Pearl: Meaning, Chemical Composition and Its Fishery

In this article we will discuss about Pearl:- 1. Meaning of Pearl 2. Chemical Composition of Pearls 3. Shape, Size and Colour 4. Formation 5. Species Used in Pearl Culture 6. Distribution of Pearl Oysters in Indian Waters 7. Pearl Fishery in Japan and Other Coun­tries.

Meaning of Pearl:  

Pearl is a spherical or irregular mass formed by the calcareous secretion of the nacre (mother of pearl) found in the glandular mantle of pearl oysters, deposited in concentric layers around a nucleus which is composed of foreign object causing irritation.

Chemical Composition of Pearls:

(i) Calcium carbonate (CaCO₃) or Aragonite. A special type of calcium crystal 88-90%.

(ii) Organic conchiolin (C₃₂H₉₈N₂O₁₁). A special type of scleroprotein 3.5- 5.9%.

(iii) Water 2-4%.

(iv) Residue 0.1-0.8%.

Shape, Size and Colour of Pearls:

The shape, size and colour of the pearls are variable in different kinds of pearls. In most cases, the pearls are irregular in shape, and perfect round-shaped pearls are rare. The formation of pearls may be considered an accident of nature. Globose pearls are occasionally produced when the invasion of a very small marine organism, such as an egg of trematode, cestode takes place in the mantle of pearl oyster.

The pearls may be dumb-bell-shaped, disc-shaped or elongated. The large, badly misshapen free pearls are called baroque that are mostly pear-shaped or rather flat. The size of pearls is also variable.

It depends upon various factors, such as age, temperature of sea water and nutritional condition, etc., Colouration of the pearls such as cream, pink, bluish brown and black are due to the nature of the organic layers (conchiolin) and the introduction of certain pigments into the argonite. The shape of the pearls depends upon the pattern of nacre deposition.

Formation of Pearl:

Pearl is secreted by the mantle as a pro­tection against foreign objects, usually the parasites or coarse sand grains. The para­sites are recorded to be the larval forms of trematodes (in most cases). Sometimes a course sand grain may act as a stimulant for the production of a pearl. The parasite gets an accidental entry into a pearl oyster body and occupies a position between the mantle and the shell.

As a response to the irritation caused by the foreign object, the nacre glands in the mantle (Fig. 16.76) begin to secrete calcareous substances which are deposited around the foreign body in thin concentric layers.

As a result a pearl is produced (Fig. 16.77). The iridescence of the pearls is pro­duced by the refraction of the light rays from various nacre layers of the pearl. Duration of the pearl growth may vary according to species and usually takes a couple of years. The giant Tridacna clam which can produce a pearl of the size of a golf-ball, may take as long as 10 years.

The pink conch Slrombus gigas can pro­duce a pink pearl, and Pinna and Atrina vexillum of Indo-Pacific can produce occa­sionally black pearl.

Species Used in Pearl Culture:

Selection of the right species (oysters) is the primary step of the pearl culture. The production of good quality pearls is consid­ered to a limited number of species. Some of the commonly used marine species are Pinctada martensis (Japanese pearl oyster), Pinctada maxima (silver lipped pearl oyster), Pinctada margaritifera (Black lipped pearl oyster), and Pteria penguin (Black winged pearl oyster).

The Japanese pearl oyster is also known as Akoya oyster and found in the seas of Japan, Malayasia, Indonesia and Australia. The oyster is the most important species producing high quality of pearls. In artificial culture the diameter of the pearl becomes 11 mm and take from four to seven years. The silver lipped and black lipped pearl oysters produce larger pearls but of inferior quality.

The silver-lipped pearl oyster is found in the seas of Myanmar, Thailand, Philippines, Indonesia and Australia. The surface is of the shell usually lamellose and the diameter of the shell is about 30 cm. About 80% to 90% silver lipped pearl oyster is fished from the Arafura Sea of Australia at a depth of 10 to 15 fathoms.

The pearl of P. maxima is called “south sea pearl” and the diameter attains about 10 mm. This oyster is widely used in Australia, Philippines, Thailand and Indone­sia for artificial culture. The black lipped pearl oyster is found in the Indo-pacific oceans. The shell is dirty mushroom becom­ing greenish towards umbo with several white radial lines.

