Background: Quercetin is a natural flavonoid with potent antioxidant, anti-inflammatory, anti-apoptotic, and immunomodulatory properties, and has recently gained increasing attention as a functional feed additive in aquaculture.
Objective: This review provides a comprehensive synthesis of the existing evidence on the effects of quercetin and its related products (nanoformulations and combinations) in aquatic animals, with a particular emphasis on its protective and disease-resistant roles.
Methods: Sixty-six experimental studies involving dietary, waterborne, injectable, and in vitro administration of quercetin in fish, crustaceans, and cell models were reviewed. Studies were classified by experimental model, species, chemical form of the active ingredient, route of administration, dietary concentration (for feeding trials), administration duration, target pathogen, and response indices.
Results: Fish accounted for the largest share of experimental models (55 studies, 82%), with common carp and Nile tilapia being the most frequently studied species. Among 39 feeding studies, dietary inclusion levels varied widely from 25 to 10,000 mg/kg, with the 101–500 mg/kg range most commonly adopted (56.4%). Across species, quercetin consistently enhanced antioxidant defenses by increasing SOD, CAT, GPx, GSH, and TAC while reducing ROS, MDA, and lipid peroxidation, an effect largely mediated through the Nrf2/Keap1 signaling pathway and coordinated with reductions in inflammation, apoptosis, and ferroptosis. As a disease-control agent, quercetin and its derivatives lowered viral load and improved survival against WSSV, TiPV, and GCRV, through stimulation of innate immunity (lysozyme, ALF, TLR, JAK/STAT signaling), and reduced bacterial burden against Aeromonas hydrophila and other pathogens by enhancing immune clearance and inhibiting virulence and biofilm formation. in vitro studies further confirmed antiviral and antibacterial activities. However, nanoformulations and co-administration with compounds such as florfenicol often confound the distinction between the independent effects of free quercetin and synergistic interactions.
Conclusion: The available evidence supports quercetin as a promising functional additive that strengthens antioxidant defense, modulates inflammation, and stimulates immunity in aquatic animals, thereby enhancing resistance to infectious diseases. Yet, substantial heterogeneity in species, chemical form, inclusion level, and experimental conditions precludes direct comparisons. Before commercial application, standardized species-specific concentration–response studies, accurate determination of active content, evaluation of feed stability, long-term safety assessment, and cost–benefit analyses are required.