Dr. Rob Dillon, Coordinator

Tuesday, February 24, 2015

Updated Maps for the FWGNA Project

We are excited to announce that a fresh set of pdf maps detailing the ranges of all the freshwater gastropod species inhabiting Atlantic drainages from Georgia to the New York line are now available for download from the FWGNA site.  This is the first significant update of FWGNA mapping coverage since ever.

As our legions of loyal users will fondly recall, the oldest sections of the FWGNA website featured downloadable pdf dot maps showing the detailed distributions of freshwater gastropods in South Carolina (2005), North Carolina (2005), Georgia (2006) and East Tennessee (2011).  But neither our Virginia site (2011) nor our Mid-Atlantic site (2013) ever featured pdf dot maps.  And our maps of Georgia and Carolina distributions have gone obsolete in the interim.
 Pleurocerid map
So the fresh set of range maps now available for download integrates seamlessly with the version of the FWGNA site we rolled out in October of 2013, based on 11,471 records of 68 freshwater gastropod species and subspecies inhabiting the Atlantic drainages of nine states [1].  These 68 taxa have been divided into 26 sets of two or three related species each (in most cases), and mapped in sets as a tool for the exploration of potentially interesting joint distribution patterns.

As an example, click on the thumbnail above to download a pdf map comparing the distributions of Pleurocera proxima, P. virginica, and P. semicarinata.  You will see very little overlap between these three widespread pleurocerids, proxima ranging throughout small softwater streams of the Blue Ridge and upper Piedmont from Georgia to Virginia, semicarinata inhabiting small hardwater streams in the Great Valley of Virginia, and virginica in the streams and rivers of the lower North Carolina Piedmont, spreading northward.

By way of contrast, click on the thumbnail below left to compare the ranges of the hydrobiids Littoridinops tenuipes, Notogillia sathon, and Spilochlamys turgida.  While populations of Littoridinops demonstrate specialization on a unique (freshwater tidal) habitat up the entire length of the Atlantic Coastal Plain, Notogillia and Spilochamys are positively associated, very strikingly tending to occur together in a narrow patch of central Georgia.
 Hydrobiid map

A complete set of updated maps every bit as interesting as these are available today from the FWGNA site for the entire freshwater gastropod fauna of US Atlantic drainages.  Visit any of the 68 separate species and subspecies covered, scroll down to the “Supplementary Resources” heading, click on the “Atlantic drainages” link, and enjoy!

We are pleased to acknowledge a great debt of gratitude to Mr. Matt Ramsey, an undergraduate archaeology student here at the College of Charleston with a bright future ahead of him.  Great job, Matt!


[1] Freshwater Gastropods of Mid-Atlantic States [30Oct13]

Thursday, January 8, 2015

What I Thought I Knew About Goo

When I posted my first essay on the egg masses of pulmonate gastropods [1] way back in September, I imagined that I was embarking on a very short series, offered as a service to the larger community of generally-trained aquatic biologists and interested amateurs who might be googling about the internet, trying to identify gelatinous blobs.  I had no idea that the series would extend into the New Year, and turn into a parable plumbing the depths of our collective ignorance regarding fundamental aspects of the biology of even the most commonplace bugs and slugs in the most mundane environments.  Or that I would expose my own personal hubris to imagine that I am somehow exempt from the folly.

In the second installment of this series, I presented evidence from Quebec suggesting that the large pulmonate gastropod Lymnaea megasoma might lay string-like gelatinous egg masses of bright green embryos.  But in my third installment, I was forced to conclude that photos depicting string-like gelatinous egg masses of bright green embryos from Maine and Nova Scotia did not illustrate the egg masses of L. megasoma.  I shared my jpeg correspondence with Mr. Tom Pelletier of askanaturalist.com, who sent me the initial photos from Maine and Nova Scotia in June, and then followed up with additional photos showing a cloud of fuzzy specks emerging, decidedly unmolluscan in character.  Mr. Pelletier hypothesized (at the time) that those specks might represent the larvae of a chironomid midge.  One might think that this would conclude the matter.

But just as I was placing a period at the end of my third installment in November, I received yet another email from the extraordinarily conscientious Mr. Pelletier, containing yet another set of jpeg attachments which would call into question even that tiny little bit I thought I knew about goo.

