Age of maturity is two years at about 50-70 mm. Cockles are usually not more than 100 mm in length but can reach 120 mm. There is a great variation in size at any age and is particularly noticeable in the first year. Growth in northern latitudes is slower than in southern latitudes, probably due to decreasing mean air temperature with increasing latitude. Food availability and time submerged under sea water are important factors influencing growth.
Cockles have the highest growth rates from subtidal areas with plenty of food.
In general, the cockle clam is widely distributed but not abundant in any one location. Subtidal populations may not be extensive. Juveniles are more predominant in intertidal area while adults are more predominant in subtidal areas. Populations in the subtidal are more fecund, but the intertidal population outnumbers the subtidal by several times. The subtidal may be a valuable seed area during years when environmental conditions render the intertidal population impotent.