Dr. Rob Dillon, Coordinator

Monday, November 6, 2017

Loved to Death?

Back in 1976, when I first joined the American Malacological Union, the community of shell collectors played an important role in the day-to-day operations of the society.  It seemed to me that the amateurs actually outnumbered the professionals at our annual meetings.  Shell clubs ran the registration table, operated the A/V equipment, and sponsored the receptions.  Receptions, heck – the Houston Shell Club threw a rip-snorting party, with music, and dancing, and pretty girls.  And (then-Treasurer) Connie Boone came through kissing everybody who held still.  Man, those were the days!

My 1974 edition of a 1966 classic
So the climax of the annual meeting of the American Malacological Union in those golden days was always the shell auction, for the benefit of the student fund.  Dick Petit would get half-tuned and wisecrack from lot to lot of colorful specimen shells, offering drop-dead gorgeous little jewels of nature to the highest bidder, and some of the prices paid were eye-popping.

Although I myself never offered any bids for any of the lovely seashells on auction, I felt a strong connection to those who did.  I was quite the hobbyist myself in my youth.  I did collect a lot of marine gastropods and bivalves for their shells, and traded shells with friends all over the world, and (yes) did indeed purchase specimen shells from shell dealers and shell shops, till the day I left home for college.  Shell collecting gave me my start in malacology.

Alas, in 1996 the American Malacological Union threw it all away.  Citing vague conservation concerns, that summer the AMU Council [1] banned the sale or trade of shells at annual meetings, implying strongly as it did that overcollecting by hobbyists posed a threat to natural mollusk populations.  The hobbyists felt as though they had been insulted, which they had, and they left the AMU and never came back.  It was the stupidest thing ever done by a roomful of mollusk people.

So last month we took a peek into the worldwide trade in living freshwater gastropods for the aquarium hobby [2].  Most of the 47 species at least occasionally available from Singapore, the primary wholesale exporter of aquarium stock to the global marketplace, are widespread and trashy, as one might expect.  But our colleague Ting Hui Ng and her coauthors [3] also identified several freshwater gastropod species as “narrowly endemic,” mostly Tylomelania and other Southeast Asian pachychilids, with a viviparid or two thrown in for diverse measure.  Quoting Ting Hui directly, “The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”

So what evidence might there be to suggest that hobbyists or collectors might drive populations of gastropods such as these to extinction by overharvest?

The discipline of fisheries management is generally considered to have been born in the early 1930s, with the development of the concept of maximum sustained yield [4].  Stated simply, MSY = Kr/4, where K is the carrying capacity of the environment, and r the intrinsic rate of natural increase.  These are difficult and near-impossible parameters to estimate, respectively, and so the concept of MSY has rarely seen application, even in the management of the most commercially valuable fisheries.  Much less snails.

But, if you’ll allow Captain Obvious to take the helm here, note that the concept of MSY depends on the assumption of density-dependent population regulation.  And, to be quite frank, as your Captain always is, it is my strong impression that essentially all our colleagues with research interests in the ecology of mollusk populations, or indeed with interests in any aspect of the biology of any invertebrate population whatsoever worldwide, carry with us a (near-universally unstated) assumption of density-independence.  In other words, we assume that the size of our study populations is not a function of r and K, but of extrinsic factors such as weather, or floods, or harvest by Indonesian locals who might want to gather up a few small but exotic-looking snails from the lakeshore to sell to feed their families. 

Well, shame on us all.  The tiny little scraps of evidence available today suggest that the regulation of freshwater gastropod populations is as density-dependent as any population of living things on this earth.  The figure below, reproduced from Chapter 5 of my (2000) book, shows the densities of five freshwater gastropod populations over records of 5 – 10 years.  My PBLR test on the longest record available (the Ancylus data set of Russell-Hunter) returned a value of t significant at the 0.01 level, showing strong evidence of density-dependent regulation [5].

Dillon (2000) Figure 5.11
The grand mean density of the Ancylus population in the stream studied by Russell-Hunter, 260 per square meter, can therefore be taken as a rough estimate of carrying capacity.  The grand means of the other populations (per meter squared) were Physa = 77, Lymnaea = 93, Hydrobioides = 41, and Pomacea = 0.11.  The Pomacea estimate did not include juveniles, however.  Shall we take, as a rough estimate of carrying capacity K for freshwater gastropod populations, about 10 per square meter?

Table 5.1 of my (2000) book offers a big compilation of demographic data for 36 populations of freshwater gastropods [5].  The median value for intrinsic rate of natural increase tabulated was around r = 1.5, but the two prosobranch values were systematically lower, just r = 0.09 for Pomacea and r = 0.24 for Melanoides.  Then if we roughly estimate the value of r for freshwater prosobranch populations as r = 0.1 per generation, and carrying capacity K = 10 per square meter, we find MSY = Kr/4 = 0.25 snails per square meter per generation.

My biological intuition suggests that Tylomelania probably mature at around age one year and reproduce continuously thereafter. And the surface area of Sulawasi’s Lake Pozo, where all the Tylomelania live, is approximately 300 square km.  I realize all the lake bottom is not equally inhabitable by snails, but you get the picture.  Back-of-the-envelope estimates suggest that any subtraction of snails by collectors from the Tylomelania populations of Lake Pozo below roughly 10 million snails per year will not ultimately lower the population size, but will be replaced.  And there is zero evidence suggesting otherwise. 

OK, I understand the justification.  My colleagues want grant funding to gather hard data on the size and demographics of Tylomelania populations, so that an informed decision can be made.  In the meantime, since we don’t know, we must err on the side of caution, yes?  Well, I agree with that first sentiment, but not the second one.  Science does not make recommendations based on the premise, “Since we don’t know.”  I understand the fear, but fear is not reason.  Fear is the opposite of reason.

The chairman of my graduate committee 1977 – 1982 was a prominent ecologist named Dr. Robert E. (Bob) Ricklefs.  At some point in my graduate career, he made a point that has stuck with me for 40 years.  Bob observed that if we assume population size is a function of density-independent effects, populations are like drunks on a subway platform, veering left and right randomly until they either go extinct or cover the earth ass deep.  I realize that sometimes it seems extinction is frighteningly common.  But if the size of freshwater gastropod populations were indeed a function of density-independent factors [6], the two phenomena (extinction and ass-deep-coverage) would be equally common, with the frequency of extinction = frequency of coverage = 0.50.  Since that is not the case, density-dependence must prevail.

Well, we’ll return to that ass-deep-earth-coverage thing next month.  But the bottom line for today is that there is no evidence that natural populations of mollusks can be driven to extinction by the love of hobbyists [7].  And a lot of evidence that the love of hobbyists can be beneficial to the work of science.  The birdwatchers are a tremendous asset to ornithology, and the stargazers a tremendous asset to astronomy.  Indeed, the vibrant amateur communities of birders and gazers are where the professional ornithologists and astronomers are born. 

Meanwhile we malacologists are so wrapped up in the remote likelihood that our study organisms might disappear into extinction that we cannot see the imminent extinction of our own profession.  And I strongly suspect the attrition rate in the halls of malacology has been well above Kr/4 for quite a few generations now.


[1]  I myself was not called to the AMS Council until 1999.  I did campaign heavily for repeal of the shell ban during my 2001-02 presidency, but could not find the votes.  The best I could do was neglect the issue entirely when I redrafted the AMS constitution & bylaws 2002-03.

