Genetic Assessment of the Spinner Dolphin ( <scp> <i>Stenella longirostris</i> </scp> ) in the Largest Offshore Sedimentary Basin in Brazil

ABSTRACT Assessing the genetic diversity of a population is critical to evaluate its resilience in the face of anthropogenic impacts. This study aimed to assess the genetic diversity and structure of the spinner dolphins ( Stenella longirostris ) that inhabit the Santos Basin (SB), south and southeast Brazil. Analyses were conducted using seven microsatellite loci and a 686 bp fragment of the mitochondrial DNA (mtDNA) control region. The genetic relationship between SB spinner dolphins with the island‐associated spinner dolphins of Fernando de Noronha and spinner dolphins distributed in the western North Atlantic, including the Wider Caribbean region, was also investigated through a 324 bp fragment of the mtDNA control region. Results indicate that the offshore group of spinner dolphins in the SB has high genetic diversity for both mtDNA and nuclear markers. Based on mtDNA, the SB group has moderate genetic connectivity to the western North Atlantic/Wider Caribbean region spinner dolphins, but high genetic differentiation from their closest neighbors (Fernando de Noronha). These findings indicate that although SB spinner dolphins occur in an area of strong anthropogenic impacts, they present a high level of neutral genetic variation, with subtle matrilineally derived structure in the offshore (not island‐associated) waters of the western Atlantic.