Dietary Green-Algae Chaetomorpha linum Extract Supplementation on Growth, Digestive Enzymes, Antioxidant Defenses, Immunity, Immune-Related Gene Expression, and Resistance to Aeromonas hydrophila in Adult Freshwater Snail, Bellamya bengalensis

Macroalgae plays a significant role in the formulation of innovative and environmentally sustainable approaches to address food challenges. Specifically, green macroalgae serve as dietary supplements aimed at improving the health, growth, and feeding efficiency of various species of marine and freshwater fishes, as well as mollusks. The effects of Chaetomorpha linum extract (CLE) on growth performance, physiological responses, and disease resistance are studied in Bellamya bengalensis against Aeromonas hydrophila. In this experiment, adult B. bengalensis (4412 ± 165.25 mg) were randomly divided into 15 rectangular glass aquariums (35 snail/aquaria; 45 L capacity) and their basal diet was supplemented with different levels of CLE, including 0 (CLE0), 1 (CLE1), 2 (CLE2), 3 (CLE3), and 4 (CLE4) g/kg for 60 days. The growth performance in the CLE3 dietary group was significantly higher that of the CLE0 group, exhibiting both linear and quadratic trends in relation to dietary CLE levels (p < 0.05). The activities of pepsin, amylase, and lipase were found to be highest in CLE3 and lowest in CLE0. Both linear and quadratic responses to dietary CLE levels in digestive enzymes were observed (p < 0.05). The activities of superoxide dismutase and catalase in the hepatopancreas were found to be elevated in snails due to the synergistic effect of the supplemented CLE diet. Among different levels of diet given, CLE2-supplemented snails showed an increase in their enzyme activity (p < 0.05). Interestingly, all the CLE-treated snails expressed elevated levels of mucus lysozyme and mucus protein when compared to control (p < 0.05). Additionally, hepatopancreatic acid phosphatase and alkaline phosphatase activity were elevated in snails consuming CLE3 (p < 0.05). The transcription levels of immune-related genes, including mucin-5ac and cytochrome, were significantly elevated in snails that were fed a diet supplemented with 2–4 g of CLE/kg. Furthermore, the transcription level of the acid phosphatase-like 7 protein gene also increased in snails receiving CLE-supplemented diets. After a 14-day period of infection, snails that consumed a diet supplemented with 3–4 g/kg of CLE exhibited a notable increase in survival rates against virulent A. hydrophila. Based on the above findings, it is suggested that a diet supplemented with 3 g/kg of CLE may enhance growth, antioxidant and immune defense, and disease resistance in the freshwater snail B. bengalensis.