Large-scale characterization of the deep megabenthic fauna along the Italian coast: diversity, distribution, ecology and vulnerability

The Mediterranean Sea is a relatively small basin characterized by a deep homeothermy, a complex water circulation system, and an intricate network of topographic features, including canyons, seamounts, hydrothermal vents, cold seeps, and abyssal plains. Despite being considered a biodiversity hotspot with a uniquely high percentage of endemic species, the knowledge about its species richness is almost completely confined to coastal ecosystems, and data on deep-sea assemblages are still limited. Over the past 25 years, the advancement of underwater equipment made access to deep-sea environments easier and more cost-effective. In particular, the use of Remotely Operated Vehicles (ROVs) turned out to be a valid method for observing habitats, identifying organisms, and understanding interactions between them and their environment in a non-invasive way. Although its widespread use in the Mediterranean mesophotic and bathyal zones since the beginning of the century, comprehensive studies targeting the characterization of the biodiversity of deep-sea ecosystems from a large-scale perspective are lacking nowadays. For this reason, the aim of the present work was to depict the large-scale biodiversity and distribution patterns of mesophotic and bathyal megabenthic and demersal taxa in the Italian seas. To do this, an ROV archive consisting of 654 dives, obtained from multiple oceanographic campaigns carried out from 2006 to date between 40 and 1825 m along the Ligurian and Tyrrhenian coasts, Sicily Channel, Adriatic and Ionian seas, and offshore localities, has been analysed. Data on species occurrence, abundance, depth, habitat characteristics (substrate and slope), ecology, population structure, and anthropic impact have been collected. The data elaboration in the present work, potentially offering a large number of case studies, was targeted to some charismatic taxa. The visual census allowed the identification of 566 taxa belonging to 13 phyla, including some rare species, like the two Atlantic echinoderms Coronaster briareus (Verrill, 1882) and Holothuria (Vaneyothuria) lentiginosa lentiginosa Marenzeller von, 1892, reported for the first time in Italian waters. Cnidarians and fish were the dominant taxa in terms of occurrence and species richness in all the considered macro-areas. Among anthozoans, the red coral Corallium rubrum (Linnaeus, 1758) resulted widespread along the Italian coast, except in the Adriatic/Ionian seas, with the highest occurrence in the Tyrrhenian Sea, probably because of the deep nutrients-rich currents in this area. Large-scale considerations could be made also for the bathymetric distribution of other taxa: some showed a marked preference for the mesophotic zone, like molluscs, bryozoans, and ascidians, while some were more abundant in the upper bathyal zone, like crustaceans and echinoderms. To disentangle the factors potentially explaining these distributions, species or groups of species must be considered separately. Brachiopods, for example, showed a peak of occurrence in the upper bathyal zone, below the shelf break, especially over outcropping and sub-outcropping rocks showing less competition with arborescent habitat-formers and higher silting levels. Interestingly, the species Novocrania anomala (O. F. Müller, 1776) was very abundant in the bathyal depth range, despite being much more commonly known in shallow submarine caves, probably showing bathophilic conditions. Fifty-three reference habitats have been identified in the studied regions, corresponding to about 80% of the recently described ones for the mesophotic and bathyal zones, including coralligenous accretions, offshore rocks and muddy areas dominated by animal forests, coral reefs, and thanatocoenoses on bathyal muds. This information increased the knowledge on deep habitats, including ones not yet listed in the SPA/RAC classification system. For example, dense aggregations of the yellow scleractinian Dendrophyllia cornigera (Lamarck, 1816) were observed on muddy planes, despite this species has always been considered a typical hard-bottom species, and no reference habitat referring to this type of facies exists yet. The potential of the ecological information included in such a large dataset has been investigated also through another case study targeting gastropods. By noting the ecological preferences of heterobranchs and their bathymetric distribution, it has been possible to hypothesize how, in this group, stenophagy may represent a factor limiting the colonization of the deep sea. With the aim to define the structure and environmental status of the populations of the main habitat-forming anthozoans, detailed data were collected for 27 structuring species. Two species studies were investigated in the present work. In the case of Corallium rubrum (Linnaeus, 1758), 17 major coral areas characterized by highly aggregated colonies were identified, and a size structure skewed towards the smaller sizes was identified in all of them, supporting an effect of the long-term harvesting pressure to which this species is subjected. In addition, high entanglement percentages and mass mortality events in mesophotic populations increased the level of vulnerability of this charismatic species. The pink fan-shaped gorgonian Callogorgia verticillata (Pallas, 1766) was present in all the Italian seas, with seven structured forests identified at mesophotic depths on horizontal rocky bottoms. The overall modal size class of the colonies was 11-30 cm, even if distinctive juvenile and adult cohorts coexisted in some sites. High entanglement percentages were reported in a few areas subjected to significant longline fishing pressure, supporting the identification of priority protection measures for these sites. Monitoring protocols and conservation strategies greatly benefit from the information extrapolated from datasets such as the one built here. Two different case studies were considered here, targeting mesophotic rocky reefs dominated by coral forests and bathyal Cold-Water Coral reefs. Considerations on the abundance of the charismatic species Astrospartus mediterraneus (Risso, 1826) could be made considering the data collected during ten years of surveys within the Marine Strategy Framework Directive, targeting the mesophotic diversity along the Ligurian coast. The variations in the abundance of the basket star resulted to be mainly related to the amount of rainfall and the density of gorgonians. Finally, pluri-annual investigations focused on two Tyrrhenian CWCs areas, coupled with photogrammetric techniques, constituted the baseline to propose a standard monitoring protocol for bathyal CWC-dominated environments. In the last century, the knowledge on the diversity, composition, distribution, and vulnerability of deep environments has become of primary interest. To employ non-invasive methods, such as ROVs, is a valid alternative to traditional destructing methods. They allow to collect a large amount of data on different aspects of the deep-sea habitats and fauna, over a wide geographic and temporal extent. This is fundamental to provide effective monitoring and conservation strategies, in a large-scale, long-term perspective.