One Goodrich Missed: The skinny simplex of Maryville is Pleurocera gabbiana

Editor’s Note.  This is the third installment in a three-part series on the discovery of a cryptic pleurocerid species in East Tennessee.  You might want to back up and read my essays of 13Sept16 and 14Oct16 if you haven’t already.  A more technical version of the present essay was published in the FMCS Newsletter Ellipsaria 18(3): 10 – 12 [pdf] if you’re looking for something citable.

Longtime readers may remember the 2007 essay I wrote on Calvin Goodrich [1], who monographed the North American pleurocerids in a series of spare papers published by the University of Michigan 1939 – 1944.  Goodrich synonymized scores of specific pleurocerid nomina without rationale, and simply neglected to mention many more.  And by the end of his six-year effort, what had been a jungle of 1,000 species of North American pleurocerids [2] had been bush-hogged down to a merely untidy 150.  While admitting that Goodrich’s scholarship might not have been the most intellectually satisfying, here’s what I wrote in 2007: 

 “And Goodrich's review has proven to be of great use to malacologists working in American freshwaters today.  My 25 years of research on the population genetics of pleurocerids in the South suggests to me that the total number of biological species in this country will prove to be far less than 500, and indeed less than 100. I haven't found a biological species that Calvin Goodrich missed.”

 So back in September I reported the discovery of two species of Pleurocera cryptic under the common and widely-distributed P. simplex in East Tennessee, “fats” and “skinnies.”  And in October, I matched the fat simplex to the type population in Saltville, Virginia.  Now what are those “skinnies?”

My first thought was to refer back to Goodrich’s (1940) paper covering that part of the world [3], to see if one of the names Goodrich synonymized under P. simplex might be resurrected.  There were only five such names: warderiana, subsolida, densa, vanuxemii, and prestoniana [4].

Goodrich’s papers were unfigured, alas, and so my next move was to pull my trusty copy of Tryon (1873) off the shelves and try to picture-match [5].  If some of the shells of those putative simplex-synonyms figured in Tryon had been skinny, and if their original authors had been so kind as to suggest a type locality, I suppose I would have been strongly tempted to visit that locality and fetch a batch home for genetic work.  But none of the figures in Tryon corresponding to any of the names in Goodrich looked skinny in the least.

Figure 1.   Holotype of Goniobasis gabbiana (center) is compared to exemplars from S1 of Indian Creek (left) and S5 topotypic P. simplex (right).  The scale bar in mm.  A = Apex height, B = Body whorl height.

So I put the question aside for a while, and diverted my attention to other questions.  The thought of describing a new species did cross my mind. But good grief, number 1,001?  Does North America need another nominal species of pleurocerid?

In 2012 the focus of the FWGNA project shifted from East Tennessee to the Middle Atlantic States, and I started paying calls to all the national collections in the big cities of the east.  So it must have been the third or fourth day of my visit to the National Museum of Natural History on Constitution Avenue, and I got getting tired of logging drawers full of Physa.  And I thought I might just have a peek at the type collection, probably more out of intellectual curiosity than anything else.  Isaac Lea described over 400 species of pleurocerids [6] between 1831 and 1869, and (I suppose) a large fraction of his types are housed at the USNM, pretty much all together in a set of locked cabinets off to the side.  And by chance, my eyes happened to fall on USNM 118991, the lectotype of Goniobasis gabbiana.

Isaac Lea first published a brief Latinate description of Goniobasis gabbiana in his (1862) description of the new genus Goniobasis, followed by a more complete (English) description and figure in 1863 [7]. His locality data were uselessly vague: “Tennessee Prof. G. Troost; Alabama Prof. Tuomey.” The species nomen was passed along as valid by Tryon (1873) but was essentially forgotten by Goodrich, who listed it as a “species in doubt” in his 1930 Alabama paper [8] but neglected it entirely in his 1940 review of the Pleuroceridae of the Ohio River drainage system [3], which covered East Tennessee. Thus, although still valid, Goniobasis (or “Elimia”) gabbiana was not included in the more recent reviews of Burch (1989) and Turgeon et al. (1998).  In addition to the lectotype, the USNM holds just two lots labeled Goniobasis gabbiana, both from the nineteenth- century malacologist A. G. Wetherby, neither matching the type. To my knowledge, no pleurocerid lots are held under the nomen gabbiana in any other North American collection.

