Edible Fungi in Functional Aquafeeds: Prebiotic and Immunostimulatory Effects in Commercially Important Farmed Fish

Aquaculture faces structural constraints linked to input sustainability, animal health, and a persistent reliance on fishmeal and fish oil, alongside the intensive use of antibiotics in high-density production systems. In this context, edible fungi have emerged as promising functional ingredients, primarily due to their content of structural polysaccharides, particularly β-glucans and chitin. This review synthesizes recent evidence on the dietary inclusion of fungal-derived compounds in aquafeeds for commercially relevant species. Overall, these bioactive components have been associated with beneficial effects on growth performance, feed efficiency, antioxidant status, and disease resistance in species such as Oncorhynchus mykiss, Salmo salar, and Oreochromis spp. However, the magnitude and consistency of these responses remain highly variable across studies. Such variability is primarily attributed to differences in the structural composition of the ingredients, their physicochemical form, processing methods, and their compatibility with host digestive physiology, all of which influence functional bioavailability and the resulting biological responses. Consequently, their biological effects are context-dependent and should be interpreted according to ingredient characteristics, host species, dietary inclusion level, and experimental conditions rather than assuming a universal dose–response relationship. Effective implementation will require advances in matrix standardization, detailed structural characterization of bioactive compounds, and validation under industry-relevant conditions.