Conservation Genetics of Freshwater Mussels: Comments on Mulvey et al.

In a recent issue of Conservation Biology , Mulvey et al. (1997) reported on the use of allozyme electrophoresis and sequencing of the mitochondrial 16s rRNA gene to identify “diagnosibly distinct evolutionary entities” within the freshwater mussel genera Amblema and Megalonaias. Each genus contains a widely distributed, common species ( A. plicata and M. nervosa ) and other described species, including several considered endangered ( A. neislerii ) or of special concern ( M. boykiniana ). Mulvey et al. calculated genetic distances for allozyme data and sequence divergence for mitochondrial DNA (mtDNA) and used these results to construct phenograms describing relationships among species within these genera. They concluded that A. neislerii is a distinct species, that A. plicata perplicata from the Coosa drainage should be recognized as A. elliotti , and that M. boykiniana is synonymous with M. nervosa. To discriminate among species within a genus, one must quantify the degree of variation both among populations within each species and among the putative species. Accurate estimation of such variation increases with sample size. The sample sizes used by Mulvey et al. were typically small, averaging 3.79 individuals per locus ( 6 0.64 SE, their Table 1) for the allozyme analysis and 2.19 individuals ( 6 0.28 SE, their Fig. 2) for the mtDNA analysis. We propose that such sample sizes are insufficient for accurate estimation of genetic variation, so the discriminatory power of their data is low. Furthermore, the use of the 16s gene is problematic because this gene may not be useful for discriminating closely related bivalve taxa (Hoeh et al. 1997). Consequently, Mulvey et al. have a high probability of committing a Type II error when they suggest including M. boykiniana within M. nervosa. Because the taxonomic status of taxa generally determines the degree of protection afforded them by state and federal agencies, it is imperative that the conclusions reached by Mulvey et al. (and other researchers who utilize molecular markers) be scrutinized. The implications of Mulvey et al.’s determinations are far-reaching. They conclude that the listing of A. neislerii as endangered is justified, that the uncertain conservation status of A. elliotti is of concern, and most important, that M. boykiniana requires no special protection. The empirical data and results published in Conservation Biology and other peer-reviewed journals are often used (and rightly so) as the basis for management decisions. Thus, it is critical that the limitations of studies such as that of Mulvey et al. be presented, especially when such a study proposes to make a taxon of conservation concern synonymous with a common taxon, an action that could lead to a loss of protection for the former. We compared Mulvey et al.’s estimates of allozyme variation with estimates we calculated from populations of A. plicata in which many more individuals were sampled. We also present Monte Carlo simulations of the effects of sample size on estimates of allele frequencies, within-population variation, and genetic distance among populations. These results and our critique of Mulvey et al.’s choice of mtDNA gene cause loss of confidence in their conclusions.