Wednesday, 16 October, 2019

Economic Zoology

Economic Zoology

Introduction.

While dealing with animals welcome across certain animals which are very beneficial for human kind. The study of such animals constitute the term economic zoology. These animals are economical in various ways either their products or they them selves are used in many ways.

Sericulture.

Sericulture is the breeding and management of silk worms for the production of silk. The silk which is produced by silk worm is of a valuable natural protein fibre. Silk worms are the larvae of silk moths. The rearing of silk worm for the production of silk is known as sericulture.

(i) History of silk : Historical account of use of silk and rearing of silk worm eggs, larvae and cocoons is available from China. It was Lotzu, the empress kwang-Ti, who for the first time discovered the silk thread and its source, the silk worm cocoon. In about 550 B.C., the sericulture technique was diffused to European countries. By about 1000 A.D., the sericulture was in practice in China, Europe and India, China was the leading country in this field.

At present, China is topmost country producing some 48% cocoons and 40.9% of raw silk. Next biggest silk producing country is Japan, India is placed at third position as far as the production of silk in terms of quantity is concerned.

(ii) Silk in India : A number of looms were in operation in Banaras, and different parts of Uttar Pradesh, Kashmir became centres for the production of cocoons and rearing of silk worm. Sporadic silk textile centres were also present in South India. It was in 1905-1906 that a scientific investigation in the field of sericulture was undertaken in India by the Indian Institute of Agricultural Research at Pusa, New Delhi. It was Lefroy who conducted research on the silk worm and potentialities of silk production in India.

 

 

Central Sericulture Station, Berhampore, Central Research and Training Centre, Mysore and Ranchi have been established. Various states have undertaken a programme of research, training and plantation of host plants under their rural development programmes. As a result of these efforts, new varieties of mulberry plants have been developed and are being cultivated. These varieties are called as M 2 and M5 varieties. They gave 100% increased yield of mulberry leaves upon which the silk worm feeds.

Different varieties of silk worm, Bombyx mori and Autherea have been developed which can be cultivated in various states. Low production and higher yield have been achieved as a result of these efforts. India is producing 4200 metric tonnes of silk per annum (1980). India is exporting some 25% to 30% of its total silk production in the form of silk garments and fabrics. Karnataka is the biggest silk producing state followed by Jammu and Kashmir and Tamil Nadu, Madhya Pradesh is also emerging on the scene of silk production. India is producing China silk, Tasar silk or Cosa silk, Muga silk and Eri silk today. Largest silk producing state of India is Karnataka. The zoological name of common silk worm is Bombyx mori which produces mulbery silk. Silk is obtained from Bombyx mori.

(iii) Systematic position :

Phylum – Arthropoda

Class – Insecta

Order – Lepidoptera

Family – Bombicidae and satarnidae

(a) Bombyx mori : It is known as China silk worm or mulberry silk worm. It is native of China. It has been fully domesticated for the production of silk. It produces quality of silk which is white silk or yellow silk. Other species of Bombyx are B. texior, B. fortunatax and B. meridionles. They are well known in our country.

(1) Habit and habitat : The silk worm distributed in temperate regions are diapause type i.e. they remain inactive for some time in winter.

 

 

(2) Adult moth : The moth measures about 25 mm in length and wing span measures about 40-50 mm in width. Female moths are larger than male moths. In general, univoltine races are of larger size than multivoltine. The body is divisible into head, thorax and abdomen. Head contains a pair of eyes and a pair of pectinated antennae specially larger in males. Thorax contains three pairs of legs and two pairs of wings covered with scales. Female moths are without mouth. The abdomen is plump. Digestive system is  poorly developed.

(b) Life History

(1) Copulation :The copulation lasts for about three hours. The female has a scent gland at the terminal end of the abdomen, which secretes volatile secretion called pheromones to attract the male.

(2) Egg : Copulation is immediately followed by egg laying which are oval and creamy white in colour. Each moth lays about 500 to 2000 eggs. The eggs are glued to the under-surface of the leaves of the host plant. In univoltine eggs, hatching takes palce after one year. In multivoltine, it takes place after 10-12 days.

