ECO-PHYSIOLOGICAL MEMORY OF FISH TO STRESS EPISODES: POTENTIAL ROLE IN AMMONIA ACCLIMATION  

Amit Kumar Sinha*, Jyotsna Shrivastava, Ronny Blust, Gudrun De Boeck
 
 Aquaculture/Fisheries Center, University of Arkansas at Pine Bluff, Arkansas-71601.
 sinhaa@uapb.edu

In animals, prior exposure to a non- lethal concentration of a contaminant may favor the evolution of adaptive mechanisms and improves tolerance to subsequent exposure through acclimation, implying that animals execute a form of 'stress-memory'. These 'stress-avoidance strategy imprints' are not solely associated with the neuro-endocrinological regulatory systems, animals can also develop priming mechanisms in an array of physiological, biochemical and transcriptional processes which can persist beyond the exposure, facilitating them to respond more quickly to future stress. Nevertheless, evidences favoring the acquisition of an adaptive repertoire as a consequence of stress memory are scarce in fish. Moreover, there is no consensus whether compensatory responses once developed in response to ammonia pre-exposure can be imprinted in fish to stimulate a faster protective adjustment to subsequent ammonia exposures.

The present study was designed to test the hypothesis whether pre-acclimation with low concentration of ammonia can prime the fish to tolerate subsequent (sub)lethal ammonia threat. For the present work, common carp (Cyprinus carpio) was used as model species. Fish were pre-exposed to 0.27 mM ammonia (~10% 96 h LC50) for 3, 7 and 14 days. Thereafter, each of these pre-exposed and parallel naïve groups were exposed to 1.35 mM high ammonia (~50% 96 h LC50) for 12 h and 48 h to assess the occurrence of ammonia acclimation based on sub-lethal end-points, and to lethal ammonia concentrations (2.7 mM, 96 h LC50) in order to assess improved survival time. We reported that fish pre-exposed to lethal ammonia had a significant longer survival time than the ammonia naïve fish (Fig. 1). The results for sub-lethal end-points showed a significant augmentation in the ammonia excretion rate among ammonia pre-exposed fish, suggesting an efficient removal of internal load of ammonia compared to the parallel un-acclimated fish. Interestingly, the increments in ammonia excretion rate among ammonia pre-acclimated fish were associated with the up-regulation of mRNA expression of Rhesus glycoproteins (Rhcg-a, Rhcg-b and Rhbg), the key ammonia transporter in fish gills facilitating ammonia removal from the blood. Our findings connote that pre-exposure to low dosage of ammonia offered protective effect as it improves the tolerance to subsequent high ammonia exposure, however, the priming effects seems to be attenuated by 14 days. Conclusively, our finding suggests that it will be possible to mitigate detrimental effects of water borne ammonia in aquaculture systems by prior artificial application of the mild ammonia stress.