Sphyrnidae, Bonaparte, 1840

Ochoa-Zavala, Maried, Mar-Silva, Adan Fernando, Pérez-Rodríguez, Rodolfo, Palacios-Barreto, Paola, Adams, Douglas H., Blanco-Parra, Pilar & Díaz-Jaimes, Píndaro, 2025, Mitochondrial DNA patterns describe the evolutionary history of the bonnethead shark Sphyrna tiburo (Linneus 1758) complex in the western Atlantic Ocean, Journal of Fish Biology 106 (2), pp. 403-419 : 411-412

4.1 | Species delineation in the Sphyrnidae family

The hammerhead family is one of the youngest living groups of sharks, diverging from Carcharhiniformes between 10 and 20 MYA (Klimley, 2013; Lim et al., 2010). They are characterized by a unique laterally expanded head or cephalofoil. It is considered that this characteristic could be related to hydrodynamic adaptation, olfactory advantages, electroreception, maneuverability, binocular vision, and prey handling advantages (Kajiura et al., 2005 and references therein; Lim et al., 2010). However, the selective pressure responsible for the considerable variation in cephalofoil size within the family remains unknown ( Aroca et al., 2022; Lim et al., 2010). Although the evidence regarding its evolutionary history is limited, the ancestor of Sphyrna was probably a large shark with worldwide distribution. Subsequently, small-bodied sharks (<1.2 m) are thought to have evolved independently within the eastern Pacific and/or WA margins. It is therefore probable that all extant hammerhead sharks descended from a large-bodied shark (> 1.5 m) (Lim et al., 2010). Cladogenesis events inferred from our phylogenetic trees indicated that small-bodied sharks evolved over time following large sharks, supporting previous hypotheses.

The mean divergence time obtained in this study indicates that the S. tiburo complex comprises the youngest taxa among the small-bodied hammerhead sharks (<1.2 m), as was indicated previously in a study with nuclear and mitochondrial genes (Lim et al., 2010). However, in our study, both the COI and D-loop independently suggested different taxa as sister species of the S. tiburo complex (Figures 3 and 4). This may result from the amount of homoplasy observed in each marker. As with our results using the D-loop, Lim et al. (2010) previously identified S. corona as the sister species using nuclear and mitochondrial genes. More recently, using the entire mitochondrial genome, Grobler et al. (2023) suggested S. gilberti as a sister species. However, these authors did not include all small coastal sharks of this genus in their analysis. For this reason, it is necessary to further evaluate the phylogenetic relationships by combining the whole nuclear and mitochondrial genome, including all members of the family, to help establish robust phylogenetic hypotheses and thereby clearly identify the sister group of S. tiburo .