Data on the factors affecting the accumulation of selenium in the cockle Anadara trapezia and mullet Mugil cephalus are presented, together with the distribution and speciation of selenium in tissues. Selenium concentration in whole cockles showed a small but significant decrease with weight. No further decrease in selenium concentration was apparent once an organism reached 0.25 g dry weight. Selenium concentration in cockles was not dependent on sex. The tissue distribution of selenium concentration in cockles was in the order gill > intestine > adductor > mantle > foot.
Selenium concentrations in liver tissues of mullet increased with the whole weight of the fish. In contrast, selenium concentrations in muscle, stomach, heart and kidney tissues were fairly low and constant in fish weighing less than 200 g (20 cm in length). Fish of greater weight and size ( > 250 g and > 30 cm) had higher and more variable selenium concentrations. No differences in selenium concentration between male and female fish occurred; however, the sex of many of the fish could not be distinguished. The tissue distribution of selenium concentration in mullet was in the order liver > stomach > heart > muscle > kidney.
Most of the selenium recovered from both the cockle tissues and the mullet muscle tissues was found to be associated with proteins and to be present as selenocysteine. A conceptual model is presented for selenium transformations in marine organisms based on the formation of selenoamino-acids and subsequent incorporation into proteins.