of its own native species whatsoever?”

Yes, I told him that was indeed the case, and suggested that we might either swap faunas, or swap governments, to solve the problem.  That former solution seems to be in the works.


[1] The actual legislative and regulatory history is much more complicated than this.  The 1990 act was amended and expanded by the 1996 “National Invasive Species Act,” and a body called the “National Invasive Species Council” was also created by Executive Order in 1999.  Exactly why the Nonindigenous Aquatic Species database landed in the US Geological Survey, as opposed to the Fish & Wildlife Service, or the USDA Department of Pests and Weeds, or the Coast Guard, or the Corps of Engineers (both of which were explicitly charged in the 1990 act) I have no idea.

[2] I do feel considerable sympathy for our long-suffering civil servants here.   It amazes me how often freshwater mollusk invasions are not single-species efforts, but rather seem to result from conspiracies of multiple species nearly indistinguishable from each other.  The zebra mussel / quagga mussel invasion is a prominent example, of course, as is the situation with the purple-Corbicula and the white-Corbicula.

[3]  Community Consequences of Bellamya Invasion [18Dec09]

[4]  Although the distribution data are vintage 2006, there are excellent illustrations of the various Pomacea species and their egg masses in a colorful PDF flyer available from the Florida Fish & Wildlife Commission:
  • Non-Native Applesnails in Florida [pdf]
[5] Science and Public Policy are not compatible, but they’re not incompatible either.  They’re two entirely different things.  See any of my essays under the menu item “Science and Public Policy” at right above for more.

[6] Toward the Scientific Ranking of Conservation Status – Part III [19Mar12]

[7] I simply blocked and copied from the “Collection Info” page(s) for each species of interest, then pasted into an excel spreadsheet.  I could have gotten a lot more data if I had requested a “custom query” from the NAS staff, but screw it.

[8] It seems possible to me that all the Viviparus populations that have spread throughout the American North and Midwest in recent decades may be a cryptic invasion of the European V. viviparus, but no actual data have been brought to bear on the question as yet.

[9] These three records from the 1970s got the sticky-note treatment.  I marked the creeks involved with sticky-notes on my Maryland map book.  Next time I’m in the area…

[10] Potamopyrgus in US Atlantic drainages [19Nov13]

[11]  Anderson, T. K. (2004) A review of the United States distribution of Melanoides tubeculatus (Muller, 1774), an exotic freshwater snail.  Ellipsaria 6: 15-18.

[12] This quote comes from Mark Twain’s (1875) Old Times on the Mississippi.  The dialogue between The Captain and the author continues:
“You—you—don’t know?"  The captain mimicked my drawling manner of speech. “What do you know?”
“I—I—nothing, for certain.”
“By the great Caesar’s ghost, you don’t know enough to pilot a cow down a lane.”

Tuesday, September 22, 2015

5:57 of Physa!

I stopped by to see our good friend Bobby Martin of Martin Microscope at the Society For Freshwater Science meeting in Milwaukee this past May. And we got to talking, and one thing led to another. And two months later, out popped a six minute video of an individual Physa acuta crawling around in circles. Enjoy!

Thursday, September 3, 2015

The Lost Thesis of Samantha Flowers

Warning.  The essay below is the fifth in a five-part series on species relationships in the enigmatic North American “stagnicoline” lymnaeids.  I will assume that you have read all four of my previous posts.  In addition, I also make explicit references to two essays in my 2012 series on this same subject, [10May12] and [4June12].  In fact, it would probably help if you started with my [20Nov06] essay on F. C. Baker, and read my [28Dec06] essay on the classification of the Lymnaeidae as well.  Stand back, I’m going to try Science! [1]

Subsequently published as Dillon, R.T., Jr. (2019b)  The lost thesis of Samantha Flowers.  Pp 95-106 in Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston.

Insofar as I was able to determine from my vantage point 820.64 miles south of Ann Arbor, Samantha made good progress on her thesis research through the 2012-13 academic year, and into the field season following.  I was pleased to see the abstract of a talk she gave at the AMS meeting in The Azores in July of 2013, although I myself was unable to attend.  And on 15Aug13 I received a very upbeat email from her, my first in over a year.  She reported that she was “currently on the last leg of my Master's journey, prepping a manuscript to recount my arduous tale of stagnicoline systematics that should be wrapped up within the next month or two.”  She also promised to keep me posted “for when the sequences are thrown up on GenBank [2].”  And that was to be the last I ever heard from her.

As that “month or two” stretched into 2014, with no reply to my repeated emails, I began to worry that something might be amiss with our promising young malacologist.  Googling around on the University of Michigan website, I was able to confirm that Samantha did indeed defend her thesis, “Inferences into species delimitation of Nearctic Stagnicola using geometric morphometric and phylogenetic methods,” on November 15, 2013.  The outcome I was unable to determine.  But surely, I thought, if her thesis were successfully defended and signed, it must ultimately appear for download (or purchase?) through some public outlet somewhere, yes?

