Dr. Rob Dillon, Coordinator

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]

Tuesday, April 4, 2017

SFS Raleigh 2017

This is just a quick note to alert all my colleagues in the Society for Freshwater Science that I am planning to attend this year’s meeting in Raleigh June 4 – 8.  Come visit me under the “Gastropoda” sign at the taxonomy fair, and bring all your vials of tiny beat-up Physa.  I’m also offering a first glimpse of our next big survey, The Freshwater Gastropods of The Ohio.  Evans, Pyron, Watters, Reeves, Kungblenu, Bailey, Whitman and I have logged over 4,500 records from an eight-state area, recording 64 species and subspecies.  Some other than Physa.  See you in Raleigh!

Tuesday, March 14, 2017

Fred Thompson, Elizabeth Mihalcik, and the Pleurocerids of Georgia

Science is a human enterprise, alas.  Last month [1] we reviewed the fitful progress of research into the systematic relationships of the Florida Pleuroceridae through the 1970s and 1980s, as a function of the personalities of two scientists, the late Dr. Fred G. Thompson and Dr. Steven M. Chambers.  In 1990 Steve Chambers published a malacological masterpiece, “The genus Elimia (= Goniobasis) in Florida and adjoining drainage basins [2],” correcting Clench & Turner’s 1956 seven-species model down to four.  He then shifted his professional interests out of the field.  Thompson, failing to cite the Chambers 1990 paper or indeed (almost) any of Chambers’ impressive body of research, published an update of his “Freshwater Snails of Florida” in 1999.  He was going in the opposite direction.

Thompson’s Second Edition [3] began with the Clench & Turner seven-species model but added an undescribed “spring Elimia” from Holmes Creek and three new species that “may occur in Florida” (Elimia taitiana, Choctawhatchee Elimia and Escambia Elimia), bringing his grand total up to 11 and foreshadowing things to come.  Simultaneously, Thompson was working with a committee of the AFS to publish a second edition of the Turgeon et al. “Common and Scientific Names of Mollusks,” which appeared in 1998 [4].  The Turgeon work was destined, in a perverse way, to become the Bible of American malacology.  Thus less than ten years after the publication of Steve Chambers’ landmark paper, the Clench & Turner model became gospel, and Chambers’ work forgotten.

Meanwhile, a new graduate student arrived at the University of Florida named Elizabeth L. Mihalcik.  She entered the laboratory of the noted herpetologist F. Wayne King, and was “introduced into the world of invertebrates through her mentor and friend, Dr. Fred G. Thompson.”  In 1996 she was awarded an EPA STAR graduate fellowship for her proposal entitled, “Analyzing morphological forms within southeastern river system species complexes.”
The Mihalcik [6] study area, showing P. floridensis
Elizabeth Mihalcik’s proposal originally focused on estimating gene frequencies at polymorphic enzyme-encoding loci in populations of what her mentor and friend referred to as “the Elimia curvicostata complex” of Central Georgia and the Florida drainages of Alabama.  And in fact, I did host Elizabeth at my lab in Charleston for several days in the late 1990s, and showed her everything I knew about allozyme electrophoresis.  But if she ever ran an allozyme gel independently, I never saw the data.

Elizabeth’s (1998) dissertation [5] focused primarily on shell morphometrics: 6 measures, 4 counts, spire angle and 8 ratios of the above in various combinations, blessedly naive of any inkling that any shell character might demonstrate any interpopulation variance for any reason whatsoever.  But the winds of change were blowing across systematic biology.  Her dissertation also included a little bit of mtDNA sequence data.

The first fully automated sequencing DNA machine had come onto the market in 1987, even as Kerry Mullis’ patent for PCR amplification was approved.  The Folmer et al primers to amplify the mitochondrial CO1 gene were published in 1994, and by the late-1990s what had been a trickle of DNA sequence data became a torrent.  Surely such data must be useful for systematic biology, if we could just settle on an algorithm to build our gene trees, yes?  The “phylogenetic species concept,” originally proposed by Cracraft in 1983, came into vogue.

So by the time Elizabeth published her dissertation with Fred Thompson in 2002 [6], her analysis was almost entirely based on mitochondrial CO1 sequence data.  She sequenced 25 individual snails from 24 Pleurocera populations scattered across Florida, Georgia and Alabama, with one individual P. catenaria from South Carolina thrown in to boot.  Her results (below) returned four clusters of roughly 5 - 7 individuals each, corresponding remarkably well to the four species that Steve Chambers documented in 1990.

