Growth-Associated Succession and Persistence of Gill and Skin Mucus Microbiota in Cultured Larimichthys crocea

External mucosal surfaces form major interfaces between fish and the aquatic environment, yet their microbial succession during host growth remains incompletely resolved. We characterized the gill and skin mucus microbiota of cultured Larimichthys crocea at three successive body-size stages (5–8 g, 39–46 g, and 160–162 g) using 16S rRNA gene amplicon sequencing. At each stage, 15 fish were sampled, and material from three fish was pooled to generate one pooled sample, yielding five pooled samples for each host-associated group. Four samples of truck-delivered seawater supplied by the same provider used throughout cultivation were included as an environmental reference. Rarefaction curves approached saturation, and amplicon sequence variant (ASV)-level community structure differed significantly among the seven groups (ANOSIM, R = 0.7521, p = 0.001). Early-stage gill and skin mucus communities occupied clearly separated regions of ordination space and showed distinct dominant taxa, whereas large-stage gill and skin mucus communities showed reduced separation and greater similarity in dominant taxonomic composition. At the genus level, 2013Ark19i characterized early-stage gills, Vibrio and Photobacterium were prominent in early-stage skin mucus, and Methylobacterium increased markedly in both large-stage mucosal communities. Although 17 phyla, 67 families, and 69 genera were shared across all groups, only 30 ASVs were universally shared, indicating extensive fine-scale turnover. Dataset-wide prevalence classification further showed that persistent ASVs contributed an increasing fraction of community abundance during growth, rising from 21.93% to 75.48% in gills and from 12.77% to 67.61% in skin mucus. Middle-stage skin mucus contained 1387 group-specific ASVs and the highest transient-ASV contribution among host-associated groups, indicating particularly extensive turnover. Overall, external mucosal microbiota exhibited tissue-specific and non-linear growth-associated reorganization, with pronounced intermediate-stage turnover and increasing abundance dominance of persistent ASVs during later growth.