A baseline of epibenthic species distribution on the northern Adriatic mesophotic biogenic reefs

In the northern Adriatic continental shelf, hundreds of mesophotic biogenic reefs, ranging in size from tens to many thousands square meters, are scattered on sandy and muddy bottoms. The study of their epibenthic assemblages, which started in the early 2000s, is continuing with the exploration of new sites at increasingly depth and distance from the coast, providing new insights into species diversity and distribution. The first intensive survey carried out in 2013-2014 from Chioggia to Grado provided data on 55 epibenthic taxa occurring in 12 sites. The second survey, carried out in summer 2017 with the same sampling effort (12 sites, 10 photographic samples per site) but extended to two remote outcrops and with only 4 sites in common with the previous survey, recorded 83 taxa (33 added and 5 not recovered). The two deepest and farthermost outcrops, 31-32 m depth and about 27 nm off Chioggia, hosted sponge gardens mainly composed by dense populations of Polymastia boletiformis (Lamarck, 1815), Polymastia mamillaris (Müller, 1806) and Ulosa stuposa (Esper, 1794). Meanwhile, some previously unexplored outcrops off Grado, close to the midline between Italy and Croatia, revealed assemblages extremely rich in sponges and colonial ascidians. Among the newly recorded taxa there were 15 sponges, 2 anthozoans, 3 bryozoans and 8 ascidians. The data currently available represent a new baseline of knowledge on the diversity of northern Adriatic mesophotic biogenic habitats. This may help to understand their resistance and resilience to human and climate change threats, and develop transboundary ecosystem-based management.