In late November Mr Pelletier introduced me to Mr. Trevor Vannatta, a graduate student working on a parasitological survey at the University of Minnesota in Duluth.  And Mr. Vannatta, by happy fortune, had spent his summer collecting a variety of local lymnaeid snails, including L. megasoma.  And at some point during captivity, possibly as a consequence of shifting into the refrigerator, one of Mr. Vannatta's L. megasoma laid an egg mass in the Petri dish in which it was contained.

It is hard to imagine anything less string-like than the gelatinous mass shown above.  The bona fide L. megasoma egg mass depicted in Mr. Vannatta's jpeg attachments, as forwarded by Mr. Pelletier, is large but otherwise indistinguishable from typical lymnaeid egg masses of the sort we figured back in September.

Is it possible that L. megasoma lays dimorphic egg masses, stringy-green in lakes and flat-typical in Petri dishes?  At 15 cm, the Quebec egg masses we depicted in October do still seem significantly larger than the insect masses we illustrated in November.  In any case, the egg mass morphology of L. megasoma needs confirmation.

And meanwhile, I have found some consolation [2] watching from the CC line as our hero, Tom Pelletier, has continued his mighty struggles to figure out exactly what did lay those gelatinous masses of bright green embryos from Maine and New Brunswick we featured in November.  After exploring the chironomid hypothesis at great length he ultimately turned to a caddisfly hypothesis, specifically large-bodied species of the genus Phryganea or Agrypnia, which lay egg masses in loops, rather than linear strings [3].  Mr. Pelletier posted his essays #2 and #3 on the entomological side of this gooey mystery at askanaturalist.com in mid-December, which make most interesting reading, from the links below [4].

What has stricken me through this entire 5 month and 4 + 3 = 7 essay saga is the insularity of all the research communities involved.  Not only are we freshwater snail people completely ignorant regarding the egg masses of aquatic insects, I don’t think the caddisfly community is especially aware of the chironomid midge community, and vice versa.   And I don’t think that the algal symbiosis community knows anything about any of us.

In November I cited a 1994 paper on the symbiotic association between algae and amphibian egg masses authored by Pinder and Friet.  A couple of excellent works [5] have been published more recently, however, including a very thorough review by Kerney in 2011.  And although the algae/amphibian symbiosis seems to have attracted a great deal of attention over 120 years of study, the (identical? analogous?) symbiotic association between algae and aquatic invertebrate eggs (of all sorts) seems to have remained completely undocumented in the primary literature.

Is the same algal species, generally identified as Oophila ambystomatis in frog and salamander eggs, involved in all these symbioses, invertebrate and vertebrate?  How do the algae become so tightly associated with all these diverse embryos, inside their diversity of gooey matrices?  And is the symbiosis mutualistic in all these cases?  Obligately?  And why do we spend billions of dollars bouncing satellites off comets when such profound mysteries quietly float at the end of the docks in our own backyards?  Mystery on top of wonder, upon wonders.


[1] Our series thus far:
  • The Egg Masses of Freshwater Pulmonate Snails [26Sept14]
  • The Egg Mass of Lymnaea (Bulimnea) megasoma [17Oct14]
  • A Remarkable Convergence of Goo, Or, Perils of a Jpeg Naturalist [11Nov14]
[2] Perhaps this is schadenfreude.  I confess.   

[3] These large, gelatinous egg loops are typically attached to debris and vegetation several feet below the surface of northern lakes.  But wait, don’t adult insects have wings?  Are female caddisflies diving into cold, northern lakes like cormorants?  Wonder on top of wonders.

[4] Tom Pelletier’s series:
  • What is this bright green string of eggs? Part 1 [19Nov14]
  • What is this bright green string of eggs? Part 2 [17Dec14]
  • What is this bright green string of eggs? Part 3 [19Dec14]
[5] Kerney, R. (2011)  Symbioses between salamander embryos and green algae.  Symbiosis 54: 107-117.  Graham, R. R., S. A. Fay, A. Davey and R. W. Sanders (2013)  Intracapsular algae provide fixed carbon to developing embryos of the salamander Ambystoma maculatum.  J. Exp. Biol. 216: 452-459.

Thursday, December 4, 2014

Spur-Of-The-Moment Workshop

In recent years I have occasionally fielded heart-felt requests to organize some sort of workshop on the biology of freshwater gastropods, especially focusing on identification skills.  I was part of an FMCS committee to conduct one such effort in Tuscaloosa in 2004 (to mixed reviews), and I myself led a workshop more narrowly focused on the Pacific Northwest gastropod fauna in Missoula in 2006 [1].  But we've had nothing in recent years.  At least, not around here.

