Recruits of Corallium rubrum in artificial caves

Restoration ecology is an emerging branch of environmental science. Current measures to protect the marine environment primarily aimed at reducing environmental stressors, for example through the establishment of MPAs, but this passive protection is not sufficient. The precious Mediterranean coral Corallium rubrum is listed in the IUCN red list: in several countries, the deep stocks are heavily harvested, and climate change poses an additional threat to the shallow populations. Therefore, basic transplantation techniques are inappropriate for this extremely slow growing species. The only successful way to obtain more colonies is through reproduction and the generation of newly born colonies. Our goal is to establish a protocol based on science that focuses on sexual reproduction as the main vector of restoration. To this aim, we set up colonies of C. rubrum in 6 artificial caves at 40 m depth in 2021 and focused on finding the ideal sex ratio and colony density to obtain an optimal reproduction.

In each cave, 9 plates could be inserted, and colonies of Corallium rubrum, already identified as male or female, were placed on each of these plates. On each tile 4 substrates were glued (smooth and rough terracotta, and red and white marble) to find out whether the larvae have a settlement preference. The sliding tiles can be removed and brought back to the laboratory, to monitor the recruitment success or the growth of the colonies. These caves constitute an underwater laboratory in semi-controlled conditions and allow the development of standardized rearing protocols that are optimized for the restoration C. rubrum populations.

Our results show that two of the 6 caves were particularly favorable to larval settlement as we counted 139 and 102 larvae attached to the substrates. Concerning the sex ratio, the presence of one or at most 3 males for 6 to 8 females was the most favorable combination. In these caves, we counted 250 recruits in total, with 84 recruits on the terracotta without distinction between smooth and rough, and 166 recruits on marble (64 on red and 79 on white marble).