[2] What’s Out There? [9Oct17]

[3] Ng Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130

[4] Russell, E. S. (1931). Some theoretical Considerations on the "Overfishing" Problem. ICES Journal of Marine Science. 6 (1): 3–20.
Graham, M. (1935). "Modern Theory of Exploiting a Fishery, and Application to North Sea Trawling". ICES Journal of Marine Science. 10 (3): 264–274.

[5]  Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.  All the details for the demographic analyses referenced above are found in Chapter 5, with the material on carrying capacity pp 202 – 207, and intrinsic rate of natural increase on pp 172 – 182.

[6] Or if such populations could be driven to extinction by harvest rates very much above Kr/4.

[7] In fact, it seems unlikely to me that even unregulated commercial harvest could drive marine shellfish populations to extinction.  And I don’t think there’s any evidence that the pearl-button industry was responsible for any of the freshwater mussel extinctions.  The only cases of human overharvest I’ve ever heard of, for any mollusk population in any environment whatsoever, are the occasionally-successful handpicking efforts to control land snail pests.  See:

Simberloff, D. 1997. Eradication. Pages 221–228 in D. Simberloff, D. C. Schmitz, and T. C. Brown, editors. Strangers in paradise. Island, Washington, D.C., USA.

Monday, October 9, 2017

What's Out There?

Last summer an interesting paper came across my desk – so interesting, in fact, that I filed it in my stack of promising subject matter for the FWGNA blog.  Where it was promptly buried.  But from which this week it has been exhumed, to stimulate my imagination afresh.

The paper is entitled, “Molluscs for sale: Assessment of freshwater gastropods and bivalves in the ornamental pet trade [1].”  The senior author, Ting Hui Ng, and her six coauthors hail almost exclusively from Singapore, the primary exporter of ornamental fishes to the global marketplace.  Ting Hui and her colleagues surveyed seven local retail shops and major aquarium-stock exporters between 2008 and 2014, documenting an impressive 47 species of freshwater gastropods and 12 species of bivalves available for sale to the hobby worldwide.  Of the gastropods, the authors considered 26 to be oriental in their origin, 7 Australasian, 4 Neotropical, and 1 North American, leaving 9 classified as “cosmopolitan.” 
Figure 1 of Ng et al. (2016)
The entire malacological zoo is depicted in the author’s Figure 1 reproduced above, with their (extensive) caption copied at note [2] below.  I hate to fuss [3], but the scales are completely messed up in this figure – not just the unmarked scale bars (which are supposed to be 10 mm) but the scale bars “indicated differently.”  With regard to the actual sizes of any of these 59 creatures, you are on your own.

The single North American species was identified as “Physa sp.”  I would have bet dollars-to-donuts that this must be Physa acuta, the world’s most cosmopolitan freshwater snail [4], which is indeed widely introduced into the waters of Singapore, until I swapped emails with Ting Hui. She called my attention to her 2015 paper reporting the discovery of a single individual physid in a Singapore aquarium shop, and a second individual in Malaysia, 22% different from anything in GenBank [5].  I'm going to fight the temptation to digress here, but see note [6] below.

The main point is that Physa (and three other gastropod species) were classified as “hitchhikers” by Ting Hui and colleagues, which means that they were discovered in Singapore incidentally transported with aquatic plants, ornamental fish or other freshwater mollusks, and not literally “For Sale,” because nobody wants them.  But at least North America wasn’t skunked entirely.  USA!  USA! USA!

The four Neotropical species were all (you guessed it) ampullariids – Pomacea canaliculata, Pomacea maculata, Pomacea diffusa, and Marisa cornuarietis.  All these various South American ampullariids are also quite familiar to us here in the USA, introduced years ago into Florida and Texas, recently knocking on our doorsteps in South Carolina [9].

Among the remainder of the gastropods for sale in Singapore, I myself was most interested by the strange and exotic-looking pachychilids and thiarids.  We are certainly familiar with #48 Melanoides tuberculata here in the USA [10].  But I was surprised not to see the oriental Tarebia granifera in the gallery compiled by Ting Hui and her colleagues.  Tarebia has been introduced to several southern US states and is sporadically common in Central America and on many Caribbean islands [11].

And speaking of noteworthy absences.  Neither Bellamya japonica nor B. chinensis appears on Ting Hui's list of 59.  I’m sure their mothers must love them, but perhaps Bellamya are too plain and clunky to find a market among aquarium hobbyists?  Or has Bellamya become so common here in North America [12] that nobody buys them out of Singapore?  It has always been my impression that they are marketed here in the USA primarily by suppliers to the backyard “water garden” trade, along with koi, water lilies, and little tumbling fountains.  Not the (indoor) aquarium shops supplied by Singapore.

The research interests of Ting Hui and colleagues seem to have been motivated by two primary concerns – that exotic freshwater mollusks might spread, and that they might not.  Let’s set aside the former concern for the time being, and focus on the latter.

Eight of the 59 freshwater mollusk species for sale in Singapore were identified by Ting Hui as “narrowly-endemic.”  These included the three spiny species of Brotia, the four Tylomelania species, and the viviparid Celetaia persculpta.  Quoting directly from Ting Hui: 
“These species appeared to fetch higher prices compared to more common species (up to US$10 per individual Tylomelania sp. compared to US$5 per individual Thiaridae or Neritidae, THN pers. obs.). The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”
Pachychilid snails of the genus Brotia are ovoviviparous but sexually reproducing – males and females apparently present in equal frequency.  Brotia armata (#34) and B. binodosa (#35) are members of what Glaubrecht & Kohler called a “species flock” in the Kaek River of central Thailand [13]Brotia pagodula (#37) is endemic to the Salween River and its tributaries on the border of Thailand and Myanmar.   Their habitat is given as “Attached to rocks in sectors with swift currents” [14]

The genus Tylomelania (#39 - 42) is another group of ovoviviparous pachychilids, these endemic to particular lakes, rivers, and streams on the Indonesian Island of Sulawesi.  Our friends Thomas von Rintelen and Matthias Glaubrecht recognized 62 species of Tylomelania the last time I checked, but I haven’t been home since morning [15].

So the paper by Ting Hui and colleagues opened with this sentence: “The ornamental pet trade is often considered a key culprit for conservation problems such as the introduction of invasive species (including infectious diseases) and overharvesting of rare species.”  Can a gastropod population – especially a population endemic to some narrowly-circumscribed patch of habitat – indeed be threatened by overharvest?  What is the evidence that hobbyists can drive populations of their favorite organisms to extinction with love?  Tune in next time.


[1] Ng Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130

[2] Fig 1. Freshwater molluscs in the ornamental pet trade.
Unless indicated differently, scale bars = 10mm. 1. Batissa similis; 2. Batissa violacea; 3. Corbicula fluminea; 4. Corbicula moltkiana; 5. Hyriopsis bialata; 6. Hyriopsis desowitzi; 7. Parreysia burmana; 8. Parreysia tavoyensis; 9. Pilsbryoconcha exilis; 10. Scabies crispata; 11. Sinanodonta woodiana; 12. Unionetta fabagina; 13. Marisa cornuarietis; 14. Pomacea canaliculata; 15. Pomacea diffusa; 16. Pomacea maculata (photograph by K.A. Hayes); 17. Bithynia sp.; 18. Clea bockii; 19. Clea helena; 20. Radix rubiginosa; 21. Clithon corona; 22. Clithon diadema; 23. Clithon lentiginosum; 24. Clithon mertoniana; 25. Neripteron auriculata; 26. Neritina iris; 27. Neritina juttingae; 28. Neritina violacea; 29. Neritodryas cornea; 30. Septaria porcellana; 31. Vittina coromandeliana; 32. Vittina turrita; 33. Vittina waigiensis; 34. Brotia armata; 35. Brotia binodosa; 36. Brotia herculea; 37. Brotia pagodula; 38. Sulcospira tonkiniana; 39. Tylomelania towutica; 40. Tylomelania sp.; 41. Tylomelania sp.; 42. Tylomelania sp.; 43. Physa sp.; 44. Amerianna carinata; 45. Indoplanorbis exustus; 46. Gyraulus convexiusculus; 47. Semisulcospira sp.; 48. Melanoides tuberculata; 49. Stenomelania offachiensis; 50. Stenomelania plicaria; 51. Stenomelania cf. plicaria; 52. Stenomelania sp.; 53. Thiara cancellata; 54. Celetaia persculpta; 55. Filopaludina cambodjensis; 56. Filopaludina peninsularis; 57. Filopaludina polygramma; 58. Sinotaia guangdungensis; 59. Taia pseudoshanensis.

