Clams reproduce by releasing gametes, or eggs and sperm, into the water. Male and female clams have no direct contact. The clams are prompted to reproduce by changes in the water's temperature and pH. In the quahog clam, spawning usually happens during neap tides, which is the lowest point of a high tide.
In a clam like the quahog, male clams are the first to release gametes into the water. The female clams then release eggs that are fertilized by the sperm. Clams like the giant clam are hermaphrodites, which means they produce both eggs and sperm. These clams still need to broadcast gametes into the water as they can't perform self-fertilization. As with other clams, giant clams are stimulated to spawn by the actions of nearby giant clams.
Within about half a day after fertilization, the eggs grow into larvae and begin to feed on plankton, such as diatoms. Over the weeks, their organs, feet and shells develop. At one point, they attach themselves to a surface to allow their shells to harden, or calcify. This only happens when the water is a certain temperature. Finally, they detach from the surface and begin life as adult clams. Biologists believe that only 10 percent of clam larvae attain adulthood.