Dr. Rob Dillon, Coordinator

Tuesday, July 14, 2015

The Type Locality of Lymnaea catascopium

Although not explicitly stated, it is traditional to assume that Thomas Say was referring to his home town of Philadelphia when he wrote, in 1817, “Inhabits the Delaware River and many other waters of the United States, in considerable numbers, and may be found plentifully, during the recess of the tide, about the small streams through which the marshy grounds are drained [1].”  Say was describing the habitat of the first “stagnicoline” lymnaeid, Lymnaea catascopium.  Standing on the Philadelphia waterfront in June of 2012, however, I found it nearly impossible to imagine how any self-respecting freshwater gastropod population of any description might ever have inhabited such a place.

Philadelphia 1702, from phillywatersheds.org
The Delaware is reliably fresh but quite tidal at Philadelphia, with a daily range of several feet.  So I gather that in the 18th and early 19th centuries, merchant ships anchored some distance offshore and transmitted their cargos over these "marshy grounds" at high tide by tender.  Then as the years advanced and the technology improved, the river must have been dredged and the fill material dumped directly onshore, creating more land, deepening the harbor, and allowing direct offloading of ships to finger-like cargo docks.  Whatever the historical scenario, however, by June of 2012 it was clear to this particular 21st-century malacologist that his efforts to sample a topotypic population of L. catascopium from the Delaware River must be re-directed upstream.

Philadelphia 1908, from phillywatersheds.org
The Academy of Natural Sciences holds one lot of L. catascopium collected by Charlie Wurtz from Pennypack Creek in 1948.  Pennypack Creek drains an entirely urban (actually, rather post-industrial) catchment inside the Philadelphia city limits, emptying into the Delaware just a few miles upstream from the docks.  And so it was to Pennypack Park that I set my GPS early in the morning of June 11, 2012.

I threw my kayak directly into the Delaware River and paddled upstream through the mouth of the creek into a zone of broad, intertidal mudflats decorated with a dense stand of arrowhead (Sagittaria).  The creek narrowed and deepened substantially as I paddled upstream, looking for the solid substrates I knew that populations of Lymnaea catascopium require.  Soon the air crackled with gunfire, as I passed alongside (perhaps “beneath” would be a better preposition) the Philadelphia Police Academy Firing Range.  After about a mile the creek had shallowed to the point I could get out and walk.

But I found no L. catascopium, nor indeed any habitat.  The steam bed was too muddy.  I found Littoridinops moderately common on floating debris, a few Physa acuta, N=1 Amnicola, and some beer-can limpets, but that was it.  So I paddled back downstream to the truck, loaded my kayak, and drove a couple miles upstream to the Verree Road Bridge.  Pennypack Creek was lovely at that point on a June afternoon, but flashy and low-nutrient, and simply not the kind of place one might expect L. catascopium.

Some nontrivial fraction of the early 19th century prosperity of Philadelphia was due to the network of canals communicating between the Delaware River and the interior of rapidly-expanding America.  In 1832 the Delaware Canal was completed to run 60 miles along the right (descending) bank of the river from Easton to the quaint old town of Bristol, PA, about 15 - 20 miles above Philadelphia.  And the Delaware and Raritan Canal was completed in 1834, connecting New Brunswick, NJ, to Bordentown, and climbing the left (descending) bank of the Delaware.  So on 12 June I checked the historic lock areas around Bristol, and also across the river at D & R Canal Lock #1, at Bordentown.  The latter spot was tough to access but afforded a pretty and diverse habitat with a disappointingly poor freshwater gastropod fauna.  The still-rather-strongly tidal environment is probably a factor.  Just a couple Physa acuta, and a Menetus or two, and I was gone.

The Delaware River passes through the fall zone at Washington Crossing, PA, famous primarily as the type locality of Physa ancillaria (Say, 1825).  Breeding results we published back in 2006 suggested that ancillaria is a fattish shell morph of Physa gyrina [2].  Most interestingly, some allozyme gels we ran in support of that research effort in 2005 returned evidence of low-frequency hybridization between P. gyrina and P. acuta at Washington Crossing – the only place (to this day) where the phenomenon has been documented.  You would think that at least a couple of the numerous historical markers one finds on both sides of the Delaware River at this point would feature such a remarkable finding.  But no.

