Dr. Rob Dillon, Coordinator


Thursday, December 4, 2014

Spur-Of-The-Moment Workshop


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

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

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

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

Looking forward to it!

Notes

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


Tuesday, November 11, 2014

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


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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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


Notes

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

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

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

Friday, October 17, 2014

The Egg Mass of Lymnaea (Bulimnea) megasoma


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

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

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


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

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

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


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

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


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

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

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

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


Notes

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

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

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

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

Friday, September 26, 2014

The Egg Masses of Freshwater Pulmonate Snails


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

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

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

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

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

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

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

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

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

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

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

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

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


Notes

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

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

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

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

Tuesday, August 5, 2014

Just Before The Bust


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

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

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

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

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

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

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

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

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

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


Notes

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

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

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

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

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

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

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

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

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

Friday, July 11, 2014

Elimia livescens and Lithasia obovata are Pleurocera semicarinata

Faithful readers of this blog will by now be familiar with cryptic phenotypic plasticity in the shell morphology of pleurocerid snails – interpopulation variance so extreme as to prompt an (erroneous) hypothesis of speciation.  We first documented the phenomenon in our (2011) survey of Pleurocera clavaeformis in East Tennessee [1] under the term “Goodrichian taxon shift,” and generalized the concept to “cryptic phenotypic plasticity” in our (2013) study of Pleurocera canaliculata ranging from New York through the Midwest to North Alabama [2].

Now in a third installment of the series, published in this month’s issue of Zoological Studies [3], we document what may be the most surprising example of the phenomenon yet brought to light.  And again, it might be easiest to understand the significance of the discovery if we back up to the 19th century and take a running start at it.

Thomas Say described “Melania semicarinata” from “Kentucky” in 1829, one year before C. T. Menke described his “Melania livescens” from “Lake Erie, New York.”  Classically, livescens populations bear short, stubby shells while the shells of semicarinata are more slender.  But the populations inhabiting most of the Midwest seem to vary tremendously in shell morphology, such that the distinction between semicarinata and livescens simply disappears.  Calvin Goodrich (1940) divided the species ranges roughly at the glacial maximum, giving the range of semicarinata as “tributaries of Ohio River, Scioto River, Ohio, to Big Blue River, Indiana; Licking River to Salt River in Kentucky; two creeks of Green River of Kentucky” and that of livescens as “tributaries of Ohio River, east of Scioto River in Ohio; Wabash River and branches, west to Illinois River.  Especially common in the St. Lawrence basin.” [4]

But one gets the impression that this division was rather arbitrary on Goodrich’s part.  Our colleagues Steve Jacquemin and Mark Pyron published a nice morphometrics paper back in 2012 [5] showing that Indiana populations bearing broader shells tend to be found in more lentic environments at higher latitudes, and that populations bearing slender shells tend to be found in lotic waters at lower latitudes.  Our Indiana friends identified all 39 of their study populations as “Elimia livescens,” but Goodrich would have identified the 16 populations they sampled from the southern half of the Hoosier State as Goniobasis semicarinata.

I began thinking about a test of the hypothesis that semicarinata and livescens populations might be conspecific in the spring of 2009.  And it was obvious to me that if I were to use the genetic tools that have stood me in such good stead for 30 years, I would need some sort of external calibration.  The combined range of nominal livescens and semicarinata together would extend across 12 states and two Canadian provinces, much larger than any area I had heretofore surveyed.  How much genetic divergence might one expect among conspecific pleurocerid populations spread across so large a region?

So (again, this was back in 2009) it occurred to me that populations of (what I called at the time) Pleurocera acuta also range from New York through the Midwest to Kentucky, and that nobody has ever questioned their conspecific status.  So along with each study population of either semicarinata or livescens, I resolved to sample a control population of Pleurocera acuta.  And if the genetic divergence among my semicarinata/livescens study populations proved no greater than that of my acuta controls, the evidence would suggest that livescens (Menke 1830) is a junior synonym of semicarinata (Say 1829).  I actually made several field trips through Kentucky, Ohio, and Indiana in the summer of 2009, sampling pleurocerid populations under this original (much simpler) study design.

