Due to the increasing discharge of pollutants into mangrove wetlands in China, a trend of heavy metal pollution has been becoming more and more widely and intensively. The mangrove mud clam Geloina coaxans (Gmelin, 1791) is a dominant macrobenthos of mangrove wetlands, and serves as common seafood in the South China coast. Owing to its wide distribution, large body size and high abundance, the mangrove mud clam would act as a potential indicator of marine heavy metal pollution.
In this study, the effects of Cd stresses (level×exposure time) on the activities of superoxide dismutase (SOD), catalase (CAT), amylase, lipase and proteinases and malondialdehyde (MDA) content in the stomach tissues of the mangrove mud clams were studied under hydrological culture. Eight Cd levels, i.e. 0, 0.125, 0.25, 0.50, 1.00, 2.00, 4.00 and 8.00 mg/L were chosen. The stomach tissues of the mangrove mud clams were sampled on 0 d (the beginning day), 1d, 3d and 7d, and Cd-release 6d respectively.
The results showed that the SOD activities in the stomach tissues of the mangrove mud clams were induced rapidly after exposure to higher levels of Cd (4.00 mg/L and 8.00 mg/L), while it needed longer time to induce the enzyme under lower Cd level. The CAT activities in all treatments were induced once exposure to Cd but inhibited with the prolonging of the exposure time. Stress-release experiments showed that these inhibition effects on the CAT activities could be reversed only under the lower levels of Cd exposure. MDA content increased once exposure to the low levels of Cd and then decreased with the prolonging of exposure time. However, under the high levels of Cd stresses, MDA contents increased with the exposure time and maintained at higher level even after the stress was released. Cd exposure had an inhibitory effect on the activities of the amylase. Significant induction of the lipase by Cd occurred at the early time of the exposure, especially under the lower levels of Cd treatments; however, this induction effect vanished and the lipase activity decreased with the prolonging of Cd stress, and returned to higher level after the stress was released. The effects of Cd stress on proteinase activity varied disorderly. These results showed that the effects of Cd exposure on the stomach tissues of the mangrove mud clam differed through the course of the experiment. Whether it would be promotion or inhibition depended on the given indexes, exposure time and Cd levels.
Our results indicated that the mangrove mud clam could be suitable for an indicator organism because some significant effect-dosage relationships occurred. When the clams were exposed to Cd stress for 1d, the SOD activity responded regularly to the stress level, however, this regular effect didn't repeat as the stress time prolonged. As to the activities of CAT and amylase and the content of MDA, the regular effects occurred on the later stage of Cd exposure. Therefore, these four biochemical indexes would be complementary if they were applied to the cadmium monitoring together, but CAT and MDA would be more practicable than others.