Editor’s Note – This essay was subsequently published as:
Dillon, R.T., Jr. (2023b) CPP Diary:
What is Pleurocera (AKA Melania, AKA Goniobasis, AKA Elimia) ebenum? Pp 17 – 24 in The Freshwater Gastropods of
North America Volume 6, Yankees at The Gap, and Other Essays. FWGNA Project, Charleston, SC.
Looking back on my long and checkered career as a malacologist,
I think I first developed my mental image of Pleurocera ebenum on a brief side
trip to the Falls of the Cumberland in the summer of 1988. There in the Kentucky State Park, above the
falls only – not below – I found the rocks covered with pale, large-bodied
pleurocerids bearing heavy, triangular shells that I assumed must be P. ebenum
[1]. See the example at far right below.
Well, they certainly matched Figure 370 [2] in Burch [3],
second from left below. And Burch must
have Goodrich’s collections at his fingertips for reference, yes? And Goodrich [4] specifically listed
“Cumberland River above the Falls” in the range of Goniobasis (aka Elimia, aka
Pleurocera) ebenum, yes?
 |
| From left: Lea's original figure of M. ebenum [9], Tryon from Burch [2], Branson [6], fresh from the Falls of The Cumberland |
So the FWGNA Project began our initial surveys of the Cumberland drainages about ten years ago, and in the last couple years, as we have
sewed up the Green River and the Kentucky River further north, The Cumberland
has increasingly come into focus.
Branson [5] promised us that P. ebenum would be “common” in Kentucky,
publishing the (rather shabby) figure 27 third from left in the figure
above. And this is the description
Branson provided in his key to identify it [6]:
“Upper whorls without carinae on periphery of whorls; shell obtusely conical, smooth, spire relatively short; aperture often with purplish tinge within; Cumberland River system”
So indeed, as our survey has progressed, we have found
populations of pale, large-bodied pleurocerids bearing heavy shells “obtusely
conical, smooth, spire relatively short” in mid-sized rivers and streams all
over the Bluegrass State, and Middle Tennessee as well.
And almost everywhere we found populations of P. ebenum, we
discovered smaller, darker-bodied populations of Pleurocera simplex in small
creeks upstream, bearing gracile shells with higher, more slender spires,
convex even to the point of being teardrop-shaped in their outline. Scroll down to Figure J at the bottom of last month’s
post to see a typical example. Upon
careful study, we simply have not been able to draw a line between simplex
populations upstream and ebenum populations downstream, anywhere. It has become apparent that the
simplex/ebenum relationship must be yet another case of cryptic phenotypic
plasticity (CPP), and that ebenum (Lea 1841) is best considered a subspecies
[7] of simplex (Say 1825), following the model laid down for us in the Duck
River Lithasia by Goodrich [8].
It was only very recently, quite late in the entire process,
that I actually read Isaac Lea’s 1843 description [9] of Melania [10] ebenum:
“Shell smooth, obtusely conical, thick, black; spire obtuse; sutures small; whorls somewhat convex; aperture rather large, ovate, subangular at base, within purplish”
The “habitat” Lea gave surprised me, a bit: “Robinson
County, Tenn. Dr. Currey.” Tennessee has
no constituent “Robinson” County, but a Robertson County was organized in 1796
about ten miles north of Nashville. The
county lies on a plateau draining west to the Red River, entering the
Cumberland at Clarksville, about 65 miles downstream from Nashville. There isn’t much mid-sized riverine habitat
especially suitable for P. ebenum in Robertson County.
I was also surprised by Lea’s remarks that his M. ebenum
were “very dark-coloured” with “convex” whorls and “usually purplish on the
whole of the inside of the aperture.”
Those characters sound much, much more like typical Pleurocera simplex
to me.
 |
| M = Springfield, N = Tyree Springs, O = Shackle Island |
So the evening of 15May19 found me checking into a cheap
motel in Goodlettsville, north of Nashville, with big plans to find Isaac Lea’s
type locality on the morrow. The “Dr.
Currey” referred to by Lea must have been Dr. Richard Owen Currey (1816 – 1865),
the prominent Nashville physician impeccably credentialed with a Presbyterian
heritage and University of Pennsylvania degree.
So, my plan was to survey the rivers and streams of southern and eastern
Robertson County first, closest to Nashville, then expand my search area north
and west as necessary.
The morning of 16May19 dawned crisp and clear. And I enjoyed lovely field conditions
surveying the pretty little springs and streams of southern Robertson County,
taking three good swings, striking out.
So I headed west, and on my second at-bat, made solid contact in
Springfield, the county seat (M, above).
I found three species of pleurocerids inhabiting Sulfur Fork at the US
431 bridge in Springfield, although none common: P. laqueata, P. troostiana,
and (sure enough) P. simplex of triangular, heavily-shelled ebenum morphology [11]. Could this be Isaac Lea’s type locality? If so, the subspecies hypothesis would seem
justified.
The only misgivings I continued to harbor about the
population at Springfield, however, were that the animals, in both shell and
body, were pale brown, with white apertures, as (I have always assumed) typical
for ebenum. They were not the “very
dark-colour” specified by Lea, nor did they sport “purplish” apertures nor
“convex” whorls. And Springfield is 30
miles north of Nashville. Is that too
far afield in 1841?
In any case, I turned my pickup back toward Nashville,
with other field objectives now rising higher on my agenda. And my route happened to take me south down
TN 258, by an historic marker for Tyree Springs. And there, at Site N, I stumbled upon a very
attractive nominee for the type locality of Melania ebenum.
All I know about the history of the place you can read for
yourself on the sign (click for larger).
