The acute and chronic effect of ammonia and nitrite were evaluated on Fenneropenaeus indicus using standard toxicological methods. Also, the effect of pH on the ammonia excretion and the ontogenetic change in oxygen consumption and ammonia excretion were studied.
The 24 and 144-h LC₅₀ values for NH₃-N were 2.16 and 0.87 mg 1^-1 for postlarvae and 1.70 and 0.73 mg 1^-1 for juveniles and for nitrite-N, the values were 62.51 and 18.73 mg 1^-1 for postlarvae and 230.80 and 21.38 mg 1^-1 for juveniles. The joint action of equitoxic mixtures of ammonia and nitrite have a synergistic-additive effect in postlarvae and an antagonistic effect in juveniles.
Chronic exposure to ammonia and nitrite significantly reduced the ingestion rate, trypsin activity and the growth rate of postlarvae. Oxygen consumption of postlarvae increased by 38% when exposed to nitrite in comparison to the rate when shrimp were exposed to ammonia. The inhibition of the energy intake induced by ammonia and nitrite could explain, at least in part, the long-term reduction of growth observed in chronically exposed postlarvae. The difference in the magnitude of the reduction of growth in postlarvae after exposure to nitrite compared with ammonia, could be explained because the energy intake was more reduced and the energy used in the metabolism increased when postlarvae were exposed to nitrite.
Differences between postlarvae and juveniles, such as the effect of the equitoxic mixture of ammonia and nitrite and the effect of nitrite on the respiration rate, make juveniles more resistant than postlarvae.
 The weight-specific oxygen consumption and ammonia excretion decreased from larvae to juveniles. The ammonia excretion of postlarvae and juveniles could contribute to an increased ammonia-N concentration in the water at a rate of 0.008 to 0.890 mg day^-1 ind^-1, dependent on prawn age and the pH.