This study presents the first record of the overall distribution of the seagrass Halophila beccarii Ascherson on the island of Hainan. Statistical and ecological methods were used to analyze the distribution of the species and the influence of environmental factors. Halophila beccarii was mainly distributed in the northwest and southeast of Hainan, in lagoons with a total area of 20.981 km², including the largest H. beccarii area in China covering approximately 9.58 km² in Xiaohai, Wanning; there were also 4.89 km² in Xinying Bay, Danzhou, approximately 2.20 km² in Huachang Bay, Chengmai, 1.88 km² in Dongzhai Port, Haikou, 0.95 km² in Xinying Port, Lingao, approximately 0.668 km² in Laoyehai, Wanning, approximately 0.363 km² in Hongpai, approximately 0.23 km² in Maniao, and approximately 0.22 km² in Huanglong Port, Lingao. The average coverage of H. beccarii measured at 7.08–56.33%, the density of stem and branch was 487.47–20,167.1 ind/m², and the biomass measured at 1.57–112.94 gDW/m². The growth distribution was mainly influenced by habitat type and, to a lesser extent, by tidal branching channels, heavy metal content (Cu, Pb, Cr, Zn) in the marine environment, and human activities in adjacent coastal areas.

In aquaculture, reduced water exchange helps constrain exogenous pathogen invasion, lower pumping costs, and improve management efficiency; nevertheless, its effects on the growth of cultured aquatic animals remain poorly characterized. This study compared two daily water-exchange rates (100% and 0%) to evaluate growth and associated intestinal responses in Babylonia areolata (ivory shell). Growth performance and intestinal enzyme activities were measured, while 16S rRNA gene sequencing and untargeted metabolomics were used to characterize microbial composition and metabolite profiles. The low-water-exchange group exhibited significantly lower weight gain rate (WGR, p < 0.0001), specific growth rate (SGR, p < 0.0001), daily increments in shell length (DISL, p = 0.0060), and shell width (DISW, p = 0.0012). Intestinal total superoxide dismutase (T-SOD, p < 0.0001) and lipase (LIP, p = 0.0011) activities were also significantly lower. The relative abundances of Vibrio, Halodesulfovibrio, and Fusibacter were higher, whereas that of Christensenellaceae_R-7_group was lower. Lipid-related metabolic pathways showed the most pronounced intestinal differences. Intestinal levels of prostaglandins (PGs), leukotrienes (LTs), and sphingosine (Sph) were significantly higher under low-water-exchange conditions. The relative abundance of Halodesulfovibrio correlated positively with PGs and LTs but negatively with lysophospholipids (Lpc) and T-SOD activity. The relative abundance of Christensenellaceae_R-7_group correlated positively with LIP activity and was significantly associated with metabolites involved in primary bile acid biosynthesis. Overall, low-water-exchange conditions were associated with coordinated changes in growth, intestinal enzyme activities, microbial composition, and metabolite profiles, although the underlying mechanisms require functional validation. These findings provide new insights into intestinal microbiota–metabolite responses to low-water-exchange conditions and may guide future validation and intervention studies in RAS-based ivory shell culture.