PEARL OYSTER FARMING AND PEARL CULTURE
TABLE OF CONTENTS
Prepared for the Pearl Oyster Farming and
Pearl Culture Training Course conducted by the
Central Marine Fisheries Research Institute at Tuticorin, India
and organized by the Regional Seafarming Development and Demonstration Project (RAS/90/002)
February 1991
Training Manual 8
PREFACE Pearls, one of the highly esteemed gems, are very valuable due to the high demand and prices for them. Several countries bordering the Indian and Pacific Oceans and some countries along the Eastern Atlantic Ocean have pearl oyster resources. Many of these countries, particularly those in Asia, are very much interested in pearl oyster farming and pearl culture. Japan stands foremost in the two fields having developed technologies and innovations in the field.
The techniques of pearl oyster farming and pearl culture are not widely known. There is a need to promote more widely the techniques and relevant information on the bionomics of pearl oysters. In India, interest in pearl culture began at the start of this century. Several studies have been conducted by the Madras Fisheries Department in the 1930s. In 1972, the Central Marine Fisheries Research Institute (CMFRI) took up intensive research on pearl culture at Tuticorin achieving a breakthrough in July 1973 when it produced free spherical cultured pearls by employing the mantle graft implementation technique. Since then intensive research has been carried out by the Institute on pearl formation, pearl oyster biology and ecology, and hatchery techniques for production of pearl oyster seed. Considerable information of applied value has been obtained. The development of the pearl oyster hatchery technology in India in 1981 opened the way for large and commercial scale culture of this bivalve species. Based on the technical know-how provided by the CMFRI, a company has been established at Tuticorin to produce cultured pearls.
In view of the keen-interest shown by countries in the region, the FAO/UNDP Regional Seafarming Development and Demonstration Project (RAS/90/002) requested the Indian Council of Agricultural Research (ICAR), New Delhi to conduct a training programme on "Pearl Oyster Farming and Pearl Culture" at the Central Marine Fisheries Research Institute in Tuticorin, to train personnel from different countries. In line with this training course, this training manual was prepared. This manual deals with various aspects of pearl oysters, pearl oyster farming, pearl production technology, etc. The Manual is designed for technicians as well as entrepreneurs.
The effort by Mr. A. Chellam, Dr. A.C.C. Victor, Mr. S. Dharmaraj, Mr. T.S. Velayudhan and Dr. K. Satyanaryana Rao in preparing and editing the manual is acknowledged. I would like to thank Mr. Chen Foo Yan, Coordinator of the Seafarming Development and Demonstration Project, and his staff, particularly Mr. Pedro Bueno, Mr. Alessandro Lovatelli and Prof. H.P.C. Shetty for further editing and publishing the manual.
Dr. P.S.B.R. James Director Central Marine Fisheries Research Institute, Cochin, India
National Coordinator Regional Seafarming Development and Demonstration Project (RAS/90/002)
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TABLE OF CONTENTS PREFACE
LIST OF FIGURES
LIST OF PLATES
CHAPTER I
Pearl culture in India
1.1 Introduction
CHAPTER II
Taxonomy and distribution
2.1 Taxonomy 2.2 Distribution
CHAPTER III
Morphology and anatomy
3.1 Morphology
3.1.1 Shell features 3.1.2 Shell structure
3.2 Anatomy
3.2.1 Mantle 3.2.2 Foot 3.2.3 Byssal gland 3.2.4 Muscular system 3.2.5 Digestive system 3.2.6 Respiratory system 3.2.7 Circulatory system 3.2.8 Excretory system 3.2.9 Nervous system 3.2.10 Reproductive system
CHAPTER IV
Biology and ecology
4.1 Biology
4.1.1 Food and feeding habits 4.1.2 Age and growth 4.1.3 Reproduction
4.2 Ecology
CHAPTER V
Hatchery techniques for seed production
5.1 Artificially reared spat
5.1.1 Hatchery building 5.1.2 Seawater management 5.1.3 Aeration
5.2 Live food production
5.2.1 Phytoplankton
5.3 Broodstock handling and spawning
5.3.1 Broodstock maintenance 5.3.2 Spawning 5.3.3 Fertilization 5.3.4 Early development and larval rearing
5.4 Larvae and spat handling
5.4.1 Larval rearing conditions 5.4.2 Spat production 5.4.3 Feeding 5.4.4 Transplantation 5.4.5 Survival
CHAPTER VI
Pearl oyster farming
6.1 Selection of culture sites
6.2 Environmental conditions
6.2.1 Temperature 6.2.2 Salinity 6.2.3 Bottom 6.2.4 Depth 6.2.5 Silt load 6.2.6 Water current 6.2.7 Primary productivity
6.3 Supply of oysters
6.4 Rearing methods
6.4.1 Raft culture 6.4.2 On-bottom culture
6.5 Rearing containers
6.5.1 Culture of mother oysters 6.5.2 Juvenile rearing
CHAPTER VII
Biofouling and predation
7.1 Biofouling organisms
7.1.1 Barnacles 7.1.2 Ascidians 7.1.3 Bryozoans 7.1.4 Molluscs 7.1.5 Sponges 7.1.6 Other organisms
7.2 Boring organisms
7.3 Predator organisms
7.4 Control measures
7.4.1 Fouling 7.4.2 Boring 7.4.3 Predation
CHAPTER VIII
Culture system
8.1 Culture operations
CHAPTER IX
The mantle
9.1 Mantle structure
9.1.1 Marginal mantle 9.1.2 Mantle isthmus
9.1.3 Pallial mantle 9.1.4 Central mantle
CHAPTER X
The surgery
10.1 Surgical instruments 10.2 Nucleus 10.3 Selection of oysters 10.4 Graft tissue preparation 10.5 Conditioning for surgery 10.6 Surgery
CHAPTER XI
Pearl formation
11.1 Natural pearl formation 11.2 Cultured pearl formation
CHAPTER XII
Post-operation culture
12.1 Culture conditions
CHAPTER XIII
Production of cultured pearls
13.1 Development of implantation technique
13.2 Nucleus retention and pearl production
13.3 Pearl harvesting
CHAPTER XIV
Improvement of pearl quality
14.1 Measures for enhancing pearl quality
14.1.1 Oyster selection 14.1.2 Narcotization of oyster 14.1.3 Graft tissue preparation 14.1.4 Implantation 14.1.5 Oyster convalescence 14.1.6 Tool maintenance
14.2 Colour of pearls
APPENDIX I
REFERENCES
LIST OF FIGURES Figure No.
1. World distribution of pearl oysters. 2. Section of the shell of Pinctada fucata. C.L.= conchiolin layer; P.L.= prismatic layer;
and N.L.= nacreous layer. 3. Anatomy of Pinctada fucata. 1) Mouth; 2) oesophagus; 3) stomach; 4) left labial palp;
5) left inner labial palp; 6) crystalline style; 7) liver; 8) digestive diverticula; 9) descending intestine; 10) ascending intestine; 11) rectum; 12) anal papilla; 13) byssal gland; 14) nucleus implanted in the gonad.
4. (A) Culture raft constructed with teak poles; (B) A FRP styrofoam buoy; (C) A mild steel buoy, and (D) Oyster long-line culture system.
5. Section of oyster mantle. (1) Central mantle; (2) Pallial mantle; (3) Marginal mantle. I.F.= inner fold; M.F.= middle fold; S.F.= shell fold; P.G.= periostracal groove; and P.S.= periostracal secretion.
6. Steps in graft tissue preparation. (A) mantle tissue when removed from an oyster (p.m.= pallial mantle and m.m.= marginal mantle); (B) trimming of the margins to remove marginal mantle and inner muscular tissue; (C) further trimming to obtain ribbons of pallial mantle; and (D) cutting of the ribbon into small sections.
7. Process of pearl formation. (A) round and half natural pearls; (B) half cultured pearl; and (C) round cultured pearl with an artificially implanted nucleus.
LIST OF PLATES Plate No.
I. (A) Pinctada fucata and (B) Pinctada margaritifera.
Cont'd. (C) Pinctada chemnitzii and (D) Pinctada sugillata.
Cont'd. (E) Pinctada anomioides and (F) Pinctada atropurpurea.
II. (A) Inside view of the C.M.F.R.I. pearl oyster hatchery in Tuticorin, (B) Male oyster while spawning and (C) Pyriform oocytes.
Cont'd. (D) Fertilized oocytes, (E) Early cleavage and (F) Morula stage.
Cont'd. (G) Trochophore larvae and (H) Straight-hinge larvae.
III. (A) Umbo larvae, (B) Eye-spot larvae and (C) Transitional stage.
Cont'd. (D) Pediveliger larvae, (E) Plantigrade larvae and (F) spat.
IV. (A) A scuba diver diving to collect pearl oysters and (B) A culture raft floated with mild steel barrels.
Cont'd. (C) A culture raft with FRP styrofoam buoys and (D) Oyster long-line culture.
V. (A) A box-cage containing pearl oysters and (B) A frame netcage with oysters.
Cont'd. (C) A netcage for rearing oyster spat of 3–10 mm in size and (D) Rearing netcage covered with velon screen.
VI. (A) Fouling organisms on adult oysters and rearing cage and (B) Oysters heavily encrusted with barnacles.
Cont'd. (C) Damage caused by a boring sponge and (D) Cymatium cingulatum, a major pearl oyster predator.
VII. Pearl oyster surgical instruments. VIII. Implantation of a pearl oyster. (A) Opening of the oyster valves and (B) Insertion of
the graft tissue.
Cont'd. Implantation of a pearl oyster. (C) Implantation of the nucleus, and (D) General view o
CHAPTER I PEARL CULTURE IN INDIA 1.1 Introduction Pearls have been known to mankind since the beginning of civilization. They are highly esteemed as gems for their beauty and splendour. These structures are secreted by the mantle (i.e., the skin) of pearl oysters in response to irritations caused by external or internal stimuli such as sand grains, molluscs eggs, parasites, detritus, and other foreign particles. Many attempts have been made to culture pearls in freshwater mussels. In the 13th century the Chinese fixed small Buddha figures inside freshwater mussels which became covered with a pearly layer. After considerable perseverance and study on the mode of pearl formation, success was achieved early in this century in Japan on the production of spherical cultured pearls. The Japanese grafted a piece of mantle with a small bead in a pearl oyster and reared the oyster in protected coastal waters with favourable environmental conditions.
India has one of the highest demand for pearls for setting in jewellry, and is particularly famous for its pearl oyster resources which yield superb pearls. The pearl oyster fisheries are located in two main areas: 1) in the Gulf of Mannar off Tuticorin coast and 2) in the Gulf of Kutch on the northwest coast of the country. The pearl oysters are found in two different environments in the two localities, at depths up to 23 meters in the Gulf of Mannar, in the intertidal zone in the Gulf of Kutch. These bivalves form large beds on hard substrata in the Gulf of Mannar, while they are sparsely distributed in the Gulf of Kutch. The pearl oyster resources in the two areas have been fished for pearls until the early 1960's.
After surveying the pearl oyster resources and fisheries in the two Gulfs at the beginning of the century, Hornell (1916) recommended that in order to maintain pearl fisheries profitably it was necessary to develop techniques to induce the Indian pearl oysters to form pearls by artificial means. In response, the then Madras Government Fisheries Department carried out preliminary research at the Marine Biological Station in Krusdai Island, Gulf of Mannar. Research focused mainly on the biology and ecology of several species. The oysters were reared in cages and induced to form pearls. That work managed to produce only two poorly shaped pearls and a half-pearl attached to the shell. Efforts in Gujarat did not meet success either.
In October 1972 the Central Marine Fisheries Research Institute started a pearl culture research project at Tuticorin. Success came in July 1973 when a perfectly spherical pearl was produced. This breakthrough was achieved by introducing a graft of the oyster mantle in the gonad of an adult specimen together with a shell bead nucleus. This is a delicate operation.
Following this success an Ad-hoc Research Scheme on pearl culture under the Indian Council of Agricultural Research (ICAR) was implemented (from 1973–78) by the CMFRI in association with the Department of Fisheries, Government of Tamil Nadu. During this Research Scheme, production of cultured pearls by multiple implantation was successfully achieved. Several aspects of pearl formation and pearl oyster biology and ecology -- highly important for successful pearl culture -- were investigated.
The CMFRI also succeeded in artificially spawning Pinctada fucata, rearing of larvae, and producing seed in the laboratory by hatchery techniques. This breakthrough is very important in light of the difficulty in obtaining sustained supplies of oysters from natural banks for culture purposes. Recently the CMFRI also produced seed of the black-lip pearl oyster, Pinctada margaritifera which produces the highly valuable black pearl.
To follow-up on the development of pearl culture technology, the Tamil Nadu Fisheries Development Corporation and the Southern Petro-chemical Industries Corporation Ltd. established in 1983 a company to produce cultured pearls, with the farm at Krusadai and the nucleus implantation centre at nearby Mandapam. Since then other companies have became interested in taking up pearl culture on a commercial scale.
The CMFRI is making efforts to promote the pearl culture technology by conducting short- and long-term training programmes. Scientific and technical personnel from fisheries institutes in all of the maritime states as well as from the Fisheries Faculties of Agricultural Universities are given the opportunity to be trained in these programmes.
This manual covers various aspects of pearl oysters, such as taxonomy, morphology, anatomy, ecology and pearl oyster seed production through hatchery techniques, and pearl oyster framing.
CHAPTER II TAXONOMY AND DISTRIBUTION 2.1 Taxonomy The true pearl oyster belongs to the genus Pinctada (Roding) under the family Pteriidae, order Dysodonta. Members belonging to the Pteriidae family are characterized by a straight hinge with 1–2 small tooth-like thickening, a cavity below the anterior angle for the byssus, and usually a scaly surface of the outer shell valves. The family includes the pearl oysters belonging to the genus Pinctada and the winged oyster shells of the Pteria genus. In Pteria spp. the shell width is much longer than the height and the hinge angle is prominent and pronounced.
In Pinctada spp. the hinge is rather long and straight, the long axis of the shell is at a right angle to the hinge, the left valve is slightly deeper than the right and there is a byssal notch on each valve at the base of the anterior ear.
