Effects of defatted black soldier fly larvae meal on growth, nutrient digestibility, hepatopancreas biochemistry, intestinal microbiota, and phosphorus discharge of giant freshwater prawn (Macrobrachium rosenbergii)

In recent years, the use of insect meal in feed has been regarded as an important alternative to fishmeal. Defatted black soldier fly larvae meal (BSFLM), derived from <i>Hermetia illucens</i>, is a promising fishmeal alternative in aquafeeds for various aquatic species. However, its potential as a protein source in diets for the giant freshwater prawn (<i>Macrobrachium rosenbergii</i>) remains underexplored. This study provides a systematic evaluation of BSFLM as a fishmeal substitute in <i>M. rosenbergii</i> diets, assessing growth performance, nutrient digestibility, hepatopancreatic biochemistry, intestinal microbiota composition, and phosphorus discharge. Five isonitrogenous experimental diets were formulated, with BSFLM replacing 0 (control, BSFLM0), 10% (BSFLM10), 20% (BSFLM20), 40% (BSFLM40), or 80% (BSFLM80) of dietary fishmeal. A total of 900 juvenile prawns (initial weight 0.25 ± 0.03 g, 40-d-old) were fed experimental diets. They were reared in 300-L tanks (60 prawns/tank, 3 replicates/group) and fed to satiation twice daily for 8 weeks. Each tank was treated as an independent experimental unit for statistical analysis. Growth performance (survival, weight gain, and feed conversion ratio) did not differ significantly among groups (<i>P</i> > 0.05). BSFLM40 and BSFLM80 groups showed significantly improved phosphorus retention efficiency (14.57% to 19.24%) and deposition rate (13.69% to 18.16%), alongside reduced phosphorus waste (<i>P</i> < 0.05). Intestinal digestive enzyme activities (trypsin, amylase, and lipase) were significantly upregulated at 40% to 80% BSFLM inclusion (<i>P</i> < 0.05). BSFLM-fed prawns exhibited enhanced antioxidant capacity (total superoxide dismutase activity was significantly elevated, <i>P</i> < 0.001) with stable hepatopancreatic malondialdehyde levels (<i>P</i> = 0.343). Transcriptomic analysis revealed differential expression of nutrient metabolism (involving in the insulin signaling pathway and mineral absorption pathway) and immune response genes (associating with phagosome activity and antigen processing/presentation) in BSFLM20 vs. control. BSFLM inclusion altered intestinal microbiota: BSFLM10 enriched Firmicutes abundance, while BSFLM40 increased <i>Agromyces</i> abundance (<i>P</i> < 0.05). BSFLM-fed prawns displayed enhanced intestinal morphology (villus height and muscularis thickness increased, <i>P</i> < 0.001). In summary, BSFLM can replace up to 80% of fishmeal in <i>M. rosenbergii</i> diets without compromising growth. Moderate replacement (10% to 20%) enhanced antioxidant capacity and optimized microbiota; higher levels (40% to 80%) improved digestive function, reduced phosphorus discharge, and stimulated chitinolytic bacteria, confirming BSFLM as a nutritionally balanced and environmentally sustainable fishmeal alternative in <i>M. rosenbergii</i> aquaculture.