Growth-Associated Reorganization of Gill and Skin-Mucus Microbiota in Cultured Decapterus maruadsi: Contrasting Occurrence-Abundance Patterns

External mucosal microbiota form important interfaces between fish and the aquatic environment, yet their variation during host growth remains poorly understood. We characterized the gill and skin-mucus microbiota of cultured Decapterus maruadsi across three successive body-size stages (4.98-6.81 g, 16.67-19.63 g, and 27.30-28.66 g) using 16S rRNA gene amplicon sequencing. Fish were sampled from the same cohort maintained in the same culture system, and two supplied-seawater samples were included as environmental references. Factorial multivariate analysis indicated significant effects of mucosal tissue, sampling stage, and their interaction on community composition. Broad taxonomic structure remained dominated by Pseudomonadota, whereas extensive differentiation occurred at finer taxonomic resolutions. Skin-mucus richness decreased across successive sampling stages, whereas no corresponding alpha-diversity difference was detected in the gill. Host-wide prevalence analysis revealed contrasting changes in the abundance contribution of recurrent ASVs between the two mucosal compartments; however, tissue-specific sensitivity analyses showed that occurrence categories depended partly on the sample set used to define prevalence. The same dominant Limimaricola ASV occurred at high abundance in one supplied-seawater sample and in middle-stage skin mucus, indicating environmental microbial input as a plausible contributor to the observed composition. Overall, this study provides a descriptive baseline for stage-associated variation in the external mucosal microbiota of one cultured D. maruadsi cohort and highlights distinct responses of gill and skin-mucus communities without attributing these changes solely to host growth.