Seascape genetics and connectivity modelling for an endangered Mediterranean coral in the northern Ionian and Adriatic seas

Context: Spatially heterogeneous oceanographic properties such as currents, waves, and biogeochemical gradients control the movement of gametes and larvae of marine species. However, it is poorly understood how such spatial dynamics may shape the genetic connectivity, diversity, and structure of marine populations. Objectives: We applied a seascape genetics framework to evaluate the relationships between marine environmental factors and gene flow among populations of the endangered Mediterranean pillow coral (Cladocora caespitosa). Methods: We modelled gene flow among locations in the Adriatic and northern Ionian Seas as a function of sea surface temperature, salinity, currents and geographic distance. Isolation by distance and isolation by resistance hypotheses were then compared using model optimization in a generalized linear mixed effects modelling framework. Results: Overall genetic differentiation among locations was relatively low (F_ST = 0.028). We identified two genetic groups, with the northernmost location segregating from the rest of the locations, although some admixture was evident. Almost 25% of the individuals analysed were identified as putative migrants and a potential barrier to gene flow was identified between the northern and central-southern basins. The best gene flow models predicted that genetic connectivity in this species is primarily driven by the movement along the coastlines and sea surface currents. Conclusions: A high percentage of self-recruitment and relatively low migration rates has been detected in the studied populations of C. caespitosa. Its fragmented distribution along the coast can be predicted by stepping-stone oceanographic transport by coastal currents among suitable habitat patches.