Analisi della variabilità genetica e della connettività tra popolazioni di due specie di Elasmobranchi: Raja clavata L.,1758 e Scyliorhinus canicula (L., 1758)

Studies concerning the status of conservation and exploitation of cartilaginous fish are essential to better understanding the biology and degree of threat of this important taxonomic group, that takes on great importance for his ecological role in the control of coastal and oceanic ecosystems. Taking into account the vulnerability of elasmobranchs, due to of their biological characteristics (low fecundity, late maturity and slow growth rates), they are mainly affected by anthropic influence, as fishing pressure. In fact, there is evidence that the elasmobranchs of the Mediterranean are declining in abundance, diversity and range due to the intense fishing activity. The genetic application to the fisheries plays an important role to define the level of intraspecific differentiation, genetic variability and the definition of stocks, proved to be a powerful tool to establish correct management plans for the marine resources. In this background, the main objective of this study is to assess genetic variability and connectivity in populations of two demersal elasmobranch species: Raja clavata and Scyliorhinus canicula, using microsatellites markers. The two species are caught by different fishing techniques like trawling, trammel nets and longline, but their commercial value is restricted to certain regions of the Mediterranean and they are primarly caught as bycatch in various demarsal fisherier. The thornback ray, Raja clavata is widely distributed in Mediterranean, Black Sea and the Atlantic coasts of Europe and Africa. The recent decreases in abundance and distribution in several areas of its geographical range led to inclusion of this taxon in the IUCN Red List as Near Threatened. Analyses were carried out on a total of 294 individuals collected from seven areas in Mediterranean (Western, Central and Eastern) and comparing them to Northeastern Atlantic using 8 microsatellite loci. Moderate levels of genetic diversity were found (uHe=0.66), with the highest values recorded in the North Sea (0.67) while the lowest in the Tyrrhenian Sea (0.57). Pairwise FST and DEST estimetes revealed genetic homogeneity between samples geographically close. North Sea samples was significant differentiated among Mediterranean populations, but Cyprus was found to be highly divergent from all the other samples, showing the highest pairwise FST and DEST values. The results of the AMOVA highlighted significant genetic differentiation comparing on three groups Cyprus/central western Mediterranean/North Sea (FCT= 0.058, P=000) Excluding eastern sample (Cyprus) ,the differentiation between the 2 groups (western Mediterranean/Atlantic Ocean), was lower but still significant. Within the Mediterranean basin, pooling samples based on geographical distribution (Western/Central/Eastern) a significant differentiation emerged (AMOVA: Fct=0.038, P<0.05), furthermore Mantel test found a significant association between genetic and geographical distance across all samples. The strong subdivision between eastern Mediterranean and other samples is also consistent with the number of cluster identified in STRUCTURE, in which Cyprus had the highest proportion in a separated cluster from other populations. According to the bottleneck results, only North Sea sample seemed to show a bottleneck signal, whereas all the other investigated populations showed a substantial demographic stability. Temporal comparison in three locations in Sardinia, sampled twice (in 2005 and 2012), highlighted no significant difference in genetic variability and differentiation. The small spotted catshark, S. canicula is one of the smallest catshark species and the most abundant in the European inshore waters. Although the species is listed as Least Concern in the red list IUCN it was considered overexploited in some areas. It is primarily caught as by catch in demersal fisheries, but also it is important as commercial species. Analyses were carried out on a total of 265 individuals sampled from ten areas within the Mediterranean Sea (Western, Central and Eastern) and one in Northeastern Atlantic, using twelve microsatellite loci. Genetic diversity was similar among populations (overall uHe=0.66), with the highest value for Morocco sample (MOR uHe=0.73). AMOVA analyses indicated a significant genetic differentiation among all locations (overall FST = 0.045;P-value=0.00). Pairwise FST and DEST values confirmed significant differentiation among all samples, with North Sea showing the highest heterogeneity. Mantel Test revealed a positive correlation between geographical and genetic distance among all samples, but no within the Mediterranean basin. AMOVA within the Mediterranean Sea found the highest values of Fct pooling samples on four groups (Sardinian samples/Morocco/Algery/Cyprus) (FCT= 0.049 P=0.000). Bayesian analysis confirmed the results of genetic differentiation testing for the K=5 in which also North Sea sample (sharing cluster with Algery) showed high genetic separation from the other Mediterranean samples. Demographic analysis revealed a bottleneck signal only for the North Sea sample, as registered for the thornback ray. Furthermore, for both species no significant differences consistent for sex-biased dispersal was found. The present study has been helpful to provide genetic data for both species in order to assess the genetic diversity and connectivity at intraspecific level. As regard R. clavata, the previous population genetic studies were limited to the Atlantic Ocean, including just a small sample in three areas in the Mediterranean and the Black Sea. For this reason it was necessary to provide new insights for thornback ray populations within the Mediterranean Sea. Results obtained instead for S. canicula provided further genetic data to compare with the scientific paper produced in recent years, and also they provided more details for population around Sardinia coasts.