Allopatric divergence and secondary contact of two weak fish species (Macrodon ancylodon and Macrodon atricauda) from the South Atlantic

The king weakfish Macrodon ancylodon (Scianidae) has been regarded as a widespread representative of Macrodon in the South Atlantic, managed as a single stock in Brazilian fisheries. Nonetheless, molecular and morphological data showed that populations from cold and subtropical waters in southeastern and southern Brazil actually corresponded to another taxon (Macrodon atricauda or Southern king weakfish). Apparently, the environmental and geomorphological heterogeneity in the South Atlantic would account for their allopatric differentiation with a putative but largely overlooked barrier in a narrow area between the northeastern and southeastern coast of Brazil. Therefore, we performed phylogenetic and phylogeographic analyses based on mitochondrial and nuclear markers in populations of M. ancylodon and M. atricauda with increased sampling efforts within and nearby their alleged range boundaries (Southern Bahia to Rio de Janeiro coast) to infer their biogeographic history and verify putative contact zones and/or introgression. Mitochondrial markers recovered both taxa as reciprocally monophyletic species that have diverged in Pleistocene (~1.5 million years ago), probably related to isolation of ancestor populations by the exposure of Abrolhos shelf (southern Bahia) and the Vitoria-Trindade chain during sea level retreats in glaciation periods. This vicariant effect combined with distinct ecological features between northeastern (warm waters) and southeastern coast (cold waters) should have favored their allopatric speciation. On the other hand, syntopic populations of M. atricauda and M. ancylodon were clearly recognized within a narrow contact zone (~60 km) between the estuaries of the Mucuri (northeastern Brazil) and Conceição da Barra (southeastern Brazil) without evidence of introgression, as a result of secondary contact. Thus, estuarine areas along this region represent a hotspot for conservation of fisheries resources, since it encompasses two differentiated stocks of king weakfish.