[3] No, I don’t.

[4] Dillon, R. T., A. R. Wethington, J. M. Rhett and T. P. Smith.  (2002)  Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra.  Invertebrate Biology 121: 226-234.

[5] Ng, TH, SK Tan, & DCJ Yeo (2015) Clarifying the identity of the long-established, globally-invasive Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in Singapore.  BioInvasions Records 4: 189 - 194.

[6]  This is fascinating.  It is certainly possible that the 22% CO1 sequence difference reported in Singapore is another case of mitochondrial superheterogeneity [7], which (we do know) occurs in North American populations of Physa acuta.  It is also possible that the "Physa sp" reported by Ng and colleagues in Singapore might indeed be a distinct biological species.

Over and over again, all over the world, we have seen invasions by species mixtures, which have been sorted out only at the site of introduction, and then subsequently corrected in their places of origin.  I do think it is possible that the American Pacific northwest may be inhabited by a species perhaps best identified as Physa concolor, but called Physa natricina in the Snake River, morphologically very similar to Physa acuta, but reproductively isolated from it [8].  Is this Physa concolor in Singapore?  Wild speculation here, but who knows?  Science is lots of fun, isn't it?

[7] For more on mtSH, see:
  • Mitochondrial superheterogeneity: What we know [15Mar16]
  • Mitochondrial superheterogeneity: What it means [6Apr16]
[8] For more on what I called "Hypothesis #2 (of 3) See:
  • The Mystery of the SRALP: A bidding [5Feb13]
[9] I’ve published quite a few essays on the North American Pomacea invasion.  See:
  • Pomacea spreads to South Carolina [15May08]
  • Two dispatches from the Pomacea front [14Aug08]
  • Pomacea news [25July13]
[10] For more on the North American populations of Melanoides, see:
  • To only know invasives [16Oct15]
  • The many invasions of Hilton Head [16Dec15]
[11] Pointier, J-P., R. Incani, C. Balzan, P. Chrosciechowski, and S. Prypchan (1994)  Invasion of the rivers of the littoral control region of Venezuela by Thiara granifera and Melanoides tuberculata and the absence of Biomphalaria glabrata, snail host of Schistosoma mansoni.  Nautilus 107: 124 – 128.

[12] For more on Bellamya in North America, see:
  • Bellamya News [6Oct05]
  • Community consequences of Bellamya invasion [18Dec09]
  • Just before the bust [5Aug14]
[13] Ng and colleagues recorded only 0.5% uncorrected pairwise 16S sequence difference between their individual B. armata and B. binodosa analyzed.  Hmm…

[14] Kohler, F, and M. Glaubrecht (2006)  A systematic revision of the southeast Asian freshwater gastropod Brotia (Cerithioidea: Pachychilidae)  Malacologia 48: 159 – 251.

[15] von Rintelen T., B. Stelbrink, R. M. Marwoto, & M. Glaubrecht (2014) "A Snail Perspective on the Biogeography of Sulawesi, Indonesia: Origin and Intra-Island Dispersal of the Viviparous Freshwater Gastropod Tylomelania". PLoS ONE 9(6): e98917.

Wednesday, September 6, 2017

Not Finding Fontigens cryptica

In July we met Mrs. Lori Schroeder, the remarkable amateur malacologist dedicated to surveying the gastropod fauna of the Bernheim Arboretum and Research Forest south of Louisville, Kentucky.  And In August we convinced ourselves that the tiny hydrobiid shells Lori discovered in debris washed along the banks of Harrison Creek were best identified as those of Fontigens cryptica, The Most Cryptic Freshwater Gastropod in The World [1].

But as of the spring of 2017, neither Lori, nor her colleagues, nor indeed a team of professional karst, cave, and groundwater biologists had found a single living Fontigens anywhere on the Bernheim property.  It was the expert opinion of Dr. Jerry Lewis that populations of Fontigens cryptica are obligately restricted to interstitial spaces in waters flowing through subterranean beds of sand and gravel, equally unlikely to be recovered from creek waters at the surface and from cave waters below.

Removing the mesh trap from a drainage pipe.

One of the many charming gags in the 1987 Hollywood movie, “The Princess Bride” is a scene where our hero and the bad guy wage a battle of wits involving a poison called iocane [2].  Iocane is odorless, colorless, and tasteless.  So the contest begins when our hero brandishes a vial of poison in front of the bad guy, challenges him to put the vial to his nose, and says, “What you’re not smelling is iocane.”

By some similar logic, I myself felt called to not see what everybody else was not seeing in the caves, springs and streams of central Kentucky.  And I also wanted to survey the complete freshwater gastropod fauna of Bernheim, not just its rarest element.  And I also very much wanted to meet my remarkable colleague, Lori Schroeder.

Note the meticulous labels
And on Saturday, 17June17, I was not disappointed at any of the three elements of my quest.  Lori and her husband Jeff met me in the Bernheim parking lot with smiles and greetings and a fresh pink & green gift bag.  And in the bag were four matching Tupperware containers.  And in one of those containers [3] were four zip lock bags.  And in each of those bags was a vial, and in each vial a meticulously-documented and beautifully formatted label and a clear pill capsule, and in each pill capsule a sample of tiny snails.

We started at Lake Nevin, right by the entrance gate.  No, of course we weren’t expecting to discover any Fontigens in a small lake impounded for landscaping purposes.  Poking around in the margins of Lake Nevin I was trying to do something else it probably isn’t possible to do – overtly correct for a known bias.

Normal sampling processes, such as the ordinary methods ecologists use to sample biological communities, are biased against rare species.  Any element with a relative abundance of less than one will be recorded as a zero.  The FWGNA databases show the opposite trend, however.  Number theoretical analysis has suggested that our lists of North American freshwater gastropods are biased for rare species, not against [4].

Longtime readers of this blog are aware that I occasionally preach little sermons [5] about a phenomenon I call “conservation-biased oversampling.”  This is the tendency, quite unapologetic in many published surveys and vivid in the systematic collections held by major museums, for researchers to focus their sampling efforts on species that they think are rare or endangered.  Such efforts, driven by the availability of research funding and the quest for more, are ultimately counter-productive.

So as of 17June17 I was carrying on the hard drive of my computer data documenting three locations where Lori had recovered shells of Fontigens cryptica in the Bernheim Arboretum and Research Forest.  And not one single datum regarding any population of any trash pulmonates that might be inhabiting the property whatsoever.  Which I felt powerfully called to fix.

