| In order to investigate ecotoxicological effects of hexavalent chromium(Cr6+)on silurus meridionalis and Ctenopharyngodon idellus,the experiments with juveniles of S.meridionalis(19.75 ± 0.12 g)and C.idellus(21.35 ± 0.29 g)exposed to waterborne potassium dichromate(K2Cr2O7)were conducted,at the constant water temperature(27.5 ± 0.5?),hardness(25 mg CaCO3/L)and pH(7.1 ± 0.2).Two series of experiments were carried out in this study: ?.Acute exposure experiment: the tested fish were exposed to five Cr concentrations,i.e.S.meridionalis at 0(control),25,40,55,70 and 85 mg/L,and C.idellus at 0(control),15,25,35,45 and 55 mg/L,respectively to estimate their median lethal concentrations in 96 hours(96 h LC50)and observe their abnormal behaviors in the acute poisoningd.?.Chronic exposure experiment: the tested fishes were exposed to Cr for 8 weeks at three concentrations of 0(control group),1/60 96 h LC50(low concentration group,LC),and 1/30 96 h LC50(high concentration group,HC),respectively to observe the growth performance,energy metabolism,biochemical composition(including moisture,fat,and protein)and energy density and measure the Cr contents in liver or hepatopancreas,kidney,gill,brain,intestine,muscle and the whole body of the tested fish.The main results were as follows:1.In the acute exposure experiment,the mortality of the two fishes increased with increasing Cr concentrations.The relationship between waterborne Cr concentration(independent variable,X),and the mortality of the tested fish(dependent variable,Y)were described by Y = 1.5232X-35.491(S.meridionalis)and Y = 2.1333X-25.127(C.idellus),respectively.96 h LC50 were estimated by method of linear interpolation,and the values for S.meridionalis and C.idellus were 56.24 mg/L and 35.27 mg/L,respectively.2.In the chronic exposure experiment,the mortality of the fish increased with increasing Cr concentrations.The mortalities at the control,LC and HC treatmens were 0,11.67% and 36.67% for S.meridionalis and were 0,5% and 66.67% for C.idellus.3.Chronic Cr exposure would restrain the growth of the tested fish.The final body weight,final body length,specific weight growth rate,and condition factor of the both fish in the two Cr exposure groups were significantly lower than those in the control group.And the all indices mentioned above,except for condition factor of S.meridionalis,at HC were significantly lower than those at LC(p< 0.05).4.Waterborne Cr exposure reduced the energy reserve of the tested fish.For S.meridionalis in HC group the moisture was significantly higher than that in the control,and significantly lower was the fat.The protein and energy density in two Cr exposure groups were significantly lower than those in the control(p< 0.05).For C.idellus in two Cr exposure group the moisture were significantly higher than those in the control,and significantly lower were the fat.The protein in the HC group was significantly lower than that in either the control or LC group(p< 0.05).5.The resting metabolic rates of S.meridionalis in the two Cr exposure groups were significantly higher than those in the control(p< 0.05),but there was no significant difference between the two groups.The resting metabolic rate of C.idellus in LC group was significantly higher than that in either the control or HC group(p< 0.05),but the difference between the control and HC group was insignificant.6.Cr contents in various organs of S.meridionalis after 8-week Cr exposure could be ordered from higher to lower as following: kidney > gill > liver > intestines > whole body > brain > muscle.Cr contents in kidney and gill at LC and HC were significantly higher than those in the other organs(p< 0.05),but there was no significant difference between the two organs.Cr content in the gill was significantly higher than that in the intestines,whole body,brain and muscle,and Cr content in the liver was significantly higher than that in the intestines,whole fish or muscle.The values in muscle and whole fish were significantly lower than those in the other organs(p< 0.05).7.Cr contents in various organs in C.idellus after 8-week Cr exposure could be ordered from higher to lower as following: kidney > liver > gill > intestines > whole body > brain > muscle.The Cr content in kidney was significantly higher than that in the other organs except the liver.The Cr content in liver was significantly higher than that in the other organs except the gill.And the Cr content in gill was significantly higher than that in intestines,whole body,brain or muscle.The value in muscle was significantly lower than that in the other organs except the brain(p< 0.05).The conclusions were suggested in this study as following:1.The value of 96 h LC50 for C.idellus was lower than that for S.meridionalis under the same experimental conditions.It suggested that the tolerance of S.meridionalis to waterborne Cr was higher than that of C.idellus.The skin tissue of S.meridionalis is compact with abundant mucus,which should be a tough defense mechanism against waterborne Cr infiltrating into the body.In addition,the metabolic rate of C.idellus is higher than that of S.meridionalis,which should suggested that more energy in the former would be expended to resist the stress from Cr exposure,and its mortality of exhaustion should be higher than that of the later.Thus the differences from the tissue texture of skin,defense mechanism of skin mucus and metabolism level between the two species might be the reasons which their tolerances to Cr exposure are different.2.The growth rates and the total energy reserves depressed for both S.meridionalis and C.idellus when exposed to waterborne Cr,which might result from lower feed efficiencies and higher metabolism.And the depressions showed different between the two species.Compared to S.meridionalis as a carnivorous fish,the grass carp as a herbivore has higher food intake and feeding frequency.The depressing effect of Cr stress on food utilization efficiency in the herbivore was stronger than that in the carnivore.Thus the decline of growth of C.idellus was more obvious than that of S.meridionalis.3.The Cr contents of kidney,liver,gill,intestines,brain,muscle and whole body in both tested fish increased with increasing Cr concentration.And the increment of Cr contents showed not only a dose-effect but also and a tissue specific-effect.The liver and kidney plays a significant role in the detoxification and excretion.Accumulated Cr would combine with metal-binding protein that would deposit in the two organs and be unable to discharge from them.It should be the reason that the Cr accumulation in the kidney and liver were significantly higher than that in other organs except the gill.Gill plays an important in waterborne metal absorbing,so that the Cr would bind to epithelial tissue of the gill via ionic bonds,which should be the main reason that gill has high Cr accumulation.4.Abundant mucus on the body skin of S.meridionalis has defensible effect against Cr penetration.The metabolic rate in C.idellus is higher,as a result the rates of turnover and accumulation of the absorbed Cr in the body would be also higher.Thus,stronger defense mechanism against Cr penetration with skin mucous and lower metabolic rate for S.meridionalis should be the reasons that its Cr accumulation is lower compared with C.idellus. |
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