So in the last couple days I've been swapping emails with a small delegation from Clemson University about hosting a freshwater gastropod workshop here in Charleston.  My schedule is flexible between now and Christmas, and so is theirs, and it just seems to have sort-of come together.

So everybody is invited to a spur-of-the-moment workshop at The College of Charleston next Monday morning, December 15, 9-12:00 noonish.   No paperwork, no registration fees.  Just zap me an email if you’d like to participate.

I’ll bring out my personal reference collection, which is really rather complete for The East.  I’ll also go out and fetch us some living critters.  We’ll do a couple dissections.  It’s not rocket science but there are a couple tricks I’ll be happy to show you.

Looking forward to it!


[1] Previous workshops:
  • FMCS Gastropod Workshop [19Dec03]
  • Report from Tuscaloosa [23Mar04]
  • Pacific Northwest Gastropod Workshop [23Mar06]

Tuesday, November 11, 2014

A Remarkable Convergence of Goo, Or, The Perils of a Jpeg Naturalist

Editor’s Note.  This is the third in (what will turn out to be) a series of four essays on the egg masses of freshwater pulmonate snails.  It will only make sense if you read my post of 17Oct14 first, and it would help to at least skim my post of 26Sept14 as well.

In the first episode of our saga, we met Mr. Tom Pelletier, the hardworking proprietor of askanaturalist.com.  It was a jpeg attachment that Mr. Pelletier sent me in the spring of 2014, as well as other correspondence I have enjoyed on the general subject of gelatinous blobs in recent years, that prompted me to post my review of pulmonate egg and egg mass morphology in September [1].  So I was not surprised to receive a second email from Mr. Pelletier this past August, with the subject line, “Another question about snail eggs.”

Mr. Pelletier wrote that he had recently received emails from two different readers bearing very similar attached jpegs depicting what might be snail egg masses, one from Nova Scotia and the second from Maine.  Here is what the Nova Scotia correspondent said:

“We are finding these slimy strings of bright green eggs (?) in the lake. They can be found on the wharf poles but the ones in the picture were on the cord for the underwater thermometer. Any idea what they are from?”

And Mr. Pellatier’s correspondent from Maine wrote as follows:  “I found this jelly blob or loop (below) in the water on the ladder to my boat. I thought it was a round blob but it was more of a loop. These dots were bright green in clear jelly.”

Well, thought I to myself, I can knock this ball out of the park!  These attached jpegs quite clearly depict the egg masses of Lymnaea (Bulimnea) megasoma, to which I devoted my entire essay of 17Oct14 [2].  They demonstrate exactly the same (very distinctive!) morphology, and have been collected from exactly the same habitat type, in exactly the same (chilly) range.  I congratulated myself that I might be the only person in the world who could, with any confidence, identify these peculiar blobs of goo as freshwater gastropod eggs with any authority.

But being the conscientious and thorough worker that he is, Mr. Pelletier had also sent similar inquiries to a herpetologist, an algae expert, and (cleverly) a researcher with expertise in chironomid midges.  And the next day, Mr. Pelletier reported to me that his chironomid guy “seemed pretty sure” that the peculiar blobs of goo in question were the egg masses of his favorite critter, not ours.

A chironomid midge, are you nuts?  I am not an entomologist, but I have waded through enough clouds of chironomids in my day to know that they are tiny little delicate things, maybe 6 – 8 mm in body length.  How could a 6-8 mm fly lay a 6-8 cm egg mass?

Well, in all candor, I had to confess to Mr. Pelletier that I had never actually laid eyes on an egg mass of Lymnaea megasoma either, nor do I have any significant field experience in lakes at any such northern latitudes, and that everything I think I might know about this entire category of question comes from second-hand reports and jpeg attachments.  So in the end, I concurred with the “chironomid guy” (some of whose correspondence Mr. Pelletier had shared) that somebody, somewhere, ought to hatch these things out.

So the results of the experiment came back in September, and my embarrassment was complete:

Although it may be difficult to identify that cloud of blurry specks emerging from the mysterious blob of goo depicted in the jpeg above, I am personally convinced that they are not hatchling Lymnaea megasoma.  The egg mass of a chironomid midge it would seem to be.

