Genetic structure of Quadrula quadrula (Bivalvia : Unionidae): Little variation across large distances

North American freshwater bivalves of the families Unionidae and Margaritif eridae represent one of the endangered faunas of the world. Effective manag ement of threatened and endangered species requires knowledge not only of a bundances of these species but also the degree of variation within species and the geographic distribution of this intraspecific variation. We used al lozyme electrophoresis to examine the genetic structure of seven Quadrula q uadrula populations from the Ohio, Tennessee, and Tensas Rivers. We then co nsidered the implications of our results for the development of effective b ivalve conservation strategies. Descriptive measures of genetic variation w ithin populations are quite high (2.1 +/-: 0.1(se) alleles per locus; 61.4 +/- 2.6% polymorphic loci; 0.24 +/-0.01 heterozygosity) relative to other u nionids. Genotype frequencies met Hardy-Weinberg expectations at all polymo rphic loci. Among-population variation was low and mostly confined to diffe rences between the Tensas River population (lower Mississippi River basin) and the Ohio River basin populations. Significant differences in allele fre quencies among populations were only detected at 3 of 10 loci; no differenc es in allele frequencies were found among Ohio River basin populations. Gen etic distances, though all small, were significantly correlated with geogra phic distance. Estimated gene flow was high among populations, but variatio n among populations did tend to follow the predictions of an isolation-by-d istance model of dispersal. The low levels of among-population genetic vari ation are remarkable given that these populations are separated by distance s as great as 2,500+ river kilometers. High levels of gene flow may ensure that within-population variation remains high and that populations do not b ecome differentiated due to genetic drift. An optimum conservation strategy for this species in the mainstem of the Ohio River would center on the pro tection of a number of large populations and maintenance of corridors for d ispersal of host fishes. Successful protection of threatened and endangered species requires conservation of both abundance and genetic diversity of u nionids. Further work is needed to characterize general patterns of genetic structure within freshwater bivalve species.