Pearl Cultivation Procedures
A matured oyster/mussel (the saltwater and freshwater mollusk is called oyster and mussel respectively) will grow for about 1-3 years before it can be nucleated. An oyster/mussel experiences several stages of growth: from the egg to embryo, from embryo to young spat, then growing into adulthood. The following charts give full illustrations on how cultured pearls are formed.
1) The birth of a mollusk
Female and male saltwater mollusks must be mixed together in broodstock to spawn, and thus millions of eggs and sperms are expelled into water. The fertilized eggs will soon develop into tiny embryos. When they are between 16 to 20 hours old, they are called larvae. However, the larvae of the freshwater mussel (glochidia) often attach to the gills and fins of fish.
2) Young mollusk - spat
Initially, the larvae swim freely, then, within a few weeks, young mollusks produce a sticky mass of threads called byssus which enables them to attach themselves to fixed surfaces, while losing their ability to swim freely. At this stage, young mollusks are called spat. The freshwater glochidia, when they grow large enough, simply drop off the fish and settle to the bottom.
3) Collection of spats or hatchery breeding
Mollusk spat can be collected from the wild during the peak spawning period, and then tended at a pearl farm within a protected environment. Now, however, good quality spats can be more efficiently obtained by hatchery breeding. Hatcheries are built with well-controlled growing conditions and are supplied with various nutrients. The entire process, from spawning to adulthood, can now all be achieved in a hatchery, while a biologist monitors the entire whole growth processes.
4) Mollusk growth
A young mollusk has to grow large and old enough to be nucleated and the age at which this can be done varies from species to species. Saltwater oysters normally grow for 2-3 years before they can nucleated, while for freshwater mussel, the maturation process is 6-12 months. During the mollusk growth, a regular cleaning process must be maintained to remove parasites, seaweed and anything else that can collect on the mollusk shell, because not only do they compete with the mollusk for nutrients, they are harmful to the mollusk’s health.
5) Preparing for nucleation
As nucleation is the key surgical process and of great importance and delicacy, great care must be taken both to ready mollusks for the procedure, and ensure their survival during the process. The basic idea is to slow down the mollusks’ metabolic process as much as possible and ensure they do not begin their spawning period. They are, for example, moved into waters with slow current to reduce their nutrient supply, or tended under colder temperatures.
6) Mollusk nucleation/grafting
In the nucleation process, saltwater oysters are pried slightly open and secured by a wedge. A donor oyster must be sacrificed to provide square pieces of mantle tissue. A nucleator carefully cuts a small incision in an oyster’s gonad and inserts a square of mantle tissue (about 3mm square) followed by a round bead made of freshwater mussel shell. The live tissue piece has to face the bead nucleus so that the tissue-irritated pearl sac can grow around the bead. This procedure needs expertise, speed and precision. To reduce mortality, nucleated mollusks must be returned to the water as soon as possible.
The nucleation process for freshwater mussels is different, as no bead is used. The live tissues from the donor mussel are inserted into the incisions of a host mussel’s mantle tissue rather than into the gonad. Typically, 12-16 pieces of tissues can be inserted into a single valve of mussel. Each tissue can initialize the formation of a single pearl. The bead-free nucleation results in almost pure solid nacre for freshwater pearls.
7) Mollusk recovery period
The nucleated mollusk normally needs 2-3 months to recover from the surgical implantation. This period is crucial for pearl sac formation and the mollusks are usually tended under carefully controlled conditions. Some hold to the belief that turning the mollusk will sometimes help produce round pearls by promoting even growth of nacre layers. Some pearl farmers use X-ray equipment to verify whether the bead nucleus has stayed in place. As many as 50-60% of nucleated saltwater mollusks expel the nucleus or die as a result of nucleation.
8) Cultured pearl growth
After the recovery period, the nucleated mollusks are returned to the pearl farms. The pearl sac has formed by this time and it is during this period that the mollusk deposits pearl nacre layer by layer until the harvest. Generally, the longer pearls are left to grow, the thicker the nacre.
Saltwater oysters are usually kept in fairly deep water, protected by a cage or mesh basket, the position of which is marked with a floating raft. Pearl farms are quite often located at remote places to provide an uninterrupted environment for pearl growth. Pearl farmers periodically pull the mollusks out to clean the surface of parasites and inspect them for signs of sickness. The water temperature, food supply and meteorological conditions are also continually monitored.
9) Cultured pearl harvesting
After the growth period, which for Akoya and Tahitian pearls can be as long as 24 months, and for South Sea pearls 30 months, the pearls are finally ready to be harvested. The growth period for freshwater pearls is longer before they can be harvested—sometimes up to seven years—because they are composed of almost pure nacre. Cultured pearls are usually harvested during the cooler months, as cold water produces an optimized luster quality.
Normally, Akoya and freshwater mollusks are disposed of when harvested. A very tiny percent of freshwater mollusks are left alive to be further nucleated or are returned to the water for reproduction, and some of them are just left to produce a second generation of pearls without nuclei. These pearls, produced by loose tissue pieces, are known as “Keshi”, regardless of their size and mollusk type, and are often characterized by their large baroque shapes consisting of pure nacre.
Tahitian and South Sea oysters can be re-nucleated up to three times in their lifetime, if they have produced a pearl and are in good health. Each time the oyster is re-nucleated, the bead used is the same size as the finished pearl that was produced in the former nucleation. However, although a larger cultured pearl can be obtained by such re-nucleation, the quality will be degraded.