In retrospect, questions of scale have continued to dog me in my long running record of folly and failure as a jpeg naturalist.  When I opened Mr. Pelletier’s August email, I had in mind a 15 cm egg mass such as depicted in last month’s essay.  But the “slimy string of bright green eggs” depicted at the top of this essay is probably no more than about 4 cm long, and a much smaller diameter as well.  And since (it seems to me) both last month’s egg masses and this month’s masses seem to contain about the same number of embryos, I think this month’s embryos may be much smaller as well.  Had the two egg masses been sitting in watch glasses on my bench top, I don’t think I would have been confused.

So have two completely independent elements of the freshwater macrobenthic fauna of the higher latitudes of North America, an insect and a gastropod, independently evolved nearly identical egg mass morphology?  And has the same green algae evolved a symbiotic relationship with both?  Could biology be any more fascinating?

Well yes, it could!  In late August our good buddy Tom Pelletier also called my attention to a 1994 paper in the Journal of Experimental Biology on oxygen transport in amphibian egg masses [3].  The amphibian researchers reported that the eggs of both wood frogs and spotted salamanders up in Nova Scotia are “cohabited by Oophila ambystomatis, a green alga found specifically in association with amphibian egg masses.”  They reviewed a large and hoary literature (going back to 1888!) suggesting that such symbiotic algae may benefit from increased ammonia or CO2 concentrations inside the amphibian egg capsules, and confirmed that, under some conditions, O2 produced by the algae seems to be required for the development of the amphibian embryos.

So the same striking egg mass adaptation may have evolved not twice, but three times independently – bug, snail, and amphibian – all three adaptations depending on the same algal symbiosis.

The sentence above looks like a conclusion, but it is not.  There is yet one more episode in this saga, and it does not end neatly.  Rather, episode #4 will make you question some not insubstantial fraction of what you have read in episodes #2 and #3.  Stay tuned!


[1] The Egg Masses of Freshwater Pulmonate Snails [26Sept14]

[2] The Egg Mass of Lymnaea (Bulimnea) megasoma [17Oct14]

[3] Pinder, A. W. & S. C. Friet (1994)  Oxygen transport in egg masses of the amphibians Rana sylvatica and Ambystoma maculatum: Convection, diffusion and oxygen production by algae.  J. Exp. Biol. 197: 17-30. [PDF]

Friday, October 17, 2014

The Egg Mass of Lymnaea (Bulimnea) megasoma

Editor's Note.  This is the second in what turned out to be a series of four posts on the egg masses of freshwater pulmonate snails. You really must read my posts of 11Nov14 and 8Jan15, because much of what I have written below will subsequently be called into question.

I characterized last month’s essay [1] as an “introductory course” in the egg masses of freshwater pulmonate snails.  That post featured photos and morphological observations comparing eggs and egg masses from typical representatives of the four major pulmonate families inhabiting the waters of North America.  This month we will explore “advanced topics.”

So in July of 2011 I received an email from Dr. Julie Moisan of the Quebec MDDEP [2] bearing the subject line “strange gelatinous mass.”  She inquired whether the image below might depict “gastropoda eggs (Lymnaeidae).”  She estimated the length of the mass as “3-5 cm” and was especially stricken (as was I) by the color of the embryos:

This was my reply: “No, that most certainly is NOT the egg mass of a lymnaeid snail, or indeed of any mollusk.  The mass is much too large, and each individual embryo is much too large.  And all the freshwater snails cement their egg masses firmly onto solid substrates.  And embryos are not green.”

My personal guess would have been amphibian eggs, but the herpetologist down the hall did not recognize the spawn depicted in that particular image either.  So ultimately I confessed my ignorance to Dr. Moisan, wished her luck, and sent her off with a request that if she ever did identify her strange gelatinous mass, to drop me another line for the sake of my own curiosity.

Imagine my great surprise (and not insubstantial embarrassment) when I opened a second email from Dr. Moisan in August 2011 containing the jpeg below.

