Editor’s
Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Fred Thompson Elizabeth Mihalcik, and the pleurocerids of Georgia. Pp 127 - 136 in The Freshwater Gastropods of North America Volume 3, Essays on the
Prosobranchs. FWGNA Press, Charleston.
Science is a human enterprise, alas. Last month [1] we reviewed the fitful
progress of research into the systematic relationships of the Florida
Pleuroceridae through the 1970s and 1980s, as a function of the personalities
of two scientists, the late Dr. Fred G. Thompson and Dr. Steven M.
Chambers. In 1990 Steve Chambers
published a malacological masterpiece, “The genus Elimia (= Goniobasis) in
Florida and adjoining drainage basins [2],” correcting Clench & Turner’s 1956
seven-species model down to four. He
then shifted his professional interests out of the field. Thompson, failing to cite the Chambers 1990 paper
or indeed (almost) any of Chambers’ impressive body of research, published an
update of his “Freshwater Snails of Florida” in 1999. He was going in the opposite direction.
Thompson’s Second Edition [3] began with the Clench &
Turner seven-species model but added an undescribed “spring Elimia” from Holmes
Creek and three new species that “may occur in Florida” (Elimia taitiana,
Choctawhatchee Elimia and Escambia Elimia), bringing his grand total up to 11
and foreshadowing things to come.
Simultaneously, Thompson was working with a committee of the AFS to
publish a second edition of the Turgeon et al. “Common and Scientific Names of
Mollusks,” which appeared in 1998 [4].
The Turgeon work was destined, in a perverse way, to become the Bible of
American malacology. Thus less than ten years
after the publication of Steve Chambers’ landmark paper, the Clench &
Turner model became gospel, and Chambers’ work forgotten.
Meanwhile, a new graduate student arrived at the University
of Florida named Elizabeth L. Mihalcik.
She entered the laboratory of the noted herpetologist F. Wayne King, and
was “introduced into the world of invertebrates through her mentor and friend,
Dr. Fred G. Thompson.” In 1996 she was
awarded an EPA STAR graduate fellowship for her proposal entitled, “Analyzing
morphological forms within southeastern river system species complexes.”
 |
| The Mihalcik [6] study area, showing P. floridensis |
Elizabeth’s (1998) dissertation [5] focused primarily on shell
morphometrics: 6 measures, 4 counts, spire angle and 8 ratios of the above in
various combinations, blessedly naive of any inkling that any shell character
might demonstrate any interpopulation variance for any reason whatsoever. But the winds of change were blowing across
systematic biology. Her dissertation
also included a little bit of mtDNA sequence data.
The first fully automated sequencing DNA machine had come
onto the market in 1987, even as Kerry Mullis’ patent for PCR amplification was
approved. The Folmer et al primers to
amplify the mitochondrial CO1 gene were published in 1994, and by the late-1990s
what had been a trickle of DNA sequence data became a torrent. Surely such data must be useful for
systematic biology, if we could just settle on an algorithm to build our gene
trees, yes? The “phylogenetic species
concept,” originally proposed by Cracraft in 1983, came into vogue.
So by the time Elizabeth published her dissertation with
Fred Thompson in 2002 [6], her analysis was almost entirely based on
mitochondrial CO1 sequence data. She
sequenced 25 individual snails from 24 Pleurocera populations scattered across
Florida, Georgia and Alabama, with one individual P. catenaria from South
Carolina thrown in to boot. Her results
(below) returned four clusters of roughly 5 - 7 individuals each, corresponding remarkably
well to the four species that Steve Chambers documented in 1990.
Mihalcik and Thompson also discovered at least three apparent
cases of mitochondrial superheterogeneity (mtSH above). I have very nearly beat this subject to death
on the FWGNA blog, but see note [7] below if you need your memory refreshed. MtSH may indeed have some utility in
identifying genuine biological species, but only if the phenomenon is
recognized, which Mihalcik and Thompson did not [8]. So setting aside their sequence data for
those three individual snails, the remaining snails demonstrate four clusters neatly
corresponding to the species Chambers called floridensis, curvicostata,
dickinsoni, and “boykiniana*.”