Figure 2.  The Goniobasis gabbiana collection at the USNM.

In Figure 3, the holotype of P. gabbiana is plotted by its apex height (A) and body whorl height (B) on that graph of Saltville fats and Indian Creek skinnies I developed last month [9]. The match in relative shell dimensions between USNM 118991 and Pleurocera population S1 of Indian Creek would appear nearly perfect. The evidence thus suggests that Pleurocera population S1 at Indian Creek and (by inference) skinny population S6s at Pistol Creek may be identified as Pleurocera gabbiana (Lea 1862), a nomen here revived after 150 years of obscurity.

The fat simplex /skinny simplex situation contrasts rather strikingly with most of my previous experience working with the systematics of the North American Pleuroceridae [10].  Typically intraspecific shell variation is so extreme that nineteenth-century taxonomists have recognized multiple nominal species, and even multiple genera, within single conspecific populations. Here’s the opposite situation, where interspecific shell variation is so slight that twentieth-century taxonomists have not distinguished any differences.

Figure 3. Shell apex (A) as a function of body whorl (B) in P. simplex topotypic population S5 and in population S1 from Indian Ck.  Exemplar shells are noted with arrows, cross marks the lectotype of Goniobasis gabbiana.

Our subsequent field surveys have uncovered 58 additional populations apparently referable to P. gabbiana -- often mixed with typical P. simplex -- locally abundant in small streams of the Tennessee River drainage from the vicinity of St. Paul, Virginia, southwest perhaps 100 km to Madisonville, Tennessee [11]. As we mount expeditions to catalog the rich biodiversity perhaps lying undiscovered in the most remote corners of the earth, we might profitably pause to examine the less exotic but equally remarkable biodiversity we have too often overlooked in our own backyards.

Notes

[1]  The Legacy of Calvin Goodrich  [23Jan07]

[2] Graf, D. L. (2001)  The cleansing of the Augean Stables, or a lexicon of the nominal species of the Pleuroceridae of recent North America, North of Mexico.  Walkerana 12: 1 – 124.

[3] Goodrich, C. 1940. The Pleuroceridae of the Ohio River system. Occasional Papers of the Museum of Zoology, University of Michigan 417:1-21.

[4] John Robinson and I added a sixth synonym in 2007, aterina (Lea 1863).  See:

• Dillon, R. T., Jr. and J. D. Robinson. 2007. The Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey.  Report to the Virginia  Division  of  Game  and  Inland  Fisheries.  25 pp. [pdf]

[5] Tryon, G. W., Jr. 1873. Land and Freshwater Shells of North America. Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253, 435 pp. Washington, D.C

[6] By my actual count, paging through Graf [2], Isaac Lea described 438 species of North American pleurocerids.  Not counting all the spelling errors and similar screw-ups by subsequent authors.

[7] Lea, I. (1862) Description of a new genus (Goniobasis) of the family Melanidae and eighty-two new species. Proceedings of the Academy of Natural Sciences, Philadelphia 14:262-272.  Lea, I. (1863) New Melanidae of the United States. Journal of the Academy of Natural Sciences, Philadelphia 5:217-356.

[8] Goodrich, C. 1930. Goniobases of the vicinity of Muscle Shoals. Occasional Papers of the Museum of Zoology, University of Michigan 209:1-25.

[9] The Fat Simplex of Maryville Matches Type [14Oct16]

[10] Papers documenting the situation where high levels of shell phenotypic variation have fooled previous authors into recognizing multiple species of pleurocerids where only one exists: Dillon, R. T., Jr. 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].  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].  Dillon, R. T., Jr., S. J. Jacquemin and M. Pyron.  2013.   Cryptic phenotypic plasticity in populations of   the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709:117-127 [pdf].  Dillon, R. T., Jr. 2014. Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata. Zoological Studies 53:31 [pdf].

[11] A detailed map comparing the distributions of P. simplex and P. gabbiana in East Tennessee and SW Virginia is downloadable from the FWGNA site.  [pdf map]