Mihalcik and Thompson also discovered at least three apparent cases of mitochondrial superheterogeneity (mtSH above).  I have very nearly beat this subject to death on the FWGNA blog, but see note [7] below if you need your memory refreshed.  MtSH may indeed have some utility in identifying genuine biological species, but only if the phenomenon is recognized, which Mihalcik and Thompson did not [8].  So setting aside their sequence data for those three individual snails, the remaining snails demonstrate four clusters neatly corresponding to the species Chambers called floridensis, curvicostata, dickinsoni, and “boykiniana*.”

Alas, Mihalcik and Thompson saw substantially more than four species in their gene tree.  They interpreted their 25 mtDNA sequences as evidence of 15 species and subspecies of Pleurocera in Central Georgia and the Florida drainages of Alabama, as follows.  Starting with baseline curvicostata, a catenaria reference from South Carolina and two flava references from Alabama, M&T resurrected four old Isaac Lea names from synonymy: induta, mutabilis, viennaensis, and ucheensis.  They raised Goodrich’s subspecies timida to the full species level, and described two new subspecies underneath it: exul and nymphaea.  And they freshly described five new species: exusta, glarea, annae, buffyae, and darwini, the last three of which Elizabeth named for her dogs. 

When one steps back to consider that the authors did not even consider six of the seven Clench & Turner species as members of the “Elimia curvicostata complex” (albanyensis, athearni, clenchi, dickinsoni, floridensis and vanhyningiana), the Mihalcik and Thompson hypothesis constitutes a staggering multiplication of pleurocerid nomina [9] for the region.

I myself was not idly sitting by in Charleston, however.  Through most of the 1980s I confess that I became distracted by hard clam aquaculture, and by the 1990s Amy Wethington and I were beginning to work on Physa.  But I never forgot my first love, which was the pleurocerid snails.  And in 2002, just a few months prior to the work of Mihalcik & Thompson, an undergraduate student and I published “A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas” [10].

We estimated gene frequencies at 8 polymorphic loci in 12 populations, picking up in NW North Carolina where my dissertation left off and extending through South Carolina to touch eastern Georgia – 4 populations of P. proxima and 8 populations of P. catenaria, including the type locality 50 km W of Charleston.  Our most interesting result touched a very familiar theme on this blog [11], ecophenotypic plasticity in pleurocerid shell morphology.  Populations of P. catenaria bearing typical shells in the Piedmont of the Carolinas tended to lose their spiral cords and costae as they ranged into the coastal plain, independently developing the shell form associated with the subspecific nomen “dislocata[12].

In 2004 Thompson published a third edition of his “Freshwater Snails of Florida,” this update strictly online [13].  From seven species of pleurocerid snails in 1984 and eleven in 1999, Thompson’s fresh online edition now recognized 12 species in Florida alone, the “Choctawhatchee Elimia” formally identified as buffyae, the “Escambia Elimia” now annae, and a new “Rasp Elimia” waiting in the wings, not as yet described.  None of Steve Chambers’ papers whatsoever were cited in Thompson’s 2004 online version.  Even the passing mention of Chamber’s 1980 paper had now disappeared.

So ten years ago, this was the situation:  Two morphologically variable species, P. proxima and P. catenaria, were understood to range from southern Virginia through North Carolina and South Carolina to the Georgia border.  At the Georgia border P. catenaria disappeared, to be replaced by over 20 rare and endemic pleurocerid species, each occupying single subdrainages, or in some cases single springs.  Does that model seem biologically plausible to you?
P. ucheensis is dislocata, the remainder are P. catenaria
In 2011 John Robinson and I extended our allozyme survey across Georgia, linking at long last my intensive dissertation work in Virginia with my buddy Steve Chambers’ extensive dissertation work in Florida [14].  John and I sampled 14 populations, including populations identified by Mihalcik & Thompson as viennaensis, darwini, mutabilis, induta and timida nymphea, reaching down to the boykiniana and floridensis populations of Chambers.  We estimated genetic divergence at 11 polymorphic allozyme encoding loci, calibrating our observed values with three populations of P. proxima, the conspecific status of which is unchallenged.

