Molecular genetic, life-history and morphological variation in a coastal warm-temperate sciaenid fish: evidence for an upwelling-driven speciation event

Aim: The Benguela Upwelling System (BUS) is a major biogeographical boundary in the south-eastern Atlantic, but little is known of the effect of historical permeability of the barrier on species distributed across the region. We used phylogenetic, life-history and morphological analyses to test the influence of regional oceanographic features on genetic divergence, evolutionary history and ecological divergence in a warm-temperate fish, Atractoscion aequidens. Location: Benguela Current, south-eastern Atlantic Ocean. Methods: Individual At. aequidens were analysed for molecular (N = 35), biological (N = 81) and morphological (N = 282) differentiation across the BUS region. To compare patterns of genetic divergence, we also included analyses of individuals sampled from three related Argyrosomus species occurring in the area. Results: Atractoscion aequidens comprises two deeply divergent lineages (average mtDNA genetic distance p = 0.049), which are separated by the boundary between the northern and southern Benguela subsystems. Divergence time was estimated at 2–3 Myr. Life-history traits, such as maximum size, growth rate and size and age at maturity, differed significantly between the two lineages, but morphological features did not. Divergent life-history traits appear to be associated with the evolution of migratory behaviour in the southern population, and thus ecological diversification across the Benguela region. Main conclusions: The oceanographic features of the BUS constitute an impermeable barrier to the dispersal of At. aequidens across the region. The estimated time since most recent common ancestor (TMRCA) points to the strengthening of the upwelling system in the last 2 Myr as the most likely mechanism driving isolation. However, the BUS is known to have periodic disruptions, which have mediated dispersal in multiple taxa. Therefore, the depth of genetic divergence and the presence of different life-history traits between the two lineages suggest that ecological divergence may have played a role in maintaining the observed isolation. Based on genealogical, geographical and ecological concordance, we suggest that At. aequidens is composed of two sibling species in the Benguela region and a full taxonomic revision of the genus is required.