(3) Larva : After hatching, a larva comes out of egg. It is called as caterpillar larva. It is 1.2 mm to 3 mm in length depending upon the race. It has grey or creamy-white colour. The body of larva is divided into head, thorax and abdomen. The head consists of three fused segments. Mouth parts are biting and chewing type or strongly mandibulate. The thorax consists of three segments. The abdomen consists of ten segments. The last and tenth segment is poorly developed. Five pairs of pseudo legs are present on 3rd, 4th, 5th, 6th and 9th abdominal segments. These are used for locomotion. Silk is the secretion of salivary gland.

 

 

(4) Silk gland : Among other visceral, organs larva contains well-developed paired glands called silk glands. Each gland is divisible into an anterior, a middle and a posterior region. The anterior parts of both the silk glands are united to form a common duct which opens through a spinneret situated on hypopharynx. The posterior coiled part of gland secretes a protein called as fibroin. It is covered and surrounded by sericin secreted by middle part. The silk is secreted in liquid form, which solidifies on coming in contact with air.

The larva is voracious eater. As the larva grows, it sheds its cuticle. This is called as moulting. The form of larva between two successive moults is called as instar. The larva has five instars. A fully-grown larva or Vth instar attains the length of 7.5 cm. It stops feeding and starts spinning the cocoon. It secretes silk thread from its spinneret and forms covering in which it encloses itself completely. It takes about 3-4 days to spin the cocoon.

(5) Pupa : The cocoon consists of silk thread. The enclosed immobile larva in the cocoon is called as Pupa. It remains inactive, but the internal organs undergo drastic changes collectively called as metamorphosis and transforms itself into imago.

(6) Cocoon : The cocoon is white or yellow in colour. It is made up of about 1000-1200 meters long silk thread. The pupal period lasts for about 10 to 12 days. A young moth comes out of cocoon.

(7) Fertilization : After the moths emerge out from cocoons one female from one lot is kept with the male from another lot. They form pair and copulate.

(8) Egg laying : After fertilization the female starts laying eggs. Egg laying is completed in about 24 hours. The laid eggs are called seeds. The eggs are transferred in sterilized tray stored at 4°C.

(c) Composition of silk : The silk is a secretory product of silk glands of the larva. Silk is composed of proteins. It consists of an inner part made up of fibroin protein (C30 H46 N10 O12) and is covered with an outer envelope made up of sericin protein (C30 H40 N10 O12). The silk thread contains 75-80% fibroin and 20-25% of sericin.

(d) Efforts made by Government of M.P. to Promote Sericulture in state : A directorate of silk has been organised under the Panchayat and Rural Development Department to make concentrated efforts. These activities have been divided in two categories :

 

 

(1) Kosa silk Area : It extends in the eastern and south eastern parts of the state. This area is predominated by tribal   population and is spread in the districts of Balaghat and Mandla.

(2) Mulberry silk Area : It is spread in the western and middle parts of the state including the districts of Indore, Dhar, Dewas, Khandwa, Ujjain, Shajapur, Rajgarh, Mandsaur, Guna and Sehore. For the promotion of the production of Kosa silk (now Mulberry silk) following efforts are being made.

(3) Kosa Seed Centre : Twelve Kosa seed centres have been established to provide scientific and technical information to the Kosa silk worm rearers. These centres also provide disinfected improved kosa seeds and caterpillars to the rearers.

(e) Kosa Guidance and Training Centre : Madhya Pradesh Government has established 67 centres which meet the basic needs of supplying disinfected improved seeds of Kosa silk and impart training and guidance to the rearers.

(1) Nursery : To meet the needs of the host plant and supply of leaves to the rearers, the government has established nurseries of Terminalia tomentosa and Terminalia arjuna.

(2) The construction of two grainage, one cold storage, one cocoon market and one reeling factory is being undertaken.

(3) Kosa Regional Research centre has been established to help the rearers to increase the yield and improve the quality of silk.

