Introduction
Early larval stages in marine fish are characterised by rapid growth, high metabolic demand, and still-developing antioxidant systems, making them particularly sensitive to oxidative imbalance. Excess production of reactive oxygen species during this phase can impair cellular function and developmental processes. While nutritional strategies in hatcheries have traditionally focused on maximising growth, there is increasing recognition that growth alone does not fully reflect larval quality. Enhancing physiological robustness, particularly through modulation of redox homeostasis, is emerging as a complementary objective in larval nutrition.
The Senegalese sole is a well-established species in marine aquaculture and a relevant model for nutritional studies during early development, where assessing physiological responses alongside growth can provide a more comprehensive evaluation of larval condition.
Algae-derived ingredients are promising functional feed components due to their richness in bioactive compounds, including pigments, vitamins, and complex sulphated polysaccharides with antioxidant properties. Microalgae such as Euglena spp. and Tetraselmis spp., together with macroalgae like Ulva spp., have been associated with improved physiological responses in aquatic organisms. However, their role in modulating oxidative status during early developmental stages remains insufficiently characterised.
Within this context, this study evaluated the effects of dietary inclusion of algae biomasses on growth performance and oxidative status in Senegalese sole post-larvae.
Materials and Methods
Senegalese sole post-larvae (initial dry weight: 6.6 mg) were reared in a recirculating aquaculture system using 8.8 L flat-bottom tanks, with three replicate tanks per dietary treatment. Water quality conditions were maintained throughout the experimental period, with dissolved oxygen above 96%, temperature at 20.4 °C, and ammonia and nitrite concentrations below 0.1 mg L-1.
From 30 to 43 days after hatching (DAH), larvae were fed one of three experimental diets: a control diet (CTRL), the control supplemented with Euglena sp. biomass (EUG), and a diet containing a blend of macroalgae (Ulva sp.) and microalgae (Euglena sp. and Tetraselmis sp.) (MIX).
At 43 DAH, larvae were sampled to assess growth performance and oxidative status. Growth was evaluated through performance indicators including final dry weight (DW), total length (TL), condition factor (K), and relative growth rate (RGR). Oxidative damage was assessed via lipid peroxidation (LPO), while antioxidant responses were evaluated by measuring superoxide dismutase (SOD), catalase (CAT), and glutathione redox balance (GSH:GSSG ratio).
Results and Discussion
At 43 DAH, no significant differences were observed among dietary treatments in growth performance. Average final dry weight reached 22.55 ± 0.45 mg and relative growth rate averaged 10% day-1, indicating that algae inclusion did not influence larval growth.
Despite similar performance, clear differences were detected in larval oxidative status. Lipid peroxidation (LPO) remained unchanged across treatments, indicating no increase in oxidative damage. However, a significantly elevated GSH:GSSG ratio in both algae-supplemented groups points to a more robust redox status, driven by the high availability of reduced glutathione (GSH). This suggests that these diets primed the larvae with a more favourable redox environment, potentially increasing their capacity to mitigate oxidative challenge.
Superoxide dismutase (SOD) activity was significantly lower in the EUG group and intermediate in the MIX group relative to CTRL, while catalase (CAT) activity remained unchanged. The reduction in SOD activity, coupled with stable LPO levels, suggests a decreased requirement for enzymatic antioxidant defences rather than impaired capacity, indicating a 'sparing effect' facilitated by the non-enzymatic glutathione system. The intermediate response in the MIX group, suggests a co-regulation of antioxidant systems, where the shift is maintained but less pronounced than with Euglena alone.
These findings demonstrate that algae-derived ingredients modulate redox homeostasis without inducing oxidative stress or affecting growth performance, supporting their role as functional dietary components during early development.
Conclusion
Algae-based diets modulate oxidative status in Senegalese sole larvae without compromising growth, highlighting their potential as functional feeds targeting physiological robustness beyond conventional performance metrics.
Acknowledgements
This work was financially supported by "Pacto da Bioeconomia Azul" (Project No C644915664-00000026) within the WP5 Algae Vertical, funded by Next Generation EU European Fund and the Portuguese Recovery and Resilience Plan (PRR), under the scope of the incentive line "Agendas for Business and Innovation" through the funding scheme C5 – Capitalization and Business Innovation; and by FCT – Foundation for Science and Technology through projects UIDB/04326/2020 (DOI:10.54499/UIDB/04326/2020), UIDP/04326/2020 (DOI:10.54499/UIDP/04326/2020), and LA/P/0101/2020 (DOI:10.54499/LA/P/0101/2020) to CCMAR. Aqsa Sharif acknowledges financial support from FCT through the PhD grant 2025.07044.BD. The authors thank the companies Flatlantic for supplying the fish and Necton SA for supplying the microalgae.