Both Physa gyrina and P. acuta are common in the rocky pools at Washington Crossing, as they are upstream for several hundred miles.  The Delaware River is one of the few places where the two species are so richly sympatric, in my experience.  Also making an initial (or possibly final?) appearance at the fall line is Pleurocera virginica.  But no evidence of Lymnaea catascopium whatsoever.

The gastropod fauna continued to richen as I collected my way north upstream the next day.  The list lengthened to include Helisoma trivolvis, H. anceps, Gyraulus, Laevapex, both species of Ferrissia, Lyogyrus, and even (ultimately, way up north) Somatogyrus.  The most memorable snapshot from my June 13 field experience was the aquaviaduct at Tohickon Creek – a genuine marvel of 19th century engineering.  Here barges plying the Delaware Canal would have passed through a covered bridge (or trough, maybe a better noun) perpendicular to and 20 feet above the rocky creek below.  I passed through the Delaware Water Gap as the sun set on the third day of my efforts to collect L. catascopium from its type locality.

Tohickon Aqua-viaduct (PA DCNR)
I remember the morning of June 14, 2012 like it was yesterday.  The Delaware River at Milford Beach was low and clear.  Wading a bit over knee-deep on the left bank under some trees I pulled up a length of woody debris and attached thereupon I found N=2 pale lymnaeids about 10 mm shell length.  Juvenile catascopium!  The shell morphology reminded me of (slenderish) Lymnaea columella, which they might easily have been mistaken for, were they sitting on a lily pad in a pond rather than grazing on a stick two feet below the surface of a river.

I really needed a sample of at least 30 individuals to estimate allozyme frequencies.  So I redoubled my efforts in all similar habitats and substrates around Milford Beach over a period of about an hour.  But alas, no additional specimens came to light.  So I drove 25 miles upstream to Shohola Bridge, where two hours’ effort netted an additional N=5 juvenile catascopium.  Further upstream at Metamoras Boat Landing and Dillontown the river did not seem as rich, and I struck out.

Delaware Water Gap
So there I stood on the riverbank, as the sun set on June 14.  Five days and 1,375 miles on the odometer since I left Charleston had yielded the N=7 pale little lymnaeids crawling implacably about in the red, half-gallon thermos jug at my feet.  I’d be lying if I said I wasn’t disappointed.  But the AMS meeting was scheduled to start on Sunday (June 16) back down the river at Cherry Hill, and I still needed to find a population of Lymnaea elodes in the Delaware Valley someplace.  As I turned my pickup south, however, I felt good about at least one thing.  The Delaware River at Milford Beach, Pike County, PA is as close to topotypic for Lymnaea catascopium (Say 1817) as can humanly be located in the modern day.

If my efforts to collect a decent topotypic sample of L. catascopium were less than successful, however, my parallel efforts to find a matching population of L. elodes were an abject failure.  I had several leads.  In fact, the naturalist at Echo Hill Environmental Education Center near Lebanon, NJ, had sent me a sample of L. elodes for identification in December of 2010.  But I stomped all around in the wooded swamp where the specimens were collected, and couldn’t find so much as a shell.

I confess that I may not have been in the best of spirits at the AMS welcome mixer Sunday evening, when up walked our good friend Tom Duda, with a nice young lady in tow.  Tom introduced her as Ms. Samantha Flowers, a new graduate student at the University of Michigan [3].  And Samantha had chosen as her research project – if you can believe this – the evolutionary relationships among the stagnicoline lymnaeids.

Have you seen me?
She related to me, as the conversation unfolded, that she planned to use a variety of approaches, including molecular phylogenetics and geometric morphometrics, and sample as broad a range of catascopium, emarginata, elodes, and exilis populations as time and resources permitted.  I’m not crazy about gene trees, I thought to myself [4], but they do work with small sample sizes.  And what can I myself do with my crappy little sample of N=7 topotypic Lymnaea catescopium except go back up the Delaware again and try to find 23 more?

And such a nice young lady!  So bright, and so eager to learn!  In five minutes not only had I decided to give her my sample of topotypic L. catascopium, I had resolved to help her with the rest of her thesis in any way I could.

