The aims of this thesis were twofold: (i) to provide quantitative baseline data for highly valuable but highly understudied coralligenous assemblages dominated by the red gorgonian Paramuricea clavata and the first assessment of P. clavata and Eunicella cavolini populations in the Eastern Adriatic Sea, as well as (ii) to explore the potential consequences of warming on the coralligenous community by examining the effects on the model organism, the red gorgonian P. clavata. Underwater photo-sampling with comparable sampling effort revealed higher species richness of the Adriatic P. clavata assemblages in comparison to the NW Mediterranean ones and significant inter-site and inter-depth variability of macrobenthic biodiversity, implying future site-based monitoring. High proportion of large adult P. clavata colonies, high abundance of juvenile colonies and low density values contrast with demographic structure reported from the NW Mediterranean. Together with results obtained for scarcely studied E. cavolini populations, these data provide new insights into the demographic structure of Mediterranean gorgonian forests. Further, conservation status of examined populations was assessed as favourable, but potential threats were also detected. The results of experimental studies implied that prolonged starvation after energetically unfavourable summer period significantly decreases P. clavata thermotolerance, while food availability promotes it. However, differences in physiological status cannot explain all of inter-colony and inter-population variability found in response of P. clavata to thermal stress. Further, thermal stress had severe negative effect on P. clavata embryos and larvae, resulting in significantly reduced survivorship, completely abnormal embryonic development and impaired metamorphosis. Their reduced viability would jeopardize the persistence of P. clavata, which relies on successful recruitment for replenishment of its populations.