| Marine pollution is one of the major issues of global concern. Large quantities of heavy metals enter the aquatic environment via natural source and anthropogenic sources every year. Heavy metals at excessive amount in marine environments may adversely affect the growth, survival, and reproduction of marine fishes. Previous studies show that early life stages of fish are more sensitive to toxicity of heavy metals than adults, which may subsequently affect the recruitment and population wellness of the next cohorts. In addition, the accumulation of heavy metals in fish tissues could also seriously threaten the safety of fish products and thus consumer health. Therefore, the establishment of biological early warning systems for marine pollution is very important for the sustainable utilization of marine fishery resource and human health.Copper (Cu) and cadmium (Cd) are two common heavy metals in the Chinese coastal waters. This study investigated the toxic effects of Cu and Cd on flounder (Paralichthys olivaceus) in different early life stages (ELS: embryos, larvae, and juveniles). The main results are as follows:(1) Acute toxicity tests: The 24- and 48-h LC50 values of Cu for embryos were 0.21 and 0.11 mg/L, whereas the 48-, 72-, and 96-h LC50 values for larvae were 0.46, 0.21, and 0.12 mg/L, respectively. The 24- and 48-h LC50 values of Cd for embryos were 7.98 and 4.65 mg/L, whereas the 48-, 72-, and 96-h LC50 values for larvae were13.64, 7.41, and 4.17 mg/L, respectively. These results suggest that Cu is much more toxic than Cd to the embryonic and larval survival; and embryos were more sensitive to these heavy metals than larvae.(2) Embryonic-larval toxicity tests: embryos were exposed, respectively, to different concentrations of Cu (0-0.12 mg/L) and Cd (0-2.4 mg/L) solutions from 3 to 130 hours post hatching (hpf) when larvae had opened mouth and were ready to initiate feeding. The results demonstrated that Cu and Cd had distinctly toxic effects on their embryonic-larval development and survival. Cu at≥0.06 mg/L and≥0.8 mg/L Cd concentrations caused low hatching success, delay in the time to hatching of embryos, reduction in yolk absorption rate of the larvae and high mortality and morphological malformations in the embryos and larvae; Neither Cu nor Cd significantly affect the heart rate of the embryos, but it significantly decreased the heart rate of the 130 hpf larvae when Cu concentration was≥0.10 mg/L and Cd concentration was≥1.2 mg/L; Total length of the larvae at the end of the tests was significantly reduced at≥0.10 mg/L Cu and≥1.2 mg/L Cd concentrations in comparison with those in the controls. The sensitivity sequence of toxicological endpoints for Cu was accumulative mortality > accumulative hatchability > morphological abnormality > growth, while that for Cd was: morphological abnormality > accumulative mortality > accumulative hatchability > growth.(3) Chronic toxicity tests: fish were exposed to waterborne Cu (0-32μg/L) and Cd (0-48μg/L) from embryonic to juvenile stages for 80 days, respectively. Activities of superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), and levels of reduced glutathione (GSH) and lipid peroxidation (LPO) were investigated at three developmental stages (metamorphosing larvae, settling larvae, and juvenile). In metamorphosing larvae, Cu exposure caused decrease in GSH level and SOD, CAT, and GST activities, but increase in malondialchehyche (MDA) concentrations. Cd exposure caused decrease in SOD and CAT activities, but increase in the levels of GSH and MDA. Cu exposure did not have significant effects on setting larvae in any concentrations, but CAT and GST activities of settling larvae were inhibited and their GSH level was elevated at the high Cd concentrations. In juveniles, both Cu and Cd caused an increase in MDA concentrations. The level of GSH and the activities of SOD and GST were increased in high Cu treatments. However, SOD activity was increased but GST activity was decreased in high Cd treatments. At the end of the tests, flounder growth was reduced in both Cu and Cd treatments and the whole body metal accumulations were elevated with increasing concentrations.(4) Short-term subchronic toxicity tests with juveniles: following subchronic Cd (control, 2, 4, and 8 mg Cd/L) exposure for 28 days, fish growth was significantly reduced at≥4 mg/L Cd concentrations compared to the Cd-free controls. Accumulation of Cd in fish was dose-dependent and tissue-specific, with the greatest accumulation in the liver, followed by the kidney, gill, and muscle. Although the gill and liver mounted active antioxidant responses to cope with oxidative stress at≥4 mg/L Cd including a decrease in GSH level and GST and glutathione peroxidase (GPx) activities, the antioxidant response failed to prevent MDA production in these organs. In the kidney, increased GPx and GST activities and decreased SOD activity were observed in fish exposed to high Cd concentrations, but no significant differences were found in MDA levels for any of the exposure concentrations. The antioxidant defense system in the kidney may have effectively scavenged ROS and thereby succeed in preventing tissue-specific oxidative damages. The gill was most sensitive to oxidative damage, followed by the liver; the kidney was the least affected tissue.Overall, this present study investigated the toxic effects of heavy metals (Cu and Cd) on development, survival, growth, accumulation and antioxidative responses in different ELSs of flounder. Some sensitive endpoints such as biological parameters (i.e., hatchability, morphological abnormality, and mortality) and oxidative stress parameters (SOD, CAT, GSH, GST, GPx, and LPO) could be used as bioindicators of marine pollution in ecological risk assessment. |