The next morning I transmitted my little sample of L. catascopium to Samantha, and told her she could keep my half-gallon thermos jug to carry them home in.  I also promised to her that I would continue to move forward on my original study design, and that I would try to send her additional samples as the summer progressed.  Looking back on it, I wasn’t entirely sure that she appreciated the potential for ecophenotypic plasticity in her chosen study organisms, or indeed that she actually understood the design of the study I (we?) were working on.

In fact, I was not entirely sure she understood the significance of the little sample of snails I handed her that morning.  Thomas Say’s (1817) nomen “Lymnaea catascopium” is the oldest available name for any of the North American stagnicolines.  Which means that regardless of all the other names invented by all the other malacologists to name all the other stagnicoline populations in all the other regions of the United States and Canada, any population matching those N=7 crappy little snails in that red jug must be Lymnaea catascopium by definition.  They were her control.  Every other sample she might acquire would be an experiment.

With the benefit of three years’ hindsight, I think that it was probably too early in Samantha’s professional career for her to take this all in.  But stay tuned!  Coming up next month - the type localities of L. elodes and L. emarginata.


[1] Lymnaea catascopium was one of a long list of species that Thomas Say described in the entry entitled, “Conchology,” which he contributed to Nicholson’s British Encyclopedia of Arts and SciencesNicholson’s Encyclopedia was published at Philadelphia in three editions: 1816, 1818, and 1819.  I gather that these works are very rare in libraries today.  And I also gather that the "1816 Edition" was actually published in 1817.  I myself only have access to W. G. Binny’s (1858) secondary reference entitled, “Complete Writings of Thomas Say on the Conchology of the United States.”  And Binny only reprints the third (1819) edition.  So that’s where I got the quote above.

[2] Dillon, R. T., and A. R. Wethington. (2006)   No-choice mating experiments among six nominal taxa of the subgenus Physella (Basommatophora: Physidae).  Heldia 6: 41 - 50.  [PDF]

[3] From this point onward in the present essay, I am assuming that you have read last month’s post,
  • Everything Changed When I Met Samantha [22June15]
[4] This is a long-running theme on the FWGNA blog, for example:
  • Gene Trees and Species Trees [15July08]

Monday, June 22, 2015

Everything Changed When I Met Samantha

Three years ago I posted a series of five essays on this blog entitled, “The Lymnaeidae 2012” [1-5].  My primary motivation was the imminent expansion of the FWGNA project into the Mid-Atlantic States, where I expected that we would come into contact with the range of an enigmatic group of lymnaeids called the “stagnicolines.”  And a couple comprehensive molecular phylogenetic studies had also recently been published that I thought might cast some light on systematic relationships in the group.

Looking back on it, however, my “Lymnaeidae 2012” series may have been something of a disappointment for my readership.  I seemed to get distracted from my primary theme by questions regarding the equally enigmatic “fossarine” group in August, and the series sort-of petered out inconclusively.  This is because everything changed when I met Samantha.

Who was Ms. Samantha Flowers?  And where has she gone, long time passing?  Our good friend Tom Duda from the University of Michigan introduced her at the AMS meeting in Cherry Hill in June of 2012, and we kept in touch until August of 2013, at which point she disappeared.  But she left behind a tangled body of potentially important research on the genetics of the stagnicolines, which we will sort through together, as this, our fresh series of essays on the Lymnaeidae unfolds.

So when last we left our story, it may be recalled that the Baker/Burch system for the classification of the North American Lymnaeidae recognizes 21 species of stagnicoline lymnaeids in two subgroups.  The dark-bodied populations of bogs, marshes and vernal ponds bearing slender shells include elodes (Say 1821), exilis (Lea 1834) and three others more recently described.  The pale-bodied inhabitants of open waters, bearing broader, more robust shells include catascopium (Say 1816), emarginata (Say 1821) and 14 others more recently described.

Half of the challenge with which we wrestled in 2012 was the relationship between our New World stagnicolines and those of the Old.  In 1951, my hero Bengt Hubendick synonymized all the dark/skinny species of North America under the European palustris (Muller 1774).  Hubendick’s figure of L. palustris is reproduced below – click for a full-sized version, with caption [6].  But by the 1960s evidence had begun to accumulate that the European palustris is a complex of several cryptic species, distinguishable only by detail of reproductive anatomy.  And in my essay of 10May12 [2], I offered evidence that at least two cryptic species of dark/skinny stagnicolines also seem to inhabit the ephemeral ponds and marshes of NW Pennsylvania.

