 There are limitations to the use of bivalves as bioindicators . The level of pollutants found in the tissues varies with species , age , size , time of year and other factors . The quantities of pollutants in the water may vary and the molluscs may reflect past rather than present values . In a study of several bivalve species present in lagoons in Ghana it was found that the findings could be anomalous . Levels of zinc and iron tended to rise in the wet season due to run @-@ off from the galvanized roofing sheets used in many of the houses . Cadmium levels were lower in young animals than in older ones because they were growing so fast that , despite the fact that their bodies were accumulating the metal , the concentration in their tissues reduced . In a study near Vladivostok it was found that the level of pollutants in the bivalve tissues did not always reflect the high levels in the surrounding sediment in such places as harbours . The reason for this was thought to be that the bivalves in these locations did not need to filter so much water as elsewhere because of the water 's high nutritional content . 
