American Oysters

Larvae of the eastern oyster (Crassostrea virginica) develop a foot just before settling to the bottom. Once settled to the bottom they will seek a substrate upon which to attach, using a rudimentary eye and chemical cues. When the substrate is found they cement themselves to it and shortly thereafter lose their foot, thus their ability to move any further. Suitable settling substrate is preferably other oyster shell but may include other types of shell, cement, rocks, pilings, even hard rubber objects like a sunken snorkeling fin! From this point on they are at the mercy of their environment so must choose their settling location carefully. As they reside on the surface of the sediment they simply open their growing shells to filter food and dissolved oxygen from the water, using their gills and mantle.
Oyster Anatomy
Oysters are also bivalves and are protandric hermaphrodites, similar to hard clams, in that they change gender during their lives, starting as males and usually ending as females. The shape of oysters can vary (long and thin, short and stout) and depends on their growing conditions.