Round cultured akoya pearls are produced by inserting mother of pearl spheres, known as nuclei, into the oyster. These spheres are ground from thick shells of the Uniondae family such as the Amblema plicata, fresh water mollusks native to the Mississippi and other gulf rivers. These nuclei are then implanted into the mollusk along with a small piece of mantle epithelium (graft) from a donor oyster. This small piece of mantle graft is responsible for the formation of a pearl sac. The pearl sac surrounds the nucleus and nacre will begin to form. The thickness of the graft also affects a pearl’s surface quality. Thinner grafts tend to produce pearls with better surface quality, while thicker ones produce poorly colored pearls. Properly sterilized equipment and water-soluble antiseptics must be used. If proper precautions are taken mortality rates are suppressed below 10%. A pearl’s color is determined by the mother of pearl deposited around the lips of the inner shell, but the area of nucleic implantation also attributes to a pearl’s color and surface quality. For instance, nuclei implanted in the ventral region of the gonad yield white or creamy pearls. Implantations on the gonad’s dorsal region yield grey or white pearls. Pearls with premium surface quality and perfect symmetry develop in contact with the softer innards of the oyster such as the liver or intestines. Pearls implanted near the retractor muscle will be baroque and unusable.
The Culturing Environment
The environment in which oysters are cultured also plays a critical role in producing premium pearls. For instance oysters cultured in waters 10-m or deeper tend to yield higher quality pearls and have less instances of disease. Lowering cultures to greater depths also helps slow the progression of “red adductor,” the disease responsible for the massive pearl oyster kill offs during the 1990’s.
The mollusks’ metabolic rate is also controlled by temperature. Higher temperatures promote faster growth and nacre production. However, lower temperatures produce thinner nacre layers, responsible for higher luster. Higher luster attributes to higher pricing so farmers will lower their cultures to greater depths towards the end of the culturing period. Farmers must be mindful of minerals in the seawater. These minerals also affect color. Silver and copper can cause creamy pearls, while higher levels of zinc and sodium may result in pink pearls.
Culturing Time Line
The culturing process begins with the birth of mother oysters. In May and June eggs are artificially fertilized to ensure the birth of premium oysters. Around August the larvae are put into a mesh bag and submersed from a raft or wharf. One bag can contain 10’s of thousands of granule-sized larvae.
II. Raising Mother Oysters
As time progresses the oysters are placed in different cages as they fully mature after ± three years.
III. Implantation Preparation
Before implantation oysters are well fed and checked to make sure that their gonads have matured. The water’s plankton and oxygen levels are strictly regulated to prevent the oyster from producing eggs or sperm, assuring room for nucleic implantation.
IV. Nucleic Implantation
V. Immediate Post Implantation Period
Oysters are neatly arranged in a cage and stored in an on site support chamber or submerged from a wharf or raft in calm waters for about a month. The oyster’s health is in critical condition so they are monitored and cleaned regularly—killing parasites and removing foreign particles.
VI. Major Culturing Period
Implanted oysters are suspended at 15-m at temperatures of 20 to 25°C. The depth is adjusted occasionally to trigger the release of eggs and sperm. This ensures enough space for pearl growth and prevents the rare occurrence of a nucleus being spat out.
After 1.5 to 2 years the pearls are removed from the oysters and sorted by size. The pearls are sized in 1-mm gradients and priced.