The endosymbiotic algae S. microadriaticum is known to live in association within C. tuberculata in the Mediterranean Sea. The dinoflagellates are typically located in the jellyfish endodermal cells within symbiosomes: A perisymbiotic membrane, derived from the host cell plasmalemma, keeps the coccoid stage of the dinoflagellate separated from the jellyfish cell cytoplasm [89]. The color of the umbrella top and the frilled edges of the oral arms, appeared greenish-brown in the living tissue where high density of zooxanthellae in the mesoglea was observed (Figures 1 and 2A,B), mostly aggregated in small clusters and showing high division rates (Figure 2C). Symbiodinum spp. was present as both coccoid and motile biflagellate cell stages. However, most of the cells were in the coccoid stage, spherical, with homogeneous size of 8.4 ± 1.0 μm in diameter, with a single, peripheral, reticulated chloroplast, occupying a large part of the cell volume. Coccoid Symbiodinium cells appeared metabolically active, since they undergo mitosis (Figure 2C); therefore, high photosynthetic activity and product accumulation it is expected at this stage [90]. It was also evident a relatively large accumulation body (3.0 ± 0.5 μm in diameter), as a round shaped, reddish-brown vacuole, which displayed an unspecific green auto-fluorescence under confocal microscope, with emission peak between 505 and 530 nm (Figure 2C), similarly to previous observationsin Symbiodinium-coral symbiotic associations [91].