(f) Mulberry silk Plans : To promote the mulberry silk production in M.P., certain efforts have been made in the direction by the Madhya Pradesh Government. These are

(1) Establishment of Nursery : To increase the production of host plant, Mulberry silk worm, the Morus indica nurseries have been established.

(2) Mulberry silk seed centres have been established.

(3) Integrated rural development projects have prepared for the production of Mulberry silk.

(4) Establishment of regional research centre and reeling factory.

(5) Demonstration and publicity plans.

 

 

Apiculture.

Apiculture is the science of rearing honeybees for obtaining honey, wax and venom. Three species of honey bees are commonly found in India vig. Apis indica (The small Indian bee), Apis florea (The little Indian bee) and Apis dorsata (the giant bee) Other important species include Apis milifera (the common European bee) and Apis adamsoni (the African bee). In India, the commonly domesticated species are Apis milifera and Apis Indica.

(i) Systematic position

Phylum – Arthropoda

Class – Insecta

Order – Hymenoptera

Genus – Apis

(a) Honeybee-Apis : Honeybee feed upon nectar and pollen of flowers, possess “sucking and chewing” mouthparts, and undergo complete metamorphosis. Each colony has its own nest called honeycomb or beehive. It comprises thousands of small, symmetrical and hexagonal chambers, called “cells”, made up of beeswax. The ”cells” are used for storing honey and pollenbreads, as well as, for rearing the brood. The polymorphic individuals are of three main types (i) a single queen (fertile female)(ii) one to a few hundred drones (fertile males) and (iii) thousands (upto 60,000) of worker bees (sterile females).

(1) The queen : She normally lives for about five years, and does nothing except laying eggs. That is why, she possesses immensely developed ovaries, a large abdomen, and a body which is nearly five times larger and about three times heavier than that of a worker bee. In other features, she is degenerate, having small wings and poorly developed legs, mouthparts, sting, brain, etc. She has no salivary or wax glands. Hence she can neither produce honey or wax nor can fly out of the hive. Although she can use her sting, but it is mainly used as an ovipositor for laying eggs. She lays about fifteen lacs of eggs during her lifetime. Normally one to three thousand eggs are laid per day. Egg-laying is a seasonal activity occurring during winters and spring in our country.

(2) The drones : These are smaller, but stouter than the queen. These also lack salivary and wax glands, and depend for food upon worker bees. Their sole function is to fertilize the queen. Hence, during breeding season, these are well-fed by the workers, and can be often seen flying near the hive, enjoying or chasing and mating with young queens in flight. After breeding season, in the following summer, the drones are neglected and eventually driven out of the hive to die of hunger and heat.

(3) The workers : These are considerably darker and smaller, and most robust with strongest mouthparts and well- developed wings. Their body is densely covered with hairlike bristles. These possess four pairs of pocket-like wax- secreting glands upon ventral surface of second to fifth abdominal segments. The wax is chewed by means of mandibles and, then used in constructing new ”cells” in the colony. The legs of worker bees are modified to collect pollen. When these bees visit flowers for sucking nectar, numerous pollen grains stick to their bristles and mouthparts. The legs are equipped with “pollen brushes” of stiff bristles which brush off the pollen from various parts of body and collect these in two ”pollen baskets”. The latter are pit like concavities upon dorsal surface of the wide tibia of hindlegs (= metathoracic legs). Due to their heavy-duty life, the worker bees live only for two to four months. Each worker bee spends its life in tireless tail. Accordingly, the worker bees of a hive fall under three major age-groups or castes as follows.

(4) Scavenger or Sanitary bees : For the first three days, each worker bee acts as a scavenger, cleaning the wall and floors of abandoned, empty ”cells” of the colony for reuse.

