Deep coral forests from the Island of Sardinia

Forests are major ecological structures of the terrestrial environment that enhance biodiversity through different kinds of specialized relations that exist between the vegetal canopy and organisms from all animal reigns. The concept of forest has recently been transplanted in the marine environment; marine forests share all ecological features of terrestrial forests. What is actually peculiar in the marine environment is the shift from the ‘vegetal’ forest in shallow depths (within the photic zone) to the ‘animal’ forest (dwelling in the circalittoral zone) throughout the depth gradient. ‘Animal forest’ concept refers to forests entirely constituted by animals, that substitute to the vegetal canopy in the three dimensional development of the habitat. Benthic suspension feeders are, in fact, the main contributors to the habitat biomass and create important three-dimensional structures that enhance the ecosystem’s overall complexity. Among factors controlling the horizontal distribution of such ecosystems, the geomorphology of the sea bottom is thought to be a key factor in the distribution of deep-sea coral as many coral species require hard substrates, particular light and current conditions. Through the combination of non-invasive techniques such as ROV footage coupled with multi-beam surveys in contrasting geological settings, this thesis aims to test (both at species and community level) if and how distribution and composition of coral forests dwelling in the mesophotic environment (from 50 to 200m depth) could be influenced by contrasting geological settings. Uni- and multivariate as well as multiple regression analyses were used to test for differences in alpha and beta biodiversity in roche du large ecosystems and canyons, which are the most common geological features along continental margins worldwide. Results revealed that, in contrast to what was previously reported in scientific literature, red coral populations are not structured along a bathymetric gradient but other intraspecific, interspecific and environmental interactions may shape the population structure. Investigations on communities emphasized a high variability at mesoscale (10s of km) that overcomes the macroscale (100s of km). In particular, tested environmental variables explain patterns of observed diversity, that is higher within the same geological feature rather than among contrasting geological settings. Finally, a ROV survey conducted over a roche du large ecosystem off Carloforte (SW Sardinia) revealed the presence of a pristine millennial black coral forest of Leiopathes glaberrima. The investigated site is a millennial deep coral oases as well as a nursery area for the spotted catshark Schyliorhinus canicula, which deserves special protection. Our results enhance knowledge on coral forests dwelling in the mesophotic environment and provide useful insights for recasting conservation strategies, both at species and community level.