Substituting fish meal with housefly (Musca domestica) maggot meal in diets for bullfrog Rana (Lithobates) catesbeiana: Effects on growth, digestive enzymes activity, antioxidant capacity and gut health

An 8-week feeding trial was carried out to evaluate the effects of replacing fish meal (FM) with housefly (Musca domestica) maggot meal (HFMM) on growth, antioxidant capacity, digestive enzymes activity, and gut health of bullfrog Rana (Lithobates) catesbeiana. A basal experimental diet was formulated using FM and soybean meal as the major protein sources, then four other diets were produced by replacing 25, 50, 75 or 100% of FM with HFMM (FM, HFMM25, HFMM50, HFMM75 and HFMM100 diets). Each diet was fed to triplicate groups of bullfrog (34.1 ± 1.10 g) to apparent satiation twice daily. No significant linear or quadratic trends were observed between FM replacement level and growth, feed utilization and survival rate (P > 0.05). A significant negative linear trend was detected between the incremental levels of HFMM and apparent digestibility coefficients (ADCs) of dry matter, protein and lipid, while positive linear and quadratic trends were found in ADC of phosphorous (P < 0.05). A negative linear trend was observed between whole-body ash content and HFMM level. Positive linear and quadratic trends were found between serum aspartate aminotransferase, alkaline phosphatase and lysozyme activities and incremental HFMM levels. Liver superoxide dismutase and catalase activities were increased linearly, and total antioxidant capacity was enhanced both linearly and quadratically by incremental levels of HFMM. In contrast, a negative linear trend was found in liver malondialdehyde concentration. A significant positive quadratic trend was found between intestinal protease, lipase and amylase activities and incremental levels of HFMM. Histological observations revealed a significant negative linear trend between HFMM level and villus height and muscular thickness. Gut microbiota analysis showed no significant changes in the operational taxonomic unit (OTU) diversity and richness indices among treatments. Positive linear and quadratic trends were found between incremental HFMM levels and expression of liver HSP70 and IGF-Ι and intestinal TNF-α, IL-1ß, IL-8 and IL-17 genes. The findings in this study showed that although growth performance was not significantly affected by total replacement of FM with HFMM, nutrients digestibility and gut health could be adversely influenced at higher replacement levels.