All hard clams start out their adult lives as males. In later years, the larger clams often switch from the production of sperm to the production of eggs, effectively changing their sex to female.
Large clams make the switch to the female sex because creating eggs takes up more metabolic energy. Large clams can devote more energy to the production of egg cells, which are larger than sperm.
Once the clams have reached a harvestable size, which is anywhere between 1 and 4 inches, there is generally a balanced ratio of male to female. The average hard clam's lifespan is approximately 30 years.