Hubendick Fig 303, Click for full caption
Although I did not mention it at the time, shortly after I published my “cryptic stagnicoline” essay of 10May12, I asked my good friends Kip Brady and Andy Turner to send samples of their enigmatic Pennsylvania populations to Charleston, which I dissected, comparing details of their reproductive anatomy to figures from the European literature.  Alas, I was unable to distinguish any of these populations anatomically, and let the matter drop.  But the cryptic stagnicolines of Brady & Turner turned out to be key to disentangling Samantha Flowers’ research results, when we were finally able to examine them in 2015.  So keep this in the back of your mind.

The other half of the challenge to working out the systematic relationships among our American stagnicolines is their great potential for ecophenotypic plasticity of shell.  Given the large body of research results such as those of Christer Bronmark on European Lymnaea peregra (aka “Radix balthica”), it is not inconceivable that the robust shells with enlarged body whorls born by populations we call catascopium or emarginata here in North America arise as an ecophenotypic response to life on solid substrates in open waters, exposed to fish predation [4].  There may be no additively heritable basis for the distinction between the broad, heavy shells of the catascopium/emarginata subgroup and the slender, gracile shells of the elodes/exilis subgroup whatsoever.  Hubendick’s figure of L. catascopium is reproduced below – click for a full sized version, with caption.

Hubendick Fig 314, click for full caption
So with the potential for cryptic speciation and shell ecophenotypic plasticity firmly in mind, in the spring of 2012 I designed a genetic survey of the North American stagnicolines.  My plan was to sample populations from the type localities of the four oldest nomina – catascopium, emarginata, elodes, and exilis.  And along with each topotypic population, I also hoped to sample a nearby population bearing the opposite shell form.  My hypothesis was that each broad, heavy open-water population would prove most genetically similar to its local slender/gracile marsh-dwelling population.

For example, the type locality of L. catascopium is the Delaware River at Philadelphia, and the type locality of L. emarginata is “Lakes of Maine.”  In 2012 it seemed likely to me that Delaware River catascopium might prove most genetically similar to the populations of (nominal) L. elodes that I expected to find in the marshes of Delaware tributaries in eastern Pennsylvania, and that Maine emarginata might prove most genetically similar to populations of nominal L. elodes sampled from the marshes and vernal ponds of Maine.  I imagine my readership will recognize this study design as the same I have used to confirm “cryptic phenotypic plasticity” in a variety of pleurocerid taxa in recent years [7].

And so I mapped out an itinerary for my 2012 field season.  I set aside six days in June for what (I presciently imagined) might be a challenging quest to re-discover L. catascopium in the Delaware River, and L. elodes in vernal habitats of the Delaware Valley, after which I planned to attend the meeting of the American Malacological Society, conveniently scheduled in the Philadelphia suburb of Cherry Hill (NJ) June 16 – 21.

Everything changed when I met Samantha.  But coming next month… “The type locality of Lymnaea catascopium.”


[1] The Lymnaeidae 2012: Tales of L. occulta  [23Apr12]
[2] The Lymnaeidae 2012: Tales from the cryptic stagnicolines [10May12]
[3] The Lymnaeidae 2012: Stagnalis yardstick  [4June12]
[4] The Lymnaeidae 2012: A clue [9July12]
[5] The Lymnaeidae 2012: Fossarine Football  [7Aug12]

[6] Hubendick, B. (1951) Recent Lymnaeidae, Their Variation, morphology, taxonomy, nomenclature and distribution.  Kungl. Svenska Vetenskapskademiens Handlingar. Fjarde Serien Band 3, No. 1.  Stockholm: Almquist & Wiksells.
See The Classification of the Lymnaeidae [28Dec06]

[7] Pleurocera acuta is Pleurocera canaliculata [3June13]

Monday, May 11, 2015

Cornhusker Freshwater Gastropods

Kudos to our good friend Bruce Stephen for his thorough review of the freshwater gastropod fauna of Nebraska, just published in the most recent issue of the American Malacological Bulletin.  A pdf download is available from the link below [1].

