Effect of chitooligosaccharides on the growth and immune capability of Crassostrea gigas larvae

• Chitooligosaccharides (COS) promoted the growth of Crassostrea gigas larvae. • COS enhanced the immune capability of C. gigas and increased its survival rate . • COS is expected to be a sustainable alternative to antibiotics in the oyster farming industry. The oyster aquaculture contributes significantly to the aquaculture in China, while recently, it has been suffered by frequent larval mortality caused by pathogenic bacteria. Developing antibiotic-free immunopotentiator is an effective strategy to enhance the immunity of oyster larvae and address the mortality issue. Chitooligosaccharide (COS), an eco-friendly prebiotic, was applied to investigate its effect on the growth and immune capability of Crassostrea gigas larvae. The growth performance of C. gigas larvae was improved by COS treatment at concentrations of 0.1, 1, 10 mg/L. After Vibrio splendidus challenge, larval survival was significantly increased only by the 10 mg/L COS treatment (64.2%). The survival rate in the 10 mg/L COS-treated group was 3.03-fold higher than that of the control group (21.2%), and 1.77-fold, 1.72-fold higher than that of the 0.1 mg/L (36.2%) and 1 mg/L (37.3%) COS-treated groups. After 10 mg/L COS treatment, the catalase (CAT) activity (3.59-fold, p < 0.05), superoxide dismutase (SOD) activity (1.25-fold, p < 0.05) and the total antioxidant capacity (T-AOC) (1.72-fold, p < 0.05) were significantly increased compared to the control group. Larval protein extracts from COS-treated group significantly inhibited the growth of V. splendidus and Staphylococcus aureus . Furthermore, COS reshaped the larval associated microbial composition by reducing the relative abundance of potential pathogenic bacteria, such as Acinetobacter and Vibrio (Proteobacteria). In summary, COS treatment promoted growth, enhanced immune capability and modulated the associated microbial composition in oyster larvae. These findings highlight the potential of COS as a sustainable alternative to antibiotics in the oyster farming industry.