We did not include any bona fide curvicostata or dickinsoni in our survey, because those two taxa seemed unambiguous to us.  But the bottom line was that all the Georgia populations we sampled were either floridensis (including induta and timida) or catenaria (including albanyensis, boykiniana, darwini, mutabiliis, and viennaensis).  There are exactly four species of pleurocerid snails in Central Georgia, Florida, and the Florida drainages of Alabama as hypothesized by Chambers: curvicostata, dickinsoni, floridensis and (*a minor correction) catenaria, all boasting extensive ranges.  And the most widespread of those species, Pleurocera catenaria (Say 1822) extends through five states, from Virginia through the Carolinas and Georgia to mimic P. floridensis at Ichetucknee Springs, bringing the discovery Steve Chambers first reported in 1978 [15] into a properly regional context.

I didn’t expect Dr. Thompson to embrace the four-species model in 2011 any more than he did in 1990.  His species concept was rooted in nineteenth-century typology, no different from such giants of American malacology as Henry Pilsbry and F. C. Baker.  Indeed, distinction of species by shell costations or spiral chords is not conceptually different from distinction by individual nucleotide substitutions, as do many of our colleagues in the present day.  But I confess, when I saw the species list published by P. D. Johnson and colleagues in 2013 [16], including as it did albanyensis, annae, athearni, boykiniana, buffyae, clenchi, darwini, exusta, glarea, inclinans, induta, mutabilis, taitiana, timida, ucheensis, vanhyningiana, and viennaensis, I was just a little bit disappointed.


[1] Fred Thompson, Steve Chambers, and the pleurocerids of Florida [15Feb17]

[2] Chambers, S. M. (1990) The genus Elimia (= Goniobasis) in Florida and adjoining drainage basins (Prosobranchia: Pleuroceridae)  Walkerana 4: 237 – 270.

[3] Thompson, F. G. (2000)  An identification manual for the freshwater snails of Florida.  Walkerana 10(23): 1 -96.

[4] Turgeon, D.D., J.F. Quinn, A.E. Bogan, E.V. Coan, F.G. Hochberg, W.G. Lyons, P.M. Mikkelson, R.J. Neves, C.F.E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione, and G.D. Williams (1998) Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks (second edition), American Fisheries Society Special Publication 26, Bethesda, Maryland, 526 pp.

[5] Mihalcik, E. L. (1998)  Elimia curvicostata species-complex within the river drainages of the southeastern United States: Morphology, DNA, and biogeography.  Ph.D. dissertation, University of Florida, 209 pp.

[6] Mihalcik, E. L. & F. G. Thompson (2002)  A taxonomic revision of the freshwater snails referred to as Elimia curvicostata, and related species.

[7] Pleurocerid populations (and a variety of other gastropod populations as well) often demonstrate “mitochondrial superheterogeneity,” where two or more members demonstrate 10% sequence divergence or greater for any single-copy mtDNA gene, not sex-linked.  For more see:
  • Mitochondrial superheterogeneity: What we know [15Mar16]
  • Mitochondrial superheterogeneity: What it means [6Apr16]
  • Mitochondrial superheterogeneity and speciation [3May16]
Mihalcik and Thompson only sampled single individuals from almost all [8] of their populations, so (strictly speaking) mtSH cannot be recognized.  But based on my personal experience with many datasets of this sort, the cases labelled flava-1, annae, and boykiniana all look like mtSH to me.

[8] The only population from which M&T sampled more than one individual was their reference “flava” from Alabama, the two individuals sequenced from which apparently demonstrating more than 20% sequence divergence.  They might have been the first researchers ever to discover mtSH in the Pleuroceridae, had their eyes been open.  Dillon & Frankis didn’t formally document the phenomenon until 2004 [PDF].

[9] Here’s something even more remarkable, if you stop to consider it.  The 15 populations M&T identified as being members of the “Elimia curvicostata complex” (explicitly excluding floridensis, dickinsoni, or boykiniana) apparently represented all four of the Chambers species (curvicostata, floridensis, dickinsoni, and boykiniana) about equally.   Shows you the value of shell morphology, doesn’t it? 

[10] Dillon, R. T. and A. J. Reed (2002)  A survey of genetic variation at allozyme loci among Goniobasis populations inhabiting Atlantic drainages of the Carolinas.  Malacologia 44: 23-31. [PDF]

[11]  Much more on cryptic phenotypic plasticity in pleurocerid shell morphology:
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
  • Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
[12] More about the pleurocerid subspecific nomen “dislocata” and its treatment by Turgeon [4] can be found at:
  • What subspecies are not [5Mar14]
[13] Thompson, F. G. (2004) on-line edition: An identification manual for the freshwater snails of Florida. Florida Museum of Natural History, Gainesville, FL [html].