The diameter of the shell is about 15 cm. Due to coarseness of the nacres, sometimes the oyster produces the inferior quality of pearls but in the seas of Polynesia and Micronesia the pearl oyster produces large and good quality of pearls. The black winged pearl oyster is found in the Indo-Pacific Oceans and produces good quality pearls (blister pearls).

Several species of pearl oysters, viz., Pinctada vulgaris (= Pinctada fucata), Pinctada margaritifera, Pinctada anomioides, Pinctada atropurpurea, Pinctada chemmitzi and Pinctada sugillata are found in Indian waters. Of these species, P. vulgaris (= P. fucata) (Fig. 16.78) is the commonest and widely distributed in the Gulf of Mannar, the Palk Bay, and the Gulf of Kachchh.

Distribution of Pearl Oysters in Indian Waters:

Pearl oysters are usually the inhabitants of the ridges of rocks and dead corals which form the pearl banks. These oyster beds are located at a depth of 10-12 fathoms (= 18- 22 m) at a distance of approximately 10 to 20 km from the shore. The pearl oyster beds are most extensive in the east coast than the west coast. In the east coast the beds extend from Kanyakumari to Rameswaram with a most productive zone near Tuticorin.

In the east coast the pearl oysters produce a best quality pearl which is spherical shaped and rainbow coloured, called lingha pearl. In the west coast the pearl oysters are found in coral reefs in the Gulf of Kachchh extending from the north of Halar district to the Jamnagar in Gujrat.

Pinctada margaritifera occurs in the Andaman and Nicobar Islands in good num­bers rather than the east and west Indian coasts. Pinctada chemnitzii is found mainly in the Gulf of Mannar.

Pinctada atropurpurea and Pinctada anomioides are found rarely in the Gulf of Mannar. Placenta (= Placuna) placenta is also found in the Gulf of Mannar, specially around Rameswaram Island, Mumbai, Kakinada (Andhra Pradesh) in which pearls of inferior quality are obtained.

Pearl Fishery in Japan and Other Coun­tries:

Pearl fishery can be divided into two types:

(1) Pearl collection, and

(2) Artificial pearl culture.

1. Pearl collection:

For centuries pearl oysters are being col­lected for pearls in many countries of the world. It has been noticed that the pearl oysters collected from the Persian Gulf and Red Sea contain valuable natural pearls.

The pearl fisheries in Sri Lanka and India flour­ished many centuries before christ. Other example recorded that an estuarine mussel, Mytilus smaragdinus of Tamil Nadu area was extensively fished for pearls before christ.

Spanish explorers recorded pearl fisheries both in the Gulf of Panama and the Gulf of Mexico. The oyster infested coastal areas of India, Sri Lanka, China, Japan, Thailand, Philippines, North Australia and a few Pa­cific Islands also yield pearls of commercial value.

The traditional method of pearl collec­tion is less profitable and uncertainty pre­vails sometimes. For example, record shows that 35,000 oysters collected by the divers from the Red Sea, only from 21 of them pearls were obtained and ironically 3 of them were valuable.

According to the report of the Fisheries Department, Govt., of India, there are 27 pearl fishery centres in the Gulf of Mannar and 17 in the Gulf of Kachchh (reported in 1967 and 1982). In Australia the pearl fishery was to­tally dependent on the pearl collection up to 1955. The pearl oysters were fished in Broome, Torres Strait, Port Darwin and Arafura Sea.

2. History of Artificial pearl culture:

There are evidences that the Chinese first developed the techniques of production of artificial pearls. Ye-Jin-Yang, a Buddhist priest who lived between 1200 and 1300, practised the method to obtain the artificial pearls. He inserted the tiny pellets of hard­ened clay or Buddha miniature made of tin within freshwater mussels and placed it be­tween the inner shell and the mantle.

These live mussels keeping in a bamboo cage were left dangling in pools or canals, and after one year pearls were collected by opening the shell. But the scientific method of the artifi­cial pearl culture who mastered the technol­ogy was Japanese scientists.

First two scien­tists Kokichi Mikimoto (1858-1954) and Tatsuhi Mise (1880-1924) put the industry on a business-like basis and pushed Japan into a forefront as a leadership of pearl production. The modern scientific techniques practised in Japan and other countries in­cluding India, were introduced by Tokichi Nishikawa (1939), a scientist of Tokyo University.

In 1893, Mikimoto first produced hemi­spherical cultured pearls and took the patent for his technique in 1896, and Tatsuhi Mise produced fully spherical cultured pearls and obtained the patent in 1907.