Six species of pearl oysters, Pinctada fucata (Gould), P. margaritifera (Linnaeus), P. chemnitzii (Philippi), P. sugillata (Reeve), P. anomioides (Reeve) and P. atropurpurea (Dunker) occur along the Indian coasts. Their morphological characteristics are as follows:
Pinctada fucata (Gould)
The hinge is fairly long and its ratio to the broadest width of the shell is about 0.85 and that to the dorsoventral measurement is about 0.76. The left valve is deeper than the right. Hinge teeth are present in both valves, one each at the anterior and posterior ends of the ligament. The anterior ear is larger than in the other species, and the byssal notch, at the junction of the body of the shell and the ear, is slit-like. The posterior ear is fairly well developed. The outer surface of the shell valves is reddish or yellowish-brown with radiating rays of lighter colour. The nacreous layer is thick and has a bright golden-yellow metallic lustre (Plate I A).
Pinctada margaritifera (Linnaeus)
The hinge is shorter than the width of the shell and has no teeth. The anterior border of the shell extends in front of the anterior lobe. The byssal notch is broad. The anterior ear is well developed while the posterior ear and sinus are absent. The posterior end of the shell meets the hinge almost at a right angle. Shell valves are moderately convex. Externally, the shell is dark grayish-brown with radially disposed white spots. The nacreous layer is iridescent with a silvery lustre except distally where it is black. This pearl oyster is also known as the Black-lip pearl oyster due to the dark marginal colouration of the shell. The width of the nacreous region at the hinge is about 2/3 that of the broadest part of the valves (Plate I B).
Pinctada chemnitzii (Philippi)
The hinge is almost as long as the antero-posterior measurement of the valves. The posterior ear is well developed and the convexity of the valves is less than in P. fucata. The anterior and posterior hinge teeth are present, the former is small and rounded and the latter prominent and ridge-like. The posterior ear and the posterior sinus are well developed. The shell valves are yellowish externally with about four or more light brownish radial markings from the umbo to the margin of the shell. The growth lines of the shell are broad. The nacreous layer is thin and bright, while the non-nacreous layer is yellowish-brown (Plate I C).
Pinctada sugillata (Reeve)
The hinge is considerably shorter than the antero-posterior axis of the shell with a ratio of 1:1.3. The anteroposterior measurement is almost equal to the dorso-ventral measurement. The anterior ear in both valves is small and the byssal notch is a moderately wide slit. The anterior ears are slightly bent towards the right. The posterior ear and sinus are poorly developed. The convexity of the valves is not prominent, especially that of the right valve. The hinge teeth are small and the posterior one is slightly elongated. The shell valves are reddish-brown with six yellowish radial markings (Plate I D).
Pinctada anomioides (Reeve)
The hinge is shorter than the width of the broadest region of the antero-posterior axis of the shell with a ratio of 1:1.2–1.5. The hinge and dorso-ventral axis have a ratio of 1:1.4. Hinge teeth are absent or poorly developed. The anterior ear is moderately developed and the byssal notch at its base is deep. The posterior ear and sinus are absent. The shell valves are translucent and externally yellowish or grayish. Some shells have faint radial markings. The nacreous layer is slightly iridescent (Plate I E).
PLATE I. (A) Pinctada fucata and (B) Pinctada margaritifera.
PLATE I. Cont'd. (C) Pinctada chemnitzii and (D) Pinctada sugillata.
Pinctada atropurpurea (Dunker)
The shell is roundish and its hinge narrow. The valves are thin, translucent and moderately convex. A poorly developed anterior hinge tooth is present in some oysters. The shell valves are copper coloured (Plate I F).
2.2 Distribution Pearl oysters of the genus Pinctada are widely distributed in the world. They occur in several seas of the tropical belt and in the sub-tropical region. Although a number of species of pearl oysters have been identified, only a few have been found to produce pearls of good quality and commercial value. Of these, P. maxima, P. margaritifera and P. fucata stand out. The gold/silver-lip pearl oyster P. maxima occurs along the north coast of Australia, Burma, Thailand, Indonesia, Philippines and Papua New Guinea at depths ranging from low tide level to 80 m. The black-lip pearl oyster, P. margaritifera is widely distributed in the Persian
Gulf, Red Sea, Sudan, Papua New Guinea, Australia, French Polynesia, Indonesia, Andaman and Nicobar Islands, Southwestern part of the Indian Ocean, Japan and the Pacific Ocean. The occurrence of this species is sporadic along the coasts of mainland India. The pearl oyster P. fucata is distributed in the Red Sea, Persian Gulf, India, China, Korea, Japan, Venezuela and Western Pacific Ocean (Fig. 1).
In the Indian waters six species of pearl oysters occur but only P. fucata has contributed to the pearl fisheries in the Gulf of Mannar and Gulf of Kutch. In the Gulf of Mannar, the pearl oysters occur in large numbers on the submerged rocky or hard substrata known as paars. The paars lie at depths of 12 to 25 m off the Tuticorin coast along a stretch of 70 km. In the Palk Bay, P. fucata occurs sporadically on loose sandy substratum attached to submerged objects in littoral waters. In the Gulf of Kutch, the pearl oysters are found as stray individuals on the intertidal reefs known as khaddas. In the southwest coast of India at Vizhinjam, Kerala coast, large numbers of spat of P. fucata have been collected from mussel culture ropes. The blacklip pearl oyster, P. margaritifera is confined mostly to the Andaman Islands where it is common in some places. From Lakshadweep, settlement of spat of P. anomioides has been observed on the ridges of rocks and corals.
PLATE I. Cont'd. (E) Pinctada anomioides and (F) Pinctada atropurpurea.
FIGURE 1. World distribution of pearl oysters.
CHAPTER III MORPHOLOGY AND ANATOMY 3.1 Morphology 3.1.1 Shell features
The shell of Pinctada fucata is about 1.5 mm thick over the greater part. The growth edge or projecting lamellae are laid down by the oyster at successive intervals on the distal border. The non-nacreous border of the inner surface of the valves are characterized by brownish or reddish patches which coincide with the external rays.
The adductor scar is elongated and sub-central. The pallial line and scars are caused by the insertion of the pallial muscles in fan-shaped bundles of fibres radiating outwards. There are 12–15 insertion scars between the umbo and antero-ventral border. Besides these distinct scars, there is a narrow continuous insertion band confluent with the posterior and ventral edges of the adductor scars. Its scar merges with that of the adductor scar. The hinge is narrow and runs along the greater part of the straight dorsal edge. Elongated ridge-like teeth are present at the anterior and posterior ends of the ligament.
3.1.2 Shell structure
The shell is composed of three layers. The very thin outer layer is uncalcified cuticular conchiolin layer or periostracum. This is an extremely delicate horny layer which allows the colour of the layer below to show through and usually becomes worn off in old shells. At the free margin of the shell the periostracum is very thin, transparent and extends beyond the calcareous matter. It is reflected to join the surface of the ectoderm cells of the mantle edge in the longitudinal groove where it is secreted.
The middle or prismatic layer shows a cellular structure formed of calcareous prisms or columns running vertically to the surface and appearing polygonal in section. The prismatic layer is deposited by the mantle epithelium near the free edge just behind the margin which forms the periostracum and many such layers on fusion are formed successively, each new one beneath the last, as the shell grows. The innermost nacreous or mother-of-pearl layer is composed of numerous fine lamellae of aragonite crystals (Fig. 2).
FIGURE 2. Section of the shell of Pinctada fucata. C.L.= conchiolin layer; P.L.= prismatic layer; and N.L.= nacreous layer.
3.2 Anatomy 3.2.1 Mantle
The free edge of the mantle lobe is thick, pigmented and fringed with branched tentacles. The pallial edge of the mantle is attached to the shell, a little away from the margin. Each pallial lobe may be divided into three parts, the central, distal or muscular and marginal mantle.
3.2.2 Foot
The foot is highly mobile, tongue-shaped organ capable of considerable elongation and contraction (Fig. 3). It arises from the anterior region of the visceral mass nearly midway between the mouth and the intestinal lobe and the anterior branchiae flanking it on either side. The dorsal portion has more chromatopores while the ventral portion has the pedal groove. The major part of the foot is composed of a network of fibres running in various directions, thus ensuring a wide range of movement. It is extensively penetrated by blood spaces and the organ is highly contractible.
3.2.3 Byssal gland
The byssus gland organ (Fig. 3) is located ventrally at the proximal end of the foot. The byssal gland lodges the common root of a bundle of stout, laterally compressed bronze-green fibres: the byssal threads. Each fibre of the byssus anchors the pearl oyster to rocks and other hard objects by means of a discoid attachment at the distal extremity. The anterior edge of the mouth of the byssal gland passes into the pedal groove (Fig. 3) extending along the whole of the remaining length of the ventral surface of the foot.
3.2.4 Muscular system
The pearl oyster is monomyarian, possessing only the posterior adductor (Fig. 3), the largest and the most important muscle in its body. The adductor muscle stretches transversely across the body from valve to valve. It is a massive wedge-shaped bundle of muscle fibres. The narrow end points upwards and lies immediately behind the ventricle of the heart. The terminal part of the rectum runs in the middle line along the posterior surface. It has two distinct regions, one a narrow tendonous strip made up of white glistening fibres forming the posterior border, the other composed of broad and massive semitranslucent fibres occupying the remainder of the mass. The power exerted by the adductor in bringing the valves together by its contraction is considerable with rapid action resembling a ratchet mechanism. The retractors of the foot are a pair of symmetrically disposed muscles lying in the horizontal plane of the body.
The muscles are V-shaped and originate from the byssal gland. Their ends are attached to the right and left valves without making a separate scar in the nacre.
The foot has four levators, two anterior and two posterior. The contraction of the anterior levator causes the foot to be retracted and raised dorsally. The posterior levators are short and insignificant, originating at level with the mouth, passing through the visceral mass to be attached to the valves behind the anterior levator scar. The branchial muscles cause the shortening of the gills and withdrawal of their posterior extremities. They run within each ctenidial axis from end to end, close to the dorsal edge. There are also muscle bundles running longitudinally down on each side of the principal filaments. The pallial muscles are retractable and together they constitute the orbicular muscle of the mantle. With the exception of heart and indistinct striations on larger portion of the adductor muscle, the muscle fibres throughout the oyster's body are non-striped.
3.2.5 Digestive system
The oesophagus, stomach, and the greater portion of the intestine lie within the viscero-pedal mass. Two horizontal lips, the labial palps, conceal the aperture of the mouth. They are smooth on the surface, and grooved on the opposite side close to the mouth aperture. The mouth is a large, slit-like depression placed transversely between the anterior levator muscles of the foot (Fig. 3). The mouth leads into a straight, dorso-ventrally compressed and ciliated oesophagus. The folds and depressions diversify the walls and floors of the stomach and break them into definite areas. The tissues consist largely of greenish-brown masses often termed as liver or digestive diverticula (Fig. 3). A peculiar (gelatinous) rod, the crystalline style, flattened and oblique, occupies a subcentral position anterior to where the postero-ventral fold disappears midway along the floor. The head of the crystalline style projects out of the sac where it is formed and across the cavity of the stomach where it bears against an irregular area of cuticle known as the gastric shield (Fig. 3).
The intestine may be divided into three sections, namely, the descending and ascending portions and the rectum (Fig. 3). The valvular folding of the intestinal ridge gives way to the ascending portion and curves backwards along the base of the visceral mass to the left of the descending intestine. From the point of intersection, the ascending intestine turns sharply upwards, running parallel and closely adjacent to the upper part of the descending portion (Fig. 3). The rectum runs posteriorly through the upper part of the pericardium (Fig. 3). Beyond this it curves vertically and passes around the posterior part of the adductor muscle in
the median line and ends by the anus in an erectile ear-like process. The anal papilla is comparatively large and slightly curved.
3.2.6 Respiratory system
The gills consist of four crescent-shaped plates, two halfgills on each side which hang down from the roof of the mantle cavity like book leaves. They represent a series of ciliated sieves, providing an efficient feeding surface. Two rows of long delicate branchial filaments are inserted at right angles along the whole length of the axis or vascular base which extends from the ventral border of the palps anteriorly curving round ventrally and posteriorly to a point opposite the anus. Its convexity extends first forwards and then downwards. Where they terminate, the mantle lobes of the two sides are slightly united by the inner mantle folds thus dividing the mantle cavity into a large inhalant chamber containing the gills and a much smaller exhalant chamber. Water enters by one and leaves by the other.
The common base of each ctenidium is a vascular ridge reaching from the anterior end of the gills. Hollow outgrowths, the inter-lamellar junctions, containing branches from the afferent vessels, convey blood from the axial trunk to the base of reflected lamellae. The blood enters the individual filaments, flows outwards to the free margin, passing over to the direct filaments, returning inward to the branchial or ctenidial axis, where it joins the efferent vessel by openings along each side. The filaments are joined chiefly by the inter-locking stiff cilia of the large ciliated discs which occur at intervals throughout their length. The normal function of the ordinary cilia on the branchiae is to create a current of water which enters the pallial chamber and passes over and through the branchial lamellae. They serve to purify the blood flowing in the filaments and to convey the food particles to the mouth.
3.2.7 Circulatory system
This system consists of a heart and a series of arteries which lie above the adductor, and contained in a pericardium. The heart consists of a single ventricle and a pair of contractile thin walled auricles, one on each side. They receive blood from the body by way of the gills and mantle, and pass it to the ventricle. Back-flow of blood is prevented by valves. Blood is driven by contractions into the anterior and posterior aorta. The latter is short and supplies blood to the adductor muscle, rectum and anus. Blood is supplied to the rest of the body by the anterior aorta through a series of minor arteries. These open into the sinuses or blood spaces in which blood circulates slowly. The aorta communicates with a pair of large blood vessels that run around the margin of each mantle lobe. The deoxygenated blood is collected in veins which carry it either into the gills or excretory organs. From the kidney it is pumped into the marginal vessel of the mantle by a pair of accessory hearts. The blood from the mantle, together with that from the gills, returns to the heart through efferent branchial vein by way of auricles. The blood of the pearl oyster is colourless.
3.2.8 Excretory system
The excretory system consists of a pair of nephridia and numerous small pericardial glands projecting from the walls of the auricles. The nephridia are two large symmetrical pouch-like sacs located on either side in the hinder half of the viscero-pedal mass. Each nephridium opens into the pericardium by a wide duct and to the exterior by a minute pore. The nephridia inter-communicate by a wide channel beneath the auricles. The external renal aperture is a minute oval opening with a sphincter muscle. It opens immediately below the genital aperture
within an inconspicuous lipped slit at the junction of the inner plate of the inner gill with the visceral mass at a point about midway between the ventral border of the latter and the base of the foot.
The passage connecting the right and left nephridia is wide and lies beneath the auricles. It has thin membranous walls and is bounded behind by the lower part of the pericardium and in front below with the body wall and forms part of the root of the adductor embayment of the suprabranchial chamber. The reno-pericardial tubules are a pair of wide lateral prolongations of the pre-cardiac part of the pericardium. They are thin walled, membranous and directed forwards. The aperture is a curved slit, with the concavity ventrally. The accessory pericardial glands on the walls of auricles have excretory function.
3.2.9 Nervous system
The nervous system is laterally symmetrical and has three pairs of ganglia, (1) the cerebral ganglia at the sides of the oesophagus, (2) the pedals joined to form a single ganglion at the base of the foot and (3) a pair of large visceral or parieto-splanchnic ganglia lying upon the anterior surface of the adductor. The stout paired cerebro-visceral connectives link the cerebral ganglia with the parieto-splanchnic ganglia, while a pair of cerebro pedal connectives join the cerebral ganglia with the pedal nerve mass. The cerebral ganglia are supra-oesophageal in position and a nerve cord or commissure forms the two parieto-splanchnic ganglia (Visceral ganglia). The cerebro-pedal connectives arise from the posterior and outer sides of the cerebral ganglia and run downwards within the visceral mass just behind the levator muscles of the foot to the pedal ganglion. Three principal nerves arise from the pedal ganglion and innervate the foot and the byssal gland. Each of the visceral ganglia receives from above the stout cerebro-visceral connective, the two ganglia themselves being united by a single transverse visceral commissure. Each branchial nerve leaves the ganglion at the anterior lateral corner, turns down into the base of the gills and then backwards to the posterior tips following the afferent vessels. The posterior pallial nerves emerge from the posterior end of the visceral ganglion. From the base of each, a stout nerve passes straight back and reaches the pigmented pallial sense organs of its respective side, a little anterior to the anus. The ramification of the pallial nerves in the muscular marginal region of the mantle and their anastomosing forms a complex network of nerves, the “pallial plexus.”
3.2.10 Reproductive system
The sexes are separate except in occasional cases. The gonads are paired but asymmetrical. They form a thick envelop covering the stomach, liver and the stomach, and the first two sections of the intestine, connecting a greater part of the outside of the proximal portion of the viscero-pedal mass (Fig. 3). No portion of the reproductive glands extends into the foot or into the mantle. The male and female gonads are indistinguishable from external appearance in the initial stages. Both are creamy yellow in colour. In the mature stage, the male gonad is pale creamy and the female gonad yellowish creamy. The gonads of the two sexes consist of branched tubules with myriads of succate caecae, the alveoli. The spermatozoa and ova develop in these. The accumulated ripe gametes fill these alveoli and tubules and later pass into three trunks which converge into one which leads to the external genital aperture. The latter is situated dorsal to the renal aperture of the same side.
FIGURE 3. Anatomy of Pinctada fucata. 1) Mouth; 2) oesophagus; 3) stomach; 4) left labial palp; 5) left inner labial palp; 6) crystalline style; 7) liver; 8) digestive diverticula; 9) descending intestine; 10) ascending intestine; 11) rectum; 12) anal papilla; 13) byssal gland; 14) nucleus implanted in the gonad.
CHAPTER IV BIOLOGY AND ECOLOGY 4.1 Biology 4.1.1 Food and feeding habits
Like other bivalves, the pearl oyster is a filter feeder. Minute food organisms in the water, enter inside the mantle cavity along with water current passing though the narrow slit formed by the inwardly directed edges of the pallial lobes. These are carried towards the branchiae which act as fine strainers arresting every particle in the water current. The food particles collected thus are carried by the cilia to the crest of the branchial lamellae and from there they are directed by the labial palps into the mouth. The labial palps have the ability to reject unwanted materials like mud particles. Unicellular organisms including infusorians, foraminifers and radiolarians have been found in the stomach of pearl oyster. Minute embryos and larvae of various organisms, algal filaments, spicules of alcyonarians and sponges were also observed. The presence of diatoms, flagellates, larvae of lamellibranchs, gastropods, heteropods, crustacean nauplii, appendages and frustules of copepods, spicules of sponges and unidentified spores, algal filaments, detritus and sand particles were also noted in the stomachs and intestines of cultured P. fucata collected off the coast of Tuticorin. Oysters from natural beds were also found to contain the same organisms in their stomach and intestine.
4.1.2 Age and growth
The age and growth of pearl oysters in the Gulf of Mannar have been studied in detail. Observations made on cultured pearl oysters collected near Krusadai Island and at Tuticorin show that the oysters can grow to a height of about 35–45 mm at the end of one year, 50–55 mm at the end of the second year, 55–60 mm at the end of the third year, 60–65 mm at the end of the fourth year and 65–70 mm at the end of the fifth year. The weight of the oysters was 10, 30, 45, 60 and 70 g at the end of the first, second, third, fourth and fifth years, respectively. Tracing the growth history of P. fucata produced in the hatchery and grown in the farm at Tuticorin Harbour during 1983 revealed that the species attains a modal size of 47.0 mm at the end of first year, 64.5 mm at the end of the second year and 75.0 mm at the end of the third year. The corresponding weights at ages 1 to 3 years were 8.3, 31.6 and 45.4 g, respectively.
The pearl oysters have been estimated to have a longevity of 5–5.5 years in natural beds, but have been observed to live up to seven years when reared in the farm.
4.1.3 Reproduction
In pearl oysters, the sexes are separate although hermaphrodite conditions have been observed in some individuals. Change of sex takes place in some oyster towards the end of spawning. Based on the external appearance, microscopic examination of smears and histological studies, five developmental stages have been distinguished in the gonads of P.
fucata off Tuticorin coast. The five stages of sexual maturity described below are based on the gonad development in female oysters.
- Stage 1: Inactive/spent/resting
The gonad is completely shrunken and translucent. In some cases it is pale orange in colour. Large vacuolated yellow (fat) cells are seen in the interfollicular spaces. The sex at this stage can hardly be distinguished.
- Stage 2: Developing/maturing
The transparent nature of the resting gonad is lost and it becomes distinguished from other visceral masses. Gametogenic materials begin to appear in the gonad. As the stage advances, the gonad begins to branch along the posterior side of the retractor muscle and advances to the anterio-dorsal region. The gametes begin to proliferate along the follicle wall. In advanced stages, the inter-follicular spaces become reduced and the lumen of the follicle may contain some free oocytes. The majority of the oocytes are irregular in shape and the germinal vesicle (nucleus) is not distinctly seen. The average size of the oocytes is 60.0 × 47.5 µm and the germinal vesicle, if present, is 20.0 µm.
- Stage 3: Mature
The gonad spreads on to most of the visceral tissues. It is mostly yellowish cream. The lumen of the follicle is filled with free oocytes. Some of them are attached to the follicular wall by means of slender stalks. The majority of the oocytes are pyriform in shape. The average size of the oocyte is 68.0 x 50 µm with a well defined germinal vesicle. The mean diameter of the nucleus is 25 µm.
- Stage 4: Partially spawned
The gonads become loose in consistency and the visceral epithelium becomes dull. The follicles shrink with the reduction of gametes in the lumen. The oocytes are free and found along the follicular wall. Most of the oocytes are spherical and nucleated. The average size of the oocyte is 51.7 µm.
- Stage 5: Spent
The gonads shrink further with a few left over gametes in the lumen of the follicles. Ruptured follicles are seen in some cases and the lumen sometimes contains ruptured cells. Oocytes, if present are few and spherical. The average size of the oocytes is 54.4 µm. The description of the spent stages applies to the oysters which have recently undergone oogenesis. Otherwise they transform to the spent resting stage quickly.
Males show the same pattern of reproductive activity. However, in stages 2 and 3, the colour of the gonad is pale cream. In other stages of gametogenesis, the gonads of males and females appear similar when observed externally.
Pinctada fucata from the Gulf of Mannar has two peak spawning seasons in a year: June-September and November-December, coinciding with the southwest and northeast monsoons,
respectively. A slight rise in water temperature may be considered as the stimulating factor for the onset of the gametogenic cycle and a slight reduction in water temperature stimulates the oysters to spawn. However a few inactive, maturing and ripe gonads are present in almost all of the months of the year. Individual oysters spawn more than once in the same spawning season as the gonads are not emptied at one stretch.
4.2 Ecology Pearl oysters are found from the low tide level to depths of about 75 m, therefore they can adapt and live in varying environmental conditions within this range. Environmental factors such as bottom topography, wind, waves, water movement, current, light, temperature, salinity, dissolved oxygen, pH, nutrient salts, and primary production play a crucial role in the settlement, growth and reproductive pattern of oysters, both in the natural beds and farms. In the Gulf of Mannar, the pearl oysters live on rocky or other hard substrata which lie roughly in a line parallel with and at a distance of 10–16 km from the coast. A rich fauna, comprising members of various groups like sponges, hydroids, polychaetes, lamellibranchs, amphipods, decapods, echinoderms, fishes etc. are usually associated with pearl oyster beds. The oysters obtained from the beds are successfully reared in shallow coastal waters with depths ranging from 4–8 m, where the sea does not become rough. In farming the pearl oysters, the preferred depth of culture is about 10 m where silting should be minimal. Unlike in Japan, the variation in temperature and salinity is not much pronounced in the Gulf of Mannar. The temperature of seawater in the natural beds varies from 27.0 °C (January) to 32.5 °C (May) whereas in the oyster farm at Tuticorin, it fluctuates between 24.0–32.5 °C. Similarly, salinity in the natural beds fluctuates between 27.4 ‰ and 35.90 ‰ with an annual range of 8.5 ‰. The salinity values in the oyster culture site at Veppalodai, ranged between 32.15 ‰ and 33.50 ‰ during 1974–76. An unusual dilution of seawater to 15.69 ‰ for short durations at the Veppalodai farm in November 1977 did not affect the oysters. If salinity level falls below 15 ‰, and if such condition is prolonged, it may lead to mortality. This may happen during unusual heavy rain and heavy discharge of fresh water from rivers in the vicinity. It is well known that the benthic ecology of the pearl culture grounds plays a vital role in the rate of production as well as quality of pearls. Rocky or gravelly bottoms are more suitable. A high amount of silt in the farm water may affect the filtration efficiency of pearl oysters. In Japan it has been observed that some culture grounds yielded pearls of good quality, while others did not. Some grounds yielded pink or white pearls while others produced only yellow and golden pearls. Repeated culture on the same ground sometimes affects the quality of pearls. Areas rich in phytoplankton which is consumed by oyster, are good but there should not be noxious blooms. A mild current of two knots per hour is necessary not only as a source of oxygen-rich seawater, but also to bring in fresh plankton as well as for the removal of metabolic products, and faecal matter. If the water current is strong, the formation of the nacreous layer is faster, but the quality of pearls is inferior. The rich nutrients discharged by the rivers into the sea increase the productivity of the water. The oysters can also directly remove the organic matter and calcium dissolved in the water. If oyster culture farms are located in places such as the vicinity of a river mouth, which is often exposed to prolonged dilution of seawater due to flooding, oyster growth will be affected.
CHAPTER V HATCHERY TECHNIQUES FOR SEED PRODUCTION 5.1 Artificially reared spat Seed of P. fucata, were produced in 1981 in the laboratory through hatchery techniques at the Central Marine Fisheries Research Institute at Tuticorin. The technology developed is helpful in overcoming the problem of insufficient supply of mother oysters for cultured pearl production. The hatchery methods developed are simple to adopt and inexpensive. The suitability of the technology has been proved by producing seed during most of the year. The hatchery is one of the most important sources of sustained supply of pearl oysters.
5.1.1 Hatchery building
The pearl oyster hatchery of the CMFRI at Tuticorin has facilities for oyster conditioning, maturation and spawning, as well as larval and spat rearing (Plate II A). The roof of the hatchery building is sufficiently high to avoid high temperature. Part of the roof of the wet laboratory has translucent fibreglass sheets to allow sufficient light for indoor phytoplankton culture. Glass panelled large windows and ventilators are provided for free passage of light and air. The concrete floor has sufficient gradient facilities for easy drainage.
5.1.2 Seawater management
The seawater is usually drawn from the sea beyond the low water mark into a well through PVC pipes. The seawater is pumped to sedimentation tanks and passed onto the biological filter which contains coarse river sand at the top, pebbles below it and charcoal at the bottom. The filtered seawater is stored in a water sump and lifted to an overhead tank for supply to the hatchery. Periodic cleaning of the filter bed keeps the seawater uncontaminated. PVC, fibreglass and stainless steel materials are used in the hatchery. The seawater sterilized by ultraviolet irradiation is used only in specific cases.
5.1.3 Aeration
Air compressors with storage tank are used to aerate seawater in the rearing tanks. The compressed air is passed through a series of filters to remove oil and moisture and is supplied to the various culture vessels through PVC pipes. Air is drawn at the required places from these pipes running the entire length of the hatchery through nozzles. The air is supplied to the tanks through diffuser stones.
5.2 Live food production 5.2.1 Phytoplankton
Flagellates measuring less than 10 µm form the main food for pearl oyster larvae. Isochrysis galbana is an important algal food for the larvae. Other microalgal cells such as Pavlova,
Chromulina and Dicrateria are also suitable for the larvae. The growth and spatfall timing varies with the different algal foods. Chromulina promotes faster growth while Dicrateria gives better spatfall.
The flagellates are grown in 4-1 Haufkin flasks as stock culture and in 20-1 glass carbuoys and 100-1 perspex tanks in mass culture. Conway or Walne's medium is used for the culture of these flagellates. They enter the stationery phase of growth in about 15 days in Haufkin flasks and are maintained for two months without aeration. In mass culture, the maximum cell concentration is reached within 5–6 days. The composition of the medium used for mass culture as well as mixed algal culture is as follows:
Potassium nitrate : 0.4 g
Potassium dihydrogen
Orthophosphate : 0.2 g
Sodium silicate : 0.2 g
Sodium EDTA : 0.2 g
Filtered seawater : 30.0 l
5.3 Broodstock handling and spawning 5.3.1 Broodstock maintenance
Oyster broodstock are maintained at a water temperature ranging from 25–28 °C in a controlled room. They are fed with a mixed algal diet at a ration of 4 l per oyster/day. The algal food is supplemented by raw corn flour at 30 mg per oyster/day. Pearl oysters with maturing gonad fed with the above food for 45 days will spawn with a 30 % response. The matured oysters can be kept for a prolonged period at 25–28 °C, while spawning of these oysters can be stimulated by raising the water temperature by few degrees.
5.3.2 Spawning
Spawning of natural oysters with mature gonads occurs when there is a simple change in the seawater environment or a mechanical shock by shell cleaning or a change in water pressure. In all cases males spawn first (Plate II B) and this induces the females to spawn within 30 minutes.