Alas, Mihalcik and Thompson saw substantially more than four
species in their gene tree. They
interpreted their 25 mtDNA sequences as evidence of 15 species and subspecies of Pleurocera in Central Georgia and the
Florida drainages of Alabama, as follows.
Starting with baseline curvicostata, a catenaria reference from South
Carolina and two flava references from Alabama, M&T resurrected four old
Isaac Lea names from synonymy: induta, mutabilis, viennaensis, and ucheensis. They raised Goodrich’s subspecies timida to
the full species level, and described two new subspecies underneath it: exul
and nymphaea. And they freshly described
five new species: exusta, glarea, annae, buffyae, and darwini, the last three
of which Elizabeth named for her dogs.
When one steps back to consider that the authors did not even
consider six of the seven Clench & Turner species as members of the “Elimia
curvicostata complex” (albanyensis, athearni, clenchi, dickinsoni, floridensis
and vanhyningiana), the Mihalcik and Thompson hypothesis constitutes a
staggering multiplication of pleurocerid nomina [9] for the region.
I myself was not idly sitting by in Charleston, however. Through most of the 1980s I confess that I
became distracted by hard clam aquaculture, and by the 1990s Amy Wethington and
I were beginning to work on Physa. But I
never forgot my first love, which was the pleurocerid snails. And in 2002, just a few months prior to the
work of Mihalcik & Thompson, an undergraduate student and I published “A
survey of genetic variation at allozyme loci among Goniobasis populations
inhabiting Atlantic drainages of the Carolinas” [10].
We estimated gene frequencies at 8 polymorphic loci in 12 populations,
picking up in NW North Carolina where my dissertation left off and extending through
South Carolina to touch eastern Georgia – 4 populations of P. proxima and 8 populations
of P. catenaria, including the type locality 50 km W of Charleston. Our most interesting result touched a very
familiar theme on this blog [11], ecophenotypic plasticity in pleurocerid shell
morphology. Populations of P. catenaria
bearing typical shells in the Piedmont of the Carolinas tended to lose their
spiral cords and costae as they ranged into the coastal plain, independently
developing the shell form associated with the subspecific nomen “dislocata”
[12].
In 2004 Thompson published a third edition of his “Freshwater
Snails of Florida,” this update strictly online [13]. From seven species of pleurocerid snails in
1984 and eleven in 1999, Thompson’s fresh online edition now recognized 12
species in Florida alone, the
“Choctawhatchee Elimia” formally identified as buffyae, the “Escambia Elimia”
now annae, and a new “Rasp Elimia” waiting in the wings, not as yet described. None of Steve Chambers’ papers whatsoever
were cited in Thompson’s 2004 online version.
Even the passing mention of Chamber’s 1980 paper had now disappeared.
So ten years ago, this was the situation: Two morphologically variable species, P.
proxima and P. catenaria, were understood to range from southern Virginia
through North Carolina and South Carolina to the Georgia border. At the Georgia border P. catenaria
disappeared, to be replaced by over 20 rare and endemic pleurocerid species,
each occupying single subdrainages, or in some cases single springs. Does that model seem biologically plausible
to you?
 |
| P. ucheensis is dislocata, the remainder are P. catenaria |
We did not include any bona fide curvicostata or dickinsoni
in our survey, because those two taxa seemed unambiguous to us. But the bottom line was that all the Georgia
populations we sampled were either floridensis (including induta and timida) or
catenaria (including albanyensis, boykiniana, darwini, mutabiliis, and
viennaensis). There are exactly four species of pleurocerid snails in Central Georgia,
Florida, and the Florida drainages of Alabama as hypothesized by Chambers:
curvicostata, dickinsoni, floridensis and (*a minor correction) catenaria, all boasting
extensive ranges. And the most
widespread of those species, Pleurocera catenaria (Say 1822) extends through
five states, from Virginia through the Carolinas and Georgia to mimic P.
floridensis at Ichetucknee Springs, bringing the discovery Steve Chambers first
reported in 1978 [15] into a properly regional context.
I didn’t expect Dr. Thompson to embrace the four-species
model in 2011 any more than he did in 1990.
His species concept was rooted in nineteenth-century typology, no
different from such giants of American malacology as Henry Pilsbry and F. C.
