Symbiotic status does not preclude hybridisation in Mediterranean octocorals

Understanding how species can form and remain isolated in the marine environment is still an active research area. Correctly delimiting species if also of interest for biodiversity conservation and for a wide range of biological studies. Here we study the differentiation and the possibility of hybridization among three temperate octocorals : Eunicella cavolini, E. singularis and E. verrucosa. These species can occur in sympatry and morphologically intermediate individuals have been observed. Among these three species, E. singularis is the only one known to show a mutualistic symbiosis with photosynthetic Symbiodiniaceae. As symbiotic relationships can be involved in species barriers, we tested here the possibility of hybridization between symbiotic and non-symbiotic Eunicella species. Through multivariate analyses and hybrid detection, we prove the existence of current gene flow between E. singularis and E. cavolini, with the observation of F1 and F2 hybrids and backcrosses. Demographic inferences indicate a scenario of secondary contact between these two species. Our data suggest an intermediate abundance of Symbiodiniaceae in the hybrids of the two species. We discuss the evolution of the Symbiodiniaceae / cnidarian symbiosis in the light of our results.