Metal toxicity and metallothionein gene expression studies in common carp and tilapia

In Hong Kong, carp and tilapia are common in ponds and rivers. Tilapia is the dominant fish species found in contaminated rivers and estuarine regions. Heavy metal pollution is one of the major problems. Here we report the use of juvenile common carp and tilapia to study: (i) lethal concentrations of copper and zinc; and (ii) metallothionein (MT) mRNA levels in liver and gills of these fish after exposure to different concentrations of metals. For tilapia, the 96-h LC 50 values for copper and zinc were determined to be 1.52 and 24.3 ppm, respectively. However, for common carp, the 96-h LC 50 values of copper and zinc were found to be 50 and 17 ppm, respectively. This indicated that tilapia is a copper-resistant species while carp is sensitive to copper ions. Juvenile carp and tilapia were exposed to different sub-lethal concentrations of copper, zinc and cadmium for 3 weeks in the laboratory, to study MT gene expression. Our results showed that exposure of common carp to these metals did not induce MT mRNA in gill and liver tissues. However, exposure of tilapia to metal ions showed significant induction of MT mRNA in the same tissues. In conclusion, carp MT level is not a sensitive biomarker for monitoring metal pollution, and tilapia MT levels may be a better biomarker for monitoring metal contamination in local waters.