In this study, we initially tested if the talitrid amphipods ate the stranded brown macroalgae and confirmed they fed on the algae, indicating that the talitrid amphipods degrade and metabolize the brown macroalgae in their guts. In order to understand the mechanism of how the amphipods degrade and metabolize the brown macroalgae, we focused on the microbes in their guts in the talitrid amphipods, initially by isolating alginate-assimilating bacteria, and then analyzing microbiota in the guts housed in the several conditions with or without antibiotics. This data are Raw fastq data of the ITS2 amplicon analysis of the gut metagenome of the talitrid amphipods housed in several conditions without antibiotics.