And such a fine field companion was Lori Schroeder!  So enthusiastic, so interested, so eager to learn.  I got a fist-bump for a limpet.  At one point about mid-morning I was poking around in the backwater of some little stream, pulled up a dead leaf, and found a single Ferrissia fragilis adhering to the underside.  Lori was so excited that she jumped up, gave a little whoop and offered me a fist-bump.  For a plain brown limpet, probably not 4 mm in maximum extent.  A human behavior unprecedented in my long experience.

What a beautiful day, what a lovely part of the world, and what excellent company!  Lori and Jeff guided me to every spring, stream, lake and pond on the Bernheim property, plus a wide assortment of bone-dry creek beds, obscure holes in the ground, and forsaken dunes of dusty organic debris.  We rode up and down the rich old fields of the Harrison Creek valley in the back of Jeff’s pickup truck, checking that series of mesh traps Lori and Jeff had wired onto the drainage pipes in March.

The picnic spread that Lori and Jeff set was spectacular.  In my ordinary life, the only time I ever eat roast beef and ham together is Christmas, maybe.  Lori offered 5 – 6 types of meats and cheeses and 10 – 12 Tupperware boxes filled with an assortment of ice-cold fruits and veggies.  And I’ve got to say, that cold bottle of Kentucky’s indigenous “Ale 8” ginger ale was among the finest things I can ever remember wrapping my lips around.
The cool, clear waters of Harrison Creek sparkled like jewels on that sun-dappled afternoon, a nice population of Pleurocera semicarinata grazing lazily across the horizontally-bedded shelves of limestone.  There wasn’t much debris of any sort in the stream, nor indeed even loose rock, cobble, or cover of any description.  At one point I stopped and looked around myself and counted, from a single vantage point, 82 crayfish.  And I paused to reflect on the selective forces that might drive a freshwater gastropod population into subterranean life – to quit competition for the rich periphyton resources potentially available in such a stream and adapt to the meager rations of the lightless hyporheic zone.

No, we did not find any Fontigens cryptica on that long, fine day at Bernheim [6].  Last I heard, Lori and Jeff were talking about assembling a Bou-Rouche sampling device from the materials available at the local Tractor Supply Company, basically a piston pump driven into the creek bed, to continue the quest.

But in my personal opinion, the world already has enough populations of pale, white, 2 mm snails crawling through interstitial spaces deep in our subterranean gravel deposits.  What this world needs is more Lori Schroeders. 


[1] My previous posts in this series:
  • Lori Schroeder’s Tiny Snails [17July17]
  • The Most Cryptic Gastropod in The World [6Aug17]
[2] The battle of wits from The Princess Bride [YouTube]

[3] If you’re curious, the other Tupperware containers held a big honkin’ Campeloma from nearby Beech Fork River, a sample of Pleurocera and Physa from nearby Sunfish Creek, and a sample of Pleurocera and Campeloma from The Land Between The Lakes.

[4]  See the analysis of species richness in my overall Synthesis of the FWGNA Atlantic drainage data here: [FWGNA Synthesis]

[5] I coined the term “conservation-biased oversampling” in my post of 19Mar12, and touched on it again 6Jan14 and 16Oct15.  See:
  • Toward the Scientific Ranking of Conservation Status – Part III [19Mar12]
  • Why Is Rarity? [6Jan14]
  • To Only Know Invasives [16Oct15]
[6] Our complete list of 17June17:  Found everywhere = Physa acuta, Menetus dilatatus, and Lymnaea humilis.  Found in the lakes and ponds = Lymnaea columella, Gyraulus parvus, and Helisoma trivolvis.  Found in the creeks = Pleurocera semicarinata semicarinata, Physa gyrina, Ferrissia rivularis, and Ferrissia fragilis (outside the gate).

Sunday, August 6, 2017

The Most Cryptic Freshwater Gastropod In The World

Through much of the 20th century, the #1 (and possibly only) expert in the biologically fascinating genus Fontigens was the legendary Mr. Leslie Hubricht (1908 – 2005).  Hubricht was trained as a repairman for adding machines and (later) for computers, but spent most of his life travelling around the United States in a van, digging through humus, rolling logs, and peering into holes [1].  He published over 150 scientific papers, primarily on land snails but also on the biota of caves.  Among these were 4 - 5 papers on spring or groundwater-dwelling hydrobiid snails, especially Fontigens [2].

So last month I shared an inspiring story about Lori Schroeder and her discovery of the tiny shells of a mysterious Fontigens species in deposits of storm water flotsam on the margins of several creeks at the Bernheim Forest in central Kentucky [3].  And I mentioned in passing that our good friend Bob Hershler, together with Leslie Hubricht and the cave biologist John Holsinger, had listed nine “recognized species” of Fontigens in their 1990 monograph [4].  Let me back up and expand that statement slightly.

Hershler and his colleagues actually listed ten species, nine of which they “recognized” and one of which was of “questionable status.”  That tenth species, mentioned in a single concluding paragraph on page 43, was Fontigens cryptica.

Leslie Hubricht described Fontigens cryptica in 1963 from under stones in a small spring along the Ohio River in southeast Indiana [5].  He seems to have had at least one living specimen in hand, because he described the animal as “translucent whitish, blind.”  But he immediately confessed, “verge unknown.”  Hershler reported in 1990 that he “was unable to find the snail during two recent visits to the type locality (and nearby localities), nor was it taken during an extensive survey of subterranean habitats in the region (Lewis 1983).”  Hence the uncertainty of its status.

Above I have scanned the 1.9 mm holotype figured by Hubricht in 1963 next to a photo of a 1.9 specimen collected last year by Lori Schroeder.

Notice also the reference to a "Lewis 1983" in the Hershler quote reproduced above.  My attention was called to the existence of this tenth, questionable-status Fontigens by none other than Dr. Julian J. Lewis himself, still very active in karst, cave, and groundwater research in Kentucky today.

By one of those strokes of fortune I have begun to take for granted in my long career, it materialized that in 2016 the Bernheim management engaged Lewis and Associates LLC to survey the subterranean fauna of its caves and springs, and that professional surveys of the entire property had been underway for several months prior to the date that Lori Schroeder first brought her Fontigens discovery to my attention.

So in April Lori offered me an electronic introduction to Dr. Julian (Jerry) Lewis, and we struck up a conversation.  And Jerry confirmed: 
“Bob Hershler and I looked for this species together at the type-locality at a spring in Clark County, Indiana and I've been there repeatedly with no luck.  The spring is high on a bluff overlooking the Ohio River and consists of a hole about the size of one's fist....not much habitat in which to search.  Subsequently I pulled shells of this species from a meter below the surface of the Blue River  -  from groundwater flowing through the hyporheic zone, in the company with a number of other non-cave subterranean species - using a Bou-Rouche sampling pumpwell. 
 I found the snails alive in the interstices of a gravels in a similar situation in a cave in Monroe County, Indiana (near Bloomington) using Karaman-Chappuis extraction. 
 So I suspect your snails are probably a groundwater species, likely living in the interstices of gravel and sand, and will require special sampling methods."
So no, Lewis and Associates LLC had not recovered any Fontigens whatsoever in their survey of the Bernheim property just recently concluded as of this spring [6].   Nor indeed was it Jerry’s expert opinion that we should expect to find any.  Jerry’s studies of the entire regional biota have led him to hypothesize the existence of a single subterranean zoogeographic province extending at least as far south as Mammoth Cave in central Kentucky and as far north as the Blue River drainage of Indiana.  But he has never seen a population of Fontigens cryptica in any cave stream he has explored, ever.  Gravel under a cave stream, yes.  But the little snail is no more an inhabitant of open flowing water under the ground than of open flowing water at the top.  Fontigens cryptica seems to be limited to a third, even more mysterious habitat: saturated interstitial spaces.

