Regarding algal polysaccharide degradation in abalone digestive gland, two scenarios are possible, involving either (i) the core dominant community only, or (ii) the core dominant community together with the less abundant and the transient community (Fig. 6). The first scenario is based on putative lateral acquisitions of CAZymes by the three dominant genera of the abalone digestive microbiota, e.g., from algal epiphytic bacteria. Earlier studies have shown the possibility of horizontal transfer of CAZyme-genes (e.g., beta-porphyranases, GH117, alginate lyases) from algal epiphytic bacteria to intestinal bacteria from human or marine fishes in relation to the consumption of seaweeds [78–80]. In this scenario, the less abundant core microbiota and transient bacteria would play only a complementary role in algal degradation. The second scenario assumes that the dominant bacteria in the microbiota are largely incapable of degrading complex algal polysaccharides, as suggested by their low CAZyme and sulfatase content found in our survey. Only Vibrio may possess a limited capacity to degrade algal polysaccharides as suggested by the analysis of V. crassostreae and V. breoganii genome, which are often found in the gut of invertebrates, and genomes of the species V. crassostreae, V. harveyi, and V. halioticoli which were previously isolated from abalone gut and described as alginate degraders [44, 81]. Also, V. halioticoli isolated from abalone are able to produce SCFA such as acetate from alginate (Additional file 5: Table S4 and Additional file 8, [16, 81, 82]). The less abundant and diet-specific core microbiota may complement the capacities of Vibrio as they also have the potential to degrade algal polysaccharides, e.g., Polaribacter, Roseobacter, or Psychromonas [54, 55, 60, 81].