The rocky shorelines of San Francisco and Marin Counties are home to a diverse collection of bivalves – mollusk species with two shells. Bivalves filter food by siphoning it from the water through gills between their two valves. Their organs, including their nervous system, are all also located between these two shells. Some bivalves have a muscular foot that can stick out of the shell to help with locomotion, attaching to rocks, or both. California mussels (Mytilus californianus) are a common bivalve species that are often found in massive clustered growths attached to rocks by strong hairlike filaments called byssal threads. The native Olympia oyster (Ostreola conchaphila) also lives all along the West Coast, though local populations are declining as a result of predation by a non-native species of whelk snail called the Atlantic oyster drill.