Oxygen consumption and ammonia-N excretion
To measure oxygen consumption and ammonia-N excretion, one shrimp from each replicate was transferred to a 1000-mL volumetric flask containing seawater of one of the five test salinities [5000, 15 000, 25 000, 35 000 (control) and 40 000 mg L−1]. The flasks were sealed and placed in a water bath at 20 ± 0.5 °C for 3 h. Dissolved oxygen was above 4.0 mg L−1 at the end of the experiment. Water samples were collected by siphoning, and DO and ammonia-N were measured at the beginning and at the end of the experiment using the Winkler method and the phenolhypochlorite method respectively (Chen 2000). Blank seawater readings (i.e. without shrimp) of DO and ammonia-N were carried out, and then subtracted from the experimental units to correct for autogenic trends. The mean differences in DO and ammonia-N were determined between the beginning and the end of this period, and the oxygen consumption (mg g−1 h−1) and ammonia-N excretion (μmol g−1 h−1) rates were corrected to the water volume of each flask, the shrimp's wet weight and the duration of exposure (h). The atomic O/N ratios of the penaeids were calculated by dividing the amount of oxygen consumed by the nitrogen excreted in each salinity treatment.