Studio delle foreste di idroidi e comunità associate nel Mar Mediterraneo con focus su una foresta di Eudendrium racemosum nello Stretto di Messina

Abstract This thesis examines the environmental role of hydrozoans (Hydrozoa) with regard to their ecological functions, trophic interactions, and symbiotic relationships within marine ecosystems in the Mediterranean Sea. Hydrozoans are highly diverse organisms and their remarkable phenotypic plasticity, coupled with undeniable biological complexity, makes them essential for the preservation of biodiversity and the functioning of benthic habitats (Bouillon, et al., 2006). Focusing on the Mediterranean Sea ecosystem, the subject of this research, hydrozoans account for approximately 10% of global species. Yet, despite their numerical relevance, they remain relatively understudied compared to other components of marine fauna (Gravili et al., 2013). To address this gap, a systematic review of the literature was conducted, analyzing the geographical distribution, life cycles, trophic strategies, and symbiotic relationships of over 200 Mediterranean hydrozoan species. This review revealed a limited number of studies on the hydrozoan microbiome, despite evidence that hydrozoans significantly influence benthic community structure. They achieve this dual role as predators and ecosystem engineers by creating complex three-dimensional habitats, such as “hydrozoan forests” (Di Camillo, et al., 2006). Building on these findings, the final section of this thesis focuses on the hydrozoan forest of Eudendrium racemosum located in the Strait of Messina. This area is of ecological interest due to its geographical uniqueness, characterized by strong currents and high biodiversity. Field research conducted along the Pellaro coast highlighted the importance of E. racemosum as a structural and functional element of the ecosystem, confirming literature findings regarding the role of hydrozoans in supporting biodiversity and benthic communities (Gravili, Boero, Alifano, & Stabili, 2012; Di Camillo, et al., 2012). In light of the gaps identified in the scientific literature on hydrozoans and their microbiome, this thesis presents the first comprehensive investigation of the subject, examining their interactions not only with the surrounding environmental matrix but also with associated predators, particularly nudibranchs. To this end, the microbiomes of hydrozoans and three common nudibranch species associated with them (Flabellina affinis, Cratena peregrina, and Dondice sp.) were analyzed with the aim of describing the bacterial taxonomic composition of this holobiont and identifying similarities and differences in the acquisition of prokaryotic symbionts among the studied organisms and their surrounding environment. No data are available in the literature regarding the microbial community associated with these three Mediterranean nudibranch species. The results demonstrate the co-occurrence of bacterial taxa specific to both nudibranchs and hydrozoans, as well as the influence of prokaryotic microorganisms from the surrounding environment, such as water and sediments. The findings further suggest that the hydrozoan-nudibranch complex acquires part of its microbiome through horizontal transmission from these environmental matrices. Additionally, specific bacterial taxa, unique to the microbiomes of hydrozoans and nudibranchs and absent from the surrounding environment, were identified, supporting mechanisms of vertical transmission of symbionts between hydrozoans and their associated nudibranchs. The most abundant phyla were Proteobacteria, Tenericutes, Chloroflexi, Thaumarchaeota and Cyanobacteria Currently. Presence of the phylum Tenericutes and the order Mycoplasmatales: Mycoplasmatales is predominant in the nudibranchs analyzed; present only in low percentage in hydroids and absent in water and sediments; probable symbiotic relationship between nudibranchs and Mycoplasmatales. Vertical transmission highlights the existence of a strong symbiotic relationship between benthic organisms and their associated fauna, contributing significantly to the understanding of microbial dynamics and marine biodiversity. Beyond the findings of this research, which further underscore the potential of hydrozoans as environmental indicators and ideal models for studying the impacts of climate change on marine ecosystems, remains a need for further studies. This is particularly true for less explored regions in the scientific literature, such as the Calabrian coasts. Such research would help fill existing knowledge gaps regarding the ecology and trophic roles of hydrozoans within marine food webs. Concurrently, these investigations could open new avenues in pharmacology, given the biological and chemical diversity of hydrozoan holobionts and their potential applications in the development of novel drugs.