 When they live in polluted waters , bivalve molluscs have a tendency to accumulate substances such as heavy metals and persistent organic pollutants in their tissues . This is because they ingest the chemicals as they feed but their enzyme systems are not capable of metabolising them and as a result , the levels build up . This may be a health hazard for the molluscs themselves , and is one for humans who eat them . It also has certain advantages in that bivalves can be used in monitoring the presence and quantity of pollutants in their environment . 