[14] Dillon, R. T. and J. D. Robinson (2011)  The opposite of speciation: Population genetics of  Pleurocera (Gastropoda: Pleuroceridae) in central Georgia.  American Malacological Bulletin 29: 159-168.  [PDF]

[15] Chambers, S. M. (1978) An electrophoretically detected sibling species of “Goniobasis floridensis” (Mesogastropoda; Pleuroceridae).  Malacologia 17: 157 – 162.

[16] Johnson, P. D. et al. (2013) Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247 – 282.  See:
  • Plagiarism, Paul Johnson, and the American Fisheries Society [9Sept13]

Wednesday, February 15, 2017

Fred Thompson, Steve Chambers, and the pleurocerids of Florida

Word has reached us of the death of Dr. Fred G. Thompson, who passed away December 27, 2016 at his home in Ocala [1].  Dr. Thompson was Curator of Malacology at the Florida Museum of Natural History in Gainesville for 40 years.  He was 82.

Dr. Thompson’s body of published work must run into the hundreds of titles – focusing primarily on the North American Hydrobiidae, but extending to include the Pleuroceridae and a broad range of terrestrial gastropod taxa as well, especially of Mexico and Latin America.  I understand that his complete necrology will soon be published in The Tentacle.

I had no personal relationship with the late Dr. Thompson.  I did attend his talks at the AMU (later the American Malacological Society) over a period of some thirty years, although I don’t recall his attending any of mine.  He did not respond to my letters or emails.  During my visit to the Florida Museum in July of 2006 he did not emerge from his office.

We did, of course, have many mutual colleagues.  The American community of freshwater gastropod workers is not a large one.  All I know about the personality and character of Dr. Thompson I have gathered from friends.  Good friends, like Steve Chambers.

Steve Chambers was an important influence on my young career.  He first met Dr Thompson on a field trip as a graduate student at the University of Florida in 1976, and became interested in the genetic relationships among Florida pleurocerid populations known at that time as Goniobasis, now Pleurocera.  Steve’s major adviser, Dr. Thomas Emmel, ran a big laboratory specializing in the promising new technique of allozyme electrophoresis.  Steve thought that the estimation of genetic divergence among populations of pleurocerid snails as a function of gene frequencies at multiple allozyme-encoding loci might elucidate evolutionary principles of great generality and importance [2].
The Clench & Turner model, as figured by Chambers [7]
The pleurocerid fauna of Florida is no less enigmatic than that of most other regions of the American southeast. Goodrich [3] recognized three species: catenaria (2 subspecies), clenchi and curvicostata, with boykiniana (3 subspecies) in nearby South Georgia.  Clench & Turner [4] took a big swing at this system, substituting floridensis for catenaria cancellata, raising catenaria vanhyningiana to the full species level, bringing one of the boykiniana subspecies down into Florida (albanyensis) and raising it to the full species level as well.  They also described two new species (athearni and dickinsoni) which, together with clenchi and curvicostata yielded seven species of Goniobasis in Florida.

It was the seven-species Clench & Turner model that Fred Thompson preferred for the Pleuroceridae of Florida, and the seven-species model is what Steve Chambers brought into his study design.  His (1977) dissertation [5] was an impressive effort to replicate the extremely influential Drosophila research that F. J. Ayala [6] had published in 1974.  Steve showed that, just as in Drosophila, populations of Goniobasis at increasing degrees of taxonomic divergence demonstrated increasing levels of genetic divergence at allozyme loci.  The paper based on his dissertation research was published in 1980 [7].

Perhaps unsurprisingly, however, Steve’s genetic data did not jive especially well with the Clench & Turner seven-species model.  Allozyme divergence suggested that vanhyningiana and clenchi be synonymized under floridensis and that albanyensis, athearni and the REF population (more about which anon) “are in the range of conspecifics,” although Chambers called for further study on that question.  Note that by the time he had published his dissertation in 1980 he had already synonymized clenchi (marked “F” in the figure above) under floridensis.

