Combined and isolated effects of ammonia and nitrite on Amazon River prawn Macrobrachium amazonicum (Heller, 1862) juveniles

This study aimed to evaluate the combined effect of total ammonia and nitrite on Macrobrachium amazonicum juveniles exposed for 96 h. Juveniles (6.3 +/- 0.7 cm and 1.79 +/- 0.60 g; n = 640) were used in a completely randomized factorial design with four concentrations of total ammonia (0, 11, 22, and 44 mg.L-1) and four concentrations of nitrite (0, 1.5, 2.5, and 5 mg.L-1) with four replicates (n = 10) for each combination. Prawn gills were collected for histological evaluation and observation of damage to the organ (Iorg). The probit method was used to calculate the isolated LC50-96 h values for total ammonia and nitrite, the results indicated concentrations of 29.85 and 2.49 mg.L-1, respectively. The LC50-96 h of the combined compounds was 20.87 mg.L-1 for total ammonia and 2.95 mg.L-1 for nitrite. The isolated safety levels of total ammonia and nitrite were 2.98 and 0.25 mg.L-1, while for the combined treatments the safety level was 2.08 mg.L-1 for total ammonia and 0.29 mg.L-1 for nitrite. Calculating the sum of additivity and the additivity index of the mixture showed the effect of the combined compounds is 0.88 times lower than the sum of the isolated effects, thus an antagonistic effect. That is reinforced by histological analysis of gills, with reduced gill damage in the combined treatments. In conclusion, M. amazonicum showed greater tolerance to total ammonia and nitrite concentrations when they were combined (antagonistic effect) rather than when subjected to the isolated compounds (total ammonia and nitrite) for 96 h.