I concluded last month’s blog post with a series of three teaser questions: “From what dark recesses of central Kentucky knobland might Lori’s tiny little snails be emanating?  And what might be their identity?  And will Lori Schroeder surrender her quest?”  This month I have addressed questions #1 and #2.  Next month I will address #3.  And here’s your teaser.  No.


[1] He stood silently in the lobby outside AMU meetings in the 1970s and 1980s, wearing a stiff dark suit.  I wish I had gotten to know him.

[2]  Gerber, J. (2010)  Leslie Hubricht (1908 – 2005), His publications and new taxa.  American Malacological Bulletin 28:15-27.

[3] Lori Schroeder’s Tiny Snails [17July17]

[4] Hershler, R., J. R. Holsinger and L. Hubricht (1990)  A revision of the North American freshwater snail genus Fontigens (Prosobranchia: Hydrobiidae).  Smithsonian Contributions to Zoology 509: 1 – 49.

[5] Hubricht, L. (1963)  New species of Hydrobiidae.  Nautilus 76: 138 – 140.

[6] Only a single freshwater gastropod was listed among the 61 species documented by Lewis and Associates - Lymnaea humilis in a seep spring.

Monday, July 17, 2017

Lori Schroeder's Tiny Snails

Mrs. Lori Schroeder manages her husband’s dental practice in Bardstown, Kentucky.  She is also a skilled and dedicated malacologist, who in recent years has expanded her interests from seashells to land snails and into the freshwater fauna.  Back in 2009 she volunteered to lead a “Bioblitz” survey of the land snails of the Bernheim Arboretum and Research Forest south of Louisville [1], collaborating with our good friends Harry Lee, the late Henry McCullagh and Bill Frank [2].  And several years ago I was blessed to strike up a correspondence.

Organic litter by Harrison Creek
Lori’s favorite method of surveying land snails is to pick through samples of forest litter, such as are often deposited along the courses of the several creeks and streams running through the 15,000 acre Bernheim property.  So it is not surprising that she occasionally finds freshwater gastropods mixed with her land snail collections.  I received my first emails from her in 2013 regarding the pleurocerids and physids of Bernheim, which I was pleased to entertain.

And I was pleased to receive another email from Lori this past January.  She sent to my attention a jpeg photo of a small planorbid, requesting guidance on the difference between Menetus and Gyraulus.  And, as an afterthought in some follow-up correspondence, she mentioned, “I also have a tiny unidentified hydrobiid species from the same lot.”

My antennae raised immediately.  I replied, “What’s this I hear about a tiny unidentified hydrobiid?  I’d be curious to see a photo of that one as well.”  And on 1 February 2017, this is the jpeg Lori sent me:

My antennae raised even higher.  Clearly a little pile of Fontigens, that diminutive inhabitant of springs, spring runs, and caves scattered widely throughout the Eastern and Midwestern United States.  I reached to the shelf beside my desk and pulled out my well-thumbed copy of the monograph by our good friend Bob Hershler and colleagues [3].  And here is the reply I sent Lori, under the subject line “OK, now you’ve got my attention:”
Dear Lori, 
The jpegs attached to your message of earlier this afternoon appear to depict a hydrobiid population of the genus Fontigens.  My first guess would be the most common species, Fontigens nickliniana.  But upon closer examination, they might look more like published photos of Fontigens aldrichi, which I’ve never personally seen on the hoof.
The genus Fontigens was marvelously-well monographed by Bob Hershler and colleagues (1990).  And among the many surprises is that Hershler did not report any records of Fontigens of any species in Kentucky, period.  F. nickliniana is widespread in the Great Valley of Virginia, spreading up to Pennsylvania, and also in western Michigan, so one might assume that they range through Kentucky.  A lot of secondary references do indeed list F. nickliniana in your state.  But I myself have never seen a F. nickliniana population in Kentucky, and don’t know of any reliable record.
All that said, the flat nuclear whorl doesn’t really look like F. nickliniana.  Picture-matching to the other eight species recognized by Hershler, they look more like Fontigens aldrichi or Fontigens antroecetes.  But those species are not known outside of Missouri.
Bottom line.  Those shells were all dead-collected, am I right?  It would be great if you could find the living population, and send a preserved sample to Bob Hershler at the USNM.  Fontigens populations are often narrowly-restricted to springs and spring runs.  There’s a lovely, clean, clear, coldwater spring right upstream from your collecting site, am I correct?  Lots of watercress, I’ll bet?  You should find your big population of Fontigens at that spring head, covering the cress and every square inch of bottom.  Go look and tell me if I’m right.  Take a Nalgene bottle or something to collect in.
Standing by,
And here is Lori’s immediate reply: 
Hello Dr. Dillon, 
I've been down the hydrobiid road with Dr. Lee and Dr. Fred Thompson at the Florida Museum previously [4].  I failed miserably to locate any living specimens.  That being said, I am more than willing to give it a second go. 
 It subsequently materialized that my speculations about a spring head with “lots of watercress” and “Fontigens covering every square inch of bottom” were very, very wrong.  And the situation much more complicated than I initially imagined.

Lori had found her little pile of Fontigens shells sorting through debris deposited along Harrison Creek by a catastrophic flood that ravaged the area in January of 2016.  Harrison Creek washes down through a lovely valley of old fields flanked by mixed hardwoods.  Yes, there are (indeed) small springs and caves on the ridges surrounding, but most issue water only in wet weather.

A further complication was that Lori had actually found Fontigens shells in similar circumstances along three different streams on the Bernheim property during the course of her extensive studies of the area, all in dry debris deposited by floodwaters, none downstream from any spring or other obvious source of groundwater.

And on 3 February 2017 Lori sent me another set of more detailed jpegs, giving a standard shell length of just 2.1 mm.  Adult F. nickliniana reach 3 – 4 mm, in my experience.

Broadly speaking, there are two categories of Fontigens in North America, which (given my Presbyterian heritage) I think of as being either “black” or “white” [5].  The black species, such F. nickliniana, F. morrisoni and F. bottimeri, are usually somewhat larger and live above ground.  The white species, such as F. orolibas, F. tartarea and F. turritella, are usually smaller and subterranean.  The shell photos Lori sent us on Feb 3 struck me as depicting a white species – not something we might expect to find at the surface.

Undaunted, Lori and her husband Jeff set out on what she called a “full-on blitz attack,” ultimately extending over several months and involving Andrew Berry, Forest Manager at Bernheim.  And I cannot remember ever witnessing, first-hand or second, a more earnest or comprehensive survey of any patch of the earth for any mollusk population, of land or sea.  Lori and colleagues surveyed every spring or cave or hole in the ground actually, potentially, or historically upstream or in the vicinity of every location where any Fontigens specimen had ever been recovered from Bernheim.  They also installed mesh traps over the openings of several culverts draining into Harrison Creek.  And the results, to quote Lori directly, were “Nothing.  Nada.  Zilch.”

From what dark recesses of central Kentucky knobland might Lori’s tiny little snails be emanating? Indeed, what might be their identity?  And will Lori Schroeder surrender her quest?  Tune in next time! 


