Genetic Assessment of Sheepshead Stock Structure in the Northern Gulf of Mexico: Morphological Divergence in the Face of Gene Flow

Abstract

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 F_ST = 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.