Aquaculture 533 (2021) 736204 Available online 25 November 2020 0044-8486/© 2020 Elsevier B.V. All rights reserved. Combined and isolated effects of ammonia and nitrite on Amazon River prawn Macrobrachium amazonicum (Heller, 1862) juveniles Claudia Caramelo Braz˜ao a, b, * , Rafael Ortiz Kracizy a, b , Fabrício Martins Dutra a, c , Ana Paula de Oliveira a, c , Rafael Itamar da Silva c, d , Eduardo Luis Cupertino Ballester a, b, c a Laborat´orio de Carcinicultura, Núcleo de Pesquisa e Desenvolvimento em Aquicultura Sustent´ avel, Universidade Federal do Paran´ a, Palotina, Paran´ a, Brazil b Programa de P´os-Graduaç˜ ao em Zoologia, Universidade Federal do Paran´ a, Curitiba, Paran´ a, Brazil c Programa de P´os-Graduaç˜ ao em Aquicultura e Desenvolvimento Sustent´ avel, Universidade Federal do Paran´ a, Palotina, Paran´ a, Brazil d Laborat´orio de Inovaç˜ ao Tecnol´ogica e Automaç˜ ao na Aquicultura, Universidade Federal do Paran´ a, Palotina, Paran´ a, Brazil A R T I C L E INFO Keywords: Ecotoxicology Amazon River prawn Nitrogen compounds Gill damage Histological change ABSTRACT 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 ran- domized factorial design with four concentrations of total ammonia (0, 11, 22, and 44 mg.L 1 ) and four con- centrations 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 LC 50 –96 h values for total ammonia and nitrite, the results indicated concentrations of 29.85 and 2.49 mg.L 1 , respectively. The LC 50 –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. 1. Introduction In 2018, worldwide shrimp aquaculture production reached approximately 6 million tons of sea shrimps and 0.51 million tons of freshwater prawns (FAO, 2020). In Brazil, commercial production of freshwater prawn is performed exclusively with Giant River prawn Macrobrachium rosenbergii (FAO, 2010), which is an exotic species introduced in 1977 (Valenti, 1998). However, working with exotic species carries great environmental risks due to possible escapes to the wild environments (Vitule and Prodocimo, 2012), therefore, farming with native species has been encouraged worldwide. For instance, although Oriental River prawn Macrobrachium nipponense is produced exclusively in China, it has achieved great productive success (FAO, 2020). In Brazil, the Amazon River prawn (Macrobrachium amazonicum) is the native species with the greatest economic and productive potential, exhibiting a rapid growth rate despite not reaching a large size (Bentes et al., 2016). However, gaps in the production of this species need to be flled, among which are some important questions about water quality and tolerance to nitrogen compounds. According to Colt and Armstrong (1981), in any type of productive aquatic system, ammonia can cause problems due to its toxicity. Shrimp can excrete part of the ammonia by passive diffusion carried out by the gills, however, when the concentration of the external environment is higher than the internal concentration of ammonia, the physiological process can be harmed or even totally interrupted, interrupting the gas exchange, the osmotic and ionic balance (Chen and Sheu, 1990). Simi- larly, high levels of nitrite in water are potential stress triggers for aquatic organisms (Lewis Jr and Morris, 1986). Studies on the effect of nitrite on shrimps have shown that high levels of this compound can * Corresponding author at: Universidade Federal do Paran´a - Setor Palotina, Laborat´ orio de Carcinicultura, Bloco IV, Rua Pioneiro, 2153, Jardim Dallas, 85950-000 Palotina, Paran´a, Brazil. E-mail address: claudiacaramelob@gmail.com (C.C. Braz˜ ao). Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture https://doi.org/10.1016/j.aquaculture.2020.736204 Received 30 August 2020; Received in revised form 19 October 2020; Accepted 21 November 2020