These are Giant Clams that I photographed on the Great Barrier Reef. These Giant Clams live in warm shallow waters in the south pacific. They can grow quite large (several feet), but most of these are less than a foot across. They are filter feeders and are open most of the day. They close very quickly if startled or if a shadow moves over them.
One of the most interesting things about these clams is that each one is a different color and pattern. Some are bright colored, others have intricate patterns, and some are relatively plain.