Three bacterial core genera, the Fusobacteria Psychrilyobacter, the Tenericutes Mycoplasma, and the Gammaproteobacteria Vibrio, dominated the abalone digestive microbiota for the whole year and for each of the monospecific algal diet. Bacteria from these genera are known as obligate (Psychrilyobacter) or facultative anaerobes (Mycoplasma and Vibrio) [31, 34, 35]. This unexpected dominance of a core microbiota through the entire year may not only be due to the initial diet composition but also to gut morphology and its physicochemical and biochemical conditions . The abalone gastrointestinal tract offers stable environmental conditions, which are microaerophilic or anaerobic, with an acidic pH of 5.3–6 in the crop and the digestive gland [36, 37]. It has been previously shown that an acidic pH together with the presence of volatile fatty acid in the rumen is toxic for some bacteria  and thus likely to select for bacteria adapted to such conditions. Indeed, anaerobic bacteria from the classes Fusobacteriia and Mollicutes are only rarely found in coastal waters or associated with macroalgae [39–41]. However, these bacterial groups seem rather typical of digestive organs of marine holobionts as they are found in the sea squirt Ciona intestinalis, the limpet Patella pellucida, and marine carnivores [21, 42, 43]. Members of Vibrio are well known as pathogen of animals, and it is the most common bacterial genus in guts of marine animals such as the intestinal mucosa of marine fishes [35, 44–46]. Vibrio is also found associated with algae such as Ascophyllum nodosum, S. latissima, and decomposing fronds of Laminaria spp. [11, 16, 47]. Other gut microbiota also contain dominant core communities, e.g., the human gut microbiota is dominated by anaerobic bacteria belonging to Firmicutes and Bacteroidetes, representing more than 98% of 16S rRNA sequences, with few other anaerobes from other phyla and only a couple of aerobic Actinobacteria [6, 48, 49]. Gut microbiota of terrestrial gastropods are also dominated by a low diversity of anaerobes, including the Gammaproteobacteria Enterobacter, some Firmicutes, and Bacteroidetes [50, 51]. Therefore, the dominant digestive microbiota constitutive of the abalone holobiont comprises bacterial taxa typically found in marine environments and these taxa are not dominant in the gut of terrestrial holobionts.