Abstract
Both natural and polluting metal ions are taken up by marine organisms. For many metal, and metal-containing ions, mechanisms exist which allow these organisms to discriminate between essential and non-essential ions, and reject the latter. In this article we examine the accumulation mechanisms and effects of metal and metal-containing ions on bivalve molluscs and tunicates (sea squirts). Special emphasis is put on the internal or physiological environment of metal ions in marine organisms. A molecular mechanism based on experimental studies is presented to explain the extraordinary accumulation of vanadium in tunicate blood cells called vanadocytes. Vanadium in the form of vanadate anion is transported into the blood cells by passage through anionic channels. Inside the cell it is reduced to cationic V(III) and V(IV) ions which are trapped inside.