## 4 Discussion
### Effect of Methomy on Antioxidant Defenses in Tilapia Testis
A similar trend in GR levels was found in liver of Nile tilapia exposed to methomyl [11] and in liver of snakehead (_Channa punctatus_) exposed to atrazine [27]. Moreover, this compensatory response of GR appears to invariably be accompanied by increases in CAT and SOD, which help prevent the production and accumulation of free radicals caused by toxic materials [28]; comparable trends in the changes in GR in relation to SOD or CAT were observed in our experiment. SOD can protect an organism's cells from the oxidative stress caused by free radicals; it can catalyze free radical of superoxide anion to and O\({}_{2}\), and then H\({}_{2}\)O\({}_{2}\) is catalyzed to H\({}_{2}\)O with the help of CAT or GPx. SOD could reportedly be induced during high production of free radical of superoxide anion [29]; therefore, the significant increase in SOD (\(p<0.05\)) in our study (Figure 1a) could be considered a direct response to the free radical of superoxide anion in response to methomyl, just as reported previously in tilapia exposed to methomyl [11] and in rainbow trout exposed to selenite [30]. CAT catalyzes hydrogen peroxide to O\({}_{2}\) and H\({}_{2}\)O; hence, the increase of CAT in the present study (Figure 1a) could reduce the oxidative stress of hydrogen peroxide produced by the methomyl exposure [28]. In our experiment (Figure 1a-d), the activities of SOD, CAT, GST and GR significantly increased (\(p<0.05\)) in testis of tilapia exposed to 2 \(\upmu\)g L\({}^{-1}\) or 20 \(\upmu\)g L\({}^{-1}\) methomyl; however, the antioxidant enzymes decreased significantly (\(p<0.05\)) in fish exposed to a concentration of 200 \(\upmu\)g L\({}^{-1}\). A similar trend was described for topmouth gudgeon (_Pseudonstora parva_) exposed to methomyl [31]. It has been reported that the dose influences the activity of antioxidant enzymes in fish exposed to pollutants [11]. It could be speculated that, because the methomyl could not be metabolized and was not completely removed from the tilapia body by the cells under conditions of chronic exposure and a high dose, the methomyl accumulated in the body and consequently destroyed the protein structures of the antioxidant enzymes, and finally manifested as decreased antioxidant enzyme activity, namely that of SOD, CAT, GST and GR [11,32]. No significant changes in the activities of SOD, CAT, GST, GR, and GPx or the contents of GSSG and GSH were found in testis of the fish exposed to 0.2 \(\upmu\)g L\({}^{-1}\) compared with the controls (Figure 1a-g), which means that 0.2 \(\upmu\)g L\({}^{-1}\) methomyl could not induce oxidative stress. In our concomitant tests, we also found no tissue damage or apoptosis of the testis of tilapia exposed to 0.2 \(\upmu\)g L\({}^{-1}\) methomyl [33]. Therefore, the methomyl concentration of 0.2 \(\upmu\)g L\({}^{-1}\) could be considered the NOAEL (No Observed Adverse Effect Level) for tilapia. Our previous acute toxicity test showed that the 96 h \(LC_{50}\) of methomyl to tilapia was 430 \(\upmu\)gL\({}^{-1}\)[33], and it is 2150 times of NOAEL. It is always thought that one-tenth of 96 h \(LC_{50}\) can be considered as safety concentration of harmful materials to fish [34], however, the present test showed that maybe shorter than one-thousandth of 96 h \(LC_{50}\) can be considered as NOAEL of harmful materials to fish.