Editor’s
Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Elimia livescens and Lithasia obovata are Pleurocera semicarinata. Pp 93 - 99 in The Freshwater Gastropods of North America Volume 3, Essays on the
Prosobranchs. FWGNA Press, Charleston.
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]
- Pleurocera acuta is Pleurocera canaliculata [3June13]
[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.
[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: