Fish exhibit different behavioural and physiological coping mechanisms to unionized ammonia (UIA). Some species detoxify ammonia to less toxic glutamine or urea, or they excrete it directly into the aqueous environment via the gills. The strategies adopted by Nile tilapia (Oreochromis niloticus) to UIA exposure, however remain vague. To investigate the coping mechanisms of O. niloticus to environmental ammonia, adult individuals were exposed to three levels of UIA, 0 (Control), 7 and 61 µM for 24,72 and168 h and assessing histological changes in the gills and alterations in haematological parameters.
ILCM
Nile tilapia with an average mass of 281.1±34.0g were exposed to 2 different levels of UIA for up to 168h. One group was exposed to 7µM NH3, and one was exposed to 61µM NH3 , and compared against a control group. Experimental fish were placed in individual PVC tubes in an experimental tank . For the control group, eight fish were used . For exposure to 7 and 61µM NH3 groups of fish (n=8) were exposed for 24, 72 or 168h respectively. At the end of the exposure period, fish were euthan ized and blood, gill, liver and white skeletal muscle was sampled from each fish .
Blood pH showed a significant (p<0.05) downregulation for each ammonia level exposure. R ed blood cells on the other hand increased from the control fish group after exposing Nile tilapia to 61µM for 168h. Nile tilapia responded to ammonia toxicity by significantly increasing plasma Cl- levels. Exposure to 61µM UIA resulted to a significant increase in plasma NH3 and Na+. Liver and muscle urea decreased significantly from the control fish group with a significant increase in excreted urea only after 24h of exposing fish to 61 µM-UIA. T he groups exposed to 7µM showed a tendency towards a reduction in interlamellar cell mass (ILCM) for all exposure durations , although the reduction was not significant. I n response to 61µM UIA, ILCM decreased significantly after 24 and 72h (Fig 1). A t higher UIA levels, gill remodelling occurs in Nile tilapia in response to ammonia exposure. It is also known from this experiment that Nile tilapia detoxify ammonia to urea but are mostly excreted in the aqueous environment.