This is clearly a hatchling Lymnaea (Bulimnea) megasoma.  I have never personally seen a living L. megasoma on the hoof, adult or juvenile, although they were not uncommon in a batch of preserved samples I got from northern Wisconsin [3] a few years ago.  F. C. Baker [4] gave the range of the species as “Northern New England, west to Minnesota, Iowa and Manitoba; northern Ohio (latitude 41⁰) northward in British America to latitude 57⁰.”  Clarke’s [5] range map confirms Manitoba, Ontario, and Quebec.  Both Baker and Clarke report broad habitat usage for populations of L. megasoma within this chilly range, “In rivers, lakes, sloughs, and ponds.”  The observations sent to me by Dr. Moisan came from Lac Vaudray and Lac Labyrinthe in the Abitibi-Temiscamingue region of Quebec, about 48⁰ N.

Dr. Moisan also sent me several additional photographs of the egg masses in August 2011, together with some additional natural history observations.

She estimated the egg mass depicted above at a whopping 15 cm length, and reported that such masses are typically attached to the substrate by their ends, to form a loop.  (I think her July estimate of “3-5 cm” may have been too low.)  She also reported that the embryos are not always green, but can vary in shades of black or brown.

This is all quite remarkable biology, but perhaps not difficult to rationalize, in retrospect.  The body mass of typical adult L. megasoma might indeed exceed the body mass of typical adult L. columella by an order of magnitude.  So an order-of-magnitude scale-up from the 11.5 mm L. columella that laid the typical (12.0 mm) egg mass we figured last month might reasonably yield the 15 cm gelatinous mass figured above. That’s my 11.5 mm L. columella at left, inspecting the (38.7 mm) shell of an L. megasoma from Wisconsin.

And what of the bright green color assumed by the embryos?  That must be chlorophyll a, which would seem to indicate a symbiotic association with algal cells, I suppose.  Is the symbiosis merely commensal – the algae finding a congenial home nestled safely inside the clear matrix of the pulmonate egg mass, the snail receiving nothing in return?  Or might the gastropod half of the association be receiving some mutualistic benefit as well?  It seems possible to me that the algae might provide oxygen to the developing embryos, in an otherwise potentially low-oxygen environment.  And possibly an initial source of food?  Is it possible that momma-snail actively provisions her embryos with an algal culture, which her newborn hatchlings might subsequently harvest for breakfast?

And aren’t we blessed to be biologists?  As far as I can determine, nobody has ever, in the history of malacology, described the egg masses of Lymnaea megasoma.  We biologists don’t need millions of dollars and superconducting supercolliders to push the boundaries of our science.  Off the end of a dock in rural Quebec, floating quietly, there lies the utter unknown.


[1] The Egg Masses of Freshwater Pulmonate Snails  [26Sept14]

[2] We gratefully acknowledge Dr. Julie Moisan of the Ministere du Developpement Durable, de l’Environnement et des Parcs in Quebec City for bringing this remarkable phenomenon to our attention, and for subsequently answering our repeated requests for additional information with great patience and care.

[3] Solomon, C. T., J. D. Olden, P. T. J. Johnson, R. T. Dillon, and M. J. Vander Zanden (2010)  Distribution and community-level effects of the Chinese mystery snail (Bellamya chinensis) in northern Wisconsin lakes.  Biological Invasions 12: 1591-1605.  [PDF]

[4] Baker, F. (1911) The Lymnaeidae of North and Middle America, Recent and Fossil. Special Publication, no. 3. Chicago: Chicago Academy of Natural Sciences.  Observations on L. megasoma may be found on pp 184 -191.  More here:
  • The legacy of Frank Collins Baker [20Nov06]
 [5] Clarke, A. (1981) The Freshwater Mollusks of Canada. Ottawa: The National Museums of Canada. 445 pp.

Friday, September 26, 2014

The Egg Masses of Freshwater Pulmonate Snails

Faithful readers of this blog will not be surprised to learn that my email inbox typically receives a rather steady stream of inquiries with attached jpeg images of freshwater snails.   But you might be surprised to discover that I also occasionally receive images of things that are not freshwater snails, but could be.

Like blobs of jelly.  Earlier this year, for example, I received an email from Mr. Tom Pelletier of askanaturalist.com, bearing the subject line “gelatinous mass.”  And the attached image was (pretty clearly) a Physa egg mass, sent to Mr. Pelletier by a correspondent who had photographed the underside of a river rock in West Virginia.  I was (of course) pleased to help Mr. Pellatier, and he wrote a very nice essay on the askanaturalist.com website [1], featuring a lot of excellent general information on the life history of Physa acuta, as well as photos and a couple video links as well.