Meanwhile, on a small farm in Kansas, a boy was growing up [8].  Well, actually, he entered the graduate program at the University of Pennsylvania in the fall of 1977, on a scientific journey remarkably similar to that of Steve Chambers, just a couple steps behind.  My adviser was also running a big lab specializing in the promising new technique of allozyme electrophoresis, and I too was interested in the genetic relationships among populations of Goniobasis, mine in the southern Appalachians.  And it turned out that my adviser, Dr. George Davis, was also the Editor of the journal to which Steve Chambers sent his first two papers, the 1980 work based on his dissertation and an earlier one on that “REF” population I mentioned above.

Steve had discovered his REF population quite by accident.  He had gone to sample Ichetucknee Springs (the source of North Florida’s Ichetucknee River) expecting to find a typical population of Goniobasis floridensis, and that’s what he thought he had, until he got back to the laboratory.  But his gels showed two reproductively isolated populations, one of which matched floridensis genetically and the other of which was much more similar to his sample populations of Goniobasis athearni and G. albanyensis.  He published “An electrophoretically detected sibling species of Goniobasis floridensis” in Malacologia in 1978 [9].

Both Steve’s 1978 and 1980 papers were highly influential in my budding career.  I struck up a correspondence with him and he was happy to share techniques, recipes, and tips.  Imagine my surprise (and dismay) when my adviser showed me a manuscript he had received attacking Steve Chamber’s work gratuitously and viciously.

Fred Thompson had submitted a Letter to the Editor for publication in Malacologia.  The journal very rarely published letters in those days.  In fact, no letters whatsoever had been published in the previous ten years.  But Dr. Thompson was livid about research that had taken place at his own home institution, research that very carefully and thoroughly documented a model of evolutionary relationships among the Florida Goniobasis he did not share.

That letter was ultimately published in 1982, along with Steve’s reply [10], over the protestations of a farm boy from Kansas [8].  It included such whoppers as “Because the Ichetucknee population of athearni is distantly related genetically to floridensis, they cannot be sibling species.”  But the amount of genetic divergence has nothing to do with sibling species.  Ernst Mayr coined that term in 1963, and he defined it as “morphologically similar or identical populations which are reproductively isolated,” and genetic divergence does not enter into it [11].  In fact, whether the observed amount of genetic divergence might correlate with the taxonomic divergence suggested by systematic biologists was the very hypothesis that Steve was trying to test.

Well, by 1982 Steve had been gone from Gainesville for five years.  He told me later that he had been a finalist for the opening at the University of Michigan Museum of Zoology left vacant when Henry van der Schalie retired, but that Dr. Thompson had written an unsolicited letter to his colleagues in Ann Arbor, scuttling Steve’s chances for that job.

But Steve was able to land a job with the FWS Office of Endangered Species in Washington, which turned out to be a plum.  From Washington he was able to publish two high-visibility works on chromosomal evolution in gastropods [12].  He also became interested in the land snail fauna of the Galapagos, describing two new species of bulimulids in 1986 [13].  And with Christine Schonewald-Cox and other colleagues he edited a very influential book on the budding discipline of conservation genetics [14].

Meanwhile up north, the farm boy from Kansas [8] accepted an AAAS congressional fellowship and moved to Washington for the 1981-82 academic year, still right on the heels of his buddy Steve.  And we struck up a personal friendship that year which I wish had lasted longer.  Toward the end of my brief sojourn in Washington Steve remarked to me, and I’m paraphrasing here, “In the entire wide community of systematic biology, malacologists have the reputation of being the most petty and venal.”  He was broadening his professional horizons, trying to shift away.

And meanwhile down south, Dr. Thompson published the first (1984) edition of his “Freshwater Snails of Florida” [15].  Perhaps unsurprisingly he clung to the Clench & Turner seven-species model, citing the work of Chambers only once, superficially: “Recent studies on isoenzymes show that, in Elimia, shell characters are conservative indicators of genetic divergence (Chambers 1980, Dillon & Davis 1980).”  Regarding Dr. Thompson’s choice of genus, see note [16] below.
From Chambers [17]
After ten years in Washington, Steve was transferred to the regional office in Albuquerque, where he became involved with federal efforts to protect endangered elements of the charismatic megafauna such as the Red Wolf.  But he had one more snail paper in the pipeline, his 1990 masterpiece “The genus Elimia in Florida and adjoining drainage basins” [17].  Here Steve combined his modern understanding of allozyme, chromosomal, and morphological divergence in an enigmatic and misunderstood freshwater gastropod fauna together with a scholar’s appreciation for the old literature and the old museum collections into a symphony of Malacological virtuosity.  He convincingly demonstrated that there are four species of Goniobasis in Florida and South Georgiaboykiniana (including athearni, albanyensis, and REF), curvicostata, dickinsoni, and floridensis (including vanhyningiana and clenchi), and the matter would seem to be settled.

