A part of gut bacteria of herbivorous marine invertebrates may utilize macroalgae components as carbon source. We hypothesized that exclusive feeding of a target alga to marine invertebrates may enrich gut bacteria involved in the degradation of the algal components. To test the hypothesis, we reared sea hares (Dolabella auricularia) and sea snails (Batillus cornutus) for a half - one month with exclusive feeding of a brown alga (Ecklonia cava) and compared their gut microbiota by sequencing of 16S rRNA gene amplicons.