The progress of urbanization and industrialization along the southern coast of Caspian Sea can threaten coastal ecosystems because of increasing metal pollution. Cadmium (Cd) is classified as toxic and persistent pollutant in the marine environment, which is also accumulated in many organisms. To obtain contamination levels, the amounts of cadmium in the shell of Amphibalanus improvisus (barnacle) and Mytilaster lineatus (mussel) were measured as biological monitoring marker of Cd in the central southern coast of the Caspian Sea. Samples were collected seasonally between October 2015 and October 2016 from three stations. The hard tissue of the samples was chemically digested and then the Cd was analyzed by atomic absorption spectroscopy. The results showed that the maximum and minimum Cd concentration for barnacle (32.96 and 1.46 μg/g dry weight) was observed in spring and winter respectively, while the Cd concentration of mussel varied between 8.16 and 0.72 μg/g dry weight. These species can be introduced as a biological indicator due to the accumulation of Cd in their shells with concentrations higher than the water and sediment.
