Genetic structure of the threatened Gray Parrotfish (Sparisoma axillare) in the Southwestern Atlantic

Despite the marine environment being typified by a lack of obvious barriers to dispersal, levels of genetic divergence can arise in marine organisms from historical changes in habitat availability, current oceanographic regimes and anthropogenic factors. Here we describe the genetic structure of the Gray Parrotfish, Sparisoma axillare, and identify environmental variables associated with patterns of genetic divergence throughout most of its distribution in Brazil. The heavily exploited Gray Parrotfish is endemic to Brazil, and there is lack of data on population structure that is needed to support sustainable management. To address this shortfall we analyzed 5429 SNPs from individuals sampled in nine locations, ranging from tropical to subtropical reef systems and costal to oceanic environments with varying levels of protection. We found low levels of genetic structure along the coast, including the oceanic island of Fernando de Noronha, and that a combination of water depth, ocean currents and geographic distance were the major drivers explaining genetic divergence. We identified a distinct genetic population around Trindade Island, 1000 km from the coast, highlighting the conservation significance of this population. Colonization of this oceanic site probably occurred during the Pleistocene periods of lower sea levels, allowing this shallow water-dependent species to use the seamount chain as stepping stones to Trindade. Our data further suggest that two protected areas, Costa dos Corais and Fernando de Noronha, likely play an important role as larval sources for much of the species distribution.