Pearl Culture in India : Types of Pearl Oysters and Formation of Pearl!
Pearl is known to human beings since ancient times. A pearl due to its delicate appearance and shine has retained its position as a costly ornamental object. Due to its properties, it occupied important place in the crown of great emperors and queens.
It was considered to be the indication of richness of the kingdom. The origin of pearl is not known to us however Chinese records show that pearl was known to them as far back as 2300 B.C.
Pearl is of animal origin and is obtained from Pearl oyster belonging to phylum mollusca. The old ancient Hindu believe, which still persists in common Indians is that the rain drops of “swati nakshatra” when fall inside the open mouth of pearl oyster become converted into pearl. But the actual nature and mode of formation of pearl inside the oyster has now been explained scientifically.
Pearl culture which is not a very old venture is restricted to certain parts of the world only. It includes the culture of oysters in restricted areas and after attaining a particular size and treated specially with the help of scientific equipments and appliances to produce pearl.
Types of Pearl Oysters and Their Occurrence:
The pearl oyster belongs to genus Pinctada, family pteriidae, and class bivalvia and phylum mollusca. The important species producing good quality of pearl are Pinctada vulgaris (schumachar), Pinctada marigavitifera (Linn), Pinctada chemnitzi (Philippi), Pinctada anamioides (Reeve) and Pinctada alropurpurea (Dunker). All these species are Marine and it is the marine forms which produce the pearl of fine quality.
Certain fresh-water forms of mussels belonging to the genera unio and Anodonta also produce pearls but they are of inferior quality and hardly of any use. However, the European species belonging to the family unionidae have been found to produce superior quality pearls.
The main sites of production of pearl are the Persian Gulf, Gulf of Manaar (Ceylon), Sulusea (near Philippines). Besides these, coast of Australia and shores of Central America (Pennama Bay, Gulf of California) including islands of south Pacific are also important productive places. Nowadays Japan has surpassed all the countries in the bulk of pearl production by invention of pearl culture techniques.
Pearl oyster beds in India are present on both eastern and western coasts. However, the east coast is more productive and extensive than the west coast as it extends from Cape Comorin to Kilakarai. Tuticorin is the most productive region of the east coast lies in the Gulf of Kutch. Hence the major sites of pearl fisheries in India are Gulf of Manaar both on Indian as well as Ceylonese sides, Gulf of Kutch, Palk Bay, Baroda and Tuticorin.
Formation and Nature of Pearl:
Pearl is secreted by the mantle of the pearl oyster as a protection against foreign objects which may be sand particles, minute larval forms or other such things. To study the mode of formation of pearl it is essential to know the structure of the shell and mantle. Shell of pearl oyster is composed of three distinct layers. They are as follows.
It is the outermost, greenish-brown, thin, translucent layer made up of an organic substance “Conchiolin”. It is secreted by mantle. It serves to protect the underlying layers from harmful effects of weak carbonic acid in water.
It is the middle layer secreted by mantle. This layer is made up of minute crystals of calcium carbonate, separated by thin layers of conchiolin. These crystals are arranged perpendicular to the shell surface. It gives strength and rigidity to the shell.
It is the inner most layer of the shell and is better known as mother of pearl”, because this layer is responsible for the formation of pearl. It consists of alternate layers of calcium carbonate and conchiolin, which lie parallel to the surface of the shell. This layer is also secreted by mantle and its function is to protect the delicate surface from harmful effects of the foreign particles.
Mantle is also made up of three layers, which are as follows:
(1) Columnar epithelium:
They contain numerous nacre secreting unicelluar glands.
(2) Connective tissue layer:
The middle, fibrous layer is mainly composed of connective tissues.
(3) Ciliated epithelium:
Inner most layer containing mucous secreting cells.
A pearl bears a central nucleus around which several layers of organic and inorganic matters are laid. A beautiful and a fine quality pearl is that in which the layers are accurately and symmetrically laid. It is also free from any sort of adhesion. Such a pearl is, however, rare and if it occurs, it fetches a good market. The chemical analysis of a pearl shows that it contains about 90% calcium carbonate, 5% organic substances and 5% water and other residues.
There are different views regarding the formation of pearl inside the body of pearl oyster. The common and most accepted view is that pearls are formed due to irritation caused to the mantle by the entry of foreign bodies. Another view is that internal parasites act as provoking agents in the process of formation of pearls. A third view says that it is the internal pathological conditions of the host which results in the formation of pearl. Only the first view is universally accepted, nowadays.
Whenever any sort of foreign body enters and gets between the mantle and shell, it becomes enclosed in a sac of mantle epithelium. This foreign body now acts as an irritant and stimulates the mantle epithelium to secrete concentric layers of nacre around this foreign body. These layers when harden becomes pearl. (Fig. 40) Size of pearl is directly proportional to the degree of irritation cause by foreign agents. Time taken for the formation of a pearl of average size is three to five years.