Total RNA was extracted using 50–100 mg of each tissue (kidney, gill and hepatopancreas) using TRI reagent (Sigma, U.S.A.) as per manufacturer's instruction. The concentration of the nucleic acid in the sample was quantified by measuring the absorbance at 260 nm using Nanodrop ND-1000 spectrophotometer (Thermo Scientific, U.S.A.). The purity of the samples was checked by measuring the ratio of OD 260 nm/OD 280 nm. Only the samples having OD ratio of 1.8–2.0 were taken for the cDNA synthesis.