At this point in my narrative, any of my readership who might be pathologically curious about the process of scientific discovery in your particularly feeble-minded correspondent might insert the essay I wrote last year regarding Pleurocera acuta, P. canaliculata, and P. pyrenellum [6].  It was only in the summer of 2010, after I had already begun work on my semicarinata/livescens project, planning to use acuta as a control, that it dawned on me that the situation with canaliculata/acuta might be as big a mess as semicarinata/livescens [7].  And since I couldn’t interpret my semicarinata/livescens data without a canaliculata/acuta calibration, the canaliculata/acuta problem had to be solved and published first.

And that’s how I stumbled upon the headline news – the relationship of semicarinata/livescens to Lithasia obovata.  Sampling the main Ohio River at the type locality for Pleurocera canaliculata in August of 2011, I encountered the first population of L. obovata I had ever personally examined on the hoof.
The hypothesis that populations which Thomas Say described as “Melania obovata” from the Kentucky River back in 1829 might be a startlingly robust, big-river morph of semicarinata/livescens did not immediately dawn on me, however.  True, juvenile obovata do look a bit like Goniobasis or Elimia, which are the genera to which semicarinata and livescens have historically been assigned. 

And indeed, in 1934 Calvin Goodrich documented the same upstream-downstream relationship between the slender, lightly-shelled “sordida form” of L. obovata and more typical, heavily-shelled “forms” in the Green River of Kentucky that he documented between the slender, lightly-shelled pinguis form of Lithasia geniculata and more typical, heavily-shelled forms in the Duck River of Tennessee [8].  It was Goodrich’s 1934 analysis of shell morphological intergradation in the L. geniculata populations of the Duck River that prompted me to coin the term “Goodrichian Taxon Shift” in his honor in 2007 [1].  And sordida (Lea 1841) was initially assigned to the genus Goniobasis by Tryon, just as pinguis (Lea 1852) was initially assigned to Anculosa.

Was Goodrich’s (1934) Plate 1 riding around in the back of my mind in 2011?  [Click the thumbnail below for a larger version.]  Figures 1 – 9 and 12 depict what Goodrich considered “forms” of Lithasia obovata, including figures 2, 6, and 12, which are all obviously Pleurocera semicarinata.  Figures 10, 11, and 14 depict his “forms” of Lithasia geniculata (Figure 11 is pinguis), which I first extracted and figured way back in 2007 as an illustration of Goodrichian taxon shift.

In the end, I’m not sure what led me to include a sample of 32 Lithasia obovata in a two-phase study already groaning with five populations of nominal acuta, four populations of canaliculata, three populations of semicarinata, three populations of nominal livescens, and a population of nominal pyrenellum from North Alabama thrown in for good measure.  But I’m glad that I did.

I have always considered myself philosophically conservative, in the sense that my initial reaction to change of any sort is negative.  And I am very sensitive to the unhappy side-effects of the taxonomic changes that I have proposed for the North American Pleuroceridae in recent years, especially with regard to the indexing function.  The best general work on the biology of the creatures to which I have dedicated my career is the 1965 paper by B. C. Dazo, “The morphology and natural history of Pleurocera acuta and Goniobasis livescens [9].”  With the publication of my most recent paper, Dazo’s study populations have now become Pleurocera canaliculata and Pleurocera semicarinata, nearly impossible to retrieve by a simple electronic search.

So let’s use subspecies, shall we?  The FWGNA Project has adopted the standard definition of the term “subspecies” as it has been understood since the birth of the Modern Synthesis: “populations of the same species in different geographic locations, with one or more distinguishing traits” [10].  Clearly some populations of Pleurocera semicarinata inhabiting big rivers bear shells so robust as to warrant distinction as P. semicarinata obovata, and other populations in more northern latitudes bear broader shells distinguishable as P. semicarinata livescens.  Such a system will preserve the indexing function of the old names, without compromising our growing storehouse of new evolutionary insights to the ever-surprising freshwater gastropods of North America.

