The clean white ring of new growth on the edge of the clam shells is always a welcome site in spring. The clams feed and begin growing as the water warms and the food supply (microscopic phytoplankton) flourishes. The rings that you see on a mollusc shell are analogous to tree rings, except they radiate out from the hinge area in a geometric spiral. Calcium is slowly added to the lip of the shell as the animal grows. The growth rate decreases in late fall so the lines are bunched together forming a distinct annual growth ring.