Linking seas: assessing the connectivity of Carcharhinus plumbeus (Nardo, 1827) from the Mediterranean Sea to global oceans

The sandbar shark (Carcharhinus plumbeus) is an endangered species inhabiting the Mediterranean Sea, where conserving coastal nursery grounds is crucial for the persistence of the population. However, identifying this species can be puzzling because of its morphological similarity to other species in the same genus, such as C. altimus and C. obscurus. This study aimed at resolving the taxonomic confusion related to the species using genetic data and examined the mitochondrial diversity of C. plumbeus juveniles collected from nursery areas in Italian waters, comparing them with specimens from other Mediterranean and global regions. A total of 89 high-quality sequences were analysed using two mitochondrial markers (NADH2 and Control Region), alongside morphometric measurements taken from 37 specimens. NADH2 revealed moderate haplotype diversity and a shared central haplotype within the Mediterranean, indicating high maternal connectivity and local genetic cohesion. On the other hand, comparisons with Atlantic and Indo-Pacific samples revealed significant genetic differentiation, suggesting limited gene flow between ocean basins and the existence of a distinct mitochondrial lineage. The Control Region confirmed reduced variability within the Mediterranean and clearly distinguished C. plumbeus from other congeneric species, thus supporting its use in species identification for this genus. Overall, these results highlight the existence of a genetically cohesive, yet vulnerable, Mediterranean population of sandbar shark, and emphasise the importance of protecting nursery areas and improving species-specific monitoring for elasmobranchs’ effective conservation.