Upcycling Terminalia catappa Leaf Waste as Phytotherapy for Red Hybrid Tilapia: Protective Effects and Mechanisms against Hypersaline Ecosystems.

Terminalia catappa leaf (TCL) is a long-undervalued derivative of the Terminalia catappa tree that commonly been regarded as natural and agricultural waste. This study novelly investigates the use of TCL as phytotherapy in aquatic ecosystem for its protective effects against hypersaline water and discusses the underlying mechanisms by using red hybrid tilapia (Oreochromis mossambicus × O. niloticus). An 8-week experiment was conducted to evaluate growth performance, lysozyme activity, neurotoxicity, and antioxidant defense in fish exposed to TCL at a sub-lethal concentration (1/10^th LC₅₀, LC₅₀ = 1.21 g/L). Red hybrid tilapias were randomly assigned to 4 groups: T1, control group; T2, TCL group; T3, saltwater group; T4, saltwater + TCL group. The growth parameters and organosomatic index were evaluated to assess the impact of TCL on the growth performance. The lysozyme levels in serum, body mucus, brain, and liver were determined to evaluate the innate immune response. Neurotoxicity was assessed by measuring acetylcholine (ACh) and acetylcholinesterase (AChE) in the brain and liver to understand the contribution of TCL in alleviating salinity stress. Oxidative defense was evaluated by measuring the activities of catalase (CAT), glutathione (GSH), glutathione reductase (GR), total antioxidant capacity (T-AOC), and superoxide dismutase (SOD) in the brain and liver. Additionally, malondialdehyde (MDA) was measured as an indicator of oxidative stress. Results show that TCL improves growth performance, enhances immune responses, and alleviates oxidative stress, neurotoxicity, and inflammation under both normal and high salinity conditions. Antioxidant activities (SOD, CAT, GSH, GR, T-AOC) increased, while MDA decreased with TCL supplementation. These findings highlight the upcycling potential of TCL (0.121 g/L) in promoting aquatic ecosystem resilience, with broad implications for improving aquaculture in saline-alkaline regions, particularly in tropical and subtropical zones.