Marine members of the Bivalvia class are molluscs that have laterally flattened bodies enclosed by a shell formed of two hinged parts. They include clams, oysters, mussels and scallops and most are filter feeders. The gills have evolved into specialised organs for feeding and breathing.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge (the hinge) by a flexible ligament that, usually in conjunction with interlocking 'teeth' on each of the valves, forms the hinge.
Most bivalves bury themselves in sand or sediment on the seafloor, making them relatively safe from predators. Others lie on the seafloor or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops, can swim.
The giant clams are members of the Tridacninae family of large saltwater clams. The two-part shell usually has a wavy opening that never properly closes and faces the sunlight. The hinged side of the shell is at the bottom, attached to a hard surface by a large mass of tough silky filaments that grow from the gap between the valves near the hinge. Some giant clams burrow into coral, with most of the shell hidden and only the shell opening facing sunlight.
The many species of spiny or thorny oysters vary considerably in appearance and are actually related to scallops, rather than oysters. Similar to oysters however, they live cemented to rocks, corals or other hard surfaces and are capable of producing pearls, depending on their environment.
Scallops are a large and varied family of plankton eating filter feeders which are found in all of the world's oceans. Most scallops live on the sandy or rubble seafloor and are capable of rapidly swimming short distances. These are known as 'free-living' scallops, however a few species live cemented to rocks or hard coral skeletons, and a few may even attach themselves to soft corals or seagrass.