Diverse defense responses to ammonia stress in three freshwater turtles

Ammonia is a common stressor in freshwater ecosystems and usually causes oxidative damage to aquatic animals at high levels. The antioxidant defense system plays an important role in protecting animals from oxidative damage. This study compared the antioxidant defense responses of three commonly cultured freshwater turtle species, namely, the three-keeled pond turtle Chinemys reevesii, the snapping turtle Chelydra serpentina and the soft-shelled turtle Pelodiscus sinensis, under harsh ammonia stress (189.52 mg/L total ammonia nitrogen (8.2 mg/ L unionized ammonia nitrogen), pH 7.8) for 96 h and subsequently recovered in the control water for 96 h, to reveal the stress response divergence of freshwater turtles that differed in morphological structure, origin and behavior, which may provide some reference for the aquaculture of freshwater turtles. We examined hematological parameters, reactive oxygen species (ROS), malondialdehyde (MDA), and antioxidant defense responses in different tissues of the three turtle species. Blood ammonia increased significantly during ammonia exposure and then recovered to the control level in all three turtles. Hemoglobin (Hb) and methemoglobin (MetHb) increased distinctly during ammonia exposure in C. reevesii and P. sinensis, while hematocrit (Hct) did not change much. In contrast, the Hct of C. serpentina decreased significantly, while Hb and MetHb levels remained constant. ROS and total antioxidant capacity (TAOC) levels in C. reevesii remained constant, but in C. serpentina, hepatic and renal ROS levels increased significantly during exposure and remained high after recovery, with hepatic TAOC declining simultaneously. In P. sinensis, both hepatic ROS and TAOC increased during exposure and then recovered. The MDA concentrations did not change much in all three species, suggesting an effective protection of the antioxidant defense system. This study found that in the process of aquaculture, a high concentration of ammonia stress will have adverse impact on freshwater turtles. When faced with the ammonia stress, the biochemistry parameter alternations and the antioxidant defense system response patterns of the three turtles were highly diverse which perhaps were in relation to their behavioral strategy and original local environment.