Why are bivalve molluscs called biological filters?

Filters pass water through themselves, squeezing out various organisms and food particles from it, and thus feed. These include bivalve molluscs. They are immobile, they do not have a special organ – a radula (or grater), which grinds food and is in active forms, so they switched to filtration. In this case, all the water is passed through their body, and all harmful substances from it can accumulate in the body of the mollusk. Therefore, it is not recommended to eat bivalve molluscs (such as mussels or oysters) that have been harvested from dirty water.