The surface waters of Nebraska drain entirely to the east, primarily through the broad, sandy Platte River, as well as through the Niobara and Republican Rivers, all tracing slow, braided paths toward the Missouri.  Glaciers extended over the greener, eastern quarter of the state about 10,000 years ago, although the western half of the state seems to have remained a bit too dry.  Today about 40% of the state is planted in row crops, dependent largely on the miracle of modern irrigation.  The other 60% of Nebraska is sand hills and semi-arid plains, typically given over to ranching. 
I am sure that there are many nice things that could be said about Nebraska and its fine citizenry, but the state does not strike me as especially rich in habitat for freshwater gastropods.  So I was mildly surprised to see that our buddy Bruce’s list extended to N = 31 species, from three primary (and hoary!) sources: Tryon 1866, Aughey 1877, and Walker 1906, a scattering of more recent references, and the collections at the University of Nebraska Sate Museum [2].

Actually, the starting number of species exceeded 80.  The biggest challenges faced by our good buddy were the pruning of over 40 synonyms from the 19th-century taxonomy, and the weeding of around 8 - 10 valid nomina apparently introduced to the fields by error.  The taxonomy of the 31 species that emerged from Bruce’s intensive efforts with hoe and hook was neat, clean, and modern [3].

Loyal followers of this blog may remember my post of 20Apr06, entitled “Surveying the Heartland” [4].  That essay was prompted by our good friend Tim Stewart’s review of the freshwater gastropods of Iowa, very similar in scope and methodology to the present work by Bruce Stephen.  And in my post of 23Jan09, I compared the (46 species) fauna of Iowa to that of Indiana in the east and Missouri in the south, identifying 20 species shared by the three boxy Midwestern states, 15 uniquely shared with the former and 6 uniquely shared with the latter [4].

The FWGNA readership may also remember my post of 26Nov08 reviewing a popular guidebook on the freshwater gastropods of Colorado, based on the (1989) work of Shi-Kuei Wu [4].  So with the publication of the present review of the Nebraska fauna, we have available an unbroken transect, extending 1,000 miles from the banks of the Mississippi River at Davenport, across the Great Plains and over the Rocky Mountains to the Colorado River at Grand Junction.

Should we expect to discover that any of our buddy Bruce’s 31 species are unique to Nebraska?  Or might we find the Nebraska fauna perfectly transitional between those of Iowa and Colorado?  The figure at left is a Venn diagram of boxy western states, composed using the same convention I pioneered in my essay of 23Jan09.  The areas of the states are adjusted relative to their freshwater gastropod faunas – the 44 species documented from Iowa [5] rendering it over twice the size of Colorado (with 20 species, ref. 6), Nebraska in the middle.

And we do indeed see a rather smooth malacological transition (perhaps “attenuation” would be more descriptive) moving west from the relatively rich fauna of Iowa to rather poor Colorado.  Setting aside the 16 cosmopolitan species shared by all three states, Nebraska shares 12 uniquely with Iowa and just one with Colorado (Lymnaea bulimoides).  But again perhaps surprisingly, Nebraska does seem to boast two freshwater gastropod species found in neither Iowa nor Colorado: Physa pomilia and Gyraulus crista.

Our buddy Bruce was a bit dubious about the Physa pomilia records, as am I.  The 15 records from S-K Wu (2004-05) predate our modern understanding of the taxon [7].  But the single G. crista record (from Taylor 1960) appears to be bona fide, this (rather boreal) species being documented from Wyoming to the northwest. 

Science is the construction of testable models about the natural world.  This definition I am in the habit of repeating once a week to my Genetics Lab 305L students, 14 weeks per semester.  Historical reviews such as the work just published by our buddy Bruce Stephens serve the important function of providing models, which we malacologists of the present day really ought to test.  The job most certainly is not over in Nebraska, or in Iowa, or indeed in most of the United States of America, sea to shining sea.  But it is well begun.


[1] Stephen, B. J. (2015)  Species composition of Nebraska’s freshwater gastropod fauna: A review of historical records.  American Malacological Bulletin 33: 61 – 71.  [PDF]

[2] The author’s online queries of the collection databases at the USNM, ANSP, UMMZ, and several museums of more regional character field to yield a single freshwater gastropod record from Nebraska.

