Is Gyrotoma Extinct?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Is Gyrotoma extinct?  Pp 113 - 116 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

It is an article of faith in the small and closely knit community of freshwater gastropod conservation biology that the Mobile Basin is home to the highest levels of freshwater molluscan biodiversity in North America [1].  According to a 1997 estimate [2], the basin (at one time) hosted 118 species of freshwater gastropods, 32% of which are now extinct, 89% of the remainder warranting conservation concern.  These numbers are not unquestioned, however.  We have devoted two series of essays, one in 2009 [3] and a second in 2016 [4] to questioning them.

A prominent fraction of the 38/118 = 32% extinction figure quoted above are all species of the nominally-endemic pleurocerid genus Gyrotoma.  Goodrich [6,7] recognized 13 species of Gyrotoma, historically ranging down the Coosa River from Greensport (under the present-day Neely Henry Lake) to Wetumpka, a total river distance of about 200 km for the 13 combined.  Goodrich’s 13 species were boiled down to six by Burch [8]: pyramidatum, pagoda, pumilium, lewisii [10], walkeri, and excisum. 

The genus is distinguished by a notch or slit or “fissure” at the posterior edge of the shell aperture, unique in the family Pleuroceridae.  See the side view of a Gyrotoma lewisii shell scanned from Goodrich [6] below, labelled #17.

Well, perhaps the Gyrotoma fissure might better be described as “sort-of unique,” or maybe “uniquish.”  Goodrich [6] wrote: 

“The affinities of Gyrotoma are with certain Goniobases which should be separated from that genus.  These mollusks… have the same wide aperture of Gyrotoma and the same microscopic sculpture.  The group has not been carefully studied, but these species unquestionably belong to it: Goniobasis impressa Lea, laeta Jay, showalterii Lea (1860), lewisii (Lea 1861) [10], bellula Lea and ovalis Lea…. Occasionally all these species develop incipient fissures.  Mr. Smith collected several specimens with fissures nearly as large as in pyramidatum and incisum and yet, in other regards, retaining their usual Goniobasic features.”

Goodrich went on to postulate that Gyrotoma pyramidatum, pagoda, and pumilium (among others) “appear to have developed from Goniobasis laeta,” and that Gyrotoma lewisii has “relations” with Goniobasis impressa that were “quite plain” to him, as indeed they were to Isaac Lea in 1869.  The hypothesis that members of a single genus might have independent origins in two different species did not, apparently, raise eyebrows in 1924.

CPP in "Gyrotoma"

The figure above compares Goodrich’s figures of Gyrotoma lewisii (16, 17) to Goniobasis impressa (18).  Goodrich [11] gave the range of Goniobasis impressa, now also extinct [12], as the Coosa River from Ten Island Shoals to The Bar, which today would extend from just below the Neely Henry Dam downstream into Lay Lake, roughly 120 km.  The range of Gyrotoma lewisii was localized around Fort William Shoals, now submerged under the waters of Lay Lake.  The striking feature of both nominal species, apparently quite plain to both Lea and Goodrich, are the fine spiral cords parallel and closely packed around the entirety of both shells, apex to aperture.

I have also added an image of my “Goniobasis WTF3” pleurocerid to the figure above, transferred from the essay posted on this blog 13Nov09, see note [3] below.  Long-time readers may remember that I discovered WTF3 together with a clavaeformis-type pleurocerid and a catenaria-type pleurocerid in Dykes Creek, a small tributary of the Coosa about 100 km upstream from the Neely Henry Dam.  

Is the relationship between WTF3 and Goniobasis impressa as plain as the relationship between Goniobasis impressa and Gyrotoma lewisii?  Might the Gyrotoma lewisii /Goniobasis impressa/ WTF3 continuum represent another case of cryptic phenotypic plasticity?  Do populations of the nominal genus “Gyrotoma,” long thought extinct, still survive in small upstream tributaries of the Coosa River system today, unrecognized by anybody?

Notes

[1] Whelan, N.V., P.D. Johnson, and P. M. Harris (2012)  Rediscovery of Leptoxis compacta (Anthony, 1854) (Gastropoda: Cerithioidea: Pleuroceridae).  PLoS One 7(8): e42499.  [html]

[2] Neves RJ, Bogan AE, Williams JD, Ahlstedt SA, Hartfield PW (1997) Status of aquatic mollusks in the southeastern United States: a downward spiral of diversity. Pp 43 – 85 In: Benz G, Colling D, editors. Aquatic Fauna in Peril: The Southeastern Perspective. Chattanooga, Tennessee: Southeast Aquatic Research Institute.

[3] Note that my 2009 series was published after I coined the term “Goodrichian Taxon Shift” to describe my 2007 observations on pleurocerid intergradation in East Tennessee, but before I formally synonymized Goniobasis under Pleurocera in 2011:

• Mobile Basin I: Two pleurocerids proposed for listing [24Aug09]
• Mobile Basin II: Leptoxis lessons [15Sept09]
• Mobile Basin III: Pleurocera puzzles [12Oct09]
• Mobile Basin IV: Goniobasis WTFs [13Nov09]

[4] This series was prompted by the remarkable paper of Whelan & Strong [5]:

• Mitochondrial superheterogeneity: What we know [15Mar16]
• Mitochondrial superheterogeneity: What it means [6Apr16]
• Mitochondrial superheterogeneity and speciation [3May16]
• The shape-shifting Pleurocera of North Alabama [2June16]
• Pleurocera clavaeformis in the Mobile Basin? [12July16]

[5] Whelan, N.V. & E. E. Strong (2016) Morphology, molecules and taxonomy: extreme incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae). Zoologica Scripta 45: 62 – 87.

[6] Goodrich, C. (1924) The Genus Gyrotoma.  Misc. Publ. Mus. Zool. Univ. Mich. 13: 1 – 32.

[7] Goodrich, C. (1944) Pleuroceridae of the Coosa River basin. Nautilus 58: 40 – 48.

[8] I am mildly irritated by this, but only mildly.  In his endnote #28 Burch [9] cited Goodrich [7] extensively as a rationale for synonymizing seven of the 13 nominal Gyrotoma species: alabamensis, amplum, cariniferum, hendersoni, incisum, laciniatum, and spillmani.  On the one hand, Burch really didn't have any fresh biological information to justify such an act.  But on the other hand, he had a point.  He probably didn’t go far enough.

[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] Note that Isaac Lea described four pleurocerid species in honor of Dr. James Lewis: Anculosa (Leptoxis) lewisii (Lea 1861), Melania (Goniobasis) lewisii (Lea 1861), Trypanostoma (Pleurocera) lewisii (Lea 1862), and Schizostoma (Gyrotoma) lewisii (Lea 1869).  Both the Gyrotoma lewisii and the Goniobasis lewisii are referred to (separately) in the essay above.

[11] Goodrich, C. (1936) Goniobasis of the Coosa River, Alabama. Misc. Publ. Mus. Zool. Univ. Mich. 31: 1 – 60.

[12] 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.