Coral restoration through the optimization of early life-stage dynamics: The case of the Mediterranean red coral (Corallium rubrum)

The recovery of ecosystem-engineering taxa, such as corals, is a critical conservation priority for maintaining habitat complexity, biodiversity, and ecosystem functioning, and requires integrated strategies that combine conservation and active restoration. In coral restoration, sexual propagation techniques may support the recovery of depleted populations by overcoming early-life demographic bottlenecks. Here, we present the first successful method for rearing the temperate octocoral, Corallium rubrum , from larval release to the juvenile stage, combining in-situ and ex-situ experiments to examine larval settlement, post-settlement growth, survival, and spatial distribution, and identify optimal conditions. Settlement success was higher ex-situ (28 ± 11%) than in natural conditions (1.5–5.5%), suggesting the possible advantages of controlled environments during the corals' earliest developmental stages. However, recruits reared in-situ exhibited higher growth and survival (43% in-situ versus 16% ex-situ after two years). By integrating ex-situ settlement with subsequent in-situ maintenance, cumulative two-year recruitment success increased to 12 ± 4.7%, representing a substantial improvement over either method alone. These findings confirm that early demographic bottlenecks strongly influence coral recruitment dynamics and underscore the importance of controlling the recruitment environment to optimize early-life stages. Our findings represent a promising pathway for restoring Mediterranean coral populations by incorporating sexual propagation techniques into current transplantation protocols. We suggest the implementation of hybrid coral restoration strategies that integrate both sexual and asexual propagation. This approach could harness the high survivorship of transplanted adults together with the high recruitment potential generated through sexual propagation, hence accelerating population recovery and enhancing the resilience of restored coral assemblages.