Cladocora caespitosa (pillow coral) is the only reef-building zooxanthellate coral in the Mediterranean Sea. According to fossil records, during the Pliocene, this species was able to build extensive reefs in the Mediterranean Sea 1, whereas now it typically forms intersperse colonies, sporadically aggregating in reef-like structures 2,3. Cladocora caespitosa generally colonizes hard bottoms, from a 5 to about 35-m depth 2, and shows different morphologies depending on hydrodynamic conditions, substrate characteristics, and seafloor topography 4,5. The hemispherical bush-like colonies can create ‘beds’ (intersperse single colonies) or ‘banks’ (aggregated colonies) covering several square meters 6,7. C.caespitosa can also occur as free-living on coarse soft bottoms, often associated with algal rhodoliths 8, with nodular morphology and spherical polyp growth (polyps growing in all directions). This species is characterized by a high trophic plasticity, shifting across a gradient from autotrophic to heterotrophic metabolism to cope with the changing environmental conditions 9,10. Calcification rates of this coral exceed 1.7 kg CaCO3 m2 a− 1 11, making C. caespitosa relevant also for carbon sequestration. As for tropical corals, the 3D structure of the carbonate skeleton produced by the pillow coral increases the habitat complexity and promotes biodiversity 12,13. C. caespitosa is being increasingly threatened by anthropogenic pressures14–16 and, during the last two decades, mass mortality events associated with heatwaves have been increasingly documented in the Mediterranean Sea 17–22. Since C. caespitosa is a long-lived, slowly growing, and low-dispersal species 7,20,23, the natural resilience of this species is unable to cope with the current loss rates 24,25. For this reason, the International Union for the Conservation of Nature (IUCN) included C. caespitosa in the endangered species list 26.
A possible solution to balance the current loss of this species is to implement its active restoration. Previous attempts on C. caespitosa, have been limited and focused on direct translocation 27. However, this approach implies a relevant impact on naturally occurring colonies and thus is not suitable for large-scale intervention. Since C. caespitosa displays high similarities with tropical reef-building corals 7, it is possible to design restoration protocols for this Mediterranean coral profiting of the extensive experience acquired from tropical reef restoration.
The approach commonly referred to as "coral gardening" is widely and successfully applied in active tropical reef restoration, even sustaining large-scale projects, with minimal to no impact on “wild” corals 28,29
The approach articulates in two phases; starting with the ex-novo generation of a coral stock by either sexual or asexual reproduction of donor materials (larval settlement, coral fragmentation, collection of corals of opportunity), which are cultured in a controlled environment (e.g. coral nurseries). This is followed by a second phase of transplantation of the nursery-grown corals 29. Since larvae, recruits, and small coral fragments are characterized by a high vulnerability24,30,31, which might impede their restoration, the coral gardening requires the use of controlled conditions30. During the nursery phase, the micro-fragmentation technique (i.e., coral fragmentation into small pieces) is used as a tool to amplify nursery stocks as it relies on the high regeneration capacities of corals along with the tendency to recover the colony’s integrity through fast asexual reproduction 32,33. Micro-fragmentation has been successfully implemented to increase growth rates in slow-growing massive corals like Orbicella faveolata and Montastrea cavernosa 34, as well as generating a large number of new corals from a limited number of donors, for endangered species 35,36.
Here we applied for the first time the micro-fragmentation on C. caespitosa using an in-situ nursery and implementing the rearing technique to develop a successful protocol for future restoration interventions. Moreover, we explored the different thermal resistance displayed by various donors, to assess their ability to survive climate-induced bleaching events.