No.  After more than a year of watchful waiting, in January of 2015 I finally emailed her major advisor, Tom Duda, to inquire about the fate of Samantha and her thesis.  Tom confirmed that Samantha’s 2013 defense was indeed successful, and that he himself was surprised not to find her thesis uploaded to the University of Michigan’s “Deep Blue” server.  Apparently The University does not have firm rules regarding the deposition of MS theses.  And Tom further confessed, “In the past we have requested that theses and dissertations be deposited in our Mollusk Division library, but regret that it was my oversight (in combination with her rapid departure and her not responding to emails after she left) that got in the way of this happening with Samantha.”

And in fact, as our conversation developed, it materialized that Tom did not have a clean, final copy of Samantha’s thesis himself.  He had apparently returned his only copy to her with written comments, and she disappeared.  I suggested that he might check with some of the other members of her committee, and he was able to locate a “near final form” version which he shared with me in April.  But Tom has asked me not to distribute the document any further, since the version from which I am working still has some errors.

Samantha’s Thesis [3] is a blockbuster. Her results simultaneously reinforce a large and growing body of research confirming the dramatic ecophenotypic effects of habitat on freshwater gastropod shell morphology, and shatter 200 years of set notions about systematic relationships in the North American stagnicolines.  Let’s digest her work in five steps.

First, Samantha’s CO1 sequence data suggest two biological species.  Perhaps some of you will recall the review of interpopulation sequence divergence in L. stagnalis I posted on the FWGNA blog in [4June12].  There I argued that the general rule-of-thumb estimate of 5% CO1 divergence often observed among biological species of pulmonate snails seems applicable to within-continent comparisons of lymnaeid populations worldwide.  This is not a law, it is a very broad-brush guideline [4].

So below I’ve reproduced a (rather heavily-edited) version of Samantha’s “Collapsed Bayesian-inferred CO1 tree,” with state and province abbreviations marking samples from ME = Maine, MI = Michigan, MB = Manitoba, and so forth.  Setting aside the single Lymnaea arctica sequence that Samantha mined from the Barcode of Life Database [5], the ABGD prior max distance bars at right seem to suggest the two clusters of stagnicolines I have labelled “V1” and “V2”.  Although the pairwise sequence differences between these two groups apparently do not consistently reach 5%, an eyeball estimate from Samantha’s scale bar, together with the plot of pairwise genetic distances Samantha offered elsewhere in her thesis, suggests to me that they probably often do.

Note that I have modified the noun “species” with the adjective “biological” here.  This is because populations of the two putative species seem to occur sympatically, at least in some cases.  More under my fourth point, below.

Second, the distinction between these two putative biological species does not coincide with taxonomic tradition, as historically based on shell morphology.  Samantha classified each of the individual snails she sampled for her C01 analysis using geometric morphometrics, digitizing their shell outlines with the large set of sliding landmarks [6] shown in the colorful figure I have reproduced at the top of this essay [7].  She recognized four nomina by shell fatness – identifying the green and red categories as emarginata, the gold as elodes and the blue as exilis.  The gold was an unfortunate color choice – nearly invisible between the red and blue in her figure. 

But in any case, the correct way to define any of these nominal taxa would have been by reference to populations sampled from their type localities.  God Knows I Tried to help Samantha with this critical component of her thesis, but for a variety of reasons, it just didn’t work out.  So I have deleted Samantha’s specific names from the CO1 tree above [8] and substituted simple color coding according to her morphometric analysis [10].  And it will be obvious that the four color categories do not correspond to the two putative biological species.  Cluster V2 shows all four colors, and cluster V1 shows three of the four.

Third, the putative biological species do coincide with Brady & Turner’s V1/V2 taxonomy.  Here I’ll ask you to open my essay of [10May12] in a separate window, and refresh your memory regarding Brady & Turner’s “cryptic stagnicoline” populations from NW Pennsylvania.  Although all four of the B&T populations inhabited fishless marshes, and all four bore dark, skinny shells typical of elodes, their “Hartstown Marsh” population demonstrated a consistently larger (and perhaps more “flat-sided”) body whorl than their Conley, Osgood, and Killbuck populations.  Kip Brady’s common garden experiments suggested that this body whorl difference seemed to be heritable.

Brady & Turner [11] considered that their Conley, Osgood, and Killbuck populations demonstrated “typical” L. elodes shell morphology, and called them V1.  They called their Hartstown Marsh population V2.  See note [12] for an interesting story about the example specimens figured at right below.

