Background: Larval settlement and intra-specific interactions during the recruitment phase are crucial in determining the distribution and density of sessile marine populations Marine caves are confined and stable habitats. As such, they provide a natural laboratory to study the settlement and recruitment processes in sessile invertebrates, including the valuable Mediterranean red coral Corallium rubrum, In the present study, the spatial and temporal variability of red coral settlers in an underwater cave was investigated by demographic and genetic approaches.

Methods: Sixteen PVC tiles were positioned on the walls and ceiling of the Colombara Cave, Ligurian Sea, and recovered after twenty months. A total of 372 individuals of red coral belonging to two different reproductive events were recorded. Basal diameter, height, and number of polyps were measured, land seven microsatellites loci were used to evaluate the genetic relationships among individuals and the genetic structure.Results. Significant differences in the colonization rate were observed both between the two temporal cohorts and between ceiling and walls. No genetic structuring was observed between cohorts. Overall, high levels of relatedness among individuals were found.

Conclusion: The results show that C. rubrum individuals on tiles are highly related at very small spatial scales, suggesting that nearby recruits are likely to be sibs. Self-recruitment and the synchronous settlement of clouds of larvae could be possible explanations for the observed pattern.