Toxic Effects Of Mercury, Lead And Zinc On Early Life Stages Of Flounder (Paralichthys Olivaceus)

Mercury, zinc and lead represent typical heavy metal pollutants in the Chinese coastal waters. In the present study, the toxic effects of mercury, zinc and lead to early life stages (ELS) of flounder (Paralichthys olivaceus) was investigated under laboratory conditions (18±1°C; 33±1‰in salinity), respectively. The aim of the present study is to explore the mechanism underlying metals toxicity to early life stages of flounder, and provide data for using flounder at ELS in monitoring metal pollution of marine environments.The acute toxicity tests indicated that the 24-h and 48-h LC50 values (median lethal concentration) of Hg2+ to embryos were 75.8 (70.1-81.5, with 95% confidence interval) and 48.1 (32.8-63.6)μg L-1; while the 24-h, 48-h and 96-h LC50 values to larvae were 99.4 (72.9-147.0), 51.2 (39.1-65.8) and 46.6 (33.3-64.8)μg L-1. The 24-h and 48-h LC50 values of Zn2+ to embryos were 22.3 (16.1-26.7) and 7.1 (6.2-8.3) mg L-1; the 48-h and 96-h LC50 values to larvae were 10.06 (7.89-12.88) and 6.77 (5.25-8.02) mg L-1. This result indicated that embryos were more sensitive than larvae to mercury and zinc exposure. In the embryonic-larval toxicity tests, embryos were exposed to 0-60μg Hg2+ L-1 and 0-4 mg Zn2+ l-1 solutions for 6 days, respectively. The results indicated that mercury exposure at concentrations≥20μg Hg2+ L-1 would lead to low hatching rate, delay in time-to-hatch, high mortality and morphological abnormality, reduced growth and inhibited yolk absorption in embryos or/and larvae. Zinc exposure at concentrations≥1 mg Zn2+ L-1 would also induce low hatching rate, delay in time-to-hatch, high mortality and morphological abnormality, reduced growth to embryos or/and larvae. In the chronic toxicity tests, fish were exposed to 0-10μg Hg2+ L-1 and 0-300μg Pb2+ L-1 solutions from embryonic to the juvenile stages for 80 days, respectively. Responses of antioxidant defense system and lipid peroxidation to mercury exposure were studied in metamorphosing larvae (20th day; 18 days post hatching, dph), settling larvae (35th day; 33 dph) and juveniles (80th day; 78 dph). The results indicated that elevated mercury or lead concentration led to increased metal bioaccumulation and reduced growth after 80 days of low concentration exposures. On the other hand, mercury exposure elevated glutathione (GSH) level in metamorphosing larvae, but decreased that in juveniles. Glutathione-S-transferase (GST) activity did not significantly vary with mercury concentration in either larvae or juveniles. However, superoxide dismutase (SOD) and catalase (CAT) activities at the three developmental stages were increased with elevated mercury concentration. Moreover, malondialdehyde (MDA) content in juveniles was significantly increased. Lead exposure significantly decreased GSH level in settling larvae, increased those of the metamorphosing larvae and juveniles. GST activity in juveniles, SOD activity in metamorphosing larvae, CAT activity in metamorphosing larvae and juvenile were increased with increasing lead concentration. Meanwhile, MDA content in settling larvae was significantly increased in lead solutions. The results indicated that these antioxidative responses varied with the types of chemical, exposure concentration and duration. On the other hand, since flounder undergo drastic physiological transitions during their early life stages, this may cause differences in the antioxidative responses to metal exposures of the flounder at differental life phases.In toxicity test on juveniles, fish were exposed to 0-160μg Hg2+ L-1 solutions for 28 days. After 7 days of exposure, SOD activity was increased, but glutathione peroxidase (GPx) activity was decreased in the gill; GSH level was elevated, while GST activity was decreased in the liver; SOD and GPx activities were increased in the kidney. MDA content in all the three tissues did not significantly vary with mercury concentration. After 28 days of exposure, SOD and CAT activities were increased, GST activity and GSH level were decreased in the gill; SOD and CAT activities GSH level were significantly increased in the liver; SOD activity and GSH level were significantly increased in the kidney. MDA content in the gill and kidney were significantly increased. On the other hand, total length, body weight and specific growth rate (SGR) of juveniles were significantly reduced while condition factor (CF) was not affected by 28 days of mercury exposure. Moreover, mercury bioaccumulation in muscle, liver, kidney, gill and bone of juveniles significantly increased with elevating exposure Hg2+ concentration. The metal accumulation efficiency in these organs was in an order of kidney≈liver > gill > bone > muscle. The results indicated that the antioxidative responses of juveniles to mercury exposure varied with different tissue types. Moreover, exposure concentration and duration could also affect the responses of antioxidants and lipid peroxidation.