Integrative restoration protocols for temperate mesophotic reefs: The case of the Mediterranean coralligenous habitats

The temperate coralligenous habitats are considered hotspots of Mediterranean marine biodiversity. These coralligenous bioconcretions are built primarily by the accumulation of encrusting coralline algae growing at low irradiance levels that host numerous long-lived macroinvertebrates with prominent ecological roles. Several pressures, such as fishing activities, invasive species, and recurrent warming-induced mass mortalities, affect coralligenous assemblages and lead to dramatic loss of habitat complexity and biodiversity. One aim of the EU-funded project MERCES, dedicated to marine ecosystems restoration, was to develop innovative active restoration protocols with a focus on coralligenous habitat-forming macroinvertebrates from three key taxonomic groups: Cnidaria/Anthozoa, Porifera/Demospongiae and Bryozoa. These protocols combined transplants from donor organisms and recruitment-enhancing devices, taking into account the life-history traits, population dynamics, and genetics of targeted habitat formers. Our results show that transplantations require low initial effort due to high survival of transplants, but decades will be needed to fully recover habitat complexity because targeted species are slow- growing. In order to speed up recovery, we explored the potential influence of facilitation processes, i.e. positive species interactions. In order to guide the choice of transplant donors, we also aimed to identify thermo-resistant populations and specimens. We thus combined a transregional common garden and whole-genome sequencing to unravel the eco- evolutionary processes driving the differential responses to thermal stress. The outcomes of these works will implement the EU directives and contribute to the strategic planning of restoration initiatives for temperate marine ecosystems in the context of global change.