Oysters eat plankton, algae and small food particles that wash over their gills. Both oysters and clams use the same type of siphoning and straining system to remove food from the water around them.
When water washes over an oyster, it pushes the water through its body and extracts any plankton or small particles of water vegetation. Oysters are called filter feeders because they essentially filter their food out of the water.
The food that an oyster eats can affect what a person finds when opening the oyster. Depending on the color of the algae eaten, an oyster may have green or red coloring inside.