Notes

[1] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]  For more, see:
  • Goodrichian Taxon Shift [20Feb07]
  • Mobile Basin III: Pleurocera puzzles [12Oct09]
  • Goodbye Goniobasis, Farewell Elimia [23Mar11]
[2] Dillon, R. T., Jr., S. J. Jacquemin & M. Pyron (2013) Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127. [html] [pdf]  For more, see:
  • Pleurocera acuta is Pleurocera canaliculata [3June13]
[3] Dillon, R. T., Jr.  (2014) Cryptic phenotypic plasticity in populations of the North American freshwater gasgtropod, Pleurocera semicarinata.  Zoological Studies 53:31. [html] [pdf]

[4] Goodrich, C. (1940) The Pleuroceridae of the Ohio River system.  Occas. Pprs. Mus. Zool. Univ. Mich. 417: 1-21.

[5] Dunithan, A., S. Jacquemin & M. Pyron (2012)  Morphology of Elimia livescens (Mollusca: Pleuroceridae) in Indiana, USA, covaries with environmental variation.  Am. Malac. Bull. 30: 127 – 133.

[6] Pleurocera canaliculata and the process of scientific discovery [18June13]

[7] And now you know the rest of the story.  In my essay of 18June13 (link above) I wrote, "Standing knee-deep in Savannah Creek (maybe 20 km N of Chattanooga) on that memorable day, it suddenly struck me that the snails crawling around my feet might be Pleurocera acuta."  This revelation was only possible because I had spent several weeks that previous summer travelling around the Midwest hunting Pleurocera acuta.  In retrospect, I think the only way I was able to make the acuta/canaliculata/pyrenellum connection in 2010 was that by that time I had personal field experience across the entire 12-state range of the species, from Yankeeland to Dixie.  This is something that Thomas Say, or George Tryon, or even Calvin Goodrich, could never have contemplated.  Good day.

[8] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae – I.  Occas. Pprs. Mus. Zool. Univ. Mich. 286: 1-17.

[9] Dazo, B.C. (1965) The morphology and natural history of Pleurocera acuta and Goniobasis livescens (Gastropoda: Cerithiacea: Pleuroceridae). Malacologia 3:1-80.

[10] For a complete review of FWGNA policies on the taxonomic level of the subspecies, see:
  • What is a subspecies? [4Feb14]
  • What subspecies are not [5Mar14]

Thursday, June 12, 2014

To Identify a Physa, 1978

Word has reached us of the death of Dr. George Ang Te, who passed away late last year in New Hamburg, NY at the age of 67.  Although I never met him, George Te’s 1970s-era research on the worldwide Physidae was a significant influence on my professional career.  In this final installment of our three-part series, we examine Te’s 1978 dissertation, and his continuing legacy.

In the 1970s everybody was in love with computers.  There was just the one computer on the Virginia Tech campus, and it was a huge IBM 360/370, with walls of blinking lights and massive tape drives.  And if you wanted to use it, you pretty much had to write the FORTRAN code yourself.  There were a few “canned” statistical packages available in those days, but even to use canned programs, you had to write your own job control language.  And you had to punch your own cards, in a big room full of keypunch machines – one line of code or data per card.  And you needed an “account” to use the machine.  (Tech undergraduates were allocated $25 per quarter.)  And you left your deck of cards at the input desk, and came back the next day, hoping to find a thick stack of tractor-feed paper with meaningful results in your cubby.  I usually found two-page bombs, with unintelligible error codes, and hefty charges to my account for the service.  I loved computers in the 1970s.

In the 1970s the world of systematic biology was completely consumed by warfare between the forces of phenetics and the forces of cladistics.  Phenetic analysis was seamlessly able to bring the power of electronic computing to bear on problems of classification.  Pheneticists told us to count, measure or score 100 characters on 100 “operational taxonomic units,” punch our cards, and let the room full of blinking machinery objectively classify our OTUs by their overall similarity.

No, cladists argued – classification can only be based on shared-derived characters, and shared-primitive characters must be discounted.  So rather than calculate some gross measure of overall similarity among our OTUs, cladists insisted that character state polarity must be taken into account.  For everything measured, counted or scored, somebody had to figure out what state was primitive, and what state was derived, somehow.   Such subjectivity did not immediately lend itself to electronic computing.