In the absence of natural spawning the technique of induce spawning is employed. In this technique thermal stimulation is adopted predominantly by gradually increasing the water temperature by several degrees (from 28.5 °C to 35.0 °C). Spawning of pearl oysters can also be effected by chemical stimulation. Different concentrations (1.532, 3.064 and 6.128 millimolars) of hydrogen peroxide in combination either with normal seawater or alkaline seawater (pH 9.1) is used in inducing spawning. Different pH media (8.5, 9.0, 9.5 and 10.0) are prepared either using Tris buffer or Sodium hydroxide pellets (NaOH) and the pearl
oysters are induced to spawn. A pH value of 9.0 in the case of Tris buffer and 9.5 in NaOH gives 78.6 % and 68.4 % of spawning, respectively. Injection of 0.2 ml of N/10 ammonium hydroxide (NH4OH) solution into the adductor muscle of the pearl oyster results in 48 % spawning.
5.3.3 Fertilization
When the eggs are released in the medium, they are pyriform in shape measuring 73.9 µm along the long axis and 45.2 µm in width. The yolk cytoplasm is heavily granulated and opaque. The egg is enclosed in a vitelline membrane and a large germinal vesicle is seen at the centre (Plate II C).
Fertilization takes place externally in the water medium. Following fertilization, the pyriform eggs assume a spherical shape with the breakdown of the germinal vesicle (Plate II D). During fertilization the polar body appears and persists on the embryo up to blastula stage.
5.3.4 Early development and larval rearing
Cleavage
The first cell division is seen 45 minutes after fertilization resulting in the formation of a micromere and a macromere. The polar body is placed at the cleavage furrow (Plate II E). During the second cleavage the micromere divides into two and the macromere divides unequally into a micromere and macromere. The stage with three micromeres and a macromere is called Trefoil stage. The macromere does not take part in further divisions. Micromeres divide repeatedly thus becoming smaller and smaller and passing through 8-cell, 16-cell, and so on until the morula stage (Plate II F). Each micromere develops a small cilium which helps in the movement of the embryo.
PLATE II. (A) Inside view of the C. M. F. R. I. pearl oyster hatchery in Tuticorin, (B) Male oyster while spawning and (C) Pyriform oocytes.
Blastula
The embryo is ball-like with transparent cells and a blastocoel. The embryos lift themselves in the water column and congregate at the surface. The floating embryos are siphoned out to clean containers and the residues at the bottom, containing broken tissues, undeveloped embroys, unfertilized eggs, sperm, etc. are discarded. Reorientation of cells starts and the blastocoel and blastopore are formed. The blastula stage is reached 5 hours after fertilization.
Gastrula
Gastrulation takes place by epiboly. The cells convolute and differentiate into different dermal layers. The archenteron is formed. The embryo is bean-shaped as there is convolution of cells. The gastrula exhibits negative photrophism. This stage is reached in 7 hours.
Trochophore larva
The minute cilia present in the gastrula stage disappear and the pre-oral and post-oral tufts of cilia develop, thus marking antero-posterior differentiation of the embryo. A single apical flagellum is developed at the anterior side. The anterior portion of the larva is broader while the posterior end is tapering like an inverted triangle. The movement of the larva is affected by the propulsive movement of the flagellum (Plate II G). The dorsal ectodermal cells secrete the embryonic shell, known as the prodissoconch I.
Veliger
A definite ‘D’ shape is obtained by the secretion of the prodissoconch I having a hinge line, mantle and rearrangement of the pre-oral tuft of cilia into a velum. The single flagellum, pre-oral and post-oral tufts of cilia disappear. The veliger larva measures 67.5 µm along the antero-posterior axis and 52.5 µm along the dorso-ventral axis (Plate II H). This stage is reached in 20 hours.
PLATE II. Cont'd. (D) Fertilized oocytes, (E) Early cleavage and (F) Morula stage.
PLATE II. Cont'd. (G) Trochophore larvae and (H) Straight-hinge larvae.
Umbo stage
Further development of the veliger to the umbo stage is gradual with the development of the shell, prodissoconch II. The typical clam shaped umbo stage is reached between 10–12 days and it measures 135 × 130 µm (Plate III A). The shell valves are equal and the mantle folds are prominent.
Eye spot stage
After attaining the full umbo stage, the larvae develop an eye spot at the base of the foot primordium. A well developed velum effects the movement of the larvae. The ctenidial
ridges develop at this stage. The minimum size at which the larva develops the eye spot is 180 x 170 µm (Plate III B) usually in 15 days.
Pediveliger stage
The foot is developed on the 18th day when the larvae measures 200 × 190 µm. The transitional stage from the swimming to the crawling phase has both velum and foot (Plate III C). Later the foot becomes functional while the velum disappears. Gill filaments are now visible (Plate III D).
Plantigrade
When the pediveliger larva selects a substratum for settlement, additional shell growth is seen along the globular shell margin except at the vertex of the umbo region, in the form of a very thin, transparent, uniform conchiolin film. In the meantime the byssal gland secretes byssal threads for attachment. Labial palps and gill filaments develop. The stage is reached in 20 days at the size of 220 × 200 µm (Plate III E).
Spat
By the repeated addition of dissoconch, the plantigrade transforms into a spat. It resembles an adult with the hinge line, anterior and posterior auricles and the byssal notch (Plate III F). The left valve is slightly more convex than the right one. The spat attaches itself to the substratum with the aid of the byssal threads. A healthy spat measures 300 µm in 24 days. The sizes and days given for the different larval and post-larval stages vary from batch to batch according to the environmental conditions.
PLATE III. (A) Umbo larvae, (B) Eye-spot larvae and (C) Transitional stage.
PLATE III. Cont'd. (D) Pediveliger larvae, (E) Plantigrade larvae and (F) spat.
5.4 Larvae and spat handling 5.4.1 Larval rearing conditions
Larval density plays a significant role in the growth of pearl oyster larvae. Under identical conditions the larvae show differential growth rate at different larval densities. At higher densities the growth and spatfall are poor. A culture density of two larvae per ml produces optimum growth and spatfall rates. The colour of the culture tanks also influences the setting of larvae. Spatfall is much higher in FRP black tanks than white and blue tanks.
Aeration during larval rearing affects growth and spatfall. The effect of aeration is more pronounced in smaller volumes of water. However, aeration is required after the setting of the pearl oyster larvae.
5.4.2 Spat production
Spat production is carried out in the molluscan hatchery at Tuticorin throughout the entire year. However, during May-August the spatfall is less due to high salinity, dustfall and warm landward wind. Sudden spurt of ciliates in the culture medium is common during this period. Such problems can be overcome by good management.
5.4.3 Feeding
The microalgal cell Isochrysis galbana is provided to the larva from the veliger stage onwards. The optimum ration for a larvae is 5,000 cells/day up to umbo stage. The dose is doubled from the umbo to the pediveliger stage and tripled afterwards up to settlement. For about 15 days after settlement each spat is fed with I. galbana at 50,000 cells/day. Mixed algal diet containing mostly Chaetoceros and I. galbana is given in a ratio of 1:1 in the following 15 days. Later the spat is supplied with a mixed algal diet.
5.4.4 Transplantation
The spat are reared in the hatchery for about two months. By then they shall have grown to 3 mm or more. They are then transferred to the farm in velon screen netcages with a mesh size of 400 µm. Mortality may occur if spat measuring less than 3 mm are transplanted. Spat growth is monitored carefully and the netcages are changed whenever necessary. The size of mesh of the rearing cages is also monitored. The oyster spat attain an average size of 40–45 mm in 12 months.
5.4.5 Survival
In the hatchery about 20 % production of spat is achieved in tanks of 500-1 capacity and 40–50 % production in 50-1 tanks. The survival of transplanted spat in the farm is about 30 % at the end of one year.
CHAPTER VI PEARL OYSTER FARMING 6.1 Selection of culture sites In any farming activity, culture site selection is of paramount importance. Technological and economic considerations play a major role in the selection process. A careful appraisal of the habits of the organism to be cultured would give a resonable level of confidence on the tolerance limits within which the various environmental parameters can vary. Due consideration has to be given to possible effects of fluctuating water flow, primary production, siltation, etc. in order to obtain the optimum level of growth and production of high quality pearls. Unsuitable levels of environmental factors such as salinity, water temperature, cold water currents and other factors such as red tides, hydrogen sulphide and pollution by industrial and domestic effluents are serious hazards to pearl culture.
Sheltered bays are ideal locations for pearl oyster farms. They offer good protection to the culture structures such as rafts and cages. Shallow coastal waters where the sea is calm most of the year can also be considered as a suitable site.
6.2 Environmental conditions 6.2.1 Temperature
In temperate regions, the water temperature plays an important role in the biological activities of pearl oysters. In Japan, the optimum temperature for oyster growth has been found to be between 20–25 °C. A temperature below 13 °C causes hybernation. Below 6 °C, the oysters die. At temperatures above 28 °C, the oysters show exhaustion. The thickness of the pearl layers are affected by the minute changes in water temperature during the day and also vary considerably according to the season of the year. The deposition of calcium stops at a water temperature of 13 °C. In the Gulf of Kutch, the oysters grow vigorously in winter months when the seawater temperature ranges between 23–27 °C. A slight decrease in temperature triggers spawning in oysters in the Gulf of Mannar. The growth-temperature relationship is presumably valid only up to a certain temperature for optimum growth.
6.2.2 Salinity
Pearl oysters tolerate a wide range of salinity from 24–50 ‰ for a short duration of 2–3 days. Salinities of 14 ‰ and 55 ‰ may cause a 100 % mortality among the oysters. The effect of salinity on the growth of pearl oyster has not been clearly investigated. However, it appears that pearl oysters tend to prefer high salinities. Oysters raised in such salinities produce pearls with a golden tint.
6.2.3 Bottom
Gravelly bottoms are suitable for pearl oyster farming, while sandy or muddy bottoms should be avoided. Oyster growth is affected by water temperature and nutritional condition of the
ground. Repeated culture on the same ground leads to some extent the deterioration of pearl quality. The chemical and physical state of the sea bottom is affected by the organic substances discharged from the oysters and fouling organisms. Periodic removal of such accumulated substances from the bottom of the culture grounds often increase production as well as quality.
6.2.4 Depth
The optimum depth for farming pearl oysters is around 15 m. At greater depths, even if the rate of nacre deposition is slower, pearls of high quality with a pinkish colouration are obtained.
6.2.5 Silt load
Pearl oysters generally prefer clear waters as high turbidity levels will affect their filtration efficiency. A decline in oyster condition was noted at Veppalodai farm due to the high silt content in the farm area throughout most of the year.
6.2.6 Water current
Culture sites should be naturally sheltered against strong winds and waves. Tidal amplitude and currents must be sufficient in order to allow replenishment of oxygenated water and fresh plankton and flush away waste materials. In strong water currents the formation of the pearl layers is usually fast, but the quality of pearls produced is affected.
6.2.7 Primary productivity
The condition of a specific culture ground depends primarily on the chemical constitution of the seawater and on the species and amount of plankton present. Rich nutrients discharged by rivers into the sea are responsible for high primary productivity. The oysters probably derive their chief source of conchiolin from the nitrogen substance of the plankton. The organic matter and calcium dissolved in the seawater are directly absorbed by the food consumption cells. The calcium passes through the mantle to be deposited on the surface of the shell or pearl in the process of their formation. The presence of trace metals in small quantities influences the colour of the nacre.
6.3 Supply of oysters In pearl oyster farming, oysters collected from the natural beds or reared from naturally collected or cultured spat are used. In the Gulf of Mannar, several pearl banks are distributed off Tuticorn at a distance of 12–15 km and at depths of 12–25 m. Pearl oysters from these beds are collected by skin and SCUBA diving (Plate IV A). Wide fluctuations in terms of pearl oysters availability have been noted in different pearl banks in this area during the last few centuries as also during the most recent years. In the Gulf of Kutch, the pearl oysters are found on the intertidal flats and the population is sparse. Collection is done by hand.
In Japan, oyster spat are collected by submerging bundles of cedar twigs near the water surface during the peak larval settlement season. Hyzez films and old fish nets are also commonly used as spat collectors. Almost the entire requirement of oyster supply to the pearl
culture industry is met by this type of spat collection. Spat collection attempts in India have not been successful, and this may be due to the distance of the pearl oyster beds from coastal waters.
However, India has recently succeeded in producing pearl oyster seed under hatchery conditions, therby providing the industry with a more dependable source of oysters.
6.4 Rearing methods 6.4.1 Raft culture
Raft culture is considered to be one of the most suitable farming methods in sheltered bays. The size of the rafts can be altered according to the convenience of the situation. A raft of 6×5 m in size can be easily constructed and floated with 4 buoys. Rafts are usually constructed with logs of teak, venteak or casuarina wood, of chosen length with the bottom of about 10 cm diameter tapering to 6 cm diameter at the tip. These logs are arranged as per the requirement and lashed with coir ropes. Floats are attached to the raft to give buoyancy. The floats can be sealed empty diesel drums of 200 l capacity with fibreglass coating, mild steel barrels painted with antisaline/anticorrosive paints or FRP styrofoam floats (Plate IV B and C). Unit raft system is found to be convenient and well suited to the Indian sea conditions. Rafts are moored with anchors at opposite sides with tested quality chains and their direction is decided according to the prevalent wind direction at the specific site (Fig. 4 A-C).
In the long-line culture method, spherical or cylindrical floats which are connected by horizontal synthetic rope or chain are used (Plate IV D; Fig. 4 D). The oyster cages are suspended from the ropes. This system is good for open sea conditions. In another method of hanging, a hole is drilled near the hinge of the pearl oyster. A small thread is put through the hole, which is then tied to a straw rope coated with tar. The straw ropes are hung from a raft.
6.4.2 On-bottom culture
Sea bottoms with a granite or coral stones composition can be used for on-bottom culture. In the Tuticorin Harbour Basin where the breakwater has been constructed with granite stones, the protected portion of the breakwater is used for culturing mother oysters. 1 m of water is available below the low water mark. Due to constant circulation of seawater, settlement of fouling organisms is poor and inconsistent. However, it has been noted that the growth of the mother oyster is slower in on-bottom culture compared to the growth of oysters cultured in raft.
6.5 Rearing containers 6.5.1 Culture of mother oysters
Box cages, measuring 40×40×15 cm, are used to rear mother pearl oysters. The size of the mesh varies with the size of the oysters to be reared. The frames of the cages are made up of 6 mm mild steel rods, coated with anticorrosive paints or coal tar. Box-cages are useful in general mother oyster culture (Plate V A).
