Optimal Tryptophan Improved Growth and Regulated Agonistic Behavior of Oriental River Prawn, Macrobrachium nipponense

Tryptophan, recognized as the third limiting amino acid, plays a crucial physiological function. The optimal tryptophan requirement for oriental river prawn (Macrobrachium nipponense) was assessed, and its impact on agonistic behavior was evaluated in this study. An 8-week feeding trial was implemented with six different tryptophan levels: 0.07%, 0.17%, 0.29%, 0.39%, 0.52%, and 0.64%. The results showed that the highest specific growth rate (SGR) was observed in prawns fed with 0.29% tryptophan. Furthermore, body protein deposition (BPD) and protein efficiency ratio (PER) were significantly elevated in the 0.39% tryptophan group than those in the 0.07% tryptophan group, while the feed conversion rate (FCR) was lowest in the 0.39% tryptophan group. Based on quadratic regression analysis of SGR, BPD, FCR, and PER, the optimal tryptophan requirement was determined to be 0.35%-0.39% of dry matter, 0.90%-1.01% of crude protein. In the 0.52% tryptophan group, crude protein and lipid contents were improved, while ash content was the lowest. The total protein (TP), cholesterol, and urea nitrogen (UN) contents in the hemolymph were observed to be the highest levels in the 0.64% tryptophan group, although there was a reduction in triglyceride (TG) and glucose (GLU) levels. Muscle tor gene expression was the highest at 0.39% tryptophan, with atf4 and atf3 expressions suppressed. Hepatopancreas tor and cpt1 gene mRNA peaked at 0.29% tryptophan, while acc, fas, and atf4 genes were inhibited. Following the feeding trial, prawns fed diets with 0.07%, 0.29%, and 0.64% tryptophan were randomly selected for the detection of agonistic behaviors. Prawns fed 0.29% and 0.64% tryptophan showed less aggression than those fed 0.07%. Serotonin levels were highest in the 0.64% group, followed by 0.29%, and lowest in 0.07%. The 5-ht1b gene expression was significantly increased in the 0.64% tryptophan group compared to the 0.29% and 0.07% tryptophan groups. Both serotonin levels and 5-ht1b expression showed the same significant difference before and after fighting behavior, with increases observed postfight across all treatments. In conclusion, the optimal dietary tryptophan requirement for the oriental river prawn was estimated to be between 0.35% and 0.39% of dry matter (0.90% and 1.01% of crude protein), which enhanced growth performance and effectively reduced agonistic behavior.