[3] Well, there are a couple smudges here and there, but I won’t quibble.

[4] My previous posts relevant to the biogeography of Western and Midwestern states, in order of appearance, have been:
  • Surveying the Heartland [20Apr06]
  • Review: Field Guide to the Freshwater Mollusks of Colorado [26Nov08]
  • The Freshwater Gastropods of Indiana [23Jan09]
[5] The fauna of Iowa has decreased by two species in nine years, due to taxonomic progress.  The lymnaeid nomen emarginata now seems to be a junior synonym of Lcatascopium, and the ancylid nomen parallela a junior synonym of F. rivularis.

[6] Wu’s (1989) monograph actually listed 30 freshwater gastropod species.  I have collapsed 15 of his specific nomina into five names as follows.  Lymnaea humilis = obrussa + parva.  Physa acuta = anatina + cupreonitans + gyrina.  Physa gyrina = elliptica + heterostropha + integra + utahensis.  Helisoma trivolvis = trivolvis + scalare + subcrenatum.  Ferrissia fragilis = fragilis + walkeri.  For more on the physid situation in Colorado, see my essay of [14Oct08].

[7]  Dillon, R. T., J. D. Robinson, and A. R. Wethington (2007)  Empirical estimates of reproductive isolation among the freshwater pulmonates Physa acuta, P. pomilia, and P. hendersoni.  Malacologia 49: 283 - 292.  [PDF]

Wednesday, April 15, 2015

The heritability of shell morphology in Physa h^2 = 0.819!

That’s the headline in the most recent edition of PLoS ONE, fresh on the news stands Monday morning.  Read all about it from the link below [1].

To place this remarkable statistic in perspective, animal breeders typically consider heritability in the range of 0.2 – 0.4 to be moderate, and heritabilities above 0.4 to be high.  The heritability of egg production in poultry is approximately h^2 = 0.10, milk yield in cattle about h^2 = 0.35, and weight gain per day in swine h^2 = 0.40 [2].  My colleague Stephen Jacquemin and I reviewed scores of estimates for the heritability of various aspects of gastropod shell morphology (marine, freshwater and terrestrial) in 14 peer-reviewed papers published since 1965, finding but one single value greater than h^2 = 0.819 ever reported [3].   For freshwater gastropods, the highest value reported to date (in a much smaller literature!) is h^2 = 0.30 [4].  How did Stephen and I arrive at our an eye-popping result?

I should imagine that most of my readership will be familiar with the shell morphology of the cosmopolitan invasive Physa acuta, figured at right above.  The shell of Physa carolinae, the recently-described [5] inhabitant of southeastern coastal plains figured at left, is distinctly more slender and fusiform by comparison.  The two species hybridize readily, although the F1 are almost entirely sterile [6].

Stephen and I crossed four pairs of acuta, four pairs of carolinae, and four axc hybrid pairs, rearing all 24 parent animals in standard conditions to age 20 weeks.  We then raised 5 F1 progeny from each of these 12 crosses to age 20 weeks in identical conditions.  And for the entire set of 24 parents plus 60 F1 offspring we took six “classical” linear measurements of the shell and digitized 11 landmark points.

The figure below shows the regression of the simple shell length of the 12 sibships on their mid-parent values, Y = 0.429x + 0.295.  The slope of the regression line estimates the narrow-sense heritability of shell length in Physa as conventionally measured, h^2 = 0.429 (s.e. = 0.139), significant at the (adjusted) 0.008 level.

Stephen and I performed similar calculations for our five other simple linear measurements, the first and second principal components extracted from both the correlation and covariance matrices of these measurements, scores on the first three relative warp axes from a geometric analysis, and centroid size.  The heritability estimates for all 14 of these variables, with their p-values adjusted for multiple comparisons, are shown in Table 2 of our paper downloadable below.  And our headline estimate of h^2 = 0.819 (s.e. = 0.073) is for variance on the first relative warp axis (RWA1). 

