Sub-Chronic Effects Of Dissolved And Dietary Cadmium In The Gift Strain Nile Tilapia (Oreochromis Niloticus)

With the fast growth of economy in China,industrial activities have released a large amount of heavy metals into the environment and heavy metal pollution has become a national problem.Heavy metals can elicit a variety of adverse effects in aquatic organisms(including fish)inhabiting in the heavy metal contaminated environment.The deleterious effects in heavy metal exposed aquatic organisms include abnormal behaviors,physiological dysfunction and tissue lesions etc.More importantly,heavy metal can also disturb the endocrine and the reproductive systems in fish.Research on the effects of environmental pollutants on fish physiology and reproductive performance can determine not only the responses of fish exposed to these contaminants but provide mechanistic explainations for the toxicity of pollutants as well.In the present study,GIFT tilapia(Oreochromis niloticus)were exposed to Cd via dissolved and dietary exposures.After the exposures,Cd accumulation in different tissues was determined.The effects on the antioxidant physiology(including CAT,GSH,GSSG,MDA and SOD in different tissues)were examined.The histopathological effects of Cd on the liver,kidney,intestine,gill and gonads were analyzed.The effects of Cd on the sex hormones(estrogen and testosterone)in serum and expression of ER,PR,GR,AR,VTG and 3?-HSD1 were revealed.Furthermore,the growth performance and reproduction of Cd-exposed GIFT tilapia were compared with the control.Finally,maternal transfer of Cd to the offspring of Cd-exposed tilapia was studied.The results of the present study provided evidence for the toxicological effects of Cd in GIFT tilapia from both dissolved and dietary Cd exposures.The results can help better understand the chronic effects of Cd in tilapia at different biological levels(from tissues,individual to population)from two exposure routes and the mechanisms for these adverse effects in particular and help the tilapia industry in general.1.Determination of the LC50 of Cd in the GIFT tilapia and the study of Cd effects on growth performance.GIFT tilapia were exposed to 8 different dissolved concentrations of Cd for 96 h to determine the LC50.LC50 was calculated using the probit analysis method.Based on this result,the concentrations used for the following chronic exposures were chosen.In addition,juvenile(2-month old)GIFT tilapia were exposed to three dissolved Cd groups(50?g/L,100?g/L,200?g/L)and three dietary Cd concentrations(50 mg/kg,100 mg/kg and 200mg/kg).Mortality and growth were calculated after 30 and 60 days of exposure.The measured Cd concentrations of the dissolved exposure were 33.02±0.12,70.32±1.0 and 143.78±2.58?g/L for their nominal concentrations of 50,100 and 200?g/L,respectively.The Cd concentrations in the diets were determined to be 39.88±1.52,78.63±2.43 and 161.15±3.54 mg/kg dry weight for the 50,100 and 200 mg/kg(nominal concentrations),respectively.Among all the treatments,mortality and biometric indices were not significantly different,except that a higher hepatosomatic index was observed in the fish exposed to 39.88 mg/kg of Cd after 60 days of exposure.2.Accumulation and tissue distribution of Cd in the GIFT tilapia tissuesAccumulation and distribution of Cd in the liver,kidney,intestine,gills and gonads of the GIFG tilapia exposed to both dissolved and dietary Cd after 30 days and 60 days were determined.The results showed that Cd was accumulated in all the tissues examined from both exposure routes.In the dissolved Cd exposed fish,Cd accumulation in different tissues were in the order:liver>kidney>intestine>gonad>gill,while in the dietary groups,Cd accumulation were kidney>intestine>liver>gonad>gill.In addition,Cd was detected in the eggs from the Cd-exposure tilapia.Interestingly,Cd level in the offspring of the Cd-exposed fish was not significantly different from that in the control offspring,indicating that most of the Cd accumulated in the eggs was in the eggs shell,which could protect the larvae from Cd exposure.3.The effects of Cd on the antioxidant system in the GIFT tilapiaActivities of the antioxidant enzymes(i.e.,CAT and SOD),non-enzymatic antioxidants(GSH and GSSG)and lipid peroxidation MDA were determined in the liver,kidney,intestine,gills and gonads of the GIFT tilapia after 30 days and 60 days exposure by Cd.The results demonstrated that that Cd reduced the GSH and SOD activities in the liver.Meanwhile,Cd stimulated the levels of MDA and CAT in the kidney and inhibited the activity of SOD.4.The effects of Cd on the histology of the tissues of the GIFT tilapiaThe histopathological effects of Cd on the liver,kidney,intestine,gills and gonads of the GIFT tilapia were examined after 30 days and 60 days exposure by Cd.Vacuolar degeneration and smaller nuclei of the hepatocyte were observed in the liver tissues in the Cd-exposed fish.In addition,a widened sinusoid and structural disorder of the liver cells were also noticed in the Cd-exposed fish.In the kidney,the arrangement of the tubules was disordered.Renal tubular epithelial cells appeared to be degenerated and necroses were found in the Cd-exposed fish.In the intestine,the villus was disordered and increased goblet cells were observed.Cd also caused edema in the intestine.Cd exposure led to the breakage of the gills,edema and fusion of the front cells.The number of primary follicles and secondary follicles was increased in the ovaries of the Cd-exposed fish.In the testis of the Cd-exposed fish,the spermatocytes and sperm cells were decreased relative to the control.5.The effects of Cd on the sexual hormones,sex-related genes and reproductive capacity in the GIFT tilapiaThe levels of the sexual hormones(estrogen in female and testosterone in male)and the expressions of ER,PR,GR,AR,VTG and 3?-HSD1(using real-time PCR)were determined after 30 days and 60 days exposure by Cd.The results showed that Cd increased the level of estrogen in the serum.However,the level of testosterone was not affected by Cd.For the sex-related genes,Cd stimulated the expression of ER in the ovary and depressed the expression of VTG.The expression of GR was up-regulated by Cd.In addition,the effects of Cd on the variables of the reproduction including fecundity,hatching rate and malformation rate were examined after 30 days and 60 days of exposure to Cd.The results showed Cd could decrease the hatchability and increase the malformation rate in the offspring of the Cd-exposed tilapia.