Phylogenetic placement and molecular dating of hammerhead sharks (Sphyrnidae) based on whole mitogenomes

Due to continued overexploitation and anthropogenic change, hammerhead sharks (Carcharhiniformes: Sphyrnidae) have experienced drastic declines over most of their geographic range. Owing to the K-selected life histories of these sharks, their population resilience and persistence, remain severely strained, further compromising ecosystem stability. Moreover, some species are largely understudied e.g. the cryptic congener, the Carolina hammerhead shark (Sphyrna gilberti), whilst specific regions, such as the South-West Indian Ocean (SWIO), remain relatively devoid of data, risking the eventual extirpation of unique hammerhead shark lineages. The aim of the present study was to verify the phylogenetic placement of the cryptic S. gilberti within the family Sphyrnidae through the inclusion of underrepresented species sequences in order to provide a more comprehensive phylogenetic perspective for understanding historical drivers of Sphyrnidae biodiversity. The present study describes the first complete mitochondrial genome of the cryptic S. gilberti originating from the US Atlantic, as well as the mitogenomes of smooth hammerhead shark (Sphyrna zygaena) and scalloped hammerhead shark (Sphyrna lewini) samples originating from the data deficient South-West Indian Ocean (SWIO). Furthermore, we estimate the phylogenetic interrelationships of the Sphyrnidae family using mitochondrial protein-coding (PCG) and rRNA genes, reaffirming the placement of S. gilberti as a sister lineage to S. lewini. The resulting phylogenetic estimate is further used to evaluate the most likely age of the first occurrence of S. gilberti, corresponding to the Late Miocene to Early Pleiocene Epoch (3.8-10.8 million years ago). Comparative analysis of these Sphyrnids between ocean basins, as well as preliminary divergence estimates for S. lewini and S. gilberti has contributed towards resolving the global hammerhead phylogeny. This has provided unique insights into the evolution of the genus, thereby aiding future efforts directed towards effective conservation and management of hammerhead populations over a larger spatial scale.