To trace the history and performance of individual oysters, frame nets are used. The frames, measuring 60×40 cm each with five compartments, meshed and hinged at one end, open as a book. The oysters are arranged in rows and held in the compartments when closed. The space available in between the two frames is about 10 mm which is sufficient for the oysters to open their valves for feeding and respiration (Plate V B).
PLATE IV. (A) A scuba diver diving to collect pearl oysters and (B) A culture raft floated with mild steel barrels.
PLATE IV. Cont'd. (C) A culture raft with FRP styrofoam buoys and (D) Oyster long-line culture.
FIGURE 4. (A) Culture raft constructed with teak poles; (B) A FRP styrofoam buoy; (C) A mild steel buoy, and (D) Oyster long-line culture system.
6.5.2 Juvenile rearing
Juvenile pearl oysters are reared in netcages (Plate V C-D). Synthetic fabric of velon screen bags whose sides are stretched with a steel rod in the form of a prism are used for rearing of juveniles. The mesh size of the screen depends on the size of juveniles to be reared. The mouth of the bag is tied with a synthetic twine which facilitates opening or closing when required. To provide further protection from predators the bags are placed in old nylon fish
net bags. Clogging by silt and by the growth of fouling organisms can be prevented by periodical replacement of the velon screen bag which can be cleaned, sun-dried and reused. Spat of up to 2 cm in size are reared in these small netcages. Box-cages which are used for rearing mother oysters can also be used for juvenile rearing by providing an additional velon screen cover inside the cage.
PLATE V. (A) A box-cage containing pearl oysters and (B) A frame netcage with oysters.
PLATE V. Cont'd. (C) A netcage for rearing oyster spat of 3–10 mm in size and (D) Rearing netcage covered with velon screen.
CHAPTER VII BIOFOULING AND PREDATION Major problems in pearl oysters farming are caused by biofouling organisms which settle and grow on the oyster shells, by boring organisms which riddle through the shells making them weak and friable, and by predators which feed on the oysters. Singly or in combination, these organisms can cause heavy mortality to the farm stock through physiological stress and diseases. The removal of foulers, borers and predators is a labour intensive activity. The seasonal variations of the dominant fouling organisms and predators have to be carefully investigated, and suitable techniques for their control should be adopted on a periodical basis.
7.1 Biofouling organisms 7.1.1 Barnacles
The cirriped Balanus amphitrite variegatus is one of the major fouling organism. Two peaks of heavy settlement of these organism, particularly in the culture sites, have been recorded in India: one from mid-June to August and the other from September to November. In the farm at Veppalodai a minimum of 2,500 barnacles (July) to a maximum of 3,460 barnacles (June) have been recorded on an area of 25 square cm during the first peak, while 1,290 (September) to 2,700 (November) in the second peak. The settlement was considerably less from January to May. Heavy settlement of barnacles cause physical obstruction to the opening and closing of the oyster valves. When the barnacles were dense they completely cover the entire surface of shell valves (Plate VI A and B). In addition to this, during the removal of barnacles, the shell margins were also damaged resulting in the recession of shell growth.
7.1.2 Ascidians
Ascidia depressiuscula, Dicarpa sp., compound ascidians belonging to the genus Diplosoma and, species of Botrilloides have been recorded throughout the year. Ascidians are particularly found in large numbers during the period from October to December.
PLATE VI. (A) Fouling organisms on adult ovsters and rearing cage and (B) Oysters heavily encrusted with barnacles.
7.1.3 Bryozoans
Species of Membranipora, Thalamoporella and Lagenipora represent the group almost throughout the year. The peak period of occurrence is usually between November and December. Other species, such as Watersipora and Bugula are more commonly found during February and June.
7.1.4 Molluscs
Among the fouling molluscs, Avicula vexillum and spat of Crassostrea sp. are numerous on the farm during April to June. The spatfall of Avicula vexillum can be so numerous in the rearing cages, that the pearl oyster spat cannot be easily separated without causing damage to or killing the spat. Modiolus metcalfei is another common fouling mollusc, usually occurring in July. The spatfall of Pinctada sp. has been recorded in the farm during May-July and November-January. The settlement of these organisms can significantly affect the culture of the mother oysters as well as the production of the cultured pearls.
7.1.5 Sponges
The profuse growth of sponges such as Callyspongia fibrosa and Haliclona exigua may result in the complete covering of an individual oyster or a cluster of oysters. The occurrence frequency of these sponges is usually low at the present Indian farm site and the damage caused to the oysters is negligible.
7.1.6 Other organisms
Besides the above mentioned groups, the fouling community may be composed of a large number of other organisms such as amphipods, hydroids and algae typically occurring in June, October and December. Major hydroids belong to genera such as Campanularia, Sertularia, Abeitinaria, Lytocarpus, Diphasia and Thuiaria. Commonly occurring algae are Gracilaria, Codium, Boergesenia and Ceramium. Other organisms such as anthozoans, juveniles of Panulirus sp., crabs, tubicolous polychaetes, Pycnogonids, polyclad worms, crinoids, alcyonarians, opisthobranchs, blennid fishes and Pinna spat may occur on the oysters and rearing facilities in certain months of the year.
7.2 Boring organisms Boring organisms comprising polychaetes, sponges, molluscs and isopods may cause considerable damage to the pearl oyster shell.
Polychaetes belonging to the families Sillinidae, Nereidae, Spionidae, Terebellidae and Cerratulidae have been found to bore pearl oyster shells. Among them the spionids Polydora ciliata and P. flava and the cirratulid Cirratulus cirratus are the most common borers. Polydora spp. typically caused simple and compound blisters on the inner side of the oyster shells. In a few cases, the blisters erupted as tumour-like protrusions, mostly near the adductor impression. Blister formation by boring polychaetes in oysters of 40 mm in length and less is usually less than in large oyster specimens. The cirratulid Cirratulus cirratus is found in furrows between the layers of periostracum of the pearl oyster shell. As a result, the furrow eventually becomes deeper and wider causing the peeling of the periostracal layer thus weakening the shell.
Major boring sponges are Cliona celata, C. vastifica and C. margaritifera. In several Indian farms over 20 % of the oysters have been found to be infected by such sponge. These borers initially attach themselves near the umbo region of the shell and later spread over the surface of the two valves. Oysters affected by these sponges have to secrete more nacre in order to
seal off the perforations. With heavy infestation the oyster shell usually becomes extremely fragile and susceptible to further infestation and damage (Plate VI C).
The pholadid bivalve Martesia sp., the mytilid Lithophaga sp. and, the isopod Sphaeroma sp. are occasionally found in the culture farms. Martesia sp. has been found to make a significant number of holes on the oyster shells.
7.3 Predator organisms Besides fouling and boring, predation is another menace encountered in pearl culture farms as well as in natural pearl oyster beds. Predators in wild beds are mainly benthic fish which feed on young oysters below one year of age, while rays, octopods and starfish feed on adult oysters. Recently, Cymatium cingulatum (Plate VI D) and Murex virgeneus have been found to be serious predators in natural oyster beds. A study on the feeding rate of Cymatium cingulatum in the laboratory showed that 20 oysters were consumed by two Cymatium 26.0 mm in size within a period of 37 days; 20 oysters in 20 days by two Cymatium of 40.5 mm and 20 oysters in 19 days by two Cymatium of 61.8 mm. Two specimens of M. virgeneus, 54.0 mm in size consumed 20 oysters in 49 days in a laboratory experiment. These gastropods have been shown to survive starvation for 57 to 125 days.
Usually in the culture sites crabs are the worst predators. These crustaceans enter the spat rearing cages during their larval phase and, as they grow, they crush and feed on the pearl oyster spat. Charybdis lucifera, Atergatis integerrisimus, Leptodius exaratus, Neptunus spp. and Thalamita spp. are some of the crabs commonly found inside the pearl oyster cages in the Indian oyster farms.
PLATE VI. Cont'd. (C) Damage caused by a boring sponge and (D) Cymatium cingulatum, a major pearl oyster predator.
Three species of parasitic trematodes have been isolated from the foot, mantle, gill, liver and gonads of pearl oysters. Their monthly percentage of infection has been recorded to vary from 3–20 % in a single farm site.
7.4 Control measures 7.4.1 Fouling
The most effective method of controlling fouling growth is by cleaning the oysters, cages and farm materials regularly. Suspending the oyster cages at depths below 5 m during the peak barnacle settlement period usually reduces the degree of settlement of this organism. In addition, periodical exposure of the oysters to sun light for a few hours results in the killing of the larvae of most undesirable settlers. Fresh water, brine and chemical treatment are also found to be effective. Finally, the peak spawning and settlement season of major fouling organisms can be also avoided by timing the introduction of the new spat stocks in the farms.
7.4.2 Boring
The boring polychaetes are easily killed by immersing the oysters in freshwater for about 6 hours. The oyster shell valves infested with boring organisms can also be brushed with 1 % formalin, dipped in freshwater and returned to the sea. The above treatment is found to be effective against sponges and Martesia sp. and partly against Polydora sp.. At a concentration of 78 %, brine has been shown to kill all polychaete species within 8 hours.
7.4.3 Predation
Periodic monitoring of the culture facilities and manual removal of the predators is the only way of containing predation on the oysters. Oyster spat can be additionally protected from fish by covering the rearing cages with old fish net.
CHAPTER VIII CULTURE SYSTEM 8.1 Culture operations Pearl culture in Japan is carried out mostly by small cooperatives or family enterprises, while only a few large-scale operations exist. In the peak period of production (1966), there were 4,710 co-operatives half of which were operating 1–14 rafts, one fifth operating 15–29 rafts, 12.0 %, 30–49 rafts and the rest more than 50 rafts. The total number of co-operatives came down to about 2,500 in 1973. This shows that small-scale operations are the mainstay in pearl culture. The Japanese pearl culturist has the advantage of being able to buy the mother oysters for his farm from those who are solely engaged in seed collection and mother oyster culture. In India also such small-scale operations at family level is possible if the production of oyster seed is done by hatcheries and supplied to the pearl culturist.
The activities, major inventory and manpower of a pearl culture establishment is summarised briefly to provide an overview for an easy understanding of the nature of this industry. Major work is in the sea involving pearl oyster collection and farming. Manpower needs and inventory items vary according to the scale of the operation.
A) Raw material: Pearl oyster (Pinctada fucata)
Oysters from natural beds
· Activity: Seasonal survey of beds and collection by diving. · Inventory: Boats; self-contained underwater breathing apparatus (SCUBA) and diving
accessories such as fins, masks, snorkel, depth-gauge, knife, belt; compressed air units (main and portable compressors); collection kit and oyster bins.
· Manpower: Boat crew, navigator, divers, diving assistants.
Oysters cultured from wild spat
· Activity: Collection of pearl oyster spat by suspending spat collectors from rafts at suitable sites.
· Inventory: Rafts, lighted buoys, anchors, chain and spat collectors. · Manpower: Same as above and farm labour.
Oysters cultured from hatchery spat
· Activity: Induced spawning, larval rearing, culture of larval food, spat production, transplantation.
· Inventory: Hatchery building, A/C larval food production room, glass carbuoys, FRP tanks 50 l and 1 ton capacity, chemicals, U.V. sterilizing unit, seawater flow-through system, glassware, plasticware.
· Manpower: Biologist, laboratory technicians, technical assistants.
B) Pearl oyster farm
· Activity: Juvenile rearing, mother oyster culture, post-operation care, farm maintenance and farm stock maintenance.
· Inventory: Log rafts, long lines, lighted buoys, floats, anchors, chain rope, juvenile rearing cages, cages, frame nets, dinghy, out-board motor, floating sheds and miscellaneous tools.
· Manpower: Farm superintendent, technical assistants, farm labour.
C) On-shore establishment
Surgical unit
· Activity: Pearl oyster surgery and convalescence. · Inventory: Surgical tools and accessories, furniture, shell bead nuclei of different
sizes, chemicals, glassware, plasticware, ultraviolet lamps and raceway. · Manpower: Chief technician and technicians.
Farm house
· Activity: Shore support for maintenance of farm and farm stock. · Inventory: Oyster cleaning tools, farm structure maintenance requirements (repairs
and maintenance of raft, long line, floats, anchors, chain, cages, etc.) and oyster tanks. · Manpower: Same as for pearl oyster farm and on-shore establishment.
D) Pearl collection centre
· Activity: Collection of cultured pearls and incidental natural pearls. · Inventory: Plasticware, chemicals, oyster knife, vats. · Manpower: Technical assistant
E) Pearl processing centre
· Activity: Cleaning, sorting and grading of pearls; treatment of pearls for removal of minor blemishes; bleaching, dyeing, and colour improvements.
· Inventory: Sorting trays, miscellaneous tools, chemicals and glassware. · Manpower: Pearl processing expert and technical assistant.
F) General services
Seawater supply
· Activity: Supply of quality filtered seawater to pearl oyster hatchery and surgery, raceway, flow-through system and other tanks, sterilized water for larval food production and larval rearing.
· Inventory: Pump house, filter bed, sump, over-head tank, supply channel with regulators.
· Manpower: Electrical supervisor and assistant.
Air supply
· Activity: To supply oil-free air to hatchery, larval food production laboratory, and pearl oyster surgical room.
· Inventory: Air blowers with air supply tubes and regulators. · Manpower: Same as above.
Power and fresh water supply
G) Laboratory
· Activity: Monitoring of oyster health and condition; maturity, seawater analysis, bacterial analysis of seawater; advice to farm superintendent and chief technician; feed-back to research system.
· Inventory: General biological laboratory equipment and analytical equipment for seawater analysis.
· Manpower: Biologist, chemist, laboratory technicians.
H) By-product unit
· Activity: Conversion of by-products of pearl culture to value-added items. · Inventory: If the unit is self-contained, all items required for utilization of shell and
meat; otherwise, collection, preservation and storage of materials, unit sale to outside agencies.
· Manpower: Specific manpower to handle by-products processing work, if self-contained, otherwise linked to other items.
I) Management and administration
· Activity: Planning, execution and administration of project. · Manpower: General manager, administration, accountants and stores staff.
CHAPTER IX THE MANTLE 9.1 Mantle structure The mantle, which is an important part of the molluscan body, is responsible for the secretion of the shell. In the pearl oyster the mantle is bilobed, each consisting of three regions: the marginal, pallial and central mantle (Fig. 5). The free edge of the marginal mantle is thick, pigmented and fringed with branched tentacles. Along the hinge line, the marginal mantles of the two lobes fuse to form the mantle isthmus. The pallial mantle is attached to the shell, a little away from the shell margin, while the central mantle is dorsal to the pallial mantle.
