Dietary tryptophan requirements of juvenile pacific white shrimp, Litopenaeus vannamei (Boone) reared in low-salinity water

An 8-week growth trial was designed to study the effect of dietary tryptophan on juvenile Pacific white shrimp Litopenaeus vannamei (Boone) in low-salinity water (0.50–0.80 g L−1). Six diets with different concentration of l-tryptophan (2.61, 3.11, 3.61, 4.11, 4.61 and 5.11 g kg−1 dry diet, defined as diet T1, T2, T3,T4, T5 and T6, respectively) were randomly assigned to triplicate groups of 30 shrimps (0.43 ± 0.005 g). The results indicated that 3.61 g kg−1 dietary tryptophan (diet T3) treatment gained the maximum weight gain, the highest protein efficiency ratio and body protein deposition, the relative muscle weight, and the lowest feed conversion ratio, hepatosomatic index, haemolymph urea nitrogen concentration and AST and ALT activities (P < 0.05). The polynomial regression calculated using weight gain, feed conversion ratio and body protein deposition indicated that the optimal dietary tryptophan requirement for L. vannamei reared in low-salinity water was 3.65–3.95 g kg−1 tryptophan of dry diet, correspondingly 8.90–9.63 g kg−1 of dietary protein. To identify the mechanism underlying tryptophan requirement, haemolymph metabolites with different treatments were analysed. The results revealed that lack or excess dietary tryptophan led to significant different metabolite profiles.