Using the rice frog (fejervarya limnocharis) as a sentinel species for cadmium contamination in Tak province, Thailand

Abstract

By utilizing the cadmium contamination history in Mae Sot District, Tak Province, Thailand, this research was developed to use a common native amphibian species, the rice frog (Fejervarya limnocharis) as a sentinel species for the problem. The objective of this research was to determine the suitability of this species as a sentinel for cadmium contamination. Frogs were collected on monthly basis during November 2007 and October 2008 from several rice fields in contaminated site (Mae Tao) and reference site (Mae Pa) in Mae Sot District, Tak Province. Frog samples were subjected to analyses for cadmium contamination level, hepatic biomarker, morphometry and gravimetry, and biological and ecological effects. The contaminant analysis showed that Mae Tao frogs had higher hepatic, renal, testicular and whole organismal cadmium than Mae Pa frogs, with the kidney as the greatest accumulator. This was mirrored by the bioconcentration factor data that showed the same trend. The biomarker analysis showed that frogs from the contaminated site had higher hepatic metallothionein and hepatic glutathione-S-transferase levels than those from the reference site. However, only glutathione-S-transferase showed a strong stressor-response correlation with hepatic cadmium. The morphometric and gravimetric analysis revealed that only condition factor, renosomatic index and female gonadosomatic index showed significant differences between Mae Pa and Mae Tao frogs. However, albeit being insignificant, Scaling coefficient and hepatosomatic index showed similar trends. In the biological and ecological effect study, it was found that Mae Tao frogs had higher macro-melanophage count in the liver. Hepatocyte swelling, possible necrotic and apoptotic hepatocytes, renal tumor-like aggregation, renal hemorrhage and more testicular ovarian follicle were also observed in Mae Tao frogs. The results showed that cadmium contamination may, either directly or indirectly, cause an increase in cadmium accumulation in the liver, kidney, testis and whole organism; an increase in hepatic metallothionein and hepatic glutathione-S-transferase; a decrease in condition factor, renosomatic index and female gonadosomatic index; and histopathological changes in the liver, kidney and testis of the rice frog. Therefore these results were able to justify the use of Fejervarya limnocharis as a sentinel species for cadmium contamination.