9.1.1 Marginal mantle
The marginal mantle consists of three folds: the inner, middle and outer fold. Even though the mantle folds look similar morphologically, functionally they are quite different. The inner fold is muscular in activity, the middle sensory, and the outer (shell) fold is secretory in function.
Inner fold
The inner fold is relatively large compared to the other two folds. The fold is lined with ciliated columnar epithelium cells 5–50 µm thick, which have a distinct basal nucleus. The margin of the fold shows a conspicuous pigmentation. Longitudinal and transverse muscle cells, which are also pigmented, are found below the epithelial layer.
Middle fold
The inner margin of the middle fold is similar to the inner margin of the inner fold. The outer margin of the middle fold has ciliated cells, 45 µm in size, columnar in shape, and also with a distinct pigmentation.
Outer (shell) fold
Close to the shell margin is the outer mantle fold. The outer surface of the fold is lined with specialized cells, which are not ciliated or pigmented. Elongated (15–30 µm) stratified columnar epithelial cells occur near the periostracal groove on the inner surface of the fold. Further towards the tip, these cells become smaller in size (10–20 um). However, the arrangement of the columnar epithelial cells, throughout the inner surface from the groove to the tip of the mantle fold, is similar in nature.
FIGURE 5. Section of oyster mantle. (1) Central mantle; (2) Pallial mantle; (3) Marginal mantle. I.F.= inner fold; M.F.= middle fold; S.F.= shell fold; P.G.= periostracal groove; and P.S.= periostracal secretion.
9.1.2 Mantle isthmus
The dorsal marginal mantle or the mantle isthmus consists of non-ciliated columnar epithelial cells. Sub-epithelial or epithelial secretory cells are totally absent in this region.
9.1.3 Pallial mantle
The portion of the mantle just dorsal to the marginal mantle is known as the pallial mantle. Here, the outer epithelium cells are non-ciliated and smaller (20–30 µm) than those of the ciliated inner epithelium, and are highly vacuolated. Secretory cells are found in both the epithelium, and the sub-epithelium layer.
9.1.4 Central mantle
Below the shell and dorsal to the pallial mantle is the central mantle, which covers the body of the animal. The outer surface of the central mantle is lined with small columnar epithelial cells (10–15 um). Histologically, the secretory cells of the inner epithelium of the central mantle look similar to those of the epithelial cells of the inner pallial mantle.
CHAPTER X THE SURGERY The two items needed to induce the formation of a cultured pearl are a piece of mantle and a nucleus. The mantle piece, taken from a donor oyster, is grafted into the gonad of the recipient oyster, along with a spherical shell bead nucleus. The different steps followed in the operation are: (1) selection of oysters, (2) preparation of graft tissue, (3) conditioning of oysters, (4) pearl oyster surgery and (5) post-operation care.
10.1 Surgical instruments A set of specially made surgical tools is used in the surgery (Plate VII). These instruments can be made to specification by any surgical instrument manufacturing company.
Knife
The knife has a blade 9 cm long and a wooden handle 11 cm long. The width of the blade is 1.2 cm at the base and 1.5 cm near the tip. The anterior portion of the blade is slightly curved corresponding to the curve of the oyster shell, so that the blade can be easily inserted between the two shells in closed condition. The blade is made by hand forging and finished by filing and grinding. The knife is used to open the unconditioned oysters by sharply cutting the adductor muscle without touching the mantle lobes.
Scissors
This is a pair of straight surgical scissors of 10 cm length. It is used for cutting a long and narrow strip of mantle from its edge. The cutting edges are sharp and the tips are finely ground so as to enable quick cutting of a strip before the mantle withdraws under the stimulus of contact.
Forceps
The forceps is usually 14 cm long. The two components are filed and ground and are provided with serrations and finely ground points at the tips. The material near the joint is ground to proper size to get required mild tension after due hardness is imparted. It is used to lift the mantle strip from the shell, to hold it while cleaning and trimming and to reverse the strip on the wooden block.
PLATE VII. Pearl oyster surgical instruments.
Spatula
The spatula is 17.5 cm long, with a round handle of 13 cm length and 4 mm diameter and a blade 4.5 cm in length and 8 cm in width. The required springiness is given to the blade by grinding and the edges are smoothened out. The spatula is used to remove dirt on the mantle strip and to smoothen the folds on it. It is also used to gently lift back the mantle, labial palps and gills of the oyster during surgery so that the foot and the main body are exposed.
Scalpel
The scalpel is flat and 17 cm long, the length of the blade portion being 3.5 cm. The scalpel is produced by forging from bar stock or blanked from sheet metal and the actual size and shape are obtained by filing and grinding. The instrument is then heat treated to get high hardness. The 2 cm broad cutting edge provided at the end has a delicate curve and is smooth and sharp. The scalpel is used for trimming the mantle strip on both edges, to remove unwanted tissue and to sharply cut the tissue into small bits of the required size. It is also used in place of scissors to cut strips from the mantle.
The second group of tools used in the actual operation are designed for the pearl culture industry operators. They are described below:
Oyster stand
The stand is used to hold the oyster in a stable position, so that the operator's hands are free to perform the surgery. It consists of two parts, the base and the clamp. The base consists of a wooden board, to which is screwed a metal square plate, 4.5 cm wide. A vertical tube of 15 cm in length and 1 cm inner diameter is welded to the basal plate. The tube has a collar at the top provided with a threaded hole for fixing a bolt to hold the shaft of the clamp tightly in position.
The clamp consists of two plates, the head-plate and a movable jaw. The head-plate is mounted on an adjustable tilting head supported by a shaft. The movable jaw is held against the head-plate by a spring. The front edges of the two plates form short, slightly curved lugs, which tend to follow the curve of the oyster shell and prevent lateral movement of the oyster. To the head-plate is fixed a curved rod, which passes through a hole in the shaft. A threaded hole and bolt are provided at this point to fix the rod in position. The plate assembly can be tilted from a vertical to a horizontal position according to the convenience of the operator. For fixing the oyster, the movable jaw is opened by applying finger pressure at the bottom of the plate and after the oyster is placed in position the pressure is released. The jaw holds the oyster firmly against the head-plate. The head-plate has a breadth of 5.3 cm and a height of 7.0 cm. The movable jaw is 5.5 cm broad and 8.5 cm high. The shaft is 11.5 cm long and its diameter is slightly less than 1 cm. The components are individually made to size and shape and are heat treated to sufficient hardness. They are then assembled to form the oyster stand.
Shell speculum
The shell speculum is used to keep the oyster open for the duration of the operation. The instrument is 14.5 cm long and consists of two components, which are made by forging from round bar stock to proper size and shape. Each component has a long straight portion and an arc. The two arms are fitted together by a male-female joint at about 5 cm from the tip. The top of the straight portion is flat and rectangular with rounded corners. The spring between the two arcs keeps the instrument in a closed position normally. A metal collar, which is provided around the straight arms, helps regulate the distance between the flat ends as desired. A maximum opening of 1.5 cm is obtained between the flat ends with the collar pushed to the bottom. This is about the distance between the two valves of the operable size oyster when the adductor muscle is in a fully relaxed condition under narcotisation. When the oyster partially opens its shells, the flat end of the speculum is inserted between the two valves. By gently closing the two arcs, the flat ends open and along with them also the shell valves. When the desired gap is obtained, the collar is slipped down to maintain the gap. The maximum possible opening between the shell valves differs from oyster to oyster.
Retractor
It is a slender, flat rod 15 cm in length, provided with a sharp bent hook at the tapered end. The retractor is used to hold the foot of the oyster in a stretched position during the operation.
Lancet-cum-graft lifting needles
There are three such needles. Each needle consists of an elongated spindle-shaped aluminium handle in the middle (6.5 cm long), with a lancet and a graft lifter, each 5.5 cm long, at the two ends. The lancet is a thin (2 mm) stainless steel tapered shank with its tip slightly curved and flattened to form an elliptical blade. The edge of the blade is rendered smooth and sharp. The graft lifter is similar to the lancet, but the tip is provided with a sharp, pointed spur. The lancet is used to make a sharp incision at the base of the oyster foot and to cut a channel through the tissues of the gonad up to the site chosen for nucleus implantation. The spurred tip of the needle is used to pick out the small graft tissue from the wooden block and to insert it into the site of implantation through the channel. The sizes of the cutting blade of the lancet and the spur are a graded series according to the size of the graft tissue to be lifted. The lancets and graft lifters are made to the desired shape and size by hand forging and finished by filing and grinding with abrasive wheels. They are polished to the required extent and fitted to the handle.
Nucleus-lifting needles
These are similar in construction to the needles described above, but are provided with hemispherical cups at both ends of the shanks. There are three such needles, each with two cups at the ends. The cups are of different dimensions to enable lifting of nuclei (spherical shell beads) of 2–8 mm diameter range. The cup shoe is initially drawn by hand forging and finished to dimensions by pressing with iron balls of proper size in the cold condition. Then the hemispherical cup is cut to the required size of slightly less than the diameter of the sphere and imparted a vacuum finish. The cup is moistened by dipping in seawater and made to touch the dry surface of the nucleus which immediately adheres to it. The cup end is inserted into the channel through the incision cut on the body of the oyster and the nucleus is placed in contact with the tissue graft. While withdrawing the needle, the nucleus is made to drop from the cup by a slight turn of the needle.
10.2 Nucleus Spherical shell beads are used as nuclei to produce round pearls. These beads are prepared out of thick shells of other molluscs, usually freshwater mussels. These shells are imported into Japan from USA, where the freshwater mussels, popularly called pig-toe shell (Tritogonia), three-ridge shell (Pleurobema) and washboard shell (Megalonais), themselves known to produce pearls, occur in the tributaries of the Tennessee River. Alternatively, locally available thick molluscan shells which have a composition akin to pearl oyster, as for example the shell of the chank Xancus pyrum, can be used after studying the specific gravity of the material and other characteristics. Molluscan shell material is preferred due to phylogenic affinity, chemical composition, binding strength and heat resistance properties which are similar to those of the nacre. The shells are processed into spherical beads of different diameters, generally 2–7 mm for Pinctada fucata, through cutting, grinding, shaping
and polishing using appropriate machines and tools. Dimensional accuracy, smooth finish and high polish are important factors. The beads should be cleaned and dried before use.
10.3 Selection of oysters The factors to be considered in the selection of oysters are their weight, reproductive stage of development and overall health. A weight of 25 g or more is the ideal size for implantation, while 20 g oysters can be considered for implantation of smaller size nuclei, i.e. 2–3 mm in diameter. Fully matured oysters are not suitable, since during surgery the gametes tend to flow out and block the visibility of the implantation site so that proper orientation of the mantle piece and nucleus cannot be ensured. Therefore, oysters in the immediate post-spawning recovery phase or those in the early phase of gametogenesis should only be selected. This factor in turn decides the annual surgery period. In addition, the oyster should not suffer from polychaete blisters, sponge borings and trematode infection. The selected oysters should be cleaned and all the fouling organisms carefully removed.
10.4 Graft tissue preparation The donor oysters do not have to be subjected to any conditioning process, which the recipient oysters have to undergo. Small pieces of the pallial mantle taken from the donor oysters are used as grafts for implantation (Fig. 6).
The donor oyster is cut open as follows:
· Hold the oyster dorsal side down and the posterior end facing the technician. · Insert the curved end of oyster knife between the two valves at the posterior side. · Push the knife straight through the oyster until the knife tip reaches the anterior end. · Press the knife straight downwards to cut through the adductor muscle. · Separate the two valves by tearing the hinge, without disturbing the two mantle lobes.
If disturbed, the lobes will shrink and cannot be used.
The steps involved in the removal of the mantle (marginal and pallial region) are given below:
· Deal with one valve, with the adhering body tissue, at a time. · Expose the mantle lobes by gently brushing aside the gills with the tip of spatula. Care
should be taken for the mantle not to shrink. · With the aid of the knife, starting from the posterior margin, cut the mantle tracing a
curve up to the anterior margin (Fig. 6 A). · Using the forceps, lift the mantle gently and place the tissue on a soft, clean, moist
wooden block without changing the side. At this stage the inner epithelium of the mantle is facing the technician.
Further steps in the preparation of graft tissues are as follows:
· Gently stretch the tissue end to end. · With a wet sponge, gently wipe out all the mucus and dirt. · Use the graft knife to cut away the marginal mantle (this mantle is characterized by
numerous folds and it is highly pigmented).
· In the same manner cut away the inner muscular portion of the mantle on the opposite side.
· Again wipe the mucus and dirt with a wet sponge. · Holding one end, lift the mantle ribbon, reverse the side (top to bottom) and place it
on the block. Now the outer epithelium is facing the technician. · Wipe the mucus and dirt softly without causing damage to the outer epithelial layer. · Trim the margins on either side until a mantle ribbon of about 3 mm width is obtained
(Fig 6 B and C). · Remove all the dirt and mucus from the wooden block. If necessary, transfer the
mantle ribbon to a new block without changing sides. · Use the graft knife to cut the ribbon into small pieces (2–3 mm). The size of each
piece has to be in proportion to the size of the nucleus (Fig 6 D). · With the use of a soft brush smear the mantle pieces with a highly diluted solution of
water-soluble eosin. · Keep the tissues moist until they are used within about 30 minutes.
Precautions
· Care should be taken to ensure that the knife does not injure the palm of the technician while inserting it between the valves of the oyster.
· Use only clean, sterilised, filtered seawater throughout the operation. · All instruments must be washed and sun dried before their use. · Sponges must be clean and moist. Different sections of the sponges must be used for
each wiping operation. · Wooden blocks must be clean, smooth and moist all the time. · Shrunken mantles should not be used due to difficulty in handling them.
10.5 Conditioning for surgery Natural conditioning is ideal and inexpensive, however it can be practised only in regions where there is stratification of seawater temperature, as well as sharp difference in food availability. This method works well in the sea conditions in Japan. Using the thermal differences, the oysters are spawned in the upper water layers which have a relatively high temperature. With the loss of the stored energy, due to spawning, the weakened oysters are further subjected to starvation by placing them at depths where phytoplankton production is low, in order to reduce their metabolic rate. Such conditioned oysters can be readily used in surgery.