But whatever else the world of systematic biology was focused upon in the 1970s, organisms were not it.  The process of classification was important, and the OTUs being classified, not so much.  And it was into this world that George Te stepped when he entered the PhD program at the University of Michigan in the fall of 1971.

So in addition to J. B. Burch, serving on George Te’s PhD committee were two much-decorated officers in the cladist army, Warren (“Herb”) Wagner and George Estabrook.  Wagner was an exellent botanist, carried to stardom by an algorithm to estimate phylogenetic relationships that became known as “Wagner parsimony.”  Estabrook was a computer scientist, entirely unsullied by any taint of organismal biology, with a head full of very original ideas for estimating cladistic classifications based on character compatibility.  And I think it was primarily Estabrook’s influence that ultimately turned George Te’s dissertation focus, and indeed George Te himself, away from biology.

But he made a promising start at it.  Last month [1] we reviewed Te’s (1975) survey of the Physidae of Michigan, which I characterized as “the zenith of classical malacology in North America.”  So broadening his scope toward a dissertation, Te expanded his regional survey to include 12,163 museum lots of worldwide physids from seven major museums, among which he initially recognized 85 “basic taxa.”  For these 85 basic taxa he scored 71 characters: 37 of shell and 34 of anatomy, each character taking some number of discrete “states” [2].

Estabrook gave George Te two sets of computer programs, one phenetic and the other cladistic.  The former was called “Simgra,” a quirky method of clustering OTUs by a matrix of similarity coefficients.  The latter was a two-part affair, Estabrook’s test for character compatibilities followed by a J. S. Farris routine to extract “cliques.”

The meat of Te’s dissertation was what can only be characterized as methodological gear-grinding, the details of which simply do not matter today.  But what ultimately popped out of the roomful of blinking machinery was a reduced set of 48 “species units,” in five groups with two outliers.   See George Te’s Figure 42, scanned above.

Throughout this retrospective I have tried to refrain, as much as humanly possible, from judging George Te by 21st century standards.  But I ordered a copy of his dissertation from University Microfilms in 1982, and my contemporary impressions of the work were profoundly mixed.

Te’s dissertation did include at least one significant advance – the description of a fifth type of penial morphology, which he called “type-bc, indicating its intermediate nature.”   This was the basis of his physid “Group V – the cubensis group.”  He misclassified most of the species in the group, including cubensis.  But his characterization of this intermediate-but-nonetheless-distinct morphology provided a critical clue to Amy Wethington and me as we began our work on the specific relationships among southeastern physids 20 years later [3].

On the other hand, however, it was as clear to me 30 years ago as it is today that the distinctions between most of George Te’s 48 “species units” were largely illusory and entirely undocumented.  Te recognized 27 species units in his “Group IV – acuta” group, for example.  For OTU-66 acuta itself, he wrote “appears to be part of the heterostropha continuum…but is geographically disjunct … and is therefore treated here as separate [4].”   For OTU-50 concolor, he wrote “forms part of the heterostropha continuum, and is the predominant taxon with the type-c penial complex in the northwest.”  He observed that OTU-60 virgata “is the principal southwestern North American component of the heterostropha continuum,” and that OTU-52 osculans “is the predominant taxon of the heterostropha continuum in Mexico,” and that OTU-68 pomilia “is part of the heterostropha continuum and is a southeastern geographic variant of OTU-70 heterostropha.”  And on and on he went.  Surprisingly, he wrote “OTU-71 integra is a predominant Group IV taxon in the Midwest, and is not part of the heterostropha continuum.”

It was extremely frustrating to me in 1982 that, although George Te explicitly stated that his observations quoted above were based on 33 nominal acuta specimens, 13 concolor, 24 virgata, 19 osculans, 29 pomilia, 28 integra, and so forth, he did not tell us where any of his samples came from, and so we who followed him could not verify his observations for ourselves.  With the benefit of 30 years’ hindsight, we now understand that acuta, heterostropha, virgata [7] and integra are conspecific.  We do not know about osculans or concolor [8].   But we do know that pomilia is a distinct species [9], correctly classified in Group V, not Group IV, and the inescapable conclusion must be that none of the 29 individual snails Te examined came from a bona fide pomilia population.

