A Holistic Approach to Taxonomic Evaluation of Two Closely Related Endangered Freshwater Mussel Species, the Oyster Mussel (Epioblasma capsaeformis) and Tan Riffleshell (Epioblasma florentina walkeri) (Bivalvia: Unionidae)
Primers for 10 polymorphic DNA microsatellite loci were developed and characterized for the endangered oyster mussel Epioblasma capsaeformis from the Clinch River, TN. Microsatellite loci also were amplified for individuals collected from the following additional populations or species: (1) E. capsaeformis from Duck River, TN; (2) E. florentina walkeri from Indian Creek, upper Clinch River, VA; (3) E. florentina walkeri from Big South Fork Cumberland River, TN; and (4) E. torulosa rangiana from Allegheny River, PA. Allelic diversity ranged from 9-20 alleles/locus, and averaged 13.6/locus for all 5 populations investigated. Average expected heterozygosity (HE) per locus ranged from 0.78-0.92, and averaged 0.86.
A genetic characterization of extant populations of E. capsaeformis and E. florentina walkeri was conducted to assess taxonomic validity and to resolve conservation issues related to recovery planning. These mussel species exhibit pronounced phenotypic variation, and are difficult to characterize phylogenetically using DNA sequences. Monophyletic lineages, congruent with phenotypic variation among species, were obtained only after extensive analysis of combined mitochondrial (1378 bp of 16S, cytochrome-b, ND1) and nuclear (515 bp of ITS-1) DNA sequences. In contrast, analysis of variation at 10 hyper variable DNA microsatellite loci showed moderate to highly divergent populations based on FST values, which ranged from 0.12-0.39. Quantitative genetic variation was observed in fish host specificity, with transformation success of glochidia of E. capsaeformis significantly greater (p<0.05) on the greenside darter Etheostoma blennioides, and that of E. f. walkeri significantly greater (p<0.05) on the fantail darter E. flabellare. Lengths of glochidia differed significantly (p<0.001) between species, with sizes ranging from 241-272 μm. Underwater photographs of mantle-pads and micro-lures of female mussels documented fixed phenotypic variation between species. The texture and color of the mantle-pad of E. capsaeformis is smooth and bluish-white, while that of E. f. walkeri is pustuled and brown, with tan mottling. Based on extensive molecular, morphological, and life history data, a population of E. capsaeformis from the Duck River, TN is described and proposed as a separate species, and a population of E. f. walkeri from the upper Clinch River, VA is described and proposed as a separate subspecies.
Genetic management guidelines were developed to assess taxonomic status, genetic variation of donor-recipient populations targeted for augmentation, and field and laboratory protocols to maximize genetically effective population size, minimize genetic changes in captive-reared progeny, and prevent the release of juvenile mussels into non-native drainages. A pragmatic approach to species recovery is advocated; one that incorporates the principles of conservation genetics into breeding programs, but prioritizes the immediate demographic needs of critically endangered mussel species.