But way back on page 261 of Chambers’ 1990 work we read: 
“R. T. Dillon has sent me shells of Elimia catenaria Say from South Carolina and suggested that my E. boykiniana may be a synonym of E. catenaria.  Although there is considerable merit in this suggestion, I decline to combine these Georgia and South Carolina populations with E. boykiniana at this time because they occur in major drainages for which genetic data are not available.”
Yes, after single years in Washington and at Rutgers, that farm boy from Kansas [8] had arrived in Charleston, SC, and begun work to tie his 1982 dissertation research in VA/NC together with the remarkable body of knowledge Steve Chambers had developed in FL/Ga.  Could direct conflict with Dr. Fred Thompson be avoided?  Tune in next time.


[1] Fred Gilbert Thompson (November 13, 1934 – December 27, 2016).  The Shell-O-Gram 58(1): 4-5.

[2] I did too, once.  When I was young.

[3] Goodrich, C. (1942) The Pleuroceridae of the Atlantic Coastal Plain.  Occas. Papers Mus. Zool. Univ. Mich. 456:1 – 6.

[4] Clench, W. J. & R. D. Turner (1956) Freshwater mollusks of Alabama, Georgia and Florida from the Escambia to the Suwannee River.  Bull. Florida State Museum 1:1 – 239.

[5] Chambers, S. M. (1977) Genetic divergence during speciation in freshwater snails of the genus Goniobasis.  Ph.D. dissertation, University of Florida, Gainesville. 59 pp.

[6] Ayala, F. J., M. L. Tracey, D. Hedgecock & R. C. Richmond (1974) Genetic differentiation during the speciation process in Drosophila.  Evolution 28: 576-592.

[7] Chambers, S. M. (1980) Genetic divergence between populations of Goniobasis (Pleuroceridae) occupying different drainage systems.  Malacologia 20: 63 – 81.

[8] Virginia, actually.  And he wasn’t born on a farm, and he never grew up, if you ask his wife.  For more, see…
  • The Clean Water Act at 40 [7Jan13]
[9] Chambers, S. M. (1978) An electrophoretically detected sibling species of “Goniobasis floridensis” (Mesogastropoda; Pleuroceridae).  Malacologia 17: 157 – 162.

[10] Thompson, F. G. (1982) On sibling species and genetic diversity in Florida Goniobasis.  Malacologia 23: 81 – 82.  
       Chambers, S. M. (1982) Sibling species and genetic diversity in Florida Goniobasis: A reply.  Malacologia 23: 83 – 86.

[11] Mayr, E. (1963) Animal Species and Evolution.  Belknap Press, 797 pp.  The definition of sibling species is found on page 34. 

[12] Chambers, S. M. (1982) Chromosomal evidence for parallel evolution of shell sculpture pattern in Goniobasis.  Evolution 36: 113 – 120.  
        Chambers, S. M. (1987) Rates of evolutionary change in chromosome numbers in snails and vertebrates.  Evolution 41: 166 – 175.

[13] Chambers, S. M. (1986) Two new bulimulid land snail species from Isla Santa Cruz, Galapagos Islands.  Veliger 28: 287 – 293. 

[14]   C. Schonewald-Cox, S. Chambers, B. MacBryde, and L. Thomas (1983) Genetics and Conservation: A Reference for Managing Wild Animal and Plant Populations.  Benjamin Cummings, Menlo Park.  722 pp.

[15] Thompson, F.G., 1984. The freshwater snails of Florida: A manual for identification. University of Florida Press, Gainesville, FL. 1-94.

[16] I suppose I should also mention that, separately but essentially simultaneously, J. B. Burch was working on his North American Freshwater Snails.  The Burch & Tottenham “Species List, Ranges and Illustrations” was published in 1980, with the full EPA Manual following in 1982 and the stand-alone separate work republished in 1989.  This work proposed a hybrid between the Goodrich and Clench & Turner classifications of the Florida and South Georgia pleurocerid fauna, which did not evolve through its extended publication history: athearni, clenchi, curvicostata, dickinsoni, induta, and boykiniana with three subspecies.  The recognition of a separate floridensis, while keeping vanhyningiana as a subspecies of catenaria, yielded a list of ten species and subspecies for the region, which Burch renamed to “Elimia.”

