Abstract
Gene flow is important for maintaining the genetic diversity required for
adaptation to environmental disturbances, though gene flow may be limited
by site fidelity in small coastal sharks. Bonnethead sharks (Sphyrna
tiburo) - a small coastal hammerhead species - demonstrate site fidelity,
as females are philopatric while males migrate to mediate gene flow.
Consequently, bonnetheads demonstrate population divergence with distance
and Atlantic populations are genetically distinct from those of the Gulf
of Mexico. Indeed, Florida forms a vicariant zone between these two bodies
of water for many marine species, including some sharks. However, while
bonnetheads are expected to have limited dispersal, the extent and rate of
bonnethead migration remains uncertain. Thus, we aimed to determine their
dispersal capacity by evaluating connectivity between disparate
populations from the Gulf of Mexico and Atlantic Ocean. Using 10,733 SNPs
derived from 2bRAD sequences, we evaluated genetic connectivity between
Tampa Bay on the Gulf Coast of Florida and Biscayne Bay on the Atlantic
coast of Florida. While standard analyses of genetic structure revealed
slight but significant differentiation between Tampa Bay and Biscayne Bay
populations, demographic history inference based on the site frequency
spectrum favored a model without divergence. However, we also estimate
that if population divergence occurred, it would have been recent (between
1,500 - 4,500 years ago), with continuous unidirectional gene flow from
Tampa Bay to Biscayne Bay. Our findings support the hypothesis that
bonnetheads can migrate over relatively large distances (>300
miles) to find mates. Together, these results provide optimism that under
proper management, a small-bodied globally Endangered shark can undergo
long migrations to sustain genetic diversity.