So it has come to my attention that reliable information on the egg masses of freshwater pulmonate snails is a rare commodity on the web.  I tried a google search on a variety of terms and combinations, and was only able to find our colleague Kathryn Perez’ dichotomous key (which is good), but which features drawings, rather than photos, and might benefit by attention to scale [2].

So welcome to “Basommatophoran Pulmonate Egg Masses 101.”  This is an introductory class.  If you are a sophomore or higher in the freshwater gastropod curriculum, feel free to take the rest of this essay off, and I’ll see you next month.

For those of you still with me.  Last week I went down to my local pond and collected adults from the three most common pulmonate populations in the Charleston area – Lymnaea (Pseudosuccinea) columella, Physa acuta, and Helisoma trivolvis.  I isolated individual snails in my standard 10 oz. plastic drinking cups, fed them green flake food, and over the following 48 hours, quite a few laid eggs.  I allowed the eggs to mature for five days, dumped the water from their cups, and trimmed representative egg masses out with a pair of scissors, still attached to their cup walls.  I then photographed one image directly down through each mass, and a second image obliquely (dewatered, propped up in a little finger bowl) to give a feeling for third-dimension thickness. 

The photo above shows a typical lymnaeid egg mass about five days old.  The clear, colorless, elongate, sausage-shaped mass depicted is approximately 12 mm in length, each of the 25 - 30 embryos it contains approximately 0.7 mm in diameter.  It is covered with a relatively tough membrane.  The standard shell length of its mother was 11.5 mm.

The second photo in this series shows a typical planorbid egg mass, also about five days old.  The 25 - 30 embryos are very similar in size to the lymnaeid embryos depicted in the first photo, but the mass is irregularly ovoid in its outline, with a maximum dimension of about 7 mm.  This mass is also covered with a relatively tough membrane.  Its mother bore a shell 13.2 mm in diameter.

Notice, interestingly, that the planorbid egg mass is tinged slightly brown or orange, in contrast with the entirely uncolored lymnaeid mass.  Planorbids are famous for their serum hemoglobin, and it seems likely to me that the slightly orange cast may indicate a bit of heme in the matrix.  The development of Helisoma embryos also seems a bit more advanced at day five than lymnaeid embryos.

The third photo in this series depicts a typical physid egg mass, similar in size and age to the lymnaeid and planorbid masses.  The matrix in the Physa egg mass is obviously much more gelatinous in its character, however, missing the relatively tough outer membrane.  The standard shell length of the mother of this brood was 9.1 mm.

To be complete, I might add a fourth image to the gallery, depicting a singleton egg of the ancylid limpet Ferrissia fragilis [3]Ferrissia reaches maturity at a much smaller size than Lymnaea, Helisoma, or Physa, and hence one might not be surprised to discover they do not lay an egg “mass” at all.  The singleton embryos of Ferrissia are a bit smaller than the embryos of the other pulmonates as well, approximately 0.60 mm diameter, and surrounded by a rather spare capsule.

I will conclude this lesson by noting that the size of freshwater pulmonate egg masses is a function of their number of embryos, which may vary greatly.  In culture it is not uncommon to see Physa egg masses with 60-80 embryos, for example, roughly comparable in total volume to that of their mother.  And production of one such egg mass every 24 hours is not unusual.

Even casual observations such as these cannot fail but impress the student with the potential for great reproductive output mounted by freshwater pulmonate snails.  Might pulmonates “over-reproduce” and expire, like spent salmon?  Readers interested in a comprehensive review of life history strategy in freshwater gastropods generally, together with a consideration of spent-salmon semelparity, are referred to my (2000) book [4].

Thus ends the introductory lecture on the egg masses of freshwater pulmonates.  Coming up next month – advanced topics!


[1] Pellatier, T. C. (2May14) What are these jelly dots under rocks?    www.askanaturalist.com

[2] Perez, K. E. & G. Sandland.  Key to egg masses of Wisconsin Snails.  www.northamericanlandsnails.com.

[3] This photo was taken by my student Jacob Herman in connection with our paper:
Dillon, R. T., Jr & J. J. Herman (2009)  Genetics, shell morphology, and life history of the freshwater pulmonate limpets Ferrissia rivularis and Ferrissia fragilis.  Journal of Freshwater Ecology 24: 261 – 271. [pdf]

[4] See pp 156 – 168 in:
Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.

