The southern flounder Paralichthys lethostigma is a key species in recreational flatfish fisheries in the Gulf of Mexico and the northwestern Atlantic Ocean. Effective management of this species relies on knowing how many populations occur within the range of the species, as knowledge of the underlying genetic structure may be used in stock assessments or in the establishment of management units. Here, the evolutionary history of the southern flounder was elicited with genetic data, and the geographic extent and long-term female effective population size (N ef ) of extant populations were estimated. Mitochondrial DNA sequence data (415 base pairs, control region) were analyzed with a combination of traditional hypothesis testing (analysis of molecular variance) and the coalescentbased maximum likelihood approach (analysis with the program Migrate). Three main findings resulted from this work. First, two distinct genetic populations of southern flounder exist in the Gulf of Mexico and western Atlantic Ocean, and there is a measured genetic divergence (i.e., F ST ) of 0.374 between them. Within ocean basins, there was little significant genetic structure among samples. Second, coalescent modeling suggests that the two populations of southern flounder have expanded rapidly in numbers in the past 10,000-22,000 years, timing that approximately coincides with the end of the last North American glacial retreat. Third, historical population growth is coupled with high estimates of long-term N ef , both of which contrast with present-day declines in abundance measured by stock assessments. Present-day southern flounder populations persist in two independent evolutionarily significant units that have divergent evolutionary backgrounds.

Strategies for conservation and management of marine species are increasingly relying on molecular genetic data for the delineation of independent fishery stocks. These data may be more reliable and less subjective than morphological or ecological variables, although their biological interpretations can be problematic. Here, we review existing morphological data synergistically with a new molecular data set for sheepsheads Archosargus probatocephalus. Two named subspecies of sheepshead exist in the northeastern Gulf of Mexico (A. probatocephalus probatocephalus and A. probatocephalus oviceps). Although these subspecies exhibit divergent morphology, it is unclear whether morphological divergence represents meaningful differentiation for management. We show that frequency distributions of each of five meristic counts are significantly different between the subspecies (P < 0.05). However, Bayesian structure analysis of microsatellite genotypes indicated that all Gulf of Mexico sheepshead populations constitute a single stock (posterior probability ≈ 0.9999). Variance partitioning of mitochondrial DNA (mtDNA) haplotypes suggests significant but limited divergence between subspecies (genetic differentiation index FST = 0.036, P < 0.005). Patterns of pairwise mtDNA genetic distance and microsatellite divergence suggest that isolation by distance, rather than subdivision among independent genetic stocks, is driving the significance of variance analyses. Overall, sheepshead molecular genetic data indicate very limited genetic subdivision between the subspecies despite considerable divergence of morphological characters. These results have multiple interpretations, each of which carries implications for the management of this species in the Gulf of Mexico.

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (FST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long-term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23-year span.

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