Biomarker and bioaccumulation responses of Asian clams exposed to aqueous cadmium.

Measured responses of biochemical or physiological indicators have been suggested to reflect thresholds where pollutants exert their initial effect. Responses in cellulolytic enzyme activity and DNA strand breakage of the Asian clam Corbicula fluminea exposed to cadmium in the laboratory were measured and metal body burdens were determined concurrently. Clams were exposed to aqueous cadmium concentrations of 3, 6, 12, or 25 ppb for 23 and 28 d. Cadmium concentrations in clam tissue were highest in lower cadmium treatments, and body burdens increased with length of exposure in only the 28-d experiment. Cellulolytic enzyme activity decreased with increasing cadmium concentrations for clams in the 28-d experiment. Induced enzyme activities were observed in cadmium treatments for both experiments and are thought to precede declines in activity through the length of exposure. Significant reductions in DNA strand lengths of cadmium exposed clams were observed by wk 3 in the 23-d exposure and by wk 2 in the 28-d exposure. Reduced DNA strand lengths in these cadmium treatments for the 28-d exposure precede significant declines in cellulolytic activity at subsequent sampling events. Combining these data with observations of mortality in higher cadmium treatments suggests that impairment of DNA structural integrity and reduced digestive enzyme activity may indicate metal-induced stress in clams.