Population structure and cryptic speciation in bonnethead sharks Sphyrna tiburo in the south-eastern USA and Caribbean

Population structure and lineage diversification within a small, non-dispersive hammerhead shark species, the bonnethead shark Sphyrna tiburo, was assessed. Sphyrna tiburo is currently described as one continuously distributed species along the Atlantic continental margins of North, Central and South America, but recent genetic analysis of an insular population (Trinidad) suggests the possibility of cryptic speciation. To address this issue S. tiburo were sampled at six sites along c. 6200 km of continuous, continental coastline and from one island location (Grand Bahama) across a discontinuity in their distribution (the Straits of Florida), in order to test if they constitute a single lineage over this distribution. A total of 1030 bp of the mitochondrial control region (CR) was obtained for 239 S. tiburo, revealing 73 distinct haplotypes, high nucleotide diversity (001089) and a pair of highly divergent lineages estimated to have separated 361-562 million years ago. Mitochondrial cytochrome oxidase I and nuclear internal transcribed spacer loci show the same pattern. Divergence is similar within S. tiburo to that observed between established elasmobranch sister species, providing further evidence of cryptic speciation. A global AMOVA based on CR confirms that genetic diversity is primarily partitioned among populations (phi(ST) = 0828, P < 0001) because the divergent lineages are almost perfectly segregated between Belize and North America-The Bahamas. An AMOVA consisting solely of the North American and Bahamian samples is also significantly different from zero (phi(ST) = 0088, P < 0001) and pairwise F-ST is significantly different between all sites. These findings suggest that S. tiburo comprises a species complex and supports previous research indicating fine population structure, which has implications for fisheries management and biodiversity conservation.