Dr. Rob Dillon, Coordinator

Tuesday, July 14, 2015

The Type Locality of Lymnaea catascopium

Editor's Note. This essay was subsequently published as Dillon, R.T., Jr. (2019b)  Malacological mysteries: The type locality of Lymnaea catascopium.  Pp 73-80 in The Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston.

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]

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