Molecular tools to gain a deeper knowledge on Sardinian Chondrichthyan species

In the marine environment, chondrichthyan species—which include sharks, rays, skates, and chimeras—are one of the most endangered taxa. Being among the slowest-reproducing vertebrates in the sea, they are extremely vulnerable to anthropogenic pressures such as overfishing, habitat modification, pollution, and climate change. Innovative molecular methods can unlock new possibilities to explore their distribution and to monitor their diversity over time. Genetic tools can especially help in addressing the challenge of precisely identifying species, a difficult task in a group characterized by a high morphological conservatism, but essential for developing effective conservation and management strategies. In this context, we applied a genetic approach to deepen our knowledge of Mediterranean chondrichthyans, with a special focus on the seas around the Sardinian Island (Western Mediterranean). A few case studies are described here, based on data collected during the building of a curated multi-marker reference library based on mitochondrial genes (COI, ND2, and 12S). In particular, we investigated the electric rays, observing the presence of potential cryptic species within the order Torpediniformes and measuring the intraspecific diversity of the three Mediterranean species (Torpedo torpedo, Torpedo marmorata, and Tetronarce nobiliana). Similarly, the application of several ‘species-delimitation’ methods allows us to find evidence of taxonomic uncertainties in the five genera of Mediterranean stingrays (Bathytoshia, Dasyatis, Himantura, Pteroplatytrygon, and Taeniurops), suggesting the urgent need for future studies and a comprehensive revision of the family Dasyatidae for its effective conservation. Moreover, sequence data allowed us to confirm the occurrence of Dasyatis tortonesei in the investigated area and to highlight that misidentification with the congeneric Dasyatis pastinaca can easily occur, suggesting the urgency for new morphological identification keys. Finally, the curated reference library allowed us to further update the Sardinian cartilaginous fish species checklist with a rare species (Rostroraja alba) and an endemic species (Leucoraja melitensis), expanding their known geographic range of distribution.