Female fecundity is high, and individuals can release 16 million to 24 million eggs per spawn, although laboratory studies often have recorded values of only 1-3 million eggs, and only a few will reach maturity. Individuals may release as many as 60 million eggs during one season. Eggs are 70 to 73 microns in diameter and are surrounded by a gelatinous membrane. Eggs and sperm of adults are expelled in the water current and fertilization occurs externally in the water column. In the Bay area, the most significant growth occurs in spring and fall, when abundant food and optimum water temperatures coincide.
Growth decreases in summer and stops altogether in winter.
Growth rate also decreases with age, and when this happens, clams become thicker, rather than increasing in shell length. The pediveliger stage is the final larval stage before settlement and metamorphosis to juveniles. At this stage the organism has a foot that extends from its shell. Once the hard clam settles to Bay bottom, it uses its foot to dig into the mud, and secretes a calcium carbonate shell that increases in size as the clam's internal organs grow. A cross-section of a hard clam's shell usually reveals a clam's age.