Clams grow by depositing calcium carbonate on the margins of their shells. The shell material is produced and laid down by the soft tissue called the mantle which is in contact with the edge of the shell. The lines that you see on clams are growth rings that are analogous to the rings on tree trunks. Each winter the growth rate slows down as the water gets cold and the clam produces a pronounced dark ring. The clams at right are between three and four years old and you can see the darker annual rings.