Gold coral (Savalia savaglia) and gorgonian forests enhance benthic biodiversity and ecosystem functioning in the mesophotic zone

The twilight or mesophotic zone is among the less explored marine regions. In coastal areas, investigations and manipulative experiments on benthic biodiversity and ecosystem functioning at depths up to >50m have been recently made possible by the progress of SCUBA techniques. In this study, we tested the effects of the presence of a gorgonian forest characterised by a large and dense population of the gold coral Savalia savaglia (Bertoloni 1819) on the benthic biodiversity (nematode species richness, and meiofauna community structure and richness of taxa), trophic guilds state (molluscs) and ecosystem functioning in the surrounding sediments. The S. savaglia colonies create elevated and complex tertiary structures. Our results indicate that the presence of these colonies was associated with a significantly increased deposition of bioavailable substrates and enhanced biodiversity, when compared with soft bottoms at the same depth but without gold corals. The higher biodiversity and altered trophic conditions resulted in higher rates of ecosystem functioning (e.g., higher benthic biomasses). These results suggest that S. savaglia should be particularly protected not only for its specific rarity, endemism and vulnerability but also because has a prominent role in sustaining high levels of biodiversity and ecosystem functioning in the surrounding benthos of the twilight zone.