Baker. Indeed, distinction of species by
shell costations or spiral chords is not conceptually different from distinction
by individual nucleotide substitutions, as do many of our colleagues in the
present day. But I confess, when I saw
the species list published by P. D. Johnson and colleagues in 2013 [16],
including as it did albanyensis, annae, athearni, boykiniana, buffyae, clenchi,
darwini, exusta, glarea, inclinans, induta, mutabilis, taitiana, timida,
ucheensis, vanhyningiana, and viennaensis, I was just a little bit disappointed.
Notes:
[2] Chambers, S. M. (1990) The genus Elimia (= Goniobasis)
in Florida and adjoining drainage basins (Prosobranchia: Pleuroceridae) Walkerana 4: 237 – 270.
[3] Thompson, F. G. (2000)
An identification manual for the freshwater snails of Florida. Walkerana 10(23): 1 -96.
[4] Turgeon, D.D., J.F. Quinn, A.E. Bogan, E.V. Coan, F.G.
Hochberg, W.G. Lyons, P.M. Mikkelson, R.J. Neves, C.F.E. Roper, G. Rosenberg,
B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione, and G.D. Williams (1998)
Common and scientific names of aquatic invertebrates from the United States and
Canada: Mollusks (second edition), American Fisheries Society Special
Publication 26, Bethesda, Maryland, 526 pp.
[5] Mihalcik, E. L. (1998)
Elimia curvicostata species-complex within the river drainages of the
southeastern United States: Morphology, DNA, and biogeography. Ph.D. dissertation, University of Florida,
209 pp.
[6] Mihalcik, E. L. & F. G. Thompson (2002) A taxonomic revision of the freshwater snails
referred to as Elimia curvicostata, and related species. Walkerana 13: 1 - 108.
[7] Pleurocerid populations (and a variety of other
gastropod populations as well) often demonstrate “mitochondrial
superheterogeneity,” where two or more members demonstrate 10% sequence
divergence or greater for any single-copy mtDNA gene, not sex-linked. For more see:
- Mitochondrial superheterogeneity: What we know [15Mar16]
- Mitochondrial superheterogeneity: What it means [6Apr16]
- Mitochondrial superheterogeneity and speciation [3May16]
[8] The only population from which M&T sampled more than
one individual was their reference “flava” from Alabama, the two individuals
sequenced from which apparently demonstrating more than 20% sequence
divergence. They might have been the
first researchers ever to discover mtSH in the Pleuroceridae, had their eyes
been open. Dillon & Frankis didn’t formally
document the phenomenon until 2004 [PDF].
[9] Here’s something even more remarkable, if you stop to
consider it. The 15 populations M&T
identified as being members of the “Elimia curvicostata complex” (explicitly
excluding floridensis, dickinsoni, or boykiniana) apparently represented all
four of the Chambers species (curvicostata, floridensis, dickinsoni, and
boykiniana) about equally. Shows you
the value of shell morphology, doesn’t it?
[10] Dillon, R. T. and A. J. Reed (2002) A survey of genetic variation at allozyme
loci among Goniobasis populations inhabiting Atlantic drainages of the
Carolinas. Malacologia 44: 23-31. [PDF]
[11] Much more on
cryptic phenotypic plasticity in pleurocerid shell morphology:
- Goodbye Goniobasis, Farewell Elimia [23Mar11]
- Pleurocera acuta is Pleurocera canaliculata [3June13]
- Elimia livescens and Lithasia obovata are Pleurocera semicarinata [11July14]
- What subspecies are not [5Mar14]
[14] Dillon, R. T. and J. D. Robinson (2011) The opposite of speciation: Population
genetics of Pleurocera (Gastropoda:
Pleuroceridae) in central Georgia. American
Malacological Bulletin 29: 159-168.
[PDF]
[15] Chambers, S. M. (1978) An electrophoretically detected
sibling species of “Goniobasis floridensis” (Mesogastropoda;
Pleuroceridae). Malacologia 17: 157 –
162.
[16] Johnson, P. D. et al. (2013) Conservation status of
freshwater gastropods of Canada and the United States. Fisheries 38: 247 – 282. See:
- Plagiarism, Paul Johnson, and the American Fisheries Society [9Sept13]