Marine ecosystems are strongly impacted by the consequences of human activities, such as habitat destruction or artificialization and climate change. In the Mediterranean Sea, sessile benthic species, and particularly octocorals, have been affected by mass mortality events linked with positive thermal anomalies. The future survival of octocoral populations impacted by global change will depend on their recolonization abilities facing local extirpation or important modification of their habitat. We studied these processes in Mediterranean octocorals in two situations: the colonization of artificial substrates (wrecks) by the red gorgonian Paramuricea clavata, and the recolonization following mortality events in the yellow gorgonian Eunicella cavolini. With microsatellite markers (seven for P. clavata, five for E. cavolini), we analyzed the genetic diversity of populations on artificial substrates and their differentiation from other neighboring populations. For P. clavata the populations on artificial substrates were not or lowly differentiated from the closest populations (1.3–1.6 km) on natural substrates, and showed similar levels of genetic diversity. Artificial substrates can then be considered as an interesting substitute for natural substrates for this species. For E. cavolini we did not detect any variation in diversity or relatedness following recuperation after mortality events. In both cases, our results suggest the input from different populations in the recolonization process, which helps in maintaining the genetic diversity. These results are useful for the management of these species and of associated ecosystems

The E. cavolini samples have been obtained thanks to an ASSEMBLE TA project granted to D. Aurelle (grant agreement n° 227799), for an access to the Stazione Zoologica Anton Dohrn in Naples. [...] The work of M. Masmoudi in France was supported by the Algerian-French collaboration program Tassili n° 12MDU853. This study has been funded by the French National Research Agency (ANR) program Adacni (ANR n° ANR‐12‐ADAP‐0016; https://adacni.imbe.fr). The project leading to this publication has received funding from European FEDER Fund under project 1166-39417. J.-B. Ledoux is funded by an assistant researcher contract framework of the RD Unit-UID/Multi/04423/2019-Interdisciplinary Centre of Marine and Environmental Research-financed by the European Regional Development Fund (ERDF) through COMPETE2020-Operational Program for Competitiveness and Internationalisation (POCI) and national funds through FCT/MCTES (PIDDAC)

14 pages, 3 figures, 4 tables, supplemental material https://doi.org/10.1007/s00227-020-03684-z.-- All genotypes have been deposited in the Zenodo database under reference: https ://doi.org/10.5281/zenod o.37240 56

With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Peer reviewed