The discovery of symbiotic associations extends our understanding of the biological diversity in the aquatic environment and their impact on the host’s ecology. Of particular interest are nudibranchs that are unprotected by a shell and feed mainly on sponges. The symbiotic association of the nudibranch Rostanga alisae with bacteria was supported by ample evidence, including an analysis of cloned bacterial 16S rRNA genes a fluorescent in situ hybridization analysis, and microscopic observations. A total of 74 clones belonging to the phyla α-, β-, γ-Proteobacteria, Actinobacteria, and Cyanobacteria were identified. FISH confirmed that bacteriocytes were packed with Bradyrhizobium, Maritalea, Labrenzia, Bulkholderia, Achromobacter, and Stenotrophomonas mainly in the foot and notum epidermis, and also an abundance of Synechococcus cyanobacteria in the intestinal epithelium. Ultrastructural analysis showed several bacterial morphotypes of bacteria in epidermal cells, intestine epithelium, and in mucus layer covering the mollusk body. The high proportion of typical bacterial fatty acids in R. Alisae indicated that symbiotic bacteria make a substantial contribution to its nutrition. Thus, the nudibranch harbors a high diversity of specific endo- and extracellular bacteria, which were previously unknown as symbionts of marine invertebrates that provide the mollusk with essential nutrients. They can provide chemical defense against predators.
