The presence of growth rings is important in distinguishing Asian clams from dreissenids. Growth lines are striations in the shell that are produced as the shell grows. In Asian clams they are typically quite pronounced, especially in the umbonal stage. In comparison, the umbonal stage of dreissenids has growth rings that are less pronounced or weak.
In addition to growth rings, there are often secondary striations or lines running parallel to the growth rings. These are present in the umbonal stage of Asian clams. When secondary lines are present, they give the shell the appearance of being pleated.