I forwarded samples of all four B&T populations to Samantha in July of 2012 [13].  And sure enough, samples from the Hartstown population (marked as PA-h on my version of Samantha’s CO1 tree) appeared genetically distinct from the Conley, Osgood, and Killbuck samples (marked PA-c,o and PA-k).  So although subtle, there does appear to be a shell morphological correlate to C01 sequence divergence between the two putative species.  The key character does not seem to be the traditional fat/skinny dichotomy, but rather the relative size of the body whorl [14].

Fourth, evidence suggests that the traditional taxonomy of North American stagnicolines may have been based on shell characters largely ecophenotypic in their origin.  The best example, ironically, comes from Douglas Lake, the home of the University of Michigan Biological Field Station.  Samantha sampled 4 individuals from the waters of Douglas Lake itself, all of these being classified as “emarginata-ovate” by her morphometric criteria, which I have marked with green letters “d” in the C01 tree above.  This small sample included three individuals belonging to putative biological species V2, and a single individual belonging to putative species V1.  Samantha also sampled 8 individuals from “Douglas Lake Pools,” presumably marginal ponds not directly connected to the lake itself.  All of these individuals were classified as exilis by Samantha’s morphometrics, and are marked with blue letters “d” above.  This included 6 individuals classified as putative species V2, and 2 classified as species V1.

Thus Samantha’s data suggest that two biological species of stagnicoline lymnaeids seem to co-occur sympatrically in Douglas Lake, both bearing fat shells of emarginata morphology in the main lake, and both bearing skinny shells of exilis morphology in marginal pools.  We search the world over, and sometimes the answers we seek are right on our own doorsteps.

And fifth, we do not actually know the correct names for either of the putative biological species.  Here I must pause, and wipe a tear from my eye.  For some reason known but to God, Samantha did not sequence that sample of topotypic L. catascopium I gave her in June of 2012.  Was this tragic oversight related to some sort of funky decision-making late in her research, regarding the taxonomy to be employed in her thesis as a whole?  See note [8] below for more.

In any case, as I have repeatedly emphasized (to Samantha, and to you all as well!), catascopium (Say 1817) is the oldest name available for any North American stagnicoline population.  One of Samantha’s two putative biological species almost certainly must be catascopium by definition, and the correct name of the other species depends.

So what to do?  Almost all of Samantha’s pale/fat snails, which might conventionally be identified as catascopium, were classified as V2.  This set included the sample I sent her from Maine, graphed as a big green triangle at the top of her C01 tree.  And almost all of Samantha’s V1 individuals demonstrated the dark/skinny shell morphology conventionally associated with elodesSo let us provisionally call the V2 species Lymnaea catascopium, leaving the name Lymnaea elodes for putative species V1.  This is admittedly a judgement call, but seems most consistent with the taxonomy currently employed by workers in the field.

Have I beat this horse long enough?  Let me conclude with two recommendations for further study.  First, the hypotheses advanced here can be tested with a good genetic survey of the stagnicoline populations inhabiting the Douglas Lake area.  Somebody needs to use microsatellites, or old-fashioned allozymes, or even older-fashioned breeding studies, to test the hypothesis that two reproductively isolated stagnicoline species are sympatric in that lake, not corresponding to the traditional fat catascopium / skinny elodes dichotomy, but rather corresponding to the new V1 elodes / V2 catascopium dichotomy.  And second, somebody needs to go back up the Delaware River and fetch us some more topotypic catascopium.  And find us some topotypic elodes at Lake Canandaigua, while shopping around in Yankeeland for lymnaeids anyway.  Not it.


[1] This catch phrase comes from the online comic, xkcd.com.  And although the xkcd logo shows a stick-figure scientist flamboyantly flourishing a beaker and a calculator, real science is at least as much theoretical as applied.  To be quite precise, science is the construction of testable models about the natural world.  This essay is real science.  Stand back.

[2] Not only did Samantha ultimately fail to make her MS thesis available from any public outlet, she also failed to “throw up” any of her sequence data on GenBank.  Alas.

[3] Flowers, S. L. (2013)  Inferences into species delimitation of Nearctic Stagnicola (Gastropoda: Lymnaeidae) using geometric morphometric and phylogenetic methods.  M.Sc. Thesis, University of Michigan, Ann Arbor.

[4] Two disclaimers.  First, gene trees are NOT species trees!  They are weak, null models of population relationship.  And second, there is no cut point for species-level sequence divergence that isn’t more exception than rule.  See, for example:
  • Phylogenetic Sporting and the genus Laevapex [20July07]
  • Gene Trees and Species Trees [15July08]
  • The Snails The Dinosaurs Saw [16Mar09]
  • What is a Species Tree? [12July11]
[5] Lymnaea arctica was one of the (only approximately ten) North American lymnaeids that Hubendick (1951) considered specifically distinct.  I wish I knew more about the taxon.  Might arctica demonstrate genetic affinities with the Old World palustris/corvus/fuscus group?  But I went over to the “Barcode of Life” database to see if I could find the PPCHU85 arctica sequence that Samantha analyzed in her CO1 tree, and came up dry.  Very frustrating.

