"Clam" is an informal term used to refer to any molluscans within Class Bivalvia.Remains first appeared in Cambrian age rocks 510 million years ago. They presently live in both freshwater and marine habitats, and range in adult size from nearly microscopic to the giant clam, which can weigh 200 kg (440 lb). Some have life cycles of only one year, while at least one has been discovered that may be over 500 years old.