There doesn’t seem to be any evidence of a resort at the site today –
just second-growth forest and the little stream that runs under the road at far
left in the photo above. And the rocks
of that little stream are covered with pleurocerids bearing thick, black
shells, with obtuse spires, small sutures, whorls somewhat convex, and
apertures rather large, ovate, and within purplish [12]. In other words, absolutely typical P.
simplex. If Tyree Springs is (indeed)
the type locality of Lea’s ebenum, the simple-synonym hypothesis must prevail.
Tyree Springs is a comfortable day trip via horse-and-buggy
from Nashville. It is easy to imagine
that a prominent physician such as Dr. Currey would be familiar with such a
place in 1841. There is just one
problem. Tyree Springs are not located
in “Robinson” County nor even in Robertson County. The springs are located in Sumner
County. The Robertson County line is
another couple miles north up the ridge.
 |
| Topotype nominees: (M) Springfield, (N) Tyree Springs, (O) Shackle Island |
Tyree Springs drain south, through Drakes Creek into the
Cumberland River above Nashville. So more
out of intellectual curiosity than anything else, I drove downstream to sample
Drakes Creek at the little town of Shackle Island [13], marked O on the map
above. And there I found a sparse
population of pleurocerids bearing the paler, heavier shells with flat whorls
and white apertures I have always considered typical of the ebenum form. The Drakes Creek population of P. simplex,
from headwaters to mouth, demonstrates a textbook case of cryptic phenotypic
plasticity.
As I pointed my pickup back toward home on the evening of
16May19, I admit to a bit of frustration.
I wasn’t any closer to answering my questions about Pleurocera ebenum
than I had been at the dawn of that long, lovely day. In fact, the picture was murkier.
And then it occurred to me that the taxonomic situation
mirrored rather beautifully the biological situation. The taxonomic validity of Lea’s (1841) nomen
ebenum is precisely as clear as the morphological distinction of those
pleurocerid populations to which the name has subsequently been applied.
In the end, I resolved to follow the rule of Sunday
afternoon. The subspecies hypothesis as
forwarded to us by Tryon, Goodrich/Burch, and Branson is the call on the
field. Against which there is not enough
evidence to overturn.
Notes
[1] Actually, the Pleurocera population at the Falls of the
Cumberland is a mixture of P. ebenum and P. semicarinata. And the two species are difficult to
distinguish here.
[2] The great
majority of the figures in Burch [3] are obviously from the pen of Mr. John
Tottenham, an accomplished scientific illustrator specifically engaged for the
project. But one of the mysteries of
Burch’s work is why many of his pleurocerid figures, in particular, were
borrowed from elsewhere. Burch
reproduced his figure of “Elimia” ebenum from Tryon’s (1865-66) Monograph of
the Family Strepomatidae.
[3] 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).
[4] Goodrich, C. (1940) The Pleuroceridae of the Ohio River
drainage system. Occasional Papers of the Museum of Zoology, University of
Michigan 417: 1 - 21.
[5] Branson, B.A. and D.L. Batch (1987) Distribution of aquatic snails (Mollusca:
Gastropoda) in Kentucky with notes on fingernail clams (Mollusca: Sphaeriidae:
Corbiculidae) Trans. Ky. Acad. Sci. 48:
62 – 70.
[6] Branson, B. A. (1987)
Keys to the aquatic Gastropoda known from Kentucky. Trans. Ky. Acad. Sci. 48: 11 – 19.
[7] I would be most gratified if you purchased my FWGNA Volume III
(Prosobranchs) [html] and read pp 77 – 91.
Or you might certainly take the easy route by clicking:
[8] Goodrich, C. (1934) Studies of the gastropod family
Pleuroceridae - I. Occasional Papers of the Museum of Zoology, University of
Michigan 286:1 - 17. For more, see:
- CPP Diary: The spurious Lithasia of Caney Fork [4Sept19]
[9] Isaac Lea published his brief, Latinate description of
Melania ebenum in 1841: “New Fresh Water and Land Shells,” Proceedings of the
American Philosophical Society 2: 11 – 15.
That paper he followed with a larger paper, offering more complete
English descriptions, in 1843: “Description of New Fresh Water and Land Shells”
Transactions of the American Philosophical Society 8: 163 – 250.
[10] Isaac Lea originally described ebenum in the genus
Melania. He created his genus Goniobasis
in 1862, formally transferring ebenum and scores of additional pleurocerid
species into it. I subsumed Goniobasis
under Pleurocera in 2011. For more, see:
- Goodbye Goniobasis, Farewell Elimia [23Mar11]
[11] Perceptive readers might note that in Springfield, a
pale, robust, heavily-shelled population of the ebenum form seems to inhabit a
mid-sized stream without a corresponding dark, lightly-shelled simplex
population upstream. Maybe there is, or
was, and I missed it? Or maybe the
Springfield ebenum population has colonized Sulfur Fork from below?
[12] In the interest
of full disclosure, it looks to me as though at least 85% of the pleurocerid
population at Tyree Springs are smallish, dark Pleurocera laqueata, and another
10% or so are smallish, dark P. troostiana.
And the apical sculpture on the shells of those two populations is not
strong, in many cases. And even I had a
hard time distinguishing the Tyree simplex from the laqueata. But I refuse to entertain the possibility
that Lea’s ebenum might be a junior synonym of Say’s (1829) laqueata or
anything else. Not gonna do it. Wouldn’t be prudent.
[13] To be only slightly more precise. My site (O) was located in Drakes Creek at the
perfectly-manicured campus of the most grotesque megachurch I have ever
seen. I counted a staff of 18
sweetly-smiling pastors and 3 directors on their website, not counting the
office support, building and grounds staff, rock musicians and stagehands.
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