Pearl Culture
Pearl is a white, highly shining globular
concretion found within the shell of an Oyster. Pearl oysters are sedentary
animals. In India it was cultured for the first time in 1973 at Thoothukudi.
Pearl oysters are found along the coast of Kanyakumari and in the Gulf of
Kutch. High quality pearls are obtained from pearl oysters of Genus Pinctada that can be cultured in the
salinity range of 30 ppt in racks, raft and long line methods. Freshwater bivalve Lamellidens
is also used in artificial pearl culture. Mostly the pearl oysters inhabit the
ridges of rocks or dead corals, forming extensive pearl banks. These pearl beds
produce best quality of pearls called as “Lingha Pearl”.
When a foreign particle accidently enters into the space between mantle and shell of the oyster, it adheres to the mantle. The mantle epithelium encloses it like a sac and starts to secrete concentric layers of nacre around it as a defensive mechanism. Nacre is secreted continuously by the epithelial layer of the mantle and is deposited around the foreign particle and over a period time the formation of repeated layers of calcium carbonate makes the hard and glossy pearl.
When the pearl enlarges the oyster
dies. The shell is then carefully opened and the pearls are manually separated
and graded (Figure 13. 12).
This can be achieved by an artificial device to
insert the nucleus as foreign particle in the shell of oyster has proved useful
for the production of pearls in greater number.
Oysters are caught by special type of cages (84 x
54 x 20 cm) by covering a heavy wire frame with two centimeter wire mesh. This
cage is dipped into a sand-cement mixture providing rough surface to the cages
to which free swimming spat get easily stuck up. These cages are suspended at a
depth of 6 meters. From July to November, where spats are easily available.
These collected oysters are now transferred to rearing cages.
The collected oysters are stocked and reared in
special type of cage called as rearing cage. These cages are well protected
from enemies of oysters like Octopus, Eel, Devil fishes etc. The collected
oysters are first cleaned and then placed into the culture cages for a
period of about 10 to 20 days to recover from the strain due to excessive
handling and for the physiological adjustment to the shallow water conditions.
In this method, a piece of mantle of living oyster
is cut off and inserted together with a suitable nucleus inside the living
tissue of another oyster. Following steps are taken for the insertion of
nucleus.
The selected oysters for the insertion of nucleus
should be healthy and strong enough to overcome the stress during operation
The piece of tissue which is inserted inside the
mantle is called as ‘GRAFT’ tissue. The outer edges of these graft squares must
be known because nacre secreting cells are found only on the outer surface of
the mantle so it is essential to keep the outer surface in contact with the
inserted nucleus.
Any small particle may function as nucleus to
initiate the pearl formation but it is reported that calcareous nucleus is the
best because the deposition of nacre was found to be more on calcarious
nucleus.
For the insertion of nucleus, oysters are fixed in
a desk clamp in the position of right valve facing upward. Mantle folds are
smoothly touched to expose the foot and the main body mass, followed by an
incision into the epithelium of the foot and a slender channel into the main
mass one graft tissue which functions as a bed for the nucleus.
Nucleated oysters are placed into cages and
suspended into sea water and attached with floating rafts to a depth of 2 to 3
metres for about 6 to 7 days to recover from the shocks due to operation. This
period of 6 to 7 days is known as ‘Recovery period’. About 3000 to 3600
nucleated oysters are kept in different cages suspended in sea water at 2 to 3
meters depth for 3 to 6 years and undisturbed except at the time of clearing
and inspection.
Pearls are harvested in the month of December to
February which may slightly vary according to climatic conditions. After the
completion of 3 years of the insertion of nucleus, pearl oysters are harvested
from the sea and the pearls are taken out from the shell.
After taking out the pearls from the oysters shell
they are washed properly, cleared with the soap solution.
Pearl comprises of water, organic matter, calcium
carbonate and the residue.
(1) Water: 2-4%
(2) Organic matter:3.5-5.9%
(3) Calcium carbonate: 90%
(4) Residue: 0.1-0.8% carbonate:
90%
The pearls obtained are of variable shapes and
sizes. They may be white, or cream red or pink red in colour. The spherical
pearls of rainbow colour are rarely found. The best quality of pearl is
obtained from marine oysters. Pearl obtained from freshwater bivalves are not
as valuable as those obtained from the marine oysters (Mishra, 1961).
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