Ecophysiological Effects Of The 3, 3', 4, 4', 5-pentachlorobiphenyl (PCB126) On Southern Catfish (Silurus Meridionalis Chen)

Two series of experiments were conducted on the southern catfish,Silurus meridionalis an obligatory carnivorous species under the laboratory conditions.In experimentâ… ,an experiment of exposure to diets contaminated with polychlorinated biphenyls(PCBs), six iso-nitrogenous,iso-lipidic and iso-carbohydrate experimental diets containing different 3,3',4,4',5-pentachlorobiphenyl(PCB126) levels of 0,50,100,200,400 and 800μg/kg were formulated,and the juvenile southern catfish were fed with experimental diets with different PCB126 levels in an indoor rearing system for 8 weeks at 27.5℃.The effects of different levels of dietary PCB126 on mortality,metabolic stress,inducibility of detoxification enzymes and oxidative stress were investigated in southern catfish,respectively.In experimentâ…¡,an experiment of exposure to water contaminated with PCBs,the effects of different aqueous PCB126 concentrations of 0, 0.1,1,5 and 10μg/L on embryonic and yolk-sac larval development were investigated in southern catfish at 22℃,respectively.The main results from experimentâ… were as follows:1.During the 8-week experiment,no mortality of the experimental fish occurred in groups fed with the diets containing PCB126 concentrations of 0,50 and 100μg/kg, whereas,the mortality was observed in fish fed with the diets containing PCB126 of 200,400 and 800μg/kg,and the median lethal time(LT₅₀) was 34 d,16 d and 11 d,respectively,which were negatively correlated with dietary PCB126 levels.2.The total amounts of PCB126 intake in southern catfish fed with the diets containing PCB126 concentrations of 0,50 and 100μg/kg were 0,30.56 and 66.66μg/kg body weight at the termination of the experiment,respectively.The total amounts of PCB126 intake in southern catfish were 90.18,92.05 and 94.11μg/kg body weight in groups of 200,400 and 800μg/kg at the median lethal time,and there were no significant differences among these three groups with mortalities of the experimental fish,but all of them were significantly higher than those in the other three groups(p＜0.05).3.The hepatosomatic index(HSI) increased with increasing dietary PCB126 levels, and HSI in PCB126-treated fish were significantly higher than control(p＜0.05).4.The resting metabolic rate(RMR) increased with increasing dietary PCB126 levels, and RMR in PCB126-treated fish were significantly higher than control(p＜0.05). RMR in groups of 100 and 200μg/kg were significantly higher than the group of 50μg/kg,and RMR in groups of 400 and 800μg/kg were significantly higher than the groups of 50,100 and 200μg/kg.5.The rate of oxygen consumption of the liver mitochondria increased with increasing dietary PCB126 levels in the groups without mortality,and the value in group of 100μg/kg was significantly higher than control(p＜0.05).The rate of oxygen consumption of the liver mitochondria decreased with further increasing dietary PCB126 levels in the groups with mortalities,and the value in group of 800μg/kg was significantly lower than the group of 200μg/kg(p＜0.05).6.The activity of cytochrome C oxidase(CCO) of the liver mitochondria increased with increasing dietary PCB126 levels in three experimental groups without mortality,and the CCO activity in group of 100μg/kg was significantly greater than control(p＜0.05).The CCO activity of the liver mitochondria decreased with further increasing dietary PCB126 levels in three PCB126-treated groups with mortalities, but was still significantly greater than control(p＜0.05),and there were significant differences in CCO activities among these three groups(p＜0.05).7.The activity of 7-ethoxyresorufin-O-deethylase(EROD) in the liver of southern catfish increased with increasing dietary PCB126 levels in all experimental groups. The EROD activities in five PCB126-treated groups,except for 50μg/kg group, were significantly greater than control(p＜0.05).There was a distinct dose-effect relationship between dietary PCB126 level and inducibility of EROD activity in liver of southern catfish,the induced level of EROD activity had a significant positive linear correlation with dietary PCB126 level(C:μg/kg) (EROD=0.0056C+1.3589,R²=0.8479,p＜0.05).EROD activity in liver of southern catfish in groups of 200,400 and 800μg/kg was 2,3 and 4 times greater than control,respectively.8.The activities of glutathione S-transferase(GST) in the liver of southern catfish in PCB-treated groups,except for 50μg/kg group,were significantly greater than control(p＜0.05).9.There was a significant positive linear correlation between the malondialdehyde (MDA) content in the liver of southern catfish and dietary PCB126 level(C:μg/kg) in all experimental groups(MDA=0.0048C+1.4576,R²=0.768,p＜0.05).The liver MDA contents in five PCB126-treated groups were significantly higher than control (p＜0.05).10.The total antioxidative capacity(T-AOC) in the liver of southern catfish in all experimental groups increased with increasing dietary PCB126 levels firstly,the greatest T-AOC appeared in 200μg/kg group,and then decreased with further increasing dietary PCB126 levels.T-AOC in groups of 50,100 and 200μg/kg were significantly greater than control(p＜0.05),whereas T-AOC in groups of 400 and 800μg/kg were significantly less than control(19＜0.05).The changes in liver superoxide dismutase(SOD) activity in all experimental groups were similar to T-AOC.11.The activities of glutathione peroxidase(GPX) and catalase(CAT) in liver of southern catfish in all experimental groups linearly increased with increasing dietary PCB126 levels(C:μg/kg) and were significantly greater than control (p＜0.05).Their regression relationships could be described as: GPX=0.0401C+38.021,R²=0.671,p＜0.05 and CAT=0.0014C+1.7546,R²=0.6081, p＜0.05.The main results from experimentâ…¡were as follows:12.Hatching rate of fertilized eggs in all experimental groups decreased with increasing aqueous PCB126 concentrations,and hatching rates in PCB126-treated groups of 1,5 and 10μg/L were significantly lower than both clean water control and solvent control(19＜0.05).Malformation rate of newly hatched larvae in all experimental groups increased