Tuesday, August 5, 2014

Just Before The Bust

The fall line as it arcs through the midlands of South Carolina is a rather indistinct region of broad shoals and rocky flats, featuring no actual falls, constituting no real line.  The Catawba River enters this region at Great Falls, SC, and, finding nothing especially remarkable, much less great, traces a lazy path about 25 miles south to the vicinity of Camden, changing its name to the Wateree River somewhere along the journey, possibly out of boredom.  Prior to the 20th century, I feel certain that this section of the river presented at least occasional rocky shoals and rapids.  But it was on the last shoal of the Catawba/Wateree, perhaps 6-8 miles north of Camden, that Duke Power built a hydroelectric dam in 1920. 

Undistinguished by its 225 foot height, but stretching a longish 3,380 ft. across the wide river channel and associated flood plain, the Wateree Dam and Hydro Station has a 56 megawatt capacity, typically generating a daily 3-5 foot cycle at the gauge in its tailwaters during the warm months.  Its 13,864 acre reservoir also sees substantial recreational use, Duke Power having thoughtfully provided boat ramps and fisherman’s accesses throughout.
Invasive populations of Bellamya (or Cipangopaludina) japonica have been established in South Carolina since at least 1995 [1].  And indeed, my good friend Bill Poly of the SCDNR alerted me to the introduction of a Bellamya population in the Wateree Dam tailwaters back in June of 2011.  So I was not surprised by the email I received last month from another of my good friends among the ranks of state aquatic biologists, David Eargle of SCDHEC.  But the way he described the gastropod situation downstream from the Wateree Dam peeked my curiosity.  David reported “Unbelievable numbers of Bellamya.  One area I thought at first was a gravel bar was all snails.  Amazing.”

So I picked out a nice Saturday late last month, loaded my kayak into the back of my pickup, and headed up the interstate northwest about 2.5 hours from Charleston to the tailwaters of the Wateree Dam.  And indeed, the adjective “amazing” describes the situation quite well.

The fisherman’s access is on the right (descending) bank of the river.  I launched my kayak and paddled maybe 20 - 30 yards across the main channel about mid-day, to the broad and (in spots) marshy region, dissected by rocky pools and shoals, that extends across the left 90% of the riverbed.  The photo below was snapped in that left-side rocky/marshy region looking downstream.  The photo at the top of this essay was snapped from the same vantage point, looking upstream.
So the third photo in this series is a typical vista looking across one of those rocky pools downstream from the Wateree Dam.  The water levels had been dropping steadily all afternoon on this particular Saturday, reaching an extreme low around 4:30 PM, at which time the horn sounded and the hydro station began to generate.  The photo below was taken around 4:00 PM.
And the next photo was snapped looking directly down through about a foot of clear water, showing that the bottom of a typical channel is essentially a “bed” of gastropods, piled on top of each other, perhaps three or four snails deep.   If you click on the image below you can download a high-resolution (4.5 mb) jpeg, zoom in,  and see that the snails are not grazing.  They are lying on their backs on the bottom of the Wateree River, apparently filter feeding like a bed of mussels.
The ability to filter-feed is fairly well-documented in viviparids [2].  In fact, if you shop in one of those nurseries that specialize in backyard water-gardens, the salesmen will often advise you to purchase “Japanese Mystery Snails” for $1.00 each (or $10 per dozen) to help “clarify” the water in your ornamental lily pond.  Which I think they probably do.

Notice in this next photo that the snails in my net seem to be strikingly uniform in size, all approximately 30 – 40 mm in standard shell length.  It was my impression that the population was comprised almost entirely of the one-year-old age class, probably born in the spring and summer of 2013.  I noticed a few young-of-the-year juveniles the afternoon of my visit, all in the 10 mm size range, and just a couple 60 mm “lunkers,” which must have been 2+ years old.  But overall, the size distribution of the snail population struck me as unusually homogeneous.
So the tide hit dead low at about 4:30 PM, and I started back through the rocky marsh, more dragging my kayak than paddling it.  And I happened to wade through one warm pool with a slightly muddy bottom, apparently not receiving as much current as some of the others.  And when I turned back to look in my wake, this is what I saw:
These are empty shells, of course.  The snail has died and rotted, and the empty shell filled with gas.  I must have kicked up several hundred such “floaters” as I walked back to the channel.