But returning to contemporary times, my bottom line impression of George Te’s 1978 dissertation was one of disappointment.  It could not be used to identify a Physa.  Which is all I ever really wanted it to do.

And can 48 meaningless OTUs be classified in some meaningful way?  Te proposed a system of two subfamilies, four genera, three subgenera, and two “sections,” which yielded the seven baskets he needed for his 48 “species units.”

But by the date of his defense, in December of 1978, it seems clear to me that George Te’s interests had strayed afield from systematic biology, and toward computer science.  Of the 324 pages comprising his dissertation, over 35% were devoted to analytical arcana.  His section on phenetic relationships, for example, was 54 pages of “Simgra” results, showing dozens of “linkage diagrams” at various “similarity levels.”  The roomful of blinking, spinning machinery was beginning to carry George Te away.

His dissertation [10] was never published.  All that ultimately emerged was a six page paper in Archiv fur Molluskenkunde, entitled “New classification system for the family Physidae” [11].  By the 1979 date of that publication, George Te was giving his address as University of Michigan Department of Computer Science.

But to his tiny, obscure 1979 paper must (in all fairness) be added the entire treatment of the Physidae in J. B. Burch’s immensely influential “North American Freshwater Snails,” which first appeared as an EPA publication in 1982.  George Te’s dissertation results became the de facto standard for the classification of the Physidae in the United States, right up in the pantheon beside Baker’s Lymnaeidae, Basch’s Ancylidae, and Goodrich’s Pleuroceridae.  And that quirky physid classification, for all its faults and failings, was the only system adopted by Burch that was, in any sense, objective.

I never met George Te.  But he was, by all accounts, a very nice man.  And he was extremely helpful to me at an early stage of my career.  In a span of just eight years, from 1971 – 1978, he was able to bring order out of taxonomic chaos in one of the most important elements of the North American freshwater benthic fauna.  He advanced our science.  Rest in peace.

Notes

[1] To Identify a Physa, 1975 [6May14]

[2] If this essay were really about George Te’s classification, we’d stop right here.  Only 14 of his 71 characters were legitimately discrete, and his character scoring system was a nightmare, and his entire analysis a house of cards.  But in 2014 it just doesn’t matter.

[3] 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]
  • True Confessions - I described a new species [7Apr10]
 [4] George Te was very, very close to making a breakthrough here.  He wrote:
“Although OTU-66 acuta is widely introduced to many parts of the world, its occurrence in North America has rarely been reported.  Considering the amount of transaction between Europe and North America, the absence of OTU-66 acuta in North America is puzzling.  The logical conclusion is that where OUT-66 acuta has been introduced in North America, it probably interbred and merged with indigenous North American “Physa” (i.e., those taxa in the heterostropha continuum).”
It was not until 2002 that we finally confirmed heterostropha to be conspecific with acuta [5].  Te was a good, conscientious worker, and his observations have substantial scientific value, in direct contrast to some of his contemporaries, who just made stuff up [6].

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

[6] See footnote #10 in this essay:
  • Red Flags, Water Resources, and Physa natricina [12Mar08]
[7] Dillon, R. T., J. D. Robinson, T. P. Smith, and A. R. Wethington  (2005)  No reproductive isolation between freshwater pulmonate snails Physa virgata and P. acuta.  The Southwestern Naturalist 50: 415 - 422.  [PDF]

[8] The nomen “concolor” is my lead candidate for the “Snake River acuta-like Physa” or “SRALP,” about which I posted a big series last year:
  • The mystery of the SRALP – A bidding [5Feb13]
  • The mystery of the SRALP – A twofold quest! [1Mar13]
  • The mystery of the SRALP – Dixie cup showdown [2Apr13]
  • The mystery of the SRALP - “No Physa acuta were found” [2May13]
[9] 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]

[10]  Te, G. A. (1978)  A systematic study of the Physidae.  Ph.D. Dissertation, University of Michigan, Ann Arbor.  324 pp.

[11] Te, G. A. (1979) New classification system for the family Physidae.  Arch. Moll. 110: 179 -184.