Although Flammulina velutipes residues are increasingly used in fish feeds, their mode of action in the gut is poorly defined. Here, we show that feeding Gymnocypris eckloni diets supplemented with F. velutipes stem‐base (FVS) or its polysaccharides (FVP) selectively enhance foregut morphology—increasing both villus height and muscle layer thickness—without affecting mid‐ or hindgut regions. Transcriptome profiling revealed th...