CULTURE OF PEARL OYSTERS

• Pearls - oldest of gems

• Used - ornamental , have medicinal value, in cosmetics and tooth pastes

• The species of pearl oysters found in India are Pinctada maxima, P. fucata and P. margaritifera

Pinctada maxima P. fucata P. margaritifera

Biology:

• Pinctada fucata - commonly used for pearl production

• attaches to hard substratum by means of byssus threads

• Stenohaline ; survives short term salinity fluctuations

• Filter feeder, mainly on phytoplankton, occasionally on bivalve eggs and copepods

• Life span - 5-6 years, maximum size 10cm

• Annual growth rate - 50mm

• Environmental factors such as depth, transparency, temperature, salinity, currents, calcium content of water, food and foulers influence growth rates

• Sexes are separate

• hermaphroditism and change of sexes from one spawning season to another are not uncommon

• Attains maturity at a size of 25-30mm

• Two peak spawning seasons- 1.during July to August; 2. from November to December

• Eggs and sperms are shed in water, fertilization is external

• Hatching takes place in about 4 hours

• Spat settle when 0.3mm in size

• Spat availability in the wild fluctuates, hence hatchery produced spat are used for culture.

Development of pearl:

• Natural pearl is formed when a foreign material enters the body of pearl oysters

• Epithelial layer of mantle forms a sac around the foreign body which secretes nacre over the nucleus (foreign body) layer after layer resulting in pearl formation

• Shape of the pearl depends on the shape of the nucleus/foreign body

• Nucleus/nuclei can be implanted along with a piece of mantle tissue as graft into the gonads oysters which results in formation of cultured pearls

Culture methods:

• Cages are stocked with seed oyster and suspended from the rafts or long lines in protected bays

• Allowed to grow for four months when they become ready to receive nucleus

• Oysters from cages are brought to the laboratory for implantation of nucleus

• They are kept in glass troughs containing filtered seawater

• A few menthol crystals are sprinkled into the troughs to anesthetize the oysters

• After one hour the oysters are ready to receive the nucleus

• Graft tissues are prepared by sacrificing a few oysters

• Pieces of mantle are cut, cleaned and trimmed to 3x2mm pieces

• They are smeared with weak eosin solution to avoid deterioration

• Nuclei are made from chank shells of shells of freshwater mussels

• Nuclei range in size from 2-8mm

• The anaesthetized oysters are mounted on a stand

• An incision is made at the base of the foot and a canal is cut through the gonad

• A graft tissue is first implanted into the gonad through the canal followed by insertion of nucleus using special instruments made for the purpose

• Depending on the size of the oyster and the nuclei, 1-8 nuclei can be inserted into the same oyster

• After implantation, oysters are returned to the farming sites in frame nets which have compartments for individual oysters

• The frame nets are suspended from the rafts/long-lines for development of the pearl

• The outer epithelium of the graft tissue grows over the nucleus and forms pearl sac in about a week

• The pearl will form in about 3 months

• 3-20 months are required to produce pearls of 3-8mm respectively•

• Six months are required to coat nacre 0.2mm thick.

• The oysters are harvested after the formation of required size of pearls

• They are sacrificed to recover pearls

• The harvested pearls are bleached in hydrogen peroxide to remove any blemishes