Editor’s
Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera shenandoa n.sp. Pp 101 - 108 in The Freshwater Gastropods of North America Volume 3, Essays on the
Prosobranchs. FWGNA Press, Charleston.
An element of serendipity was introduced into both studies
when the genetics of the “well-characterized” control populations turned out to
be as surprising as the experimental populations. The additional research this necessitated,
for P. simplex (reported in Sept – Nov 2016) [3] and P. canaliculata (reported in
June 2013) [4] yielded additional insights into the evolutionary biology of an
enigmatic group of organisms that becomes more fascinating to me every time I
step in the creek.
So,
there were actually two sorts of controls for the studies referenced above,
calibration standards and mobility standards.
In the very first study of allozyme polymorphism I ever published [5],
way back in 1980, I set Pleurocera simplex population WYTH as a mobility
standard, defining the mobility of the most common allozyme band in that
population as 100 SDEMM (standard Dillon electrophoretic mobility
millimeters.) And in every study of
allozyme polymorphism in pleurocerid populations I have published since that
date, a total of 13 in all, I have included a population linked somehow to my
1980 population WYTH, calibrating every allele at every locus by its mobility
relative to that standard.
 |
| Figure 4 of Dillon [2] |
So
in both my 2013 study of cryptic phenotypic plasticity in Pleurocera
canaliculata [4], and in my related 2014 study of P. semicarinata [2], my
mobility standard was population SV.
This was the same population of P. semicarinata I called “PINE” in my
1980 paper, which I ran beside population WYTH thirty-five years previous,
hence calibrating the mobilities of all allozyme bands in the 2013 and 2014
papers in units of SDEMM.
Now
look at Figure 4 of my 2014 study, reproduced above. “Standard” Virginia semicarinata population
SV is more genetically similar to P. canaliculata than to any member of the
(nominally conspecific) semicarinata/livescens/obovata cluster. In fact, my P. semicarinata “standard” seems
to be the most genetically divergent population in the entire study!
What
is this semicarinata population from Virginia I called SV in 2014, which I
called PINE in 1980? And what made me
think that it was Pleurocera semicarinata in the first place?
Let’s
flip the calendar back another five years to 1975, when the young Rob Dillon
was a sophomore at Virginia Tech, blissfully bumping around on the backroads of
the upper New River drainage with boots, nets and buckets thrown in the back of
a state pickup truck. You may recall,
from my essay of May 2014 [6], that my undergraduate research thesis was
entitled “Factors in the distributional ecology of upper New River mollusks
(Va/NC).”
Goodrich’s
papers [7,8] were available in the university library in 1975, but this was
before the publication of Burch’s EPA key [9], so I had no pictures. Nor (of course) any access to reference
collections. So, I confess that I simply
guessed. Goodrich listed just two
species of Goniobasis from the area, Goniobasis proxima as a trans-Appalachian
inhabitant of the highlands of North Carolina and G. simplex in the Bluestone
River of West Virginia, apparently captured over from the Tennessee
drainage. That suggested to me that my
softwater species in the New River must be G. proxima, and my hardwater species
must be G. simplex.
Even
a college sophomore knew crappy science when he saw it, and he was not happy
about that, at all. I do remember
thinking, 45 years ago, that there seemed to be at least TWO hardwater species
of Goniobasis in the upper New River drainage in addition to the single
softwater species but had no idea how to start solving the problem.
A
method to start solving the problem presented itself in grad school at the
University of Pennsylvania. My advisor
at the ANSP, Dr. George Davis, had a big grant to work out the systematics of
unionid mussels with the brand-new technique of allozyme electrophoresis, and
was generous enough to allow me to bring my samples of pleurocerids into his
lab. And absorbed onto the little paper
wicks applied onto the butt ends of the first gels I ever ran was the
proteinaceous goo of ground-up Goniobasis from the upper New River drainage.
Our
gels clearly showed one species in the softwater and two species in the hard [5]. And by now I did have a research collection
at my fingertips. The softwater species
was (indeed) Goniobasis proxima, upon which I focused my dissertation, and most
of the rest of my career, and one of the two hardwater species was (indeed)
Goniobasis simplex. That other hardwater
species seemed to match Goniobasis semicarinata.
 |
| Figure 2 of Dillon & Davis [5] |
Of
course, this was just shell-matching.
