Environmental stressors as well as the direct or combined effects of pollutants could be harmful to the populations living in a marine environment and the reproductive and nutritive processes could be impaired in a deteriorating environment. Sublethal effects of pollutants were studied in the blue mussel Mytilus edulis L., a good bioaccumulator of contaminants. Blue mussels of 3.5 cm were sampled on a rocky substrate at Pointe-Mitis (48°40' N, 68°02' W) along the coast of the St-Lawrence estuary. Mussels were placed in experimental tanks, fed, supplemented with mineral salts and continuous sea water flow and kept 72 h before the exposure to 0.01 Ig 1and 0.3 g 1-methylmercury hydroxide in the presence or absence of selenium, at a concentration of 125 tg 1-', a possible antagonist of methylmercury. The contamination protocol was performed during 45 days and a 14 day period of recuperation was allowed. The stress caused by the transplantation of mussels in the laboratory tanks and/or by the presence of pollutants was evaluated by a general indicator of stress developed in our laboratory, the measure of the lysosomal membrane fragility (LMF) of the digestive gland, according to the method developed by Moore (1976). The effects of contamination on metabolism were measured by the study of the variations of the malate dehydrogenase activity (MDH), a key enzyme of the aerobic metabolism. The first days of the contamination period led to an increased metabolism in the mantle and to a detoxifying mechanism in the hepatopancreas. At days 22 and 29 of the experiment, the affinity of the MDH was greatly decreased with both concentrations of methylmercury and selenium, suggesting a competitive inhibition of the enzymatic activity by the pollutants. LMF increased as the mussels were kept longer in the tanks. Methylmercury increased the stress undergone by the mussels. LMF gives information about the degree of stress of the organism while the biochemical indicator informs about the metabolic effects of sublethal concentrations of pollutants.
