Oysters live in large communities called reefs or beds, usually established near seashores or river mouths. Oysters use their reefs for breeding and feeding. They are also a source of food and shelter for other animals including anemones and mussels.
Oysters help filter and clean the water around them by consuming the phytoplankton and algae and filtering out harmful toxins and pollutants. These shellfish are a keystone species, since their presence or absence is an indicator of the health of particular ecosystems. As of 2015, there is a decline in the oyster population, due to over-harvesting. Several coastal areas are working to manage and replenish declining oyster reefs.