Low-Fishmeal Dietary Supplementation with Crayfish By-Product Protein Hydrolysate Affects Growth Performance, Appetite-Related Metabolic Signaling and Intestinal Microbiota of Pacific White Shrimp (Litopenaeus vannamei)

Background/Objectives: Low-fishmeal diets are widely adopted to improve sustainability in shrimp aquaculture, yet reduced palatability and metabolic stress frequently suppress feed intake and growth. We evaluated whether a crayfish (Procambarus clarkii) by-product protein hydrolysate (CBPH) could mitigate low-fishmeal-induced performance losses by modulating feeding-related metabolic signaling and gut microbiota features in Pacific white shrimp (Litopenaeus vannamei). Methods: In an 8-week feeding trial, 360 juveniles (initial body weight 0.46 g) were assigned to three diets (four replicates per diet): a commercial control (CON), a low-fishmeal diet (LFM), and LFM supplemented with 2% CBPH (CBPH). Growth, feed utilization, whole-body composition, hemolymph biochemical indices (TP, TG, GLU, AST, ALT), intestinal appetite-related gene expression (5-HTR, CART, CCK1R, D2-like, NPY), and intestinal microbiota profiles (full-length 16S rRNA sequencing, V1–V9, PacBio) were assessed. Results: Compared with the LFM group, CBPH supplementation increased feed intake and improved feed conversion, restoring final body weight and growth rates to levels comparable to CON. CBPH also alleviated low-fishmeal-associated metabolic stress, including reduced AST and ALT activities and lower glucose levels. The LFM diet induced upregulation of anorexigenic genes (5-HTR, CART, D2-like) and downregulation of NPY in the shrimp intestine, whereas CBPH supplementation reversed these transcriptional changes. In addition, microbiota richness indices (ACE and Chao1) were elevated by CBPH relative to LFM, accompanied by compositional shifts at the phylum and genus levels. Conclusions: CBPH effectively alleviated low-fishmeal-induced reductions in feeding and growth, accompanied by coordinated changes in feeding-related gene expression, systemic biochemical markers, and gut microbiota composition, supporting its potential as a functional ingredient to stabilize metabolic responses in low-fishmeal shrimp feeds.