Pleurocera acuta is Pleurocera canaliculata

Editor's Note - This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Pleurocera acuta is Pleurocera canaliculata.  Pp 65 - 70 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs.  FWGNA Press, Charleston.

I am pleased to report that the second paper in what I expect will be a continuing series on Goodrichian taxon shift in the North American Pleuroceridae has recently reached formal publication [1].  In this most recent installment, the team of Dillon, Jacquemin and Pyron uses a combination of genetic and shell morphometric techniques to demonstrate that the specific nomina acuta (Rafinesque 1824) and pyrenellum (Conrad 1839) are junior synonyms of Pleurocera canaliculata (Say 1821).  As “Pleurocerus acutus” was designated the type of the genus in ICZN Opinion #1195 of 1981 [2], this finding will be of more than the usual taxonomic interest.

Our analytical approach was patterned after that used by Dillon (2011) on the Pleurocera clavaeformis populations of East Tennessee [3].  We used allele frequencies at nine polymorphic allozyme loci to show that two populations of nominal P. acuta (from Indiana and Kentucky) and one population of nominal P. pyrenellum (from north Alabama) were each more genetically similar to the P. canaliculata population immediately downstream than any of the six populations was to any nominal conspecific.  We then used landmark-based morphometics to explore one of these “Goodrichian” shifts in greater detail, the historically important acuta-to-canaliculata transition found in the Wabash River of Indiana.

On at least two occasions Calvin Goodrich himself expressed doubts about the distinction between P. acuta and P. canaliculata, specifically mentioning the Wabash populations in 1937 [4].  So we borrowed 18 lots of historically-collected Wabash Pleurocera from the University of Michigan Museum of Zoology [5] and documented a significant correlation between shell robustness (as measured using relative warp analysis) and river size at point of collection, as estimated by catchment area.  The seven museum lots bearing more slender, high-spired shells (collected above Wabash River kilometer RK500) were identified by Goodrich as acuta, and the six lots bearing broader, heavier shells collected downstream from RK400 were identified as canaliculata.  Goodrich recognized a mixture of the two nominal species in the five lots collected in the 100 km immediately downstream from RK518, where the Tippecanoe River joins the Wabash, essentially doubling the catchment area (sites 8 - 12 in the map below).

Situations such as we document in the RK400-500 region probably explain why Calvin Goodrich, the man for whom Goodrichian taxon shift was named, did not synonymize acuta under canaliculata himself back in 1939.  The two nominal “species” sometimes seem to mix and retain a degree of distinctiveness.  The phenomenon appears even more dramatically in places where very small streams communicate directly with large mainstem rivers, such as that marked “12” in the map below.  It is not unusual to find snails bearing very slender, high-spired shells washed down into much broader, more robustly-shelled populations inhabiting the main river at spots such as this, looking for all the world like a pair of bona fide, reproductively-isolated species.

Way back in 2007 I defined "Goodrichian Taxon Shift," as “intraspecific variation in freshwater gastropod shell morphology along an environmental cline of such magnitude as to prompt the erroneous recognition of multiple nominal taxa” [6].  The phenomenon was conceived as narrowly applicable to freshwater snails, as Calvin Goodrich himself focused his own research.

But after some soul-searching, my colleagues and I have decided to broaden the concept and coin a new term for it.  So in our paper just published, we propose the term “cryptic phenotypic plasticity,” which we define as “intrapopulation morphological variation so extreme as to prompt an (erroneous) hypothesis of speciation.”  We think that the sort of insight Calvin Goodrich brought to the study of pleurocerid snails in the 1930s and 1940s has the potential to make a contribution to the understanding of evolutionary processes beyond our small community of freshwater gastropod cognoscenti, even in the present day.

Goodrich himself was strongly influenced by the research of A. E. Ortmann on unionid mussels.  Ortmann described a correlation between river size and shell robustness way back in 1920, on the basis of which he sank quite a few unionid taxa into synonymy [7].  And Ortmann credited the idea to Wilson & Clark (1914) [8].  So in some sense it might not be fair to continue to call the phenomenon “Goodrichian” anything, as we forward cryptic phenotypic plasticity onward to evolutionary science as a whole.

The extent to which cryptic phenotypic plasticity may occur in the biotas of land and sea remains to be seen, however.  It seems unlikely to me that marine and terrestrial environments manifest themselves as variably to the populations that inhabit them as fresh waters, at least for the molluscan fauna with which I have any professional familiarity.  Were the Nucella (“dog whelk”) populations of the Pacific intertidal an (historic) example of cryptic phenotypic plasticity?  What component of the baroque taxonomy of Cerion populations in the Bahamas might be attributable to ecophenotypic plasticity in their shells [9]?  Our colleagues with expertise in environments such as these are hereby invited to take notice.

Notes

[1] Dillon, R. T., Jr., S. J. Jacquemin & M. Pyron (2013)  Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata.  Hydrobiologia 709: 117-127.  [html] [pdf]

[2] Joe Morrison and the Great Pleurocera Controversy [10Nov10]

[3] Dillon, R. T., Jr. (2011)  Robust shell phenotype is a local response to stream size in the genus Pleurocera.  Malacologia 53: 265-277.  [pdf]

• Mobile Basin III: Pleurocera Puzzles [12Oct09]
• Goodbye Goniobasis, Farewell Elimia [23Mar11]

[4] Goodrich, C. (1937)  Studies of the gastropod family Pleuroceridae – VI.  Occas. Pprs. Mus. Zool. Univ. Mich. 347: 1 – 12.

Goodrich, C. (1939)  Pleuroceridae of the St. Lawrence River basin.  Occas. Pprs. Mus. Zool. Univ. Mich. 404: 1 – 4.

[5] The UMMZ was Goodrich’s home institution from 1924 to 1944.  All the pleurocerid lots curated during this period would have been identified by him, if not collected by him personally.  See “The Legacy of Calvin Goodrich.” [23June07]

[6]  Goodrichian Taxon Shift [20Feb07]

[7] Ortmann actually did this backwards.  In his (1918) “The Nayades of the Upper Tennessee Drainage, with Notes on Synonymy and Distribution” (Proc Am Phil Soc 57: 522) he wrote, “A large number of the ‘species’ described by Lea… and of those listed by Lewis… and subsequently described by various authors…are mostly synonyms.”  And as evidence he cited, “a rule…that one and the same shell assumes different shapes in the large rivers and in small streams and headwaters, a rule the existence of which will be shown elsewhere.”  Then in 1920 Ortmann published his “Correlation of shape and station in freshwater mussels (naiades)” (Proc Am Phil Soc 59: 269-312).

[8]  Wilson, C. B. & H. W. Clark (1914)  The mussels of the Cumberland River and its tributaries.  Bur. of Fisheries 781: 1 – 63.

[9]  I thought Woodruff & Gould pretty much answered this question back in the 1980s (eg, Biol. J. Linn. Soc 14: 389-416).  But Harasewych (Nautilus 126: 119-126) seems not to have been paying attention.