Now if overall “shell size” is defined as the summed distance from each of the 11 landmarks to their joint centroid, variance on RWA1 is significantly correlated with shell size at the 0.001 level.  So are almost all of the other 12 variables we measured (see Table 1 of our paper).  This implies that, as strikingly heritable as variance on RWA1 most certainly is, such variance may not be especially useful for inferring evolutionary relationships among natural populations of freshwater gastropods, which are inevitably composed of mixed ages.

Thus the second-most interesting result reported in the paper by myself and my buddy Stephen may not be the heritability of RWA1, but the h^2 = 0.312 (s.e. = 0.123) we estimated for score on RWA2.  This variance is not apparently correlated with overall shell size, and hence might (with some disclaimers) find application to populations of freshwater gastropods sampled from the wild.

And what might our first-most interesting result be?  I think it is the fact that Stephen and I were able to uncover any significant heritability in any aspect of freshwater gastropod shell morphology whatsoever.  The phenomenon of ecophenotypic plasticity is so pervasive in populations of freshwater gastropods as to warrant a separate heading in the blog index at right above, with 13 entries to date.  My stack of peer-reviewed papers documenting plasticity of shell morphology in freshwater snail populations stands 35 deep.  So quoting directly from the concluding paragraph of this our most recent addition to the literature, “Against such a background, the results reported in the present work can be viewed as providing a small measure of balance.”


[1] Dillon, R. T., Jr. & S. J. Jacquemin (2015)  The heritability of shell morphometrics in the freshwater pulmonate gastropod Physa.  PLoS ONE 10(4): e0121962. [html] [pdf]

[2] Falconer, D. S. & T. F. C. Mackey (1996) Introduction to Quantitative Genetics, Fourth Edition.  Essex: Pearson Education Ltd.

[3] Conde-Padin, P. et al (2007)  Genetic variation for shell traits in a direct-developing marine snail involved in a putative sympatric ecological speciation process.  Evolutionary Ecology 21: 635-650.

[4] Chaves-Campos, Coghill, Al-Salamah, DeWitt & Johnson (2012) Field heritabilities and lack of correlation of snail form and anti-predator function estimated using Bayesian and maximum likelihood methods.  Evol. Ecol. Res. 14: 743-755.

[5] TRUE CONFESSIONS: I described a new species. [7Apr10]

[6] Wethington, A.R., J. Wise, and R. T. Dillon (2009) Genetic and morphological characterization of the Physidae of South Carolina (Pulmonata: Basommatophora), with description of a new species. The Nautilus 123: 282-292. [pdf]

Monday, March 30, 2015

See You In Milwaukee?

 SFS 2015

The Society for Freshwater Science will be meeting on the sun-kissed shores of America’s Dairyland May 17 -21.  And once again yours truly has volunteered to man the “Gastropoda” booth at the Taxonomy Fair Wednesday afternoon the 20th.  So dig all those vials of juvenile physids out of that box under your sink, toss them loose into your checked luggage, and type MKE into your favorite online travel site today.

The scientific program looks super, as usual.  Our good buddy Sean Sullivan of Rithron Associates has organized a special session on “Invertebrate Systematics and Faunistics” that promises to be a rip-snorter!  There’s still time to register, but you must hurry.  Check out the meeting website (linked from the image above) for all the details.

Tuesday, February 24, 2015

Updated Maps for the FWGNA Project

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

As our legions of loyal users will fondly recall, the oldest sections of the FWGNA website featured downloadable pdf dot maps showing the detailed distributions of freshwater gastropods in South Carolina (2005), North Carolina (2005), Georgia (2006) and East Tennessee (2011).  But neither our Virginia site (2011) nor our Mid-Atlantic site (2013) ever featured pdf dot maps.  And our maps of Georgia and Carolina distributions have gone obsolete in the interim.

 Pleurocerid map
So the fresh set of range maps now available for download integrates seamlessly with the version of the FWGNA site we rolled out in October of 2013, based on 11,471 records of 68 freshwater gastropod species and subspecies inhabiting the Atlantic drainages of nine states [1].  These 68 taxa have been divided into 26 sets of two or three related species each (in most cases), and mapped in sets as a tool for the exploration of potentially interesting joint distribution patterns.

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

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

 Hydrobiid map

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

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


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

Thursday, January 8, 2015

What I Thought I Knew About Goo

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

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

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

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

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

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

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

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

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

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


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

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

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