Zoological Studies

Vol. 53, 2014

Cryptic phenotypic plasticity in populations of the North American freshwater gastropod, Pleurocera semicarinata

Robert T Dillon Jr

Department of Biology, College of Charleston, Charleston, SC 29424, USA

Background: Phenotypic plasticity is termed ‘cryptic’ when it becomes so extreme as to prompt an (erroneous) hypothesis of speciation. Populations of pleurocerid snails nominally identified as Pleurocera (‘Goniobasis’ or ‘Elimia’) livescens are common elements of the macrobenthos in rivers and on lakeshores from Vermont through northern Ohio and northern Indiana to Wisconsin, USA. Small streams in southern Ohio, southern Indiana, and Kentucky are inhabited by Pleurocera semicarinata, and larger rivers by Lithasia obovata. The three nominal species differ only in qualitative aspects of shell morphology - P. semicarinata demonstrating a slender shell with a small body whorl,
L. obovata a robust shell with a large body whorl, and P. livescens intermediate.

Here I use allozyme electrophoresis to estimate genetic divergence over 11 enzyme loci among three populations of P. livescens, two populations of P. semicarinata, and one population of L. obovata sampled across 650 km of their combined range. With each of these six populations I sample a control population of Pleurocera canaliculata, demonstrating that the genetic divergence among the six known conspecific populations is comparable to that observed among the six study populations.
Conclusions: The specific nomina L. obovata and P. livescens would appear to be junior synonyms of P. semicarinata. The shell morphological differences by which these taxa have heretofore been distinguished would appear to result from ecophenotypic plasticity, driven perhaps by predation, substrate, or current.

Key words: Snail; Pleuroceridae; Population genetics; Inducible defenses; Shell morphology; Allozyme electrophoresis; Predation.

*Correspondence: E-mail: DillonR@cofc.edu