Where such techniques do not work, such as in Indian waters, chemical conditioning is resorted to. Menthol crystals are sprinkled over the seawater in the tanks in which the oysters are placed. In about 60–90 minutes, the oysters become narcotised and relax their adductor muscles and open the valves. The time of response varies with the water temperature, however once narcotised the oysters become almost non-responsive to touch. The conditioned oysters are usually operated within the next 10 to 15 minutes as prolonged exposure causes swelling of tissues, copious secretion of mucus and mortality. A duration of about 30–40 minutes after response is the safe limit, and therefore the pearl oysters should be treated in batches.
FIGURE 6. Steps in graft tissue preparation. (A) mantle tissue when removed from an oyster (p.m.= pallial mantle and m.m.= marginal mantle); (B) trimming of the margins to remove marginal mantle and inner muscular tissue; (C) further trimming to obtain ribbons of pallial mantle; and (D) cutting of the ribbon into small sections.
10.6 Surgery The number of nuclei to be implanted in one oyster is usually decided before the operation. Single and double implantations are common while multiple implantations are usually carried out when large numbers of small pearls of about 2–3 mm are required. Large diameter nuclei, in the range of 6–7 mm, are generally used in single implantation, one nucleus in each oyster. Nuclei with diameters ranging from 4–6 mm are used for double implantation, a large and a
small one in each oyster. Nuclei with diameters of 2–3 mm are generally used in multiple implantations, five or more nuclei in one oyster.
The best site for nucleus implantation is the gonad, particularly in its ventral portion. Single implantation is always done at this site. In double implantation, the above site is used for the larger nucleus while a site in the dorsal region of the gonad, close to the hepato-pancreas, is used for the smaller nucleus.
The steps involved in pearl oyster surgery are as follows:
· Insert the end of the speculum through the posteroventral corner of the oyster and open it by sliding backward the gap-regulator ring (Plate VIII A). Attention should be paid not to open the oyster too much as the adductor muscle may snap and kill the oyster.
· Mount the oyster with the speculum on the clamp. The oyster should be placed correctly between the two plates of the clamp so that it does not slip. The speculum is now towards the left-hand side of the technician.
· Hook the tip of the foot with the needle with the left hand and gently pull it so that the base of foot is slightly elevated. Hold the needle in position until the operation is completed.
· With the oval knife end of the incision-cum-grafting needle in the right hand, make a sharp incision at the base of the foot. Through this opening, passing the needle below the outer skin, steadily and gently cut a passage through the gonad connective tissue up to the site of implantation. Gently withdraw the needle.
· Pick a piece of graft tissue, already on the block, with the tip of the needle (same needle as above but reversed) and gently insert it through the passage cut in the gonad. On reaching the site, gently deflect the needle and allow the graft tissue to drop. Withdraw the needle. Now the outer epithelium of the graft tissue is facing the passage (Plate VIII B).
· Lift a nucleus with the moistened cup end of the appropriate nucleus-implanting needle and gently insert the nucleus through the gonad (Plate VIII C and D). On reaching the site, deflect the needle gently so that the nucleus drops. At this stage the nucleus must be in contact with the outer epithelium of the mantle tissue which was grafted into the gonad by the preceding step. Withdraw the needle gently through the passage.
· Smoothen the incision with the cup end and let the two margins of the incision come in contact. The nucleus implantation operation is now over.
· Remove the oyster from the clamp, withdraw the speculum by slipping the gap-regulator ring forwards and place the oyster in clean seawater.
Precautions
· Before and after use, wash the instruments in clean seawater. · Adjust the pressure on the foot in a way that the foot does not get torn while pulling it
with the needle hook. · The incision should be a sharp cut and of the required length for the size of nucleus to
be inserted. If the incision is too large the nucleus may slip out. If there is too much tear of tissues, do not proceed. Place the oyster back in fresh seawater and return to the farm for future use.
· While cutting the passage through the gonad, if there is copious flow of gametes, do not proceed but return the oyster to the farm.
· Do not cause damage to the vital organs such as stomach, intestine and heart during the surgery.
· The orientation of graft tissue (outer epithelium) and nucleus should be correct. · Skill and patience are the key factors for a successful operation.
PLATE VIII. Implantation of a pearl oyster. (A) Opening of the oyster valves and (B) Insertion of the graft tissue.
PLATE VIII. Cont'd. Implantation of a pearl oyster. (C) Implantation of the nucleus, and (D) General view of oyster surgery.
CHAPTER XI PEARL FORMATION 11.1 Natural pearl formation The principal causative factor in pearl formation in a pearl oyster is the presence of a nucleus. It can be of organic or inorganic origin, such as parasites adults or larvae, molluscan eggs, decaying parts of plants, sand grains, epithelium or blood cells of the same animal, etc. These tiny particles or organisms enter the oyster when the shell valves are open for feeding and respiration. These foreign bodies may become embedded between the shell and mantle. In response to this stimulus, the foreign body is invaginated by the outer epithelium of the mantle and a pearl-sac is formed around it (Fig. 7 A).
Pearls are not produced without the formation of the pearl-sac. The pearl-sac is derived from the internal or external layer of the apithelium of the mantle or of the gill plates. The epithelial cells of the pearl-sac secrets the nacre which becomes deposited over the foreign body, forming a pearl in due course of time. These pearls are produced either within the mantle, in other soft tissues of the oyster, or between the mantle, and the interior surface of the shell. Such pearl production is accidental and occurs very rarely. They are generally small and irregular. Large and spherical pearls are still rarer to find. When the extraneous matter becomes fixed to the shell, only the exposed portion becomes covered by the pearl-sac resulting in a blister pearl.
11.2 Cultured pearl formation Cultured pearls are formed in a pearl oyster, thanks to human interference. In any pearl formation, two things are required, the outer epithelium of the mantle lobe and core substance or nucleus. It was found that cut pieces of the mantle epithelium would provide the pearl secreting cells and that processed shell beads would be accepted by the oyster as the foreign body. Through careful surgery, the mantle piece graft tissue and the shell bead nucleus are implanted together, side by side, into the gonad of the oyster.
The oysters are then returned to sea for further growth. The outer epithelial cells of the graft tissue proliferate and rearrange themselves over the shell bead nucleus, forming a pearl-sac. The inner epithelium and connective tissue of the mantle disintegrate and become absorbed by the surrounding tissue. The cells of the pearl-sac derive their nourishment from the surrounding tissues and soon reassume their function of nacre (mother-of-pearl) secretion which is deposited over the nucleus in the form of concentric micro-layers (Fig. 7 C). The nacreous matter consists of thin alternate layers of aragonite and conchiolin deposited around the nucleus. The conchiolin is organic in nature and consists of mucopolysaccarides. It forms the binding layer for the aragonite crystals. The aragonite layers are 0.29–0.60 mm thick and are made of calcium carbonate in the form of highly laminated crystals. In cultured pearls the nacre quality and the process of pearl formation are the same as in the formation of natural pearls. Cultured half-pearls (Fig. 7 B) are produced by affixing many nuclei on the inner surface of the shell valves. The outer epithelium of the mantle forms the pearl-sac on the free surface of the nucleus and the halfpearl is formed.
FIGURE 7. Process of pearl formation. (A) round and half-natural pearls; (B) half-cultured pearl; and (C) round cultured pearl with an artificially implanted nucleus.
CHAPTER XII POST-OPERATION CULTURE
12.1 Culture conditions Freshly operated oysters should be reared undisturbed for a few days. If kept in the laboratory, they should be placed in plastic troughs or FRP tanks, where seawater is allowed to flow gently. If no flow-through system is available, the seawater has to be changed frequently to overcome the narcotizing effect of menthol. When normalcy is resumed, the oysters slowly re-open their valves and commence their pumping and filtering activity. If the sea is not calm, it is desirable to rear the operated oysters under laboratory conditions till the wound heals completely. The normal duration of wound healing is only a day or two. However, if the surgery is rough or the incision is large, the nucleus could slip out of the oyster, if cultured in rough waters.
In Japan, newly operated oysters are hung in deep and calm waters for a period of 2–3 weeks, by which time they recuperate fully. Afterwards they are hung as in normal culture practices. However, in areas where the inshore waters are not calm, as in India, it is always advisable to keep the operated oysters for 3–4 days in the laboratory under observation and then transfer to the farm. If the oysters are suspended in rough sea immediately after the operation, they will be subjected to undue stress, which may lead to the dislodging of the nuclei.Some Japanese pearl culturists examine the pearl oysters individually after recuperation by fluoroscopy, in order to check the condition of the inserted nucleus. Only those with the nucleus in the proper position are further cultured for pearl production, while the rejected ones will be retained as mother oyster for future use.
The quality of the pearl to be formed is influenced by several hydro-biological factors, such as primary production, temperature, current, trace metals content, etc. Coastal waters deeper than 5 m are usually favourable for the formation of high quality pearls. However, the culture site and depth at which the hanging culture structures should be fixed must be carefully chosen to have the most favourable conditions for the culture of the pearl oysters.
During the post-operation rearing period, the oyster density in the culture cages and culture grounds should be kept at a minimum. Over-crowding may cause adverse effects such as production of low quality pearls, slow formation of the nacre layer, shell damage and physical stress, as well as oyster mortality from diseases and parasites.
The oysters must be suspended in areas of high phytoplankton production and also at greater depths than the mother oysters. Settlement of undesirable organisms and silt on the oysters is responsible for the formation of poor quality pearls, slow formation of the pearl layer and oyster mortality. Hence the oysters should be monitored periodically and fouling organisms thoroughly eliminated. In Japan, when the seawater temperature drops below 10 °C during winter, the rafts are towed to warmer water bodies to allow the pearl oysters to over-winter.
The length of the culture period following the operation phase depends on the size of the nuclei inserted and the desired size of the pearls to be obtained. The Culture period ranges between 3–24 months for 2–7 mm diameter nuclei under tropical conditions. Periodic sampling of each oyster batch will give the basis for deciding on when to harvest. Pearls with a thin nacreous layer usually will not have good lustre and iridescence, and therefore command a lower market price.
CHAPTER XIII PRODUCTION OF CULTURED PEARLS Cultured pearls are produced by pearl oysters as a result of delicate techniques developed by scientists over many years. However, the human role in the pearl production process stops with the successful completion of surgery for nucleus implantation and in providing suitable environmental conditions through careful selection of the culture sites. The actual pearl formation, which results due to an internal biological process, is left entirely to the pearl oyster itself. The quality of pearls with regard to their shape and colour, which are important factors in terms of their market value, are not only influenced by external environmental factors, but also by the inherited capabilities of individual oysters.
13.1 Development of implantation technique Free spherical cultured pearls are produced with the help of spherical shell beads, known as nuclei. These beads are manufactured from the shell of a freshwater mussel species found in the Tennessee and Mississippi Rivers in the U.S.A. These shells are collected and exported to Japan, where they are cut and processed into precisely spherical beads of different diameter.
In the initial phase of the development of the implantation technique, the shell bead nuclei were wrapped in mantle tissue and implanted into the visceral mass of the pearl oyster. Later it was found that a small piece of the mantle epithelium would suffice for the formation of the pearl-sac, which is responsible for the secretion and deposition of the mother of pearl on the nucleus itself. A small piece of mantle section measuring 2×3 mm in size, is sufficient for nuclei of 2–6 mm in diameter. The thickness of the epithelium layer plays an important part in the formation and quality of the pearls. The inner epithelium and the connective tissue rapidly disintegrate, leaving only the outer epithelium layer to proliferate and form the pearl-sac. Water soluble eosin and mercurochrome are some of the chemicals used as sterilising and colouring agents to enable the operator to place correctly the graft tissue, in terms of its orientation, at the time of insertion.
Oysters may die in the culture farm due to a variety of reasons. A common cause of death is serious infection of the wounds inflicted at the time of the implantation operation. However, diseases, biofouling, shell boring and pollution may also be responsible for oyster mortality. Proper farm management procedures usually keeps the oyster mortality rate below 10 %.
13.2 Nucleus retention and pearl production Nuclei ejection does not commonly occur, particularly in the case of large ones. However, this miss-happening can be kept to the minimum by improved surgical methods. Discharged nuclei are usually thrown away. However, in some cases they are retained for re-implantation as no nacre has been deposited over them, probably due to the non-formation of the pearl-sac. Defects in surgery and incorrect orientation of the nucleus and graft are two of the reasons for this defect. In cases of non-formation of pearls, the nucleus may either remain as it is or become eroded.
As in the case of natural pearls, the production of cultured pearls is also a result of a biological process within the pearl oyster itself. Therefore, the quality and the rate of cultured pearl production are only partly controlled by the activities of the pearl culturists. The production rate can be improved by adopting appropriate technology and care during surgery and subsequent culture. ‘Gross production’ usually refers to the number of cultured pearls produced by oysters which have survived the implementation of the shell bead. In India, the highest recorded production rate achieved in single implantation was about 65%, while in multiple implantation, a rate up to 180% has been obtained. This rate can definitely be improved by professional technicians with considerable knowledge and practical experience.
The gross production of pearls includes all kinds of pearls, from the finest quality to trash. Usually a small percentage of the pearls produced have an outstanding colouration and perfectly round shape, while a large proportion are inferior and some totally valueless as gems. This is a common feature in all pearl culture centres around the world. Generally the success of a pearl culture industry depends greatly on a high production of good quality pearls as a percentage of the total numbers produced.
Shirai (1970) has categorised the cultured pearls as follows:
Class A
Features: flawless, one flaw, small flaws, small stain marks, pink, silver or light cream in colouration. These pearls are further classified into:
A-1:
Top pearls-perfectly round, pink, flawless and lustrous. This class may also include pearls with small blemishes of the size of a pin point.
A-2:
First class pearls - with slightly larger pits and protuberances. When treated these pearls become indistinguishable from the pearls categorized as A-1.
Class B
Features: fairly large flaws, stain marks, creamy in colouration, and irregularities in the shape.
Class C
Features: wild shaped, badly coated, heavily marked, clayey lumps, half good and half bad. These pearls are usually referred to as ‘trash pearls’.
Generally, in a commercial pearl culture farm the combination of class A and class B pearls should account for at least 60 % of the gross production of cultured pearls for it to run economically. The remaining 40 % of class C pearls are usually rejected. However, their nuclei can be often salvaged and reprocessed.
13.3 Pearl harvesting Harvesting of the cultured pearls is usually carried out manually. However, the process can be automated with the use of simple machines. In the case of manual extraction the pearls are collected by initially separating the two shell valves, by cutting the adductor muscle, making
an incision on the gonad and squeezing the pearl out. The machines used for pearl extraction usually work by dissolving the oyster soft body parts with the use of chemicals while the pearls remain as they are and become easily extractable. In case the oysters need to be re-used for a second time, the pearls are carefully removed by opening the pearl-sac through the gonad taking care not to damage nor stress the oyster. The oysters are then returned to the culture site for recovery, and after a certain length of time they can be operated for a second time to produce additional pearls. In Japan, harvesting is done during periods of low temperature and pH as the pearls tend to be of higher quality due to a thick and compact outer nacreous layer.