(5) House or Nurse bees : From the fourth day onwards, each worker bee feeds the earlier brood, like a foster mother, with a mixture of honey and pollen. From the seventh day, the maxillary glands of a worker bee begin to function. These secrete ”royal jelly” with which the bee now starts feeding young larvae, the queen and those older larvae which are destined to develop into future queens. From the twelfth to the eighteenth day, each worker bee develops wax glands and works upon the architecture of the hive. Wax is secreted in the form of  thin scales. These bees also repair old cells, filling, cementing and varnishing cracks and crevices of these cells by means of a bee-glue called propolis. Propolis is prepared from resins collected by the bees.

(b) Life History : Queen lays about 2,000 eggs a day. The eggs are laid in the comb, one in each cell. They hatch out into larvae in three days. They are fed on royal jelly for a few days. But the larva which develops into the queen will be fed on royal jelly continuously. During breeding the queen bee flies in the air along with the males. This phenomenon is called nuptial flight. During nuptial flight the queen copulates with a male Copulation occurs in the air. Then the bees return to the comb and the queen starts laying eggs.

(1) Bee-Hive : The artificial box where the bee colony is maintained and managed is called hive. The place where hives are kept and managed is called apiary. There are different models of hive; but the most common model in use is Newton’s hive designed by Rev. Fr. Newton. A set of bees with a queen is introduced into a hive. Honey is collected in the combs of super and the eggs, larvae and the young ones are kept in the combs of brood chamber. When all the cells are filled with honey, the cells are capped or closed by a thin layer of wax.

(2) Honey extraction : Honey is stored in combs of super frames. It is extracted from the comb by a simple machine called honey extractor.

(3) Honey : Honey is a viscous sugary fluid formed from the nectar within the stomach of the honey bee.

(4) Formation of honey : The bees visit flower, suck the nectar, store it in the stomach and return to the hive. In the stomach the nectar is processed. It is regurgitated and swallowed repeatedly for about 240 times. Then the processed nectar is deposited in the comb cells. This processed nectar is called unripe honey or green honey. It contains about 80% water. The unripe honey is converted into ripe honey by evaporation. The ripe honey contains less than 20% water. When the honey becomes ripe, the cells are capped or closed. The honey in the unsealed cell is unripe.

(c) Chemical composition : Honey contains nearly 80 different substances of importance to human beings. The important chemicals are as follows. Water 15-20%, fructose 40-45%, Glucose 32-37%, Sucrose-12%, Dextrin, Vitamin, Protein and minerals 1-2% specific gravity 1.45 to 1.48. The vitamins present in honey are A, B1, B2, B3 , B6, C, E and K. A variety of enzymes are present in honey. They include diastase, invertase, saccharase, catalase peroxidases and lipases. It contains many organic acids. The most important organic acid is formic acid; other organic acids are malic acid, citric acid, tartaric acid and oxalic acid. It contains a variety of minerals like Ca, Na, K, Mg, Fe,Cl,P,S etc.

(d) Bee wax : Bee wax is secreted by the abdominal gland of bees. It is used for the construction of comb. It is an yellowish solid insoluble in water. It is used for the preparation of paints, varnishes, candles, models, etc. It is used as a ground substance for the preparation of ointments, creams etc. It has many industrial uses. It is used extensively in engineering industries, railways, textiles, leather industries etc.

(e) Bee venom : Bee venom is secreted by the poison-glands of stings. Bee venom is a curative toxin in humans. It is transparent and it has a bitter burning taste. It is acidic in nature. It contains formic acid, histamine, tryptophan, sulphur, many proteins, volatile oils, enzymes like hyaluronidase and phospholipase and magnesium phosphate.

(f) Waggle Dance of Honeybees : The communication of honeybees is remarkable because the so-called language of the bees uses a variety of stimuli to impart information about the environment. Karl von Fritsch, famous ethologist, carried out many detailed bee experiments in the 1940s and was able to determine that when a foraging bee returns to the hive, it performs a waggle dance. He got Noble prize in 1975 to discover Waggle dance.

Lac culture.