[17] Chambers, S. M. (1990) The genus Elimia (= Goniobasis) in Florida and adjoining drainage basins (Prosobranchia: Pleuroceridae)  Walkerana 4: 237 – 270.

Wednesday, January 11, 2017

A Previously Unrecognized Symbiosis?

Last month’s post on the aerial dispersal of freshwater gastropods [1] turned out to be one of my more popular in recent memory.  Thank you all for your kind emails.  Winning the award for most charming was our colleague Lusha Tronstad of the Wyoming Natural Diversity Database, who sent me the photos below:

And here is the story, verbatim as I received it from Lusha: 
“My son, Everett Tronstad (he just turned 6) caught this beetle (likely Dytiscus gigantus) in our barnyard this past summer.  We live northwest of Laramie, Wyoming in the foothills of the Snowy Mountains.    My son spends hours every day collecting invertebrates, both aquatic and terrestrial.  He caught this beetle on our car who probably thought the shiny paint was water.  Everett picked it up and we immediately saw the limpets attached.  We had just enough time to snap two photos before the beetle took off.  We watched the beetle fly off into the distance and we marveled at how invertebrates can hitchhike on other critters.  The date was 30 July 2016.”
I also received a cordial email from Chris Davis of the Pymatuning Lab up in Linesville, PA.  He called my attention to a note published by Andrea Walther and a host of colleagues in 2008 [2], with the striking figure reproduced below.

Andrea and her colleagues collected nine water bugs from a pond at the University of Michigan’s Edwin S. George Reserve, two of which bore five limpets each.  Note that the bugs were swimming at the time of their collection.  Andrea wrote, “Given the positioning of the L. fuscus on the dorsal surfaces of the B. flumineum, it is uncertain if the insects were able to lift their hemelytra to take flight.”

So those of you in my email address book [3] may remember the challenge I issued when I announced last month’s post, back on 15Dec16:  “Quick, quick!  How many reports of freshwater limpets on flying water bugs would you expect to find in the published literature?”  The answer I was looking for was 12, the ten listed by W. J. Rees [4] plus the two I found published between 1965 and 1991.  So here are two more [5], making 14.  At some point, a stack of related observations becomes a phenomenon, don’t you agree?

And here’s another remarkable observation, which I’m not sure has at yet been remarked.  There were three Ferrissia on Everett’s dytiscid beetle.  And there were five Laevapex on the back of each of Andrea’s belostomatid bugs.  On surface areas no more than 4 square cm??  I feel sure the densities of limpets on the insects must be orders-of-magnitude greater than their densities in the general environment.  The limpets are almost certainly aggregated on the bugs, don’t you think?

I have developed an hypothesis to account for the phenomenon of high limpet densities on large aquatic beetles and bugs.  This hypothesis depends on one obvious assumption, one commonplace observation, and one trivial fact, with a couple paragraphs of gratuitous speculation plated on the bottom.

The obvious assumption is that large aquatic beetles and bugs must remain motionless for extended periods of time, in contact with limpet habitat.  I have very little first-hand knowledge upon which to draw here, but my (admittedly superficial) google searches have returned the impression that both the dytiscid beetles and belostomatid bugs demonstrate a wide variety of life habits, but that the big species, in any case, often seem to hunt by stealth.  There are a couple YouTube videos [6] showing periods of both active swimming and quiescent waiting.  Belostomatids seem to rest anywhere, in contact with the bottom or the surface, although dytiscids seem to rest primarily at the surface.  Possibly under lily pads?  Lily pads seem to be prime limpet habitat.

My commonplace observation is that freshwater limpets tend to aggregate on smooth surfaces.  Anybody who has ever hunted for them knows that limpets are more common on smooth rocks than on rough rocks, and less common on the parts of waterlogged woody debris covered with bark than on the parts that have lost it.  When I arrive at a collecting site, one of the first items on my agenda is to wade under the bridge and look for beer bottles.  Limpets reach maximum abundance in old bottles, hard plastics, and smooth litter discarded by fishermen and motorists.  In fact, I have developed the abbreviation “bbl” for my field notes, which means “beer-bottle limpets.”  This means that limpets (of whatever species) were so uncommon as to be found only on smooth litter.

