Three-dimensional habitat-forming species in scleractinian mesophotic ecosystems along the Apulian coast (Italy)

Mesophotic bioconstructions are highly dynamic communities starting at 30-40 meters in depth and extending to over 150 meters formed as a result of the multilayer calcium carbonate depositions of benthic organisms. It is established that they are highly diverse, hosting also three-dimensional habitat-building taxa, which can have a direct influence on the habitat structural complexity. Knowledge of the habitat's physical structure is critical for its conservation since they are integral to marine ecosystems' organization, function, and resilience. Nevertheless, only recent and still fragmented data are available on the Mediterranean mesophotic zone. This study aimed to investigate the composition of three-dimensional habitat-forming benthic species at two sites along the south-Adriatic mesophotic zone, where was reported the existence of the scleractinian corals Phyllangia americana mouchezii and Polycyathus muellerae as the main biogenic constructors. Underwater videos were taken at each site using a Remotely Operated Vehicle (ROV) in order to estimate the covering values of the main benthic taxa. Both studied sites host bioconstructions developed within a bathymetric range of 35 to 53 meters. Quantitative surveys of the mesophotic substrate revealed a rich epibenthic community and a diverse structure of the three-dimensional habitat builders species, with different covering values. The present study provides insight into the role of macrobenthic invertebrates as main and secondary builders of complex bioconstructions, as well as on their structural complexity. Our results represent key elements to comprehensively assess biogenic reef dynamics and evaluate the health status of these habitats.