2.2. Experimental design Specimens of O. niloticus and G. brasiliensis were acquired froma local fish farm. The fish were acclimatized for 60 days in tanks of250 L with filtered water, constant aeration, average temperature of22◦C, photoperiod of 12 h, and daily feeding. Two weeks before thebeginning of the experiment, 105 fish were randomly assigned toseven aquaria of 108 L (15 specimens per aquarium) in conditionssimilar to the tanks (water, aeration, temperature, etc.). One aquar-ium was assigned as a negative control group (NC), one as positivecontrol (PC), exposed for 24 h to 0.5 mg kg−1of methyl methanesulfonate (MMS) via intraperitonial injection, and the other 5 astreatment groups. We made two separate experiments for eachspecies and in any time, either during acclimatization or exposure,fish of different species shared the same environment.After a 96 h hydric exposure to mesotrione in a static bioassay,fish were anesthetized with benzocaine 10%. Blood was sampledthrough caudal vein puncture, and then the fish were rapidly killedby spinal cord section. The liver was removed and placed in a petridish. A sample of the organ was separated with a scalpel and placedin a microtube containing 0.5 mL of fetal bovine serum (FBS, Invit-rogen). The other part of the liver was stored at – 80◦C for thebiochemical analysis. The third gill arch of the right side of eachfish was excised and placed in a petri dish and washed in phos-phate buffer solution (PBS, pH 7.4). The bone arch was removedwith a scalpel and only the lamellae were transferred to a microtubecontaining 0.5 mL of FBS.