Diversity and distribution of the bathyal sponge facies of the Ligurian Sea

The Mediterranean spongofauna is highly diversified, accounting for more than 700 species mainly reported from shallow waters. New technologies recently led to an increasing number of discoveries also in the deep sea. Two Ligurian bathyal campaigns (2017-2021) allow to provide here a taxonomical characterization of the deep spongofauna inhabiting canyons and seamounts (from 200 to 1825 m depth) and a description of the main sponge- dominated facies. A total of 41 species are identified. The majority of them (62%) are characterized by miniaturized growth form (< 2 cm) and are generally associated with white coral frameworks. Large sponges account for 16 species, occasionally reaching high abundances and creating characteristic facies, especially in the upper bathyal (200-500 m). The fan- shaped sponge Pachastrella monilifera Schmidt, 1868 is a common species, forming high- density aggregations in the Deiva Marina Canyon (450 m) and on the St. Lucia Seamount (220 m). The volcano-like sponge Characella pachastrelloides (Carter, 1876) creates distinct facies on the Ulisse (500 m) and Occhiali (300 m) seamounts. A peculiar facies of the lollipop sponge Stylocordyla pellita (Topsent, 1904) is reported from the Penelope Seamount (480 m). Pseudo-encrusting species of intermediate size occasionally reach high abundances, as Haliclona (Gellius) cf. bioxeata on the Occhiali Seamount coral thanatocoenoses (320 m) and the hexactinellid Tretodictyum reiswigi Boury-Esnault, Vacelet & Chevaldonné, 2017 on seamounts rocks (450-520 m). Other characteristic species include Leiodermatium pfeifferae (Carter, 1873), Atergia corticata Stephens, 1915, Phakellia spp., and Desmacella annexa Schmidt, 1870. Aggregations of the hexactinellid Farrea bowerbanki Boury-Esnault, Vacelet & Chevaldonné, 2017 (Janua Seamount, 830 m) represent the only sponge-dominated facies of the lower bathyal (500-3000 m) found so far. These findings are discussed considering the most updated international deep-sea habitat classification systems, while comparison with other basins allows to obtain information on their large-scale geographic distribution.