[1] The Bernheim Arboretum and Research Forest was originally developed in 1929 and endowed by the German philanthropist Isaac W. Bernheim “as a gift to the people of his new homeland.”  It is open to the public during regular daylight hours, with a small admission charge on the weekends.
Website: [html]

[2] The Kentucky land snail list is available at [Jaxshells-Kentucky].
Photos of many elements of the Bernheim freshwater gastropod fauna are also available from the jaxshells.org website, if you try the search button at the bottom of the front page.

[3] Hershler, R., J. R. Holsinger and L. Hubricht (1990)  A revision of the North American freshwater snail genus Fontigens (Prosobranchia: Hydrobiidae).  Smithsonian Contributions to Zoology 509: 1 – 49.

[4] Yes, it materialized that the 2016 specimens were not the first Fontigens Lori had found in her nine-year survey of Bernheim.  Several years previous she had corresponded briefly with Fred Thompson about the little snails, but Fred was not interested unless she could find live material, and the matter dropped.

[5]  For an illustration of the striking black/white dichotomy see:
  • Springsnails of The Blue Ridge [26July06]

Tuesday, June 27, 2017

The Freshwater Gastropods of The Ohio: An Interim Report

By popular demand!  Several of you have requested copies of the presentation I contributed at the Society for Freshwater Science meeting in Raleigh earlier this month.  A pdf download is available for the clicking below.

My collaborators on the work were Ryan Evans of KYDOW, Mark Pyron of Ball State, Tom Watters of Ohio State, Will Reeves of the USDA in Ft. Collins, Richard Kugblenu of SUNY-Albany, and Jeff Bailey & Mike Whitman of WVDEP.  Here’s the abstract: 
We report preliminary results from a survey of the freshwater gastropod fauna from the Ohio River basin above Paducah, including western Pennsylvania and most of West Virginia, Ohio, Kentucky, and Indiana.  Our database of 4,746 records was almost entirely drawn from state natural resource agencies (36%), museums (30%) and our own original collections (30%), almost all personally examined and verified.  We report 66 species and subspecies of freshwater gastropods [1], the most common of which are Physa acuta (959 incidences), Ferrissia rivularis (536), and Pleurocera semicarinata (all subspecies combined 528).  Nine species were collected in but a single population, four of which seem to be legitimately rare, the remainder peripheral.  The distribution of commonness and rarity appeared log-2 bimodal, with mean 3.84 (14.3 incidences) and standard deviation 2.79 (6.9 incidences).  Our Ohio River basin results are compared to similar databases previously assembled from the Atlantic drainages and from East Tennessee, and the 99 species of the combined 13-state fauna ranked by their incidence.
 I should emphasize that the results as I offered them in Raleigh were very preliminary.  Subsequent to the submission of the abstract above, our GIS analysis suggested that a couple subregions of our study area had been oversampled, resulting in a prune of our database back from 4,746 records to 4,570.  And I have just returned from two weeks in the field, adding quite a few sample sites from northern Tennessee [2] and central Kentucky, as well as a very large and impressive slug of data from Illinois [3].

Several of you have also asked me when the finished FWGO site might be expected to appear online.  That date, to be precise, is Not Quite Yet.  But we’ll keep you posted… 


[1] We recognize 6 subspecies of freshwater gastropods in the FWGO study area: the pair Campeloma decisum crassulum and C. decisum (ss), the pair Pleurocera simplex ebenum and P. simplex (ss), the pair Pleurocera canaliculata acuta and P. canaliculata (ss), the pair Pleurocera laqueata alveare and P. laqueata (ss), and the trio Pleurocera semicarinata livescens, P. semicarinata obovata, and P. semicarinata (ss).  Thus the total species we analyzed on slide #7 of our June presentation was 60.

[2] Yes, the Green River drainage includes a sliver of north central Tennessee.  The faunal similarity between Kentucky and Tennessee has been one of the biggest surprises of the FWGO survey.

[3] A tip of the hat to our good friend Kevin Cummings for his gracious hosting at the Illinois Natural History Survey.

Friday, May 26, 2017

Freshwater Snails and Passerine Birds

Editor’s Note – This is the fourth in a series of essays about the aerial dispersal of freshwater gastropods.  But no, you do not have to read the other three, which are entirely independent of this one.  My previous essays were posted 15Dec16, 11Jan17, and 24Apr17, if you’re curious [1].  But that material won’t be on the test.

So back in April of 2015 I received an intriguing email from our good friend Nora Foster, way up in Fairbanks, Alaska.  She wrote: 
"A colleague of mine has just brought me a whole sh-- load of tree swallow nests. It seems that the little birds select pond snails, presumably for food, there's some indication that it's a response to low calcium availability. I've only found one paper on this [2].  I wonder if you know of any other work done on freshwater snails being consumed by small birds. The shells are intact, by the way, I'm not sure if the swallows are spitting out the shells, or just what's going on."
From a swallow nest in Alaska
My quick answer was no, that I was not aware of any other work on the consumption of freshwater snails by small birds.  Stacks of papers have been published about ducks and other waterfowl, of course.  We’ve reviewed some of that work in recent months.  And there’s the Everglades kite, which is a raptor, feeding on Pomacea.  But no, Nora’s email of 4/15 was the first report I had ever heard that little dicky-birds might eat freshwater snails.

So Nora and I struck up a correspondence that extended a couple weeks.  Nora found one reference suggesting that momma swallows may bring freshwater snails back to their hatchlings and regurgitate them as food provisions [3].  Perhaps the empty shells we subsequently find in swallow nests were food offerings rejected by the hatchlings, or lost?  Not swallowed?  Sorry, I couldn't resist.
L. atkaensis?

In a sense this phenomenon would seem to be the complement of the situation we reviewed in December and April.  Waterfowl are very likely to ingest freshwater snails, but the snails are very unlikely to survive.  Passerine birds are certainly much less likely to ingest freshwater snails, but the likelihood of snail survival may be much greater.  Perhaps the products of the two sets of probabilities are comparable?

Now imagine a shot of a calendar flipping 17 pages.  This past September I was pleased to receive a cordial email from our good friend Bob Prezant, introducing me to Dr. T. J. Zenzal of the Migratory Bird Research Group at the University of Southern Mississippi.  Dr. Zenzal had a photo, a question, and an interesting story to tell.

Each spring migratory birds arrive on the southern Louisiana coast after long and perilous flights across the Gulf of Mexico [4].  Dr. Zenzal has worked many migration seasons and has captured (he estimates) over 20,000 birds.  Several years ago, while he was a graduate student, one of his labmates made the remarkable collection figured below from the body of an indigo bunting.

I responded to Bob and TJ that I was “about 80% sure” those snails are Lymnaea cubensis, with “maybe a 5% chance” that they could be fat, squat Lymnaea humilis.  (I’d have to count the cusps on their first marginal to rule out the humilis hypothesis.)  I also left 15% chance that those little snails might be referable to some Central or South American taxon, like viator/viatrix, neotropica, or cousini, and that any of those nominal species from South America really is specifically distinct from cubensis.  I confessed that I have zero experience with any element of the South or Central American fauna, however, and concluded with my (gratuitous) impressions that the South American lymnaeids are terribly oversplit, and that our understanding seems to be worsening with each new research paper published on the subject [5].

But the most interesting question (by far!) is not what those snails are, but how the heck did those snails get there?  Looks like 8-9 snails, from the photo.  Buried in the breast feathers, according to TJ’s note currently in press [6].  My mind was first called back to my 2015 correspondence with Nora Foster.  Apparently passerine birds do, at least sometimes, eat snails.  Possibly as a calcium provision?