Lac is the resinous secretion produced by lac insect as protective covering around its body. It belongs to genera Laccifera or Trachardia Laccifera lacca is the common Indian lac insect. It lives on the trees of fig family namely kikar, ber (Zizyphus mauritiana), babul (Acacia nilotica), dhak or palas (Butea monisperma), kusum (schleichera oleosa), Katha or khair (Acacia catechu), peepal (Ficus religiosa) and gular (Ficus glomerata). Lac insect feeds upon the sap of its host plant like any other sap sucking insect. It is found in India and Philipine islands.

Male and female chambers : The adult male and female insects live on the tree twigs enclosed in thick capsules or chambers separately. The male chambers are elongated and cigar- shaped. Each male chamber has a branchial aperture in its anterior part. There is an opening in the posterior part of the chamber which is covered by an operculum.  The male insect can crawl out through this opening.

The female chamber : It is smaller and rounded. It has a branchial aperture in its anterior part and a tubercular or anal opening in the posterior part. A ridge extends in the mid-dorsal line of female chamber, which indicates the posterior end of the last larval skin.

(i) Male and female lac insects : The lac insects have a sluggish and almost sedentary life, living inside the chambers. Therefore , these have become degenerated, without wings and distinct legs. However, the female is more degenerated. It has a bag -like body with a small reduced antenna. The eyes legs and wings are lost during metamorphosis. The male lac insect is red in colour. It has an incipient head with antennae and eyes. The thorax has three pairs of legs and abdomen carries genital sheath, penis and a pair of long caudal setae, one on either side of genital sheath. The wings may be present or absent. Because of the absence of mouthparts, the insect is incapable of feeding.

(ii) Life-cycle : The male lac insect crawls out of its chamber by pushing open the operculum reaches the female chamber and fertilizes the female through the anal or tubercular opening of female shell. The male dies soon after copulation. The female secretes more resin forming a large sized chamber. Thus the secretion by females mainly contributes to lac.

(iii) Oviposition takes place into a space inside the female chamber made by the contraction of the body of female. This space is called incubating chamber. Each female lays 200-300 eggs. The eggs hatch into red coloured larvae. These crawl out of the female’s incubating chamber. The mass emergence of larvae is called swarming. Each larva is boat-shaped in appearance and is about 1/2 mm in length. Its head bears paired antennae and the ocelli. The mouthparts are of piercing and sucking type with maxillae and mandibles together forming the sucking tube or proboscis. Its thorax is three segmented and each thoracic segment carries a pair of walking legs. The abdomen bears a pair of long caudal setae.

(iv) Attachment of larvae to new shoots : The larvae, on emergence, crawl on the twigs of any one of the host trees mentioned earlier and settle down on the undersurface of new shoots. These prefer young succulent shoots. These force their proboscis through the bark and insert it into the phloem tissue and start feeding. Here, these metamorphose into the adult insects and by secreting lac, enclose themselves into the chambers.

(v) Secretion of Lac : The secretion forms a shining layer over their bodies in the beginning but hardens and becomes opaque later on. The secretion is produced by the cutaneous glands of the skin and is deposited around three openings, the two branchial apertures at the anterior end and anal opening at the posterior end. The secretion is in the form of waxy filaments which have a woolly white appearance. On coming in contact with air, these join to from a continuous covering. Further, lac secretion continues inside this coating so that lac deposition adds to the thickness of the coating. With growth of larva and addition of lac the adjacent chambers of different larvae encloses with one another forming a more or less continuous encrustation.

(vi) Lac Cultivation : In order to obtain lac, lac insects are cultured and the technique of lac production is known as the lac culture. It involves proper care and regular pruning of the host plants, propagation of insects, and collection and processing of lac. For the purpose of propagation, the older branches containing crusts are tied with new branches and this method is called inoculation. When new crusts are formed, the old twigs are removed (approximately 20-30 cm long) and this is known as harvesting.

After inoculation, lac insects come out of the old crusts. At this stage they are known as nymphs. The nymphs hatch out from eggs laid by the females in the old crusts. The coming out of nymphs from the old crusts is known as swarming, some of the nymphs become winged or wingless male and others become female. These nymphs explore new branches. The thousands of nymphs settle side by side, and the resinous secretion builds up around them and completely encases them. The nymphs undergo several moults. Most of them develop into females and some into males. The females remain in small cavities in the resinous mass from which they never come out.

