Baseline genetic distinctiveness supports structured populations of thornback ray in the Mediterranean Sea

Abstract

1. The thornback ray (Raja clavata) is the most important chondrichthyan in terms of landings in the Mediterranean Sea. Intense harvesting may induce negative genetic effects reducing the resilience of overfished species. For this reason, genetic diversity information should be considered in fisheries management and conservation policies.2. Microsatellite markers were used to unravel the genetic features (variability, connectivity, sex-biased dispersal) of R. clavata populations, both at the small (around the coast of Sardinia, western Mediterranean Sea) and larger spatial scales (at the pan-Mediterranean level, and between the Atlantic Ocean and the Mediterranean Sea).3. Individual clustering, multivariate and variance analyses rejected the hypothesis of genetic homogeneity, with significant genetic differences between Mediterranean and Atlantic rays, as well as within the Mediterranean Sea between its western and eastern basins. The data indicated that both the Strait of Gibraltar and the Sicilian Channel seem to be effective in limiting the dispersal of thornback ray individuals, but a further structuring was identified, with the significant genetic differentiation of the populations located in the Algero-Provençal and Tyrrhenian basins. Such a fine-scale arrangement suggests the occurrence of additional barriers to species dispersal. A lack of significant genetic differentiation, stable over the years, was measured at a local scale among R. clavata Sardinian samples. 4. Several possible mechanisms, both biological and abiotic (e.g. migratory behaviour, waterfronts and oceanographic discontinuities), are discussed. Overall, the genetic data presented, both at the local and regional level, could represent the baseline information for the temporal monitoring of populations and assessing the effects of present or future fisheries-related management actions.Riccardo Melis and Laura Vacca share first authorship.

To Be, or Not to Be: That Is the Hamletic Question of Cryptic Evolution in the Eastern Atlantic and Mediterranean Raja miraletus Species Complex

“…This suggests that the Strait of Gibraltar did not represent an effective barrier to gene flow, rather than an accession gate to ancient refugia . In contrast, Melis et al [ 113 ] found moderate significant population differentiation between the Mediterranean and the Atlantic Ocean in the congeneric thornback skate R. clavata , suggesting an effective role of the Strait in limiting the dispersal of individuals.…”

“…The combination of these features could have limited gene flow between western and eastern populations and driven the differentiation of the Eastern Mediterranean samples (ADR, ISR and LEV; Figure 1 ). The specific habitat and depth preferences, the less pronounced migratory behaviour and the limited dispersal capability of taxa belonging to the R. miraletus species complex are rather common characteristics among skates [ 62 , 73 , 113 , 114 ], although other congeneric species did not show such evidence of deep differentiation at both nuDNA and mtDNA markers [ 10 , 56 , 72 , 110 ]. In detail, the Mediterranean starry ray R. asterias showed a strongly structured population with three geographical clades corresponding to the western, central–western and central–eastern Mediterranean areas [ 62 ].…”

“…In detail, the Mediterranean starry ray R. asterias showed a strongly structured population with three geographical clades corresponding to the western, central–western and central–eastern Mediterranean areas [ 62 ]. R. clavata showed a weak but detectable phylogeographic structure in the Levantine Sea [ 73 ] and a finer structuring located off the Algerian coasts and Tyrrhenian basins, suggesting the occurrence of additional barriers to dispersal [ 113 ]. On the contrary, R. polystigma showed a slightly differentiated Adriatic haplotype but a near panmictic population in the central–western part of the basin [ 56 ].…”

Looking into the prevalence of bycatch juveniles of critically endangered elasmobranchs: a case study from pelagic longline and trammel net fisheries of the Asinara Gulf (western Mediterranean)

“…The literature gives several examples about the limited contact between shark and ray populations at both large and small scales. For example, recent studies applying the molecular approach have highlighted the main role of water circulation and temperature in sharks (Di Crescenzo et al, 2022;Melis et al, 2023a) and rays (Catalano et al, 2022;Melis et al, 2023b). Similarly, evidence based on molecular data showed how biotic and abiotic features such as bathymetry, hydrological constraints, and prey abundance can influence the presence, movement, and dispersion of sharks and rays (Catarino et al, 2015;Di Crescenzo et al, 2022).…”