Dr. Rob Dillon, Coordinator

Tuesday, September 15, 2009

Mobile Basin II: Leptoxis Lessons

Editor's Note.  This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Mobile Basin II: Leptoxis legacy.  Pp. 27-33 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

Through the early history of American malacology, the freshwater gastropod fauna of the Mobile Basin was not seen as exceptionally diverse. Isaac Lea, T. A. Conrad, and J. G. Anthony described about 20 species of Leptoxis (1) from the Mobile Basin from 1834 - 1860, for example, which Tryon (1873) boiled down to about 12 (2). This would not seem to be a significantly greater tally than the approximately 20 species of Leptoxis described from the Tennessee River system during that era.

Indeed, the apparent diversity of the Mobile Basin mollusk fauna may not so much be a consequence of evolutionary history, but rather history of a very human sort. It is best, perhaps, understood as the story of my professional hero, Calvin Goodrich (3).

Some of my audience may recall that Calvin Goodrich began his career as a newspaperman, and that he was influenced by A. E. Ortmann to take up malacology around the years 1913 - 1917. One of Ortmann's closest professional colleagues was Herbert H. Smith, who retired in 1903 from the Carnegie Museum in Pittsburgh to become curator of the Alabama State Museum in Tuscaloosa. When H. H. Smith died in 1919, a review of Smith's collections of Alabama pleurocerids was passed by Ortmann to Goodrich.

"The Anculosae of the Alabama River Drainage" was Calvin Goodrich's first substantial contribution to science (4). It is essentially a tribute to Smith (and indirectly to Ortmann), adding 11 new H. H. Smith nomina to a list of 14 transferred forward from Conrad, Lea, and Anthony. Goodrich described one new species himself, Anculosa smithi (named for guess who), bringing the total inventory up to 26 Leptoxis for the drainage. A concatenation of Goodrich's Plates I and II, illustrating all 26 species (5), is shown at left (and click to enlarge).

This work long predated Goodrich's (1934-41) "Studies on the Pleuroceridae," in which our hero came around to an understanding of the ecophenotypic nature of the shell morphological characters upon which pleurocerid taxonomy has historically been based. It also (of course) predated his (1939-1944) reviews of the Pleuroceridae of North America, in which Goodrich synonymized a huge fraction of the old 19th century taxonomy, combining (for example) the 26 nomina of Leptoxis (1) described from the Ohio/Tennessee basin down to nine (6). But apparently the old newspaperman could not edit his own copy. When he came back to the "Pleuroceridae of the Coosa River Basin" twenty years later (7), he preserved 18 Leptoxis species, saying, "A re-examination in 1943 has confirmed most of the decisions of 1922" (8).

By the 1940s, however, Calvin Goodrich's editorial skills were of little consequence to the Mobile Basin Pleuroceridae. Because as early as 1914 the first of seven dams was closed on the Coosa River, covering miles of the rocky rapids which were the habitat of Leptoxis with silty slackwater. And over the next 53 years, as the Coosa was almost completely impounded by Alabama Power, the Corps of Engineers was improving navigation on the Alabama River downstream with channelization, locks and dams.

Entering the 1990s, Leptoxis populations were believed to have survived in only four regions of the Mobile Basin - one tributary of the Black Warrior River (7), the upper reaches of the Cahaba River, the lower reaches of three mid-sized tributaries of the Coosa, and (amazingly) the main Alabama River downtream from the Claiborne Lock and Dam. Perhaps not surprisingly, conventional wisdom accorded snails from these four regions four different specific nomina - Leptoxis plicata, L. ampla, L. taeniata, and L. picta (respectively). The first three of these species were approved as candidate species in 1995, and after study and comment, entered the Federal lists in 1998 - L. plicata as endangered, L. taeniata and L. ampla as threatened (9). Leptoxis picta was not considered for listing.

We interrupt this extended history lesson for a brief spasm of science. I was sent large samples of all four of these nominal species (8 populations, 30 individuals per population) by our colleague Chuck Lydeard in 1996. The sample of L. picta I received was of larger, older, and more heavily-shelled animals (10), but otherwise no morphological difference between L. picta, L. ampla, and L. taeniata was apparent. My sample of Leptoxis plicata was distinctive by their higher-spired shells, bearing slight carination.

I did a proper allozyme study, comparing divergence at 9 loci among the 8 populations to 3 populations of Leptoxis praerosa, the common and well-characterized species widespread in Tennessee drainages to the north (11). The levels of genetic divergence among the picta, ampla, and taeniata populations were negligible. So it was quite clear, as of 1996, that all the Leptoxis populations known to have survived in the Alabama/Coosa River system were conspecific, L. picta being the oldest name for the group. Leptoxis plicata (of the Tombigbee/Black Warrior system) would appear to be a valid biological species.