And I also recalled W. J. Rees’ charming “presidential address” of 1965, on the aerial dispersal of the Mollusca [7].  Rees retold a story from one “Prof. G. E. Beyer” regarding upland plovers: 
“When the birds arrive in Louisiana, they invariably carry ten to thirty snails under their wings.”  “The occurrence was so regular and confirmed so often after many years, that I expected the habit to be generally known.” 
Prof. Beyer hypothesized that plovers deliberately carried snails under their wings as food provisions.  But the plover is a ground-nesting bird, and so (I suppose) a skeptic might suggest an alternative hypothesis that all those snails had climbed into all those plover arm pits accidentally, if they were nesting, which I don’t suppose they probably were. 

But an indigo bunting?  A male, by the way, who wouldn’t need augmented calcium supplies, who wouldn’t be feeding hatchlings?  Does that sound like a food provision?  I suggested this idea to TJ, and he replied, quite reasonably: 
“The Indigo Bunting is primarily a seed eater, however birds can show flexible foraging during migration - leaving it possible for this species to eat snails.  However, I am not sure the bird would willingly carry snails just to consume them later - especially if the bird made a trans-Gulf flight.”
Fair enough.  So for now, at least, I myself have filed the remarkable observation of eight Lymnaea hitchhiking on an indigo bunting breast in my very large folder labelled, “queerer than we can suppose.”

I’m not sure if it’s a blessing or a curse, but the attentions of practicing biologists are often called to “one-of” phenomena.  While such phenomena do not lend themselves to scientific inquiry, they often register extremely high values on the importance-meter.  The origin of life was such a one-of phenomenon.  And so are we all.


[1]  The three previous essays were:
  • Freshwater gastropods take to the air, 1991. [15Dec16]
  • A previously unrecognized symbiosis? [11Jan17]
  • Accelerating the snail’s pace, 2012. [24Apr17]
[2] St. Louis, VL and L Breebaart (1991)  Calcium supplements in the diet of nestling tree swallows near acid sensitive lakes.  Condor 93: 286-294.

[3]  I can’t open this link, but for the record:

[4] Many songbirds most certainly do make the 1,000 km crossing directly over the Gulf of Mexico.  But some apparently go around.  The possibility that any particular bird might have circumnavigated the Gulf, rather than crossing it, cannot be ruled out.

[5] The Lymnaeidae 2012: Fossarine football [7Aug12]

[6] Zenzal, TJ, Jr, EJ Lain, and JM Sellers (in press) An Indigo Bunting (Passerina cyanea) Transporting Snails During Spring Migration.  The Wilson Journal of Ornithology

[7] Rees, WJ (1965)  The aerial dispersal of mollusca.  Proc. Malac. Soc. Lond. 36: 269 - 282.

Monday, April 24, 2017

Accelerating The Snail's Pace, 2012

My faithful readership may recall a review I posted last December on the ever-intriguing phenomenon of passive molluscan transport entitled, “Freshwater gastropods take to the air, 1991 [1].”  That essay was focused entirely on unpublished material I developed for my (2000) book over 20 years ago.  What have we learned since then?

A lot, actually.  But before training our intellectual laser beams on recent research progress in the avian dispersal of freshwater gastropods in detail, I should spend a paragraph to acknowledge a pair of more general reviews contributed in the mid-2000s by Figuerola and Green [2].  The F&G reviews are especially useful to place our admittedly-narrow focus on mollusks into the larger perspective. Most research published on the broader subject of avian dispersal in recent decades has been directed toward protozoans and microcrustaceans, as well as algal and plant propagules.  The charming review of Rees [3] was skipped over entirely by F&G, although they did cite the works of Boag [4] and Malone [5].  I found it interesting that across the entire subdiscipline of avian dispersal, more attention seems to have been directed toward internal transport in the gut passage than to external hitchhiking.

So in 2012 I was pleased to find the PhD thesis of a promising young scientist from The Netherlands named Casper van Leeuwen delivered to my snail-mail inbox.  That beautifully-printed 175 page work, with the whimsical cover illustration reproduced above, turned out to be a treasure trove of research on the avian dispersal of freshwater gastropods.  Some of the material in its eight chapters had just been published elsewhere as of 2012, other material was still on editor’s desks around the world.  Casper’s entire thesis, as well as the five journal articles it ultimately yielded, are all available from his professional website, see note [6] below.

Casper opened his studies with a general introduction, and a meta-analysis of 81 previously-published papers on the gut transport of macroinvertebrates and seeds, updating and refining the work of Figuerola & Green.  He got down to business in Chapter 3.

Casper reported the results of a set of feeding experiments involving mallards and four small species of aquatic gastropods common in The Netherlands: the little planorbid Bathyomphalus contortus and the hydrobioids Hydrobia ulvae [7], Potamopyrgus antipodarium, and Bithynia leachii.  The birds were fed 100 – 300 living snails and their feces collected for 24 hours.  Casper’s observations confirmed those of Malone, zero survivorship for Bathyomphalus, Potamopyrgus, or Bithynia.  But Casper did recover approximately 21 viable Hydrobia individuals, of 6,600 fed to the ducks.  Given the vast flocks of ducks that must visit the coastal marshes of Europe, the vast populations of Hydrobia that inhabit those marshes, and the vast expanses of time ducks have been eating snails, flying elsewhere, and pooping along the way, even a 0.32% survival rate is not negligible, I suppose.

The ducks Casper used for his Chapter 3 studies were all held in individual pens for the 24 hours following snail ingestion.  So in Chapter 4, he reported the results of an experiment to examine the effects of subsequent locomotion. Six mallards were fed 300 Hydrobia ulvae each and their feces collected during one of the three treatments that followed: isolation, wading, or swimming.  The entire experiment was repeated four times, for 4 x 3 x 300 = 3,600 snails ingested.  Casper recorded a total of 29 (0.81%) Hydrobia surviving the experiment, marginally better than his Chapter 3 results, but still “too low to compare between treatments.”  My eyeball impression of his Table 4.S1 suggests to me, however, that swimming was more harsh on the gut contents than wading, and wading more harsh than isolation.  Which makes me wonder if both of Casper’s estimates of survivorship, 0.32% and 0.81%, might be biased above their natural values.

As unlikely as all these highly-unlikely events most certainly are, I really think transport of viable freshwater gastropods on the outside of birds is less unlikely than their transport on the inside.  So in Chapter 5, Casper reported results from several experiments designed to evaluate the likelihood that snails might adhere to the feet, feathers, and bills of ducks and survive their subsequent dehydration.  The most interesting of these experiments involved three small European pulmonates: Lymnaea peregra (aka “Radix balthica[8]), Gyraulus albus, and Anisus vortex.  Casper constructed cages with shallow removable trays in the bottom, into which he introduced a mixture of aquatic macrophytes and snails at four densities [9].  Individual mallards were placed in these cages for 60 minutes, totaling 48 ducks over 4 days.  Each duck was then released to walk through a 3 meter tunnel, washed and brushed, and given a final inspection.

The bottom line was that snails were transported out of the cages in 34 of the 48 trials – more of the little planorbids than the Lymnaea.  Casper reported that most snails were shed off the birds during their tunnel walk, but that some remained attached through brushing and washing to final inspection [10].

In his Chapter 6 Casper flew the barnyard for the field, picking up his genetic tool box as he flapped out the back gate.  The Donana National Park is a protected wetland region in southwest Spain, featuring marshes, dunes, shallow streams, and over 3,000 hydrologically-isolated ponds.  Roughly half of these ponds dry during an ordinary summer, and roughly half of those remaining are inhabited by populations of Physa acuta.