I feel sure this is an adaptation for defense.  Limpets are better able to make a seal with the lips of their shells, and indeed better able to create suction with their foot, on a smooth surface than a rough surface.  It is very nearly impossible to remove a limpet from a beer bottle.  They may slide around on the surface of the bottle, but they do not come off.  For this reason, I keep a scalpel in the pocket of my collecting vest (sheathed in an open plastic sample vial).  The only way to remove a limpet from a really smooth surface without damaging it is to insert a scalpel blade and lift.

My obvious assumption that big aquatic beetles and bugs must hold still, plus my commonplace observation that freshwater limpets tend to aggregate on smooth surfaces, plus the trivial fact that the backs of big beetles and bugs are smooth, can yield a symbiotic association between freshwater limpets and large aquatic insects.  I feel pretty sure that the relationship is positive for the limpets, at least initially, since smooth surfaces yield better protection from predators.  I imagine the relationship is slightly negative for the insects, since the limpets must add a bit of weight, and a bit of drag.  More negative for the bug if attached limpets interfere with its ability to fly, as Andrea suggests.

What might the unusually high densities of limpets we sometimes observe on the backs of aquatic insects eat?  It would be lots of fun to imagine that the limpets are commensal with the bugs – sharing or somehow benefiting from the meat diets of their hosts.  Dytiscid beetles seem to be such sloppy eaters [7] that one might hypothesize increased concentrations of bacteria and fungi on their bodies.  The excellent studies of Calow [8] from the mid-1970s, however, convince me that European Ancylus populations graze rather exclusively on periphytic algae, especially diatoms.  I just cannot find any warrant to imagine that limpets straying onto the backs of carnivorous insects might reasonably switch to anything else.

Which means that the limpets rapidly become hungry.  My gratuitous speculation is that they graze every last diatom cell off the backs of their hosts in a matter of hours, and subsequently ride around in sullen misery, regretting their decision to hitchhike, looking for any opportunity to get off.  Which brings us back to the phenomenon that brought us here, which, I seem to recall, was not symbiosis, but rather transport.

So after this brief but (I hope you’ll agree) interesting digression, next month we’ll return to the theme I introduced last month, which was the aerial dispersal, broadly, of freshwater gastropods, generally.


[1] Freshwater Gastropods Take To The Air, 1991 [15Dec16]

[2] Walther , A. C.,  M. F. Benard, L. P. Boris , N. Enstice , A. Tindauer-Thompson & J. Wan (2008) Attachment of the Freshwater Limpet Laevapex fuscus to the Hemelytra of the Water Bug Belostoma flumineum.  Journal of Freshwater Ecology, 23:2, 337-339, DOI: 10.1080/02705060.2008.9664207.

[3] If you’d like to receive regular alerts from the FWGNA, email me at DillonR@fwgna.org

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

[5] Okay, fine, for you sticklers out there!  I understand that the first 13 were records of aquatic Coleopteran beetles, not technically Hemipteran “bugs.”  And I aso realize that the fourteenth wasn’t literally a “flying water bug,” because Andrea collected her Belostoma as they were swimming in the pond.  So technically, the answer to my query of 15Dec16 might still be zero.  I’ll bet you all really irritated your tenth-grade Biology teachers, didn’t you?

[6] Here’s a montage of videos of large belostomatid bugs hunting.  The first one shows a bug holding very still on a bottom of smooth stones (interestingly) waiting for its prey to swim by.  There’s also some footage of another belostomatid hunting from the surface, and then a sequence suggesting ambush from submerged vegetation:
As a bonus, around minute 2:30 there’s a sequence showing a belostomatid capturing and eating a big individual Melanoides.  The bug apparently lunges right by a fat, juicy Helisoma to snatch the bony Melanoides.  I can’t imagine why.

[7] Here’s a YouTube video of a Dytisid making very sloppy work of a minnow:
But we should probably also note simultaneously that belostomatids seem to be very neat eaters, inserting their proboscis and sucking their prey clean from the inside.

[8] Calow, P. (1973) The food of Ancylus fluviatilis Müll., a littoral stone-dwelling, herbivore. Oecologia (Berl.) 13, 113–133.  Calow, P. (1975) The feeding strategies of two freshwater gastropods, Ancylus fluviatilis Müll. and Planorbis contortus Linn. (Pulmonata), in terms of ingestion rates and absorption efficiencies.  Oecologia 20: 33-49.