When spawning is completed, an oyster is almost translucent. Gradually the space which was occupied by the eggs or sperm becomes filled with animal starch or glycogen, giving the body a smooth, creamy-white surface. The separate sexes of the American oyster ripen in early summer. The spawn is not released all at once but at intervals over a period of four to six weeks. During the spawning season, a single female, by clapping her shells gently, will puff out many millions of buoyant eggs and a male will release an even greater number of sperm.
Fertilization takes place in the open water and cell division begins.
The fertilized egg develops into a microscopic larva. Within 24 hours it forms a shell and develops a swimming and feeding organ consisting of a disc covered with vibrating hairs which can be thrust out and withdrawn at will, allowing free movement. For about three weeks, the little larva swims and drifts in the tidal currents and may travel far from the spawning area. It feeds on microscopic plants of the plankton community of which it is a part. The mortality rate is very high and only a small fraction of 1 percent of the young larvae reaches the next stage of development.