The nearest source for all these hundreds of isolated Physa populations is the Guadalquivir River, and especially a set of rice field connected to the river, 10 km east of the park.  Here’s a thought-experiment.  How could you distinguish vector-mediated dispersal from dispersal by sheetwater flooding in such a situation using a microsatellite survey?

Casper and his colleagues sampled the Physa populations at 21 sites, including 16 in the isolated ponds of Donana Park and 5 in the rice fields of the Guadalquivir River and estimated genetic variability using six previously-published microsatellite markers.  And as you might expect regardless of dispersal mechanism, the 16 park populations showed significant subdivision, while the 5 rice field sample sites did not.

But here’s the key.  Casper was able to distinguish two types of Physa ponds within the park, those that were often visited by large mammals (primarily cattle) and those that were rarely visited [11].  The high-cattle ponds showed less genetic subdivision than the low-cattle ponds.

The graph below shows a measure of genetic divergence plotted as a function of distance across all 16 populations [12] in the park.  These are all-pairwise distances, so the data are not independent.  But a Mantel test returned a significant correlation between geographic distance and genetic divergence in the low-cattle ponds, not in the high-cattle-ponds.

This observation has two implications.  Most importantly, it suggests that dispersal has been vector-mediated, since there is no reason to expect sheetwater dispersal to differentiate high-cattle ponds from low-cattle ponds.  It also suggests that at least sometimes, large mammals [13] transport Physa.

These results are reminiscent of a similar study Amy Wethington and I published back in 1995 involving 10 Physa acuta populations sampled here in the South Carolina lowcountry [14].  We documented a similar correlation between genetic divergence and simple overland distance across local “sea islands,” despite intervening brackish tidal creeks.  And extrapolating the results of Van Leeuwen and his colleagues about as far as you can go, they also remind me of my dissertation research on 25 populations of Pleurocera proxima in the Southern Appalachians, published way back in the dark ages [15].

The correlation between genetic divergence and simple geographic distance overland was strong for P. proxima too, over a 200 km study area extending into three states, completely independent of drainage system or mountain ranges.  Of course, levels of genetic divergence among pleurocerid populations are much more profound than among physids, and the duration of isolation must be far longer.  I continue to think my P. proxima populations evolved tens, if not hundreds of millions of years ago, possibly at the Appalachian orogeny, and have been diverging since [16].  But I also think this isolation has been punctuated by very rare, but nevertheless significant, airlifts of individual wildebeest into the bison herds [17].

As if all the research reviewed above weren’t enough to earn Casper a Ph.D. four times over, he actually finished his thesis with a Chapter 7 survey of ITS1 rDNA sequence divergence in Lymnaea (Galba) truncatula sampled over four continents.  But let’s defer that study to another day, shall we?  I feel as though the time has come to sum up.

That freshwater gastropods most certainly can be passively dispersed over extensive distances overland has been established for at least 50 years, indeed longer [18].  More recent generations of scientists have shifted our research focus away from the whether, and increasingly toward the how.  Birds can plausibly serve as vectors for the transport of viable freshwater snails over substantial distances, more likely on their outsides than their insides.  Genetic markers suggest that dispersal across terrestrial barriers, probably by birds but sometimes by other agents, has significantly influenced rates of interpopulation divergence in freshwater gastropods.  I, for one, have become increasingly intrigued by observations of their absence than their presence.


[1] Freshwater Gastropods Take To The Air [15Dec16]

[2] Figuerola, J. and A. J. Green (2002)  Dispersal of aquatic organisms by waterbirds: a review of past research and priorities for future studies.  Freshwater Biology 47: 483-494.  Green, A. J. and J. Figuerola (2005) Recent advances in the study of long-distance dispersal of aquatic invertebrates via birds.  Diversity and Distributions 11: 149 – 156.

[3] W. J. Rees (1965) The aerial dispersal of Mollusca.  Proc. Malac. Soc. Lond. 36: 269-282.

[4] Boag, D. A. (1986) Dispersal in pond snails: Potential role of waterfowl. Can. J. Zool. 64: 904-909.

[5] Malone, C. R. (1965a) Killdeer (Charadrius vociferus) as a means of dispersal for aquatic gastropods. Ecology 46: 551-552.  Malone C.R. (1965b) Dispersal of aquatic gastropods via the intestinal tract of water birds. Nautilus, 78: 135–139.

 [6]  Casper’s professional website is www.caspervanleeuwen.info/  From there pdf downloads are available for:
  • Van Leeuwen, C.H.A. (2012) Speeding up the snail´s pace: bird-mediated dispersal of aquatic organisms.  PhD dissertation: Radboud University Nijmegen, The Netherlands.
  • Van Leeuwen, C.H.A., G. van der Velde, J.M. van Groenendael and M. Klaassen (2012).  Gut travellers: internal dispersal of aquatic organisms by waterfowl.  Journal of Biogeography 39(11): 2031-2040.
  • Van Leeuwen, C.H.A., M.L. Tollenaar, and M. Klaassen (2012).  Vector activity and propagule size affect dispersal potential by vertebrates.  Oecologia 170(1): 101-109.
  • Van Leeuwen, C.H.A., G. Van der Velde, B. Van Lith, and M. Klaassen (2012).  Experimental quantification of long distance dispersal potential of aquatic snails in the gut of migratory birds.  PLoS ONE 7:e32292.
  • Van Leeuwen, C.H.A. and G. van der Velde (2012). Prerequisites for flying snails: external transport potential of aquatic snails by waterbirds. Freshwater Science 31(3): 963-972.
  • Van Leeuwen, C.H.A., N. Huig, G. van der Velde, T.A. van Alen, C.A.M. Wagemaker, C.D.H. Sherman, M. Klaassen and J. Figuerola (2013)  How did this snail get here? Multiple dispersal vectors inferred for an aquatic invasive species.  Freshwater Biology 58(1): 88-99.
[7] Hydrobia ulvae is not a freshwater snail.  But sometimes we bend the rules in this blog, a little bit.

[8] See footnote [6] here:
  • The Lymnaeidae 2012: A clue [9July12]
[9] Casper did not count the snails in these experiments.  The four densities were created using four different volumes of mixed weed, assuming uniform distribution of the three pulmonate populations (by ratio) within.

[10] It is difficult to estimate the actual numbers of snails transported from Casper’s graphs.

[11] Casper and his colleagues evaluated the intensity of large mammal visitation using the densities of “droppings” surrounding each pond.  This is the third time my essay has touched on the subject of poo.  A new record.

[12] Not all, actually.  Casper’s figure shows high/high comparisons and low/low comparisons only.  The high/low comparisons are not plotted.

[13] Including us humans.  The large-mammal hypothesis will be weaker if the high-cattle ponds also tend to be high-waterfowl ponds as well.  Perhaps the high-cattle ponds were consistently larger?  Data regarding the relative sizes of the sample ponds would have been welcome here.

[14] Dillon, R.T., and A.R. Wethington (1995) The biogeography of sea islands: Clues from the population genetics of the freshwater snail, Physa heterostropha. Systematic Biology 44:401-409.  [PDF]

[15] Dillon, R.T. (1984) Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33:69-82.  [PDF]

[16] Dillon, R T. and J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11.  [PDF]  See:
  • The Snails The Dinosaurs Saw [16Mar09]
[17]  This is the “jetlagged wildebison” model I proposed last spring:
  • Mitochondrial Superheterogeneity: What it means [6Apr16]
 [18] Charles Darwin, Freshwater Malacologist [25Feb09]