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International audience; Deep-sea environments in the Mediterranean Sea have been widely degraded over the past decades and thus there is an urgent need to implement protection measures. Within European Union's Mediterranean waters vast extensions of the deep-sea ecologically relevant ecosystems have been protected by integrating them to the Natura 2000 network. Inside the French Exclusive Economic Zone, this network is currently expanding to off-shore areas, beyond 12 nautical miles. In this study, we characterized megabenthic biodiversity on target features located inside a designed site of the south west Corsican margin, particularly on a volcanic promontory on the south flank of the Ajaccio submarine canyon and a wall of the Valinco Canyon, between 1690 and 2250 m depth. By means of a fine scale three-dimensional photogrammetric analysis, four megabenthic assemblages segregated by substrate were identified. Hard substrate assemblages were characterized by the presence of sponges, polychaetes, gastropods and the squat lobster Munida tenuimana while soft sediment assemblages were characterized by the elpidiid holothurian Penilpidia ludwigi and an unidentified sabellid. Highest densities were registered on serpulid thanatocoenoses, which hosted widespread patches of an unidentified encrusting sponge. Lebensspuren had a diverse origin likely deriving from the activity of epibenthic, infaunal and bathypelagic organisms. In general, litter was not abundant at these offshore sites: in the Valinco site, it had both a marine and land-based origin while at the even more offshore Ajaccio site, it was mainly marine based. More extensive surveys would be advisable to confirm this trend. This study provides new insight on the ecology and diversity of Mediterranean deep-sea environments in the 2000 m bathymetric zone, highlighting the enduring knowledge gap surrounding orographically complex deep-sea escarpments in the basin.

Seamounts represent ecologically important marine environments and undeniably play a crucial role in the functioning of the deep-sea ecosystems, thanks primarily to their contribution in supporting offshore benthic and pelagic biodiversity. In the last decades, these features have received attention in all oceans of the world, but they were relatively overlooked in the Mediterranean Sea. In particular, the Ligurian Sea hosts six major structures, whose summits, ranging from 150 to nearly 2000 m depth, are virtually unknown from the biological point of view. The summit region of the Janua Seamount, located approximately at 40 nautical miles off the harbour of Genoa at 810 m depth, was explored for the first time using Remotely Operated Vehicles. The sea bed topography is characterized by a heterogeneous hard bottom, dominated by sub-fossil thanatocoenoses of Lophelia pertusa (Linnaeus, 1758) covered in a thick Ferrous-Manganese crust, and colonized by a scattered bathyal assemblage dominated by hexactinellids and benthic cnidarians showing a distinct regionalization around the summit. Cnidarians observed in this site include a rare Atlantic bamboo coral (Isididae, Chelidonisis aurantiaca Studer, 1890), described herein for the first time for the Mediterranean Basin. These findings provide new information on the biogeographic patterns of colonization of the bathyal Mediterranean fauna but also highlight the large knowledge gap that still exists for deep-sea biodiversity of this basin.