Rhodolith beds (RBs) are globally distributed, biodiversity-rich habitats formed by free-living coralline algae that function as ecosystem engineers and important carbonate reservoirs. Despite their ecological and biogeochemical significance and their increasing vulnerability to climate change and ocean acidification, RBs remain understudied. This preliminary study integrates morphological, taxonomic and genetic approaches to characterise RBs in the Tavolara-Punta Coda Cavallo Marine Protected Area (Sardinia, Italy) and assess their role in promoting biodiversity.
RBs were investigated through remotely operated vehicle (ROV) observations and grab samples, following the Marine Strategy Framework Directive monitoring protocol. RBs were identified as a continuous area covering at least 500 m² and based on the spatial distribution of 15 ROV video transects, six distinct RBs were mapped between 46.6 m and 77 m depth.
Genetic analyses were performed on rhodoliths representing different morphotypes to capture species diversity and morphological variability within the sampled beds. Morpho-molecular analyses conducted on three of the six investigated beds revealed that: i) one bed (RB1) was primarily composed of Ramicrusta sp., a genus previously unreported in the investigated area; ii) a second bed (RB2) was characterised by a relatively high abundance of branches morphology; iii) a third bed (RB3) was dominated by pralines morphology. Species belonging to Lithophyllum, Lithothamnion, Phymatolithon and Spongites were also identified within the collected material. The associated benthic and necto-benthic assemblages were investigated by annotating video footage with BIIGLE, a web-based annotation platform, yielding 2193 observations. Organisms were assigned to major taxonomic groups, including Algae, Demospongiae, Cnidaria, Polychaeta, Mollusca, Ophiuroidea, Echinodermata, Tunicata and Teleostea enabling the characterisation of assemblage structure and variability among RBs. Demospongiae, Ophiuroidea and Teleostea were the most abundant groups, highlighting the importance of RBs as biodiversity hotspots supporting diverse benthic and necto-benthic assemblages.