Aggressive behavior, Serotonin immunoreactivity and Melanomacrophage centers (MMCs) formation in the Nile tilapia (Oreochromis niloticus L.) following exposure to chlorpyrifos insecticide

Two treatment set ups were established to determine the effects of exposure to CPF with varying residual levels and observation periods on aggressive behavior, brain serotonin and melanomacrophage proliferation on Nile tilapia: dose response (0.016 ug/L, 0.16 ug/L, 1.6 ug/L, 2.8 ug/L and 4.2 ug/L) for 24 hours and time course (1.6 ug/L (PF for 3, 5, 10, 15 and 20 days). The dominant fish was characterized as having red-colored caudal fins and flared dorsal fins while the subordinate fish has dark body coloration that appears zebra-like. No significant skin darkening was noted among the fishes exposed to residual concentrations of CPF while observations from fishes exposed to 1.6 ug/L CPF at different periods (days) showed that skin coloration was dependent on time lengths. Aggressive behavior observed were tail chasing, side butting, and uncoordinated movements such as lateral swimming with its body touching the bottom of the tank. Subordinate fishes usually stay at a corner of the tank, close to the walls while the dominant fish swims freely below often attacking subordinate ones. Serotonin immuno-reactive (5HT-ir) structures were generally found in four areas of the tilapia brain: telencephalon, diencephalon, mesencephalon and brain stem. A negative correlation between the number of 5HT-ir neurons and the different residual concentrations of CPF was observed. Histolopathological findings showed hepatocytes with lipid droplets while and some degenerated hepatocytes, characterized by the destruction of the cytoplasm and absence of nucleus. The distribution of hemosiderin was observed to be solely restricted to the melanomacrophage centers in the spleen. In addition to this pigment, melanin and lipofusin have been identified as well. No significant relationship between the exposure period and number of agonistic encounters, body weights, presence of hemolysis, size and number of MMCs as well as between the number of agonistic encounters per minute, body weights, hemolysis, number of 5 HT-ir neurons, size and number of MMCs and dose levels have been observed.