That is all that pleurocerid taxonomy had ever been, as of 1980. And Goodrich [7] gave the range of Goniobasis
semicarinata as “Tributaries of Ohio River, Scioto River to Big Blue River,
Indiana; Licking River to Salt River in Kentucky,” saying nothing about
Virginia at all. I assumed at the time,
and continued to assume for many years, that the range of G. semicarinata must
extend from Ohio through West Virginia up some 200 km of Kanawha River drainage
into the upper New River basin, unbeknownst to Goodrich.
Yes,
the Roanoke River is an Atlantic drainage.
And in subsequent years, as the FWGNA survey extended throughout all the
Atlantic drainages from Georgia to the New York line, I discovered populations
of what I called Goniobasis or Pleurocera semicarinata north down the length of
the Great Valley of Virginia, in tributaries of the Roanoke, James, and
Shenandoah Rivers as far north as Waynesboro, my home town [10].
This
went on for many years. And at no time,
from my initial judgement call in 1980 to maybe perhaps 2010, had I ever
studied at any length a bona fide, Midwestern population of semicarinata on the
hoof.
In
2010 the FWGNA project began to devote an increasing amount of our attention to
the freshwater gastropod fauna of Ohio River drainages. This led to the population genetic surveys
that ultimately disentangled the evolutionary relationships between what had
historically been called Goniobasis semicarinata, G. livescens, Pleurocera
canaliculata, P. acuta, and Lithasia obovata.
Perhaps the most striking result of those studies was the tremendous
ecological adaptability of what we now call Pleurocera semicarinata –
populations now known to inhabit the entire range of waterbodies in the
Midwest, from the smallest creeks to the grandest rivers, including lake shores
of both sand and rock. In addition to
this, or perhaps because of this, populations of bona fide P. semicarinata
demonstrate tremendous plasticity of shell morphology.
That
plasticity certainly extends to cover the shell morphology demonstrated by what
I had been calling semicarinata in the Great Valley of Virginia. But 30 years of field experience had
impressed upon me that Virginia populations seem to be entirely restricted to
small creeks. They do not extend into
larger rivers or lentic bodies of water of any sort, nor do they demonstrate
anywhere near the range of shell phenotypic plasticity. There also seemed to be body color
differences – the Midwestern populations typically demonstrating a brighter
orange coloration and the Virginia populations darker. I’m not entirely convinced of this, but it is
worth mentioning.
And
in 2014 we extended the FWGNA survey into the Ohio drainages of West
Virginia. There really are no Pleurocera
populations of any species inhabiting any tributaries of the Kanawha river
through most of the state. The Virginia
populations I had been calling semicarinata appear to be isolated by over 200
km of uninhabited waters from the nearest bona fide semicarinata population in
Ohio.
So
all that, together with the genetic results shown in 2014 Figure 4 reproduced
above, combined to force me into describing a new species, in the appendix of
the first hardcopy volume of the FWGNA project, coming soon [11]. This new species, Pleurocera shenandoa, is
locally quite common and widely distributed throughout a long-settled part of
the world, where one might reasonably expect the biota to be well-known and
well-characterized.
It
is not. Perhaps the primary theme of my 20
years of blog posts, and the entire set of four FWGNA volumes to be published
soon, is that even here in the 21st century, in the home of the best
science the world has ever known, we remain stunningly ignorant of even the
most commonplace. We spend billions of
dollars shooting space probes to the moons of Jupiter, and not a nickel to
understand the little brown snails in the creeks behind our own houses. Shame on us all.
Notes
[1]
Dillon, R. T. Jr. (2011) Robust shell
phenotype is a local response to stream size in the genus Pleurocera
(Rafinesque 1818). Malacologia 53: 265-277. [PDF] See:
- Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
- The cryptic Pleurocera of Maryville [13Sept16]
- The fat simplex of Maryville matches type [14Oct16]
- One Goodrich missed: The skinny simplex of Maryville is Pleurocera gabbiana [14Nov16]
- Pleurocera acuta is Pleurocera canaliculata [3June13]
- Pleurocera canaliculata and the process of scientific discovery [18June13]
[6]
Sweet, gauzy memories of my college days:
- To identify a Physa, 1975 [6May14]
[8]
Goodrich, C. (1942) The Pleuroceridae of the Atlantic Coastal Plain. Occas. Pprs. Mus. Zool. Univ. Mich. 456: 1-6.
[9]
This is a difficult work to cite. J. B.
Burch's North American Freshwater Snails was published in three different
ways. It was initially commissioned as
an identification manual by the US EPA and published by the agency in
1982. It was also serially published in
the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in
1989 (Malacological Publications, Hamburg, MI).
[10]
Sweet, gauzy memories of Waynesboro, VA. Well, not so much:
- The Clean Water Act at 40 [7Jan13]