(vii) Extraction of Lac : The largest yield of lac and dye are obtained by harvesting the infested twigs while females are still living. The harvesting is done twice a year in June and November. The encrusted twigs are pruned and lac scrapped from them. This is known as stick lac. It is grounded and sieved. The resulting granular lac is called seed lac, and the fine particles the dust lac. The seed lac is washed, melted, spread out in a thin layer and dried thus forming the shellac of commerce. The dust lac is used for making toys, shellac is used in the preparation of varnishes, paints and polishes; in making gramophone records and pots; and in filling ornaments like bangles and bracelets. It is used as insulating material.

(viii) Composition of lac : Resin-68%, Pigmented material-10%, Wax-6%, Glue-5.5%, Sugar 4%. Lac insects are highly useful to man. They yield lac, the utility of which is discussed above. Besides this, a red dye is obtained from the body of female. The dye is used by women to colour the soles of the their feet, skin. Lac insects are also used for curing lung and stomach troubles.

(ix) Economic importance of Lac : Lac is used in the preparation of sealing wax (shellac), paints, varnish, the manufacture of photographic materials, electrical goods. Lac is also used in the preparation of bracelets, buttons, toys and in filling hollow gold ornaments. Lac is also utilized in confectionary trade and in artificial leather and pottery. Gramophone is industry used to consume 30-40% of the annual production in the preparation of records.

(x) Cultivation of Lac in India : India has monopoly in the production of lac. It is about 75% of the world’s total output. Approximately 40 lakh ponds of lac is produced. Bihar, M.P. and West Bengal are major lac producing states in India. Thailand is major competitor of India as it shares 25% of the total exports. India exports about 1,80,400 kg, of lac. The use of lac is being gradually replaced by plastic.

Poultry.

The term poultry is used for a wide variety of birds which can be reared under domestication (e.g. chicken, turkeys, ducks, pheasants, pigeon, etc.) India has the sixth place in poultry farming in the world. Domestic fowl (Gallus domesticus) constitutes the major poultry bird. Poultry exclusively grown for meat are called Broilers (e.g. Plymouth Rock). Many varieties of high egg-yielding (layers) and meat-yielding (broilers) birds have been produced by artificial selection. There are two types of fowl – (1) Indigenous (desi type)   (2) Exotic (improved type)

Some indigenous breeds of fowl

Aseel Karaknath Ghagus
Brahma Busra Chittagong

 

Some exotic breeds of fowl

White Leghorn Plymouth Rock New Hampshire
Rhode Island White Cornish

 

The Aseel fowls are used in cock-fighting. The indigenous breed are crossed with exotic breeds for improving egg production. Ducks (Anas) comprise 6 percent of total poultry population in India. Ducks are abundant in the southern and extern parts of India. Turkey (Meleagris) is the most recent domestication. Turkeys belong to the breeds Narfold, British White, Broad Breasted Bronze and Beltsville small White.

(i) Poultry Diseases

(a) Encephalomalacia : Deficiency of vitamin E causes softening of brain tissue in young poultry.

(b) Coccidiosis : The protozoan Eimeria causes coccidosis in fowls. It causes bloody diarrhoea.

(c) Bacterial diseases : Fowl cholera, salmonellosis, coryza.

Viral diseases

Ranikhet disease Fowlpox
Infectious bronchitis Lymphoid leukosis

 

Fungal diseases

Thrush Brooder pneumonia
Aflatoxicosis Aspergillosus

Fisheries.

Fishes are a valuable and easily accessible source of food, rich in protein, highly nutritious and easily digestible. They are abundantly available from sea, rivers, lakes, ponds and marshes. Aquaculture is the production of useful aquatic plants and animals such as fishes, prawns, shrimps, lobsters, crabs, molluscs by the proper utilization of small and large bodies of water. Pisciculture is the production and breeding of fishes by man in ponds. India has abundant marine and inland fish resources. It has a coast line extending to 4667 Km long and a continental shelf of 2,59,00 square Km offering good scope for fish production. The fish production has increased many folds since India got independence. During 2000-01, the annual fish production of our country has been 46 lakh tonnes. The per capita consumption of fish in India is estimated at 1.51 Kg/year. India is, at present, the 6th foremost sea food producing nations in the world.

