The Ostrea shell is made up of a bottom concave valve, and an upper flat valve. Ridges on the lower valve helped the oyster anchor itself in sediment or next to other shells. The top valve then acted as a lid which opened when the oyster pumped water across its gills, extracting oxygen, and food. These oysters were often utilized by boring worms, sponges such as Clione, and other epifauna (organisms that live on the outside of other living organisms).
M. J. O'Donnell