The downside of filter feeding is that they filter out the toxins and poisons in the water. It is good for the water because the oyster filters out the toxins but it isn't good for the oyster. The toxins are usually in the water during the summer times of the year and therefore are not a good time of the year to be eating oysters. They also have gills and get their oxygen from the water.
Sometimes, dirt, sand grains, or other particles can become lodged within the mantle tissue. These foreign objects are irritating to the oyster. To compensate for this cells from the mantle stick to the foreign object that has become lodged within the mantle tissue, and they secrete around it, which over time, forms a pearl. Commercial oysters lack the ingredient in its secretion to form a pearl of any value. There can be more than one pearl inside an oyster at any given time. .
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Oysters spawn from the warm weather in late spring to early fall. Eggs are discharged by females and released into the water and are fertilized by males who discharge sperm into the water. This can be productive and not so productive at the same time, it depends heavily on the tides. If the tide is slack than this can be a productive event, however is the tide is moving than the oyster would be lucky if one of the eggs became fertile.
A fertilized egg quickly develops into a swimming, "shell-bearing" larval oyster, which is also known as a "Veliger." During the "Veliger" stage, which lasts approximately two to three weeks, larval losses are very high. The tide distributes them throughout the bay or their surrounding waters. They could be carried to waters that are too fresh or waters that are too salty. Predators feed especially on these larvae. Pollution from industrial waste is also a leading factor that kills oysters. The small percentage that live can result in a tremendous number of tiny oysters.
Larvae that have settled to the bottom and attached themselves to a hard surface are called "Spat.