So in summary, two of the three nominal species of Leptoxis currently on the US endangered species list are junior synonyms of a third species, which is not listed. What lessons can be learned from this mess?

Once again we see a vivid demonstration that science and public policy are two entirely different things. Most of you have heard me preach this sermon before, so I won't preach it again (12). Scientists have a language, culture, value system, and assumptions about the world that are completely different, no better or worse, from the language, culture, values, and assumptions brought by politicians, lawyers, and the natural resource managers who put laws into practice.

The adjective "endangered" is not scientific - it can't be measured, quantified, verified, or falsified. A designation of endangerment is the result of a political process, and by that process Leptoxis plicata (for example) is endangered and Leptoxis picta is not.

Now we read that yet another nominal species of Leptoxis from the Mobile Basin has been proposed for protection under the US Endangered Species Act (13). Field surveys undertaken in the upper Coosa in the late-1990s led to the discovery of a pleurocerid population in the Oostanaula River of North Georgia that has been identified as Leptoxis downei or L. foremani. In the last ten years, this population has become the target of an extensive recovery effort, our colleague Paul Johnson initiating a captive propagation program at the Tennessee Aquarium Research Institute in 2000, which he carried with him to the Alabama Aquatic Biology Center in 2005 (14).

But is Leptoxis foremani a valid biological species? Tryon synonymized foremani under L. picta in 1873, and the taxon was only resurrected by Goodrich in 1922 on the slenderest of threads (15). So given the tortured history of Leptoxis systematics in the Mobile Basin through 150 years, are we surprised that so much time, money and effort has been spent on conserving "L. foremani," while no effort whatsoever has apparently been directed toward establishing its biological reality? Nope. Science and public policy are two entirely different things.


(1) I'm going to fight the urge to digress into a discussion of the genus-level taxonomy here. Maybe one day soon. In the mean time, for the purpose of this essay, I'm lumping Anculosa and Nitocris/Mudalia together under Leptoxis.

(2) Tryon G. W. (1873) Land and Freshwater Shells of North America. Part IV, Strepomatidae. Smithsonian Miscellaneous Collections 253: 1 - 435.

(3) "The Legacy of Calvin Goodrich" See my Post of January '07.

(4) Goodrich, C. (1922) The Anculosae of the Alabama River Drainage. Misc. Publ. Univ. Mich Mus. Zool. 7: 1-57.

(5) The suviving taxa depicted on Goodrich's (1922) Plates I and II are as follows: Figs 3 - 5 are L. ampla, Figs 18-19 are L. foremani, Figs 34 - 35 are L. picta, Figs 36-38 are L. plicata, Figs 46 - 49 are L. taeniata.

(6) Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage system. Occas. Pprs. Mus. Zool. Univ. Mich 417:1 - 21.

(7) Here's another digression I hate to take. The Mobile Basin is composed of two halves - the Alabama/Coosa and the Tombigbee/Black Warrior. Most of the literature we are reviewing here is for subsets. Goodrich's (1944) "Pleuroceridae of the Coosa River Basin" would include just a geographical (not taxonomic) subset of his (1922) "Anculosae of the Alabama River," for example, and neither work would include populations like L. plicata of the Black Warrior. A map of the Mobile Basin is available [here].

(8) Goodrich, C. (1944) Pleuroceridae of the Coosa River basin. Nautilus 58: 40-8.

(9) Endangered status for three aquatic snails, and threatened status for three aquatic snails in the Mobile River Basin of Alabama. 63 FR 57610-57620 [PDF]

(10) Goodrich himself prominently noted the relationship between river size and pleurocerid shell morphology on many occasions. See my post of February '07, "Goodrichian Taxon Shift." We will return to this subject very soon.

(11) Dillon, R.T., and C. Lydeard (1998) Divergence among Mobile Basin populations of the pleurocerid snail genus, Leptoxis, estimated by allozyme electrophoresis. Malacologia 39: 111-119. [PDF]

(12) For example, "Idaho Springsnail Panel Report" (December '05), or "Red Flags, Water Resources, and Physa natricina." (March '08).

(13) Mobile Basin I: Two pleurocerids proposed for listing. Post of August '09.

(14) Interrupted Rocksnail Reintroduced to the Coosa River. Outdoor Alabama, February 2004, p. 33 [PDF]

(15) "In shell characters this species (L. foremani) is closer to A. picta Conrad than is A. formosa Lea. But while the operculum of picta and formosa are much alike, that of foremani is like the operculum of neither. The similarity of the shells of formosa and foremani, picta out of consideration, varies strangely with locality, the resemblances and differences seeming to play a game of see-saw as the collector travels down the Coosa River." (Goodrich 1922: 18).

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