Deepwater corals biodiversity along roche du large ecosystems with different habitat complexity along the south Sardinia continental margin (CW Mediterranean Sea)

Rocky pinnacles arising from soft bottoms of the circa-littoral zone may host oases of deepwater coral biodiversity: Engineer species that enhance ecosystem’s three-dimensional complexity often acting as shelter for associated fauna that can comprise commercially relevant species. Although variations in beta diversity of these coral oases were recently documented at different spatial scales, information on the role of geomorphogical features of rocky outcrops can be considered mostly an unaddressed issue. To comply with this task, five rocky pinnacles showing different geomorphological features (average slope, ruggedness, sediment accumulation), dwelling between 120 and 170 m depth along south Sardinia continental margin, were investigated by means of ROV imaging. We tested the null hypothesis that coral abundance and diversity [estimated using four black corals (Antipatharia) and seven gorgonians (Alcyonacea) as target species] did not differ among investigated sites. Overall, total coral abundance varied from 1.35 to 7.23 col m2; Callogorgia verticillata, followed by Eunicella cavolinii and Corallium rubrum were the most abundant gorgonians, while Antipathella subpinnata and Parantipathes larix were the most abundant black corals. Through uni- and multivariate analyses, significant differences in corals biodiversity were identified, allowing us to reject the null hypothesis, emphasizing a remarkable variability in beta diversity, apparently following variations in geo-morphological features of investigated pinnacles. Moreover, a visual census of anthropogenic debris revealed that lost fishing gears represented the most frequently observed debris, stressing the need to identify further and more focused strategies of conservation to protect these oases of coral biodiversity.