Oysters are called bivalves, meaning they have 2 shells that open and close and feed by filtering the plankton they require through their gills from the water around them. Oysters pump through a large amount of water to gain the necessary food for growth. Oysters are not fed any supplementary diet.
By filtering the water the oysters help maintain a healthy marine environment by reducing the nutrient load in waterways. One oyster can filter up to 40 liters of water per day. In the Chesapeake Bay, one of the largest inland saltwater waterways in the world a program is underway to repopulate the bay from hatchery-produced oysters. This has been considered necessary to counter algae blooms that threaten other fish species in the bay by turning the waters acidic.
Deepwater shellfish farms with long lines and cages can replicate a reef habitat and can act as Fish Aggregation Devices where larger fish feed on smaller shrimp-like animals.
Oysters are currently rated as the Number 1 sustainable choice seafood by the UK-based Marine Conservation Society in their publication titled Fish online.
Oysters are also rated as a Better Choice category by the Australian Marine Conservation Society which is the highest rating category by that organization.