Low-Molecular-Weight Chitosan Produced by Gamma Irradiation and Hydrogen Peroxide Promotes Immune Readiness, Antioxidant Responses, and Survival in Photobacterium damselae-Challenged Asian Seabass (Lates calcarifer).

Low-molecular-weight chitosan may exhibit improved dispersion and biological accessibility compared with high-molecular-weight chitosan, but direct comparisons in Asian seabass remain limited. This study evaluated dietary low-molecular-weight chitosan produced by combined gamma irradiation and hydrogen peroxide depolymerization. The untreated and degraded preparations had apparent GPC-derived molecular weights of approximately 85 and 10 kDa, respectively. Juvenile Asian seabass (Lates calcarifer) were fed a Control diet of 1.0% high-molecular-weight chitosan or low-molecular-weight chitosan at 0.25%, 0.5%, or 1.0% for four weeks. Growth, serum antioxidant and humoral immune parameters, tissue-specific gene expression, and resistance to Photobacterium damselae were evaluated. No treatment significantly affected growth or feed utilization. Both high- and low-molecular-weight chitosan reduced serum MDA relative to the Control, whereas selected antioxidant and immune responses differed among inclusion levels. The 0.5% low-molecular-weight treatment increased GSH and GPx activity, the 1.0% low-molecular-weight treatment produced the highest CAT activity, and the 0.25% low-molecular-weight treatment produced the highest total serum IgM. Low-molecular-weight chitosan generally produced broader tissue-specific transcriptional responses than 1.0% high-molecular-weight chitosan. Following bacterial challenge, all chitosan treatments improved survival relative to the Control, with the highest numerical RPS observed in the 0.5% low-molecular-weight group. These results indicate that molecular-weight reduction influenced the biological response to dietary chitosan. Among the tested levels, 0.5% produced the most consistent combined response, although the optimal level was endpoint-specific and requires longer-term validation.