A genomic assessment of movement and gene flow around the South Florida vicariance zone in the migratory coastal blacknose shark, Carcharhinus acronotus

South Florida has been identified as a genetic break for multiple mobile marine taxa but the mechanisms that impede gene flow largely remain unknown. To understand how South Florida functions as a barrier for blacknose shark, a highly migratory species that has genetically diverged Atlantic and Gulf populations, patterns of genetic variation were assessed in 212 individuals sampled from the Atlantic, eastern Gulf, and Florida Keys at 2213 nuclear-encoded SNP-containing loci. Results support divergence between the Gulf and Atlantic (F-ST similar to 0.002, P<0.05), and 51 individuals caught in the Keys were assigned to the Gulf, as compared to only two individuals that assigned to the Atlantic, indicating that Florida Keys is largely composed of Gulf individuals. Further, two to three migrants were identified, all of which were Gulf individuals captured in the Atlantic. The results indicate that South Florida does not prevent individual movement between the Gulf and Atlantic and that the Keys may be a seasonal mixing zone. However, the Gulf and Atlantic remain genetically independent, suggesting that region-specific reproductive behavior/compatibility, or aspects of movement ecology, such as swimming energetics or temperature-driven interannual variability in migratory range, may maintain divergence rather than a physical barrier in South Florida.