In the marine environment, mercury is accompanied by selenium in all investigated species of mammals, birds, and fish--possibly due to a normal homeostatic regulation. It therefore seems likely that selenium will exert its protective action against mercury toxicity in the marine environment, decreasing its detrimental effects on reproduction, behavior, growth, etc. of the organisms and thus protect the population and ecosystem. On the other hand, the increased retention of mercury caused by selenium may lead to a higher level of biomagnification in the food chain and a higher burden in the individual. This might counteract the positive effect of decreased intoxication.