with increasing aqueous PCB126 concentrations, and malformation rates in groups of 1,5 and 10μg/L were significantly higher than solvent control(p＜0.05).Incubation duration of fertilized eggs in all experimental groups prolonged with increasing aqueous PCB126 concentrations,and incubation duration in groups of 1,5 and 10μg/L were significantly longer than both clean water control and solvent control(p＜0.05).Hatching rate and incubation duration of fertilized eggs as well as malformation rate of newly hatched larvae in solvent control were not significantly different from those in clean water control.13.The survival rates of yolk-sac larvae hatched from the embryos in clean water control were negatively correlated with aqueous PCB126 concentrations during the 6-day yolk-sac larval development.The survival rates in groups of 1,5 and 10μg/L were significantly lower than both clean water control and solvent control(p＜0.05). There was not a significant difference in survival rate of yolk-sac larvae between clean water control and solvent control during experiment.The survival rates in all experimental groups decreased with time during the 6-day experiment.14.The survival rates of yolk-sac larvae hatched from the PCB126-treated embryos with concentrations of 0.1,1,5 and 10μg/L,as well as from clean water control and solvent control were negatively correlated with aqueous PCB126 concentrations during the yolk-sac larval development.The survival rates in four treatment groups were significantly lower than both clean water control and solvent control(p＜0.05).There was not a significant difference in survival rate of yolk-sac larvae between clean water control and solvent control during experiment.The survival rates in all experimental groups decreased with time during experiment.15.The survival rates of yolk-sac larvae priorly affected by different concentrations of aqueous PCB126 in their embryonic development in all experiment groups were lower than those in yolk-sac larvae priorly unaffected by aqueous PCB126 in their embryonic development and treated with corresponding PCB126 concentrations during larval experiment.The conclusions suggested in this study were as follows:1.A distinct dose-effect relationship was found when dietary PCB126 caused mortalities in southern catfish,LT₅₀ of southern catfish were negatively correlated with dietary PCB126 levels.Consequently,it was suggested that PCB126 did not cause mortality in southern catfish until the accumulative amounts of PCB126 intake reached a critical value,and this lethal critical value of total PCB126 intake was approximately 90.18μg/kg body weight in southem catfish.2.It was found that HSI in southern catfish increased significantly with increasing dietary PCB126 levels,which suggested that PCB126 exposure via diets can cause some of the morphological alterations such as oedema or tumefaction in the fish liver.3.It was found that the resting metabolic rate(RMR) in southem catfish was positively correlated with dietary PCB126 levels.It was hereby suggested that the metabolic capability of fish can be physiologically regulated to reach a higher level, so as to meet increased energy demands for the physiological processes such as detoxification and antioxidation under persistent organic pollutants(POPs) stress.4.It was found that there was a significant dose-effect relationship between induction of liver EROD activity and dietary PCB126 level,induction level of EROD had a significant positive linear correlation with dietary PCB126 level,but no significant dose-effect relationship between induction level of liver GST activity and dietary PCB126 level.Consequently,it was suggested that liver EROD detection in southem catfish can be considered as a potential biomarker in environmental monitoring for contamination with the organic contaminants in aquatic ecosystem.5.It was found that there was a significant positive linear correlation between the degree of oxidative damage and dietary PCB126 level.It was hereby suggested that the determination of MDA content in liver of southern catfish can be used as a biomarker for bio-monitoring of aquatic environment pollution.High level of dietary PCB126 exposure can cause greater degree of oxidative damage,which may be one of the most important causes resulting in mortalities in southem catfish.6.It was found that changes in liver T-AOC and SOD activities exhibited a biphase trend:the activities first increased and then decreased with increasing dietary PCB126 levels in southem catfish,whereas a monophase trend of continuous increase for liver GPX and CAT activities.It was suggested that there were at least two effect patterns in which dietary PCB126 exposure caused changes of several important antioxidative parameters. 7.A higher tolerence to toxicity of PCB126 was found in embryos of southern catfish. PCB126 would not significantly affect embryonic development of southern catfish until aqueous PCB126 concentrations were at least higher than 0.1μg/L.It was suggested that the fertilized southern catfish eggs usually had larger size and thicker egg membrane with a stronger tenacity,which may be one of the most important reason that the embryos had a higher tolerance to aqueous PCB126 exposure in southern catfish.8.It was found that the southern catfish had a lower tolerance to aqueous PCB126 during yolk-sac larval development than during embryonic development.The yolk-sac larvae priorly affected by aqueous PCB126 exposure during their embryonic development had a lower tolerance to aqueous PCB126 compared with the larvae priorly unaffected by aqueous PCB126 exposure during their embryonic development.Aqueous PCB126 exposure of the fertilized eggs during their embryonic development can affect yolk-sac larval development and decrease larval survival.9.Based on the observed results in this study,an inference was made that even if concentration down to 0.1μg/L,aqueous PCB126 can also cause almost complete mortalities of larvae once the yolk-sac larval development reached a certain timein southern catfish.A further experimental design for verifying the toxic effects of aqueous PCB126 on embryos and larvae of southern catfish was hereby suggested.