The surface structures and mucus layers that form an interface between the epithelial layer of organisms and their external environment were studied in the bloom-forming moon jellyfish (Aurelia solida, Scyphozoa) from the northern Adriatic. The surface of the polyps revealed epithelial ciliated cells and numerous nematocysts, both non-discharged and discharged. Cilia were also the most prominent features on the surface of adult medusa, protruding from the epidermal cells and with microvilli surrounding the base. Histochemical methods and various microscopy techniques (light/epifluorescence and electron microscopy) confirmed the presence of abundant mucus around polyps and on the surfaces of adult medusa, and that the mucus contained acidic and neutral mucins. The observed mucus secretions on the exumbrella surface of the medusae were in the form of granules, flocs, and sheets. Scanning electron microscopy and transmission electron microscopy analyses confirmed the presence of various microbes in the mucus samples, but not on the epithelial surfaces of the polyps or the exumbrella of the medusae.

This study is the first to investigate bacterial community associated with live medusa Aurelia sp . in the Gulf of Trieste (northern Adriatic Sea) using both culture independent and culture-based methods. We have analysed bacterial community composition of different body parts of medusa: exumbrella surface, oral arms (‘outer’ body parts) and of gastric cavity (‘inner’ body part) and investigated possible differences in medusa associated bacterial community structure at the time of jellyfish population peak and during senescent phase at the end of bloom, when jellyfish start to decay. Based on 16S rRNA clone libraries and denaturing gradient gel electrophoresis (DGGE) analysis, we demonstrated significant difference between bacterial community associated with Aurelia and the ambient seawater bacterial assemblage. Comparing bacterial community composition between different Aurelia medusa body parts, communities differed significantly, especially the one within the gastral cavity. The pronounced difference is dominance of Betaproteobacteria ( Burkholderia, Cupriavidus and Achromobacter ) in gastral cavity of medusa and Alpha - ( Phaeobacter, Ruegeria ) and Gamma - proteobacteria ( Stenotrophomonas, Alteromonas, Pseudoalteromonas and Vibrio ) on ‘outer’ body parts. This suggests that body-part specific bacterial association might have an important functional roles for the host. The results of bacterial isolates showed the dominance of Gammaproeteobacteria , especially Vibrio and Pseudoalteromonas in all body parts. Finally, comparison of medusa associated bacterial community structure, at the time of jellyfish population peak and during senescent phase at the end of bloom showed increased abundance of Gammaproteobacteria , especially Vibrio . Our results suggest members of Vibrio group are possible commensal opportunistic visitors, later becoming consumer of moribund jellyfish biomass and that the structure of jellyfish bacterial community might be affected by anthropogenic pollution in the marine environment.