 

Zoological name Common Name Areas of availability
(a) Fresh water fishes
1. Catla catla Catla All over India, common in Krishna and Godavari rivers
2. Labeo rohita Rohu North, East and South India
3. Labeo calbasu Calbasu North and South India
4. Cirhinus mrigala Mrigal North and South India
5. Mystus singhala Singhala All over India
6. Heteropneustes fossilaris Singhi All over India
7. Wallago attu Malli North, east and South India
8. Clarius batrachus Fresh water shark magur All over India
(b) Brackish water fishes
9. Chanos chanos Milk fish A.P.coast
10. Mugil cephalus Grey mullet East coast
11. Laters calcorifer Perch East coast
(c) Marine fishes
12. Sardinella longiceps Oil sardine West and south coasts
13. Harpodon heherius Bombay duck Maharastra coast
14. Hilsa ilisha Hilsa/ Indian shed Coastal India
15. Stromateus sinensis Pomphret Indo pacific coast
16. Anguilla anguilla Eel Coastal India
17. Aluitheronema Salmon East and west coast
18. Cyano-glossus semifas- ciatus Flat fish East coast of India

 

 

Pearl Industry.

Pearl is a concretion formed by molluscs. It consists of nacre or mother of pearl. It is characterised by iridescence and translucence. Pearls are produced by the marine molluscs such as pearl oyster and mussel

(i) Types of pearls : Pearls are of seven types. They are the following

(a) Lingha pearl : This is the best quality pearl obtained from marine oysters.

(b) Seed pearls : The small pearls are called seed pearls.

(c) Baroque pearls : These are spherical pearls formed inside the body.

(d) Blister pearls : These are pearls attached to the shell. They are half-spherical in shape.

(e) Oriental pearls : These are true pearls with a great lustre, beauty and a smooth surface.

(f) Natural pearls : These are the pearls obtained from pearl oysters of deep oceans.

(g) Cultured pearls : These are the pearls obtained from cultivated species of pearl oysters.

(ii) Composition of pearl : The pearl is formed of nacre. The nacre is formed of two substances namely a calcium carbonate which is in the form of argonite or calcite and an albuminoid substance called conchiolin.

(iii) Pearl-producing animals : Pearls are produced by bivalve molluscs. There are marine as well as fresh water animals are Pinctada vulgaris , Pinctada fucata, Pinctada chemnitzi, Pinctada margaritifera, Pinctada anomioides, Pinctada atropurputea, Haliotis, Mytilus, Placuna blacenta, Placuna maxima, and Unio margaritifera.

(iv) Cultivable species : Pearls are intensively produced by cultivating pearl oysters. The most important molluscs cultivated for pearls are Pinctada vulgaris.

(v) Biology of pearl oysters : Pearl oysters are sedentary animals. They are attached to rocks. They have two values. One valve is cemented to the rocks and the other free. They spawn twice in a year. The eggs are hatched into free swimming larvae. The larvae sink to the bottom of the water and develop into young oysters called spats. They grow to their maximum size in four or five years.

(vi) Pearl formation : The pearl oysters produce pearl as an adaptation against outside materials. When a foreign material such as a sand grain or a parasite happens to enter the body it adheres with the mantle. The mantle epithelium at once grows over the material in the form of a sac and encloses it. This mantle epithelium starts secreting concentric layers of nacre around the foreign material. The completed structure is called pearl.

(vii) Culture of pearls : The culture of pearls is a complex but sensitive process. It involves the following steps.

(1) Collection of oysters.

(2) Preparation of graft tissue

(3) Preparation of nucleus.

(4) Implantation.

(5) Rearing of oysters

(6) Harvesting

 

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