Effect of live yeasts and essential oils as functional feed additives on growth performance and stress response in juvenile cobia

The intensification of aquaculture introduces significant challenges, including increased exposure to environmental stressors that promote disease outbreaks, historically managed through antimicrobial use. As sustainable alternatives, functional feed additives are being explored for their potential to enhance fish performance and resilience. This study screened six candidate functional feed additives, including two autochthonous probiotic yeasts ( Candida haemuloni C27 and Debaryomyces hansenii C28) and four essential oils (EOs): lemongrass, ginger, garlic, and oregano, to evaluate their potential benefits and adverse effects on growth performance, physiological status, stress response, and serum antipathogenic activity in juvenile cobia ( Rachycentron canadum ), a benthopelagic marine species with high aquaculture potential. Fish were fed functional diets for four weeks, maintained under controlled physicalchemical conditions. Overall, most additives had no significant positive effect ( p > 0.05) on productive parameters, except garlic EO, which reduced feed intake and was associated with lower growth performance and histological alterations (shorter intestinal folds and damaged enterocytes and hepatocytes). However, serum from juvenile cobia supplemented with C. haemuloni C27 and ginger EO exhibited inhibitory activity against three strains of Vibrio harveyi , pathogens of fish. Additionally, cobia supplemented with C. haemuloni C27, ginger EO, garlic EO, and oregano EO displayed reduced serum cortisol levels following hypoxia-induced stress. Overall, the response to functional additives was additive-specific. C. haemuloni C27 and ginger EO emerged as the most promising candidates, enhancing serum antipathogenic activity and stress resilience without adverse effects on growth or tissue integrity, whereas garlic EO negatively affected feed acceptance, growth performance, and tissue condition at the inclusion level tested. These findings highlight the importance of evaluating both efficacy and safety when selecting functional feed additives for juvenile cobia.