[6] Samantha really should have digitized more than the shell outlines.  In particular, the relative sizes of the shell whorls, especially the body whorl, seem to contain a great deal of heritable information in freshwater gastropod populations [14], which may be difficult to recover without landmarks on the suture lines or aperture.  See for example:
  • Dunithan A, Jacquemin SJ, Pyron M (2012) Morphology of Elimia livescens (Mollusca: Pleuroceridae) in Indiana, U.S.A. covaries with environmental variation. Am Malac Bull 30:127–133.
  • Dillon, R. T., S. J. Jacquemin & M. Pyron (2013)  Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127.  [PDF]
  • Dillon, R. T. & S. J. Jacquemin (2015)  The heritability of shell morphometrics in the freshwater pulmonate gastropod Physa.  PLoS ONE 10(4) e0121962. [PDF]
[7] Samantha’s original Figure 5 had a nasty little error in its legend, which I have fixed by deleting her legend in its entirety.

[8] Although Samantha did not explicitly cite any reference works to support her taxonomy, it is my impression that her choices of the category names emarginata-ovate, emarginata-narrow (“canadensis”), elodes and exilis follow the 1992 work of Burch & Jung [9]. Whatever the origin, her taxonomy is most unfortunate.  For unexplained (indeed unexplainable) reasons, Samantha seems to have dropped the oldest specific nomen available for stagnicoline lymnaeids, catascopium (Say 1817), from her methods and results sections in favor of emarginata (Say 1821).  But (I’m guessing here) the decision may have come late in her project?  Because a couple samples remain identified as “catascopium” in her draft Table 1, with purple coding in her draft Figures 10, 11, and 12.  What a mess.

[9] Burch, J. B. & Jung, Y. (1992) Freshwater Snails of the University of Michigan Biological Station Area.  Walkerana 6(15): 1 – 218.

[10] It is my broad-brush impression, based on nothing more than inspection of the figures in Samantha’s draft thesis, plus the sets of stagnicoline shells figured by Burch and Jung, that samples our Michigan colleagues tend to call emarginata-narrow (“canadensis”) and elodes may tend to represent putative species V1, and the samples our colleagues call emarginata-ovate or exilis may tend to represent V2,  But since Samantha didn’t digitize the shell aperture or suture lines, the distinction was not recovered by her morphometrics.  So let’s just focus on Samantha’s color coding, and set her taxonomy aside to the extent possible.

[11] Brady, J. K & A. M. Turner (2010) Species-specific effects of gastropods on leaf litter processing in pond mesocosms.  Hydrobiologia 651: 93-100.

[12] For several weeks during the spring and early summer of 2012, I held cultures of the four B&T populations here in Charleston, dissecting samples to hunt for anatomical distinctions that I ultimately did not find. During that period both the Killbuck (V1) and Hartstown (V2) populations laid eggs.  These I hatched and reared for quite a few months in my standard plastic aquarium boxes, at densities that were certainly too high, largely neglecting them, changing their water infrequently.

In any case, my results seem to confirm those of Kip Brady.  My standard culture water here in Charleston is almost certainly much softer than that to which stagnicoline lymnaeids are usually adapted, yielding the chalky appearances of the two example shells figure above.  Yet the V1 offspring did indeed seem to develop relatively smaller body whorls than the V2 offspring.

[13] And here’s another little confusion.  Unknown to me, Kip Brady sent Jack Burch samples from a couple of his stagnicoline populations several years prior to my shipment to Samantha.  Kip never heard anything further.  But apparently 5 of Kip’s earlier samples were sequenced at some point along with 8 of the individuals I sent to Samantha in 2012, and all 13 appear graphed side-by-side in Samantha’s C01 tree, under two different labelling schemes.

[14] Body whorl differences of this subtle sort seem to be quite heritable in freshwater pulmonates as a general rule.  In fact, body whorl differences were the way we initially distinguished Physa carolinae from Physa acuta back in 2009 [15].  For more, see:
  • The Lymnaeidae 2012: A clue  [9July12]
  • The heritability of shell morphology in Physa h^2 = 0.819  [15Apr15]
[15] Wethington, A.R., J. Wise, and R. T. Dillon (2009) Genetic and morphological characterization of the Physidae of South Carolina (Pulmonata: Basommatophora), with description of a new species.  The Nautilus 123: 282-292.  [PDF]