Although local correlations between shell phenotype and stream size have often been documented in freshwater mollusks, the species- and even genus-level taxonomy of pleurocerid snails has historically been based almost entirely on aspects of the shell. Here I test the hypothesis that lightly shelled pleurocerid populations inhabiting smaller rivers in east Tennessee and north Georgia, variously assigned to the genus Goniobasis or Elimia, may be local variants of heavily shelled Pleurocera populations downstream. Populations of the nominal species Goniobasis (“Elimia”) acutocarinata, G. clavaeformis, and Pleurocera unciale were sampled from the Powell, Little, and Hiwassee subdrainages of the Tennessee River, and populations nominally Goniobasis carinifera and Pleurocera vestita sampled from the Coahulla subdrainage of the Mobile Basin. A population of Goniobasis simplex was sampled from each of the four subdrainages to calibrate expected levels of genetic divergence. Gene frequencies at ten polymorphic allozyme-encoding loci (15 populations, 30 individuals per population) revealed that each population of Pleurocera was more closely related to its local populations of Goniobasis (or “Elimia”) than to any other population of Pleurocera. All nine populations identified as G. acutocarinata, G. clavaeformis, and P. unciale appear to be conspecific, their minimum genetic identity of 0.771 much greater than the 0.356 minimum identity among the four G. simplex controls. The specific relationship between the nine Tennessee populations and populations of G. carinifera and P. vestita from the Mobile Basin is ambiguous, with identities ranging down to 0.284. This larger set of 11 populations is here referred to as the carinifera group. Evidence that intraspecific variation in shell morphology has risen to the level of the genus suggests that Goniobasis, Elimia, and several other generic nomina be subsumed under Pleurocera (Rafinesque, 1818).
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