Bioaccumulation, Histological and Structural Changes in the Gills of the Nile Tilapia (Oreochromis niloticus) Exposed to Heavy Metals

         Heavy metals are hazardous substances that affect aquatic organisms, especially the Nile tilapia, which is considered an important commercial fish. In addition, it has the ability to withstand and spread in Iraqi rivers and lakes of water. Therefore, this study aimed to assess the toxic effects of cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) on the gills of tilapia exposed to sub-lethal concentrations for 96 hours. The accumulation of metals among fish organs for cadmium, copper and zinc was in the following order: liver > gills > muscles, while for lead it was in the order of gills > liver > muscles. The toxicity test of the elements at sub-lethal concentration was performed on the gills, and histopathological investigations showed an increase in the severity of gill tissues over 96 hours of exposure. In conclusion, tilapia fish are severely affected by the toxic and cumulative effects of toxic elements in gill and liver tissues, highlighting the urgent need for careful monitoring and consideration.