It seems likely to me that the Bellamya population downstream from the Wateree Dam is on the verge of a bust.  Dramatic die-offs of invasive viviparid populations are not uncommon in the southeastern United States [3], occasionally even reaching the attention of the popular press.  I got telephone calls about a stinking Bellamya die-off in Lake Murray on the other side of Columbia a couple years ago, and there was a huge mess on the Neabsco River up in Virginia in 2010, necessitating the deployment of heavy equipment.  But in all cases of my personal experience, the phenomenon is discovered after the crash.  Last month’s observations were the first I have ever personally made during the population flush phase.

The population age distribution was especially interesting to me.  So I loaded my kayak back in my pickup and drove around the fisherman’s access roads to Lake Wateree just above the dam, maybe 300 yards upstream from where I had spent most of the afternoon.  The size distribution of the Bellamya population in the shallows of the reservoir seemed dominated by big lunkers 50-60 mm in shell length, which is normal year-round in the Carolinas, in my experience.  Typically very few animals aged one year and younger will be apparent upon a causal census of a Bellamya population around here.  I took this as evidence that the Bellamya population downstream from the Wateree Dam had reproduced explosively in the last year or so.

“Boom-and-bust” or “flush-crash” population dynamics are a familiar aspect of invasive species biology.  But it is my impression, after a couple hours of poking around the published literature, that about 95% of everything we know is anecdotal [4].  Danielle Haak and her colleagues [5] suggested that a 17-39% die-off of the adult Bellamya population in a Nebraska reservoir was due to “an extreme drought event, which was coincident with abnormally hot weather.”  Moore and colleagues [6] documented the community effects of a Potamopyrgus boom-and-bust cycle in California, reviewing the three most obvious hypotheses to account for the bust (weather, intraspecific competition and predators/pathogens), but not ultimately selecting a favorite [7].

Simberloff and Gibbons [8] conducted an “exhaustive” review of the worldwide literature on population crashes of established introduced species, together with systematic “queries to experts on invasives by particular taxa.”  They observed:
“even quantitative data documenting perceived declines were exceedingly scarce, while the great majority of proposed explanations were simply more-or-less reasonable ad hoc suggestions with no supporting evidence.”
Across the N=17 case studies Simberloff considered the best-documented, four of the putative causes were “competition with other introduced species,” with one case each for “parasitism by subsequently introduced species,” “adaptation by native herbivore,” and “exhaustion of resource.”  And for ten of the 17 best-documented invasive species population busts [9], “there is no strong evidence suggesting a cause.”

One would think, with all the dump trucks full of money being spent to study the causes and consequences of biological invasions worldwide – all the weeds, all the bugs, and all the slugs combined – at least a little funding would be available to study why this problem, at least occasionally, solves itself.  Or am I wrong, again?


[1] This is the fifth post I have authored in the last ten years on Bellamya invasion.  If you’re interested in digging into the phenomenon, perhaps the best approach would be to first go to the FWGNA pages on Bellamya japonica and the closely-related Bellamya chinensis, read the general biology, and then follow the links from the “Essays” sections at the bottom of those two species pages back to this blog.   So start here:
  • Bellamya japonica [FWGNA]
  • Bellamya chinensis [FWGNA]

[2] See pp 99 – 100 in my book:
Dillon, R. T. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.

[3] Invasive viviparids in South Carolina [19Oct03]

[4] The irony that I myself am adding yet another anecdotal report here does not escape me.

[5] Haak, D. M., N. M. Chaine, B. J. Stephen, A. Wong, & C. R. Allen (2013)  Mortality estimate of Chinese mystery snail, Bellamya chinensis in a Nebraska reservoir.  BioInvasions Records 2: 137-139.

[6] Moore, J. W., D. B. Herbst, W. N. Heady and S. M. Carlson (2012)  Stream community and ecosystem responses to the boom and bust of an invading snail.  Biological Invasions 14: 2435-2446.

[7] Have there been similar (local) crashes of Potamopyrgus populations throughout the American West?  This was the impression that I took from the Snake River below Minidoka Dam in 2010, mentioned in footnote #4 here:
  • The Mystery of the SRALP: A Twofold Quest! [1Mar13]

[8] Simberloff, D. & L. Gibbons (2004)  Now you see them, now you don’t – Population crashes of established introduced species.  Biological Invasions 6: 161-172.

[9] Including the giant African land snail Achatina on Pacific islands.