Finally, the harvested pearls are washed in distilled water, polished with refined salt and sorted for sale according to size, colour, shape, lustre and other external characteristics.
CHAPTER XIV IMPROVEMENT OF PEARL QUALITY 14.1 Measures for enhancing pearl quality Quality commands a premium price in pearls. The success of the pearl culture industry depends on the high rate of production of quality pearls. Therefore, considerable attention is paid to this aspect. The value of a pearl is decided by its quality, size, shape, colour and lustre. Exceptional pearls command special premium price.Being a product of biological origin, individual variations are bound to occur in each and every pearl. The secretion of the mantle or the pearl-sac, which leads to the formation of the pearl, may be organic or inorganic in origin, or a combination of both, with unpredictable and subtle variations in structure and composition. The final product may range from the finest to the trash due to such variations.Even under the highest possible human control, the quality of cultured pearls cannot be controlled absolutely, but can be considerably improved through appropriate care in surgery and during farming. Only the size and shape of the cultured pearls are under the control of the pearl culturist. However, the colour and lustre, which depend on the secretion of the pearlsac, can also be improved to some extent by proper understanding of individual biological and physiological factors as well as the environmental conditions of the culture farms, which influence the formation of pearls.
To achieve a high rate of production of quality pearls, the following factors are required to be taken care of:
14.1.1 Oyster selection
Large oysters, in terms of size and weight should be selected. They must be free from a heavy fouling load and blisters caused by sponges and polychaetes. The oysters should be healthy as can be judged from the colour of the visceral mass and gills.
14.1.2 Narcotization of oyster
The amount of menthol required to narcotize, and the duration of narcotization should be carefully adjusted depending on the volume and weight of the oysters.
14.1.3 Graft tissue preparation
The graft tissue is one of the most critical factors in controlling the rate of pearl production. The donor oyster should be of the desirable size with a well developed and healthy mantle. Extreme care should be taken in selecting, stretching, cleaning, trimming and cutting of the donor mantle tissue. Good water quality and correct level of the chemical agents should be used in maintaining the tissue pieces.
14.1.4 Implantation
The nucleus implantation is one of the most important factors in cultured pearl production. Its success greatly depends on the selection of the correct site and skill of the technician. The
positioning and orientation of graft tissue in contact with nucleus is also critical and should be carried out with great skill and patience. Multiple nucleus implantation requires still greater care and patience.
14.1.5 Oyster convalescence
Oysters can be made to recover from the effect of narcotization through periodic changes of water or gentle flow-through. Sufficient time must be allowed for the incision wound to heal before taking the oysters to the sea for further farming.
14.1.6 Tool maintenance
The tools must be sharp, rust-free and should have been either sterilized or suitably cleaned and sun-dried.
14.2 Colour of pearls Different molluscs produce pearls of different colours. The colour of a pearl is usually similar to the colour of the shell nacre of the mollusc which produces it; this character is genetically controlled. This is very clearly shown by Pinctada margaritifera (black or steel grey), P. maxima (silvery white), abalones (green) and freshwater mussels (pink). However, in the case of P. fucata, the colour of the pearls produced may be golden yellow, pink, white or cream, depending on slight differences in the site of nuclei implantation. The pearls produced in the ventral region of the gonad are white or golden, while those produced in the dorsal region of the gonad, in proximity to the hepato-pancreas, are usually grey or white. Flawless pearls of regular form are frequently seen among the pearls developed in contact with internal organs, such as liver, byssal gland and intestine. Pearls produced close to the retractor muscle tend to be baroque in shape with irregular protrusions and with a distinct black colouration.
The thickness of the epithelium of the graft tissue is also considered to be responsible in determining the quality of pearls. A thin graft (2–10 µm) usually produces pearls with a good surface, while thicker ones (>20 µm) tend to produce dull and badly coloured pearls.
A number of environmental factors plays a predominant role in determining the colour and lustre of the pearl nacre. Water depth is one of the most important factors, as quality pearls tend to be produced in waters below 10 m. Fouling and boring problems and siltation are considerably less at depths of 10 m or more.
The pearl culture grounds also play a significant role in determining pearl quality, and repeated culture on the same ground has been shown to affect the quality of pearls. Organic substances discharged by the pearl oysters and fouling organisms are deposited on the sea bottom and their build-up eventually affects the chemical and physical state of the water. Periodic removal of these deposits increases the production of pearls with desirable quality. During the final “make-up culture” period, pearl oysters are shifted to places of potential quality pearl yielding grounds.
Temperature controls the metabolic rate of the molluscs. Higher temperature leads to faster growth in oysters and higher rate of nacre deposition. But this affects the quality of pearls. Thinner laminar nacreous layers, which result from low temperature and pH, are desirable at
least in the later phase of the culture period, since the thinner mineral laminae in the upper layers of the pearl give a better lustre to the pearls.
The physiological state of the pearl oyster and the condition of the culture ground have bearing on oyster growth and the size and colour of pearl. This depends principally on differences in chemical composition of the seawater, as well as the kind and amount of plankton in the area where the pearl oysters are reared. The chief source of conchiolin are the nitrogenous substances of the plankton, which influences the colour of pearls.
Minerals and trace elements in the seawater are important, as these also influence the colour of pearls. It has been found that the golden and cream coloured pearls contain more copper and silver, while skin coloured and pink pearls contain more sodium and zinc. The golden coloured pearls have been found to contain more metallic elements than green pearls. The pearl colour varies according to the amount of porphyrins and metalloporphyrins present in them. Iron-bound peptide in the nacre favours the formation of yellow pearls. The organic substances deposited at the beginning of the pearl formation also would influence the colour. Good quality blue pearls are of this origin.Short duration culture practice is another cause for inferior quality of pearls. Pearls should be allowed to reach maturity in proper time. Pearls with 0.5 mm nacre are accepted in the market.
The rate of nacre growth is dependent on the size of the nucleus. The pearl growth rate obtained at Tuticorin is as follows:
Nucleus diameter (mm) Thickness of nacre (mm) Duration of culture (days)
3 0.22 94–108
3 0.26 161
3 0.32 191
4 0.31 161
5.8 0.26 159
The rate of pearl growth in relation to the size of oysters, as obtained at Tuticorin, is as follows:
Oyster size (mm)
Nucleus diameter (mm)
Thickness of nacre (mm)
Duration of culture (days)
40–50 3 0.609 388
40–50 4 0.692 402
50–60 3 0.929 395
50–60 4 0.732 404
60–70 5 0.590 318
The quality of mantle of the donor oyster also influences the quality of the pearls. Utmost care should be taken in the selection of donor oysters and in the process of graft tissue preparation.
Processing of cultured pearls through bleaching and dyeing is a highly specialised technique for value addition. It is managed by the pearl processing technicians according to the market needs of various trading centres. It is believed that most of the cultured pearls in the market go through some kind of processing to remove minor defects and improve colour.
The structure and composition of pearls reveal that several formations are possible during the development of cultured pearls. Only those formed by aragonite crystals in tabular form, presenting a regular laminar brick wall-like structure with micro-layers of elemental mineral lamellae alternating with homogeneously deposited organic matrix in concentric layers around the inserted nucleus, would qualify as gems. The rest of the formations will have less or no commercial value.
The lustre of the pearl is due to the play of light on the laminated aragonite layers of the nacre due to absorption and reflection of the waves of incident light. Homogeneity, thinness and smoothness of these layers are responsible for the great play of lustre. Therefore, it is very important that the final phase of post-operation culture of every batch of oysters should be done under ideal conditions.
Cultured pearls are broadly classified into:
1. nacreous layer pearls 2. prismatic layer pearls, and 3. organic layer pearls
These are determined by the quality of secretion of the pearlsac. Nacreous layer pearls are composed of aragonite crystals of calcium carbonate and they alone are valued as gems. The prismatic layer pearls are formed by calcite crystals and the organic ones by proteinous layers of conchiolin.
Appendix References Publications and Documents, RAS/90/002
APPENDIX I TRAINING PROGRAMMES
The Central Marine Fisheries Research Institute is the nerve centre of pearl culture technology in India. In accordance with the policy of the Indian Council of Agricultural Research on transfer of technology, the above technologies developed by the Central Marine Fisheries Research Institute have been disseminated through training courses offered to candidates sponsored by different governments, universities and other agencies. The aim of the training programmes has been to extend the technical know-how to end-users.
Since fisheries development in India is the responsibility of the States/Union Territories, the target group for training consists of technical officers of fisheries departments of the maritime States and Union Territories. The scientists of the Fisheries Colleges of Agricultural Universities and other Research Institutes are also given training.
15.1 Training programmes in pearl culture
The technology of pearl culture was originally developed in Japan, which naturally captured the world market for cultured pearls. Japan has helped some other countries, like Australia, Myanmar, Indonesia, Malaysia, Philippines and Thailand, in developing pearl culture practices. The Central Marine Fisheries Research Institute, after developing the technology indigenously, has adopted an open policy of imparting the technology through training courses, not only to Indian citizens but also to foreign technicians who are sponsored by their respective Governments.
15.1.1 Long-term training course
A long-term training course of six months duration was conducted once in 1976–77 by the CMFRI for managerial and supervisory personnel. This was a comprehensive course which dealt with pearl oyster resources, oyster biology, mother oyster culture, pearl collection and farm management. Nine trainees from maritime states of India participated in that programme.
15.1.2 Short-term training course
The above long-term course has been replaced by a shortterm course of 4–6 weeks duration. Four such programmes were conducted between 1977 and 1986, in which 25 personnel were trained. The content was developed for technicians. The course is more popular among the interested organizations. The course curriculum is confined to mothe-oyster culture, pearl oyster surgery and pearl collection. Details of the curriculum are given below:
Introduction
Theory: Morphology and anatomy of pearl oyster; functions of mantle; pearl-sac formation; mechanism of production of cultured pearls.
Mother oyster culture
Practical: oyster raft culture; construction of rafts and holding baskets; pearl oyster collection and farming; farm maintenance; care of oysters.
Pearl oyster surgery
Practical: Handling of surgical instruments; selection and conditioning of oysters; graft tissue preparation; nucleus implantation; post-operation care of oysters.
Pearl collection
Practical: Bleaching; collection of pearls, cleaning of pearls; sorting of pearls.
Refresher training courses have also been conducted as and when requests were received.
Apart from the above nominees who underwent training in the long-term, short-term and refresher courses in pearl culture, a batch of 16 participants of a Summer Institute in Culture of Edible Molluscs, were also trained on the techniques of pearl culture and pearl oyster surgery.
15.1.3 Training programme in pearl oyster hatchery
This is a 4-week course. This is useful not only to those concerned with pearl culture, but also to the molluscan aquaculturists who wish to raise marine bivalve stocks in hatchery. The course curriculum includes infrastructure facilities needed for a hatchery, biology of pearl oyster, controlled maturation and spawning, microalgal food production, larval rearing, larval feeding, water quality management, disease control, spat settlement, spat collection and juvenile rearing.
The first training course was conducted during 1986.
15.1.4 Training for mariculture students
An intensive one-week training on the techniques of pearl culture and pearl oyster hatchery is given to each batch of students of M.Sc. and Ph.D. courses in mariculture under the post-graduate teaching programme in mariculture of CMFRI every year.
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PUBLICATIONS AND DOCUMENTS OF THE REGIONAL SEAFARMING DEVELOPMENT AND DEMONSTRATION PROJECT RAS/90/002 (RAS/86/024)
Working Papers RAS/86/024
NACA-SF/WP/87/1. Lovatelli, A. Status of scallop farming: A review of techniques. 22 pp.
NACA-SF/WP/88/2. Lovatelli, A. Status of oyster culture in selected Asian countries. 96 pp.
NACA-SF/WP/88/3. Lovatelli, A. and P.B. Bueno, (eds). Seminar report on the status of oyster culture in China, Indonesia, Malaysia, Philippines and Thailand. 55 pp.
NACA-SF/WP/88/4. Lovatelli, A. Status of mollusc culture in selected Asian countries. 75 pp.
NACA-SF/WP/88/5. Lovatelli, A. and P.B. Bueno, (eds). Seminar report on the status of seaweed culture in China, India, Indonesia, Rokorea, Malaysia, Philippines and Thailand. 79 pp.
NACA-SF/WP/88/6. Lovatelli, A. and P.B. Bueno, (eds). Seminar report on the status of finfish culture in China, DPRKorea, Indonesia, ROKorea, Malaysia and Singapore. 53 pp.
NACA-SF/WP/88/7. Lovatelli, A. Seafarming production statistics from China, Indonesia, ROKorea, Philippines, Singapore and Thailand. 37 pp.
NACA-SF/WP/88/8. Lovatelli, A. Site selection for mollusc culture. 25 pp.
NACA-SF/WP/88/9. Lovatelli, A. and P.B. Bueno, (eds). Seminar report on the status of finfish netcage culture in China, DPRKorea, Indonesia, ROKorea, Malaysia, Philippines, Singapore and Thailand. 56 pp.
NACA-SF/WP/88/10. Chong, K. C. Economic and social considerations for aquaculture site selection: an Asian perspective. 17 pp.
NACA-SF/WP/89/11. Chen J. X. and A. Lovatelli. Laminaria culture - Site selection criteria and guidelines. 30 pp.
NACA-SF/WP/89/12. Chen J. X. Gracilaria culture in China. 18 pp.
NACA-SF/WP/89/13. Seafarming Project, RAS/86/024. Site selection criteria for marine finfish netcage culture in Asia. 21 pp.
NACA-SF/WP/89/14. Lovatelli A. Seafarming production statistics from China, India, Indonesia, ROKorea, Philippines, Singapore and Thailand. 47 pp.
NACA-SF/WP/89/15. Chong K.C. and D. B. S. Sehara. Women in aquaculture research and training. 20 pp.
Working Papers RAS/90/002
SF/WP/90/1. Chen J. X. Brief introduction to mariculture of five selected species in China. 32 pp.
SF/WP/90/2. Lovatelli, A. (ed.). Selected papers on mollusc culture. 74 pp.
SF/WP/90/3. Lovatelli, A. Artificial propagation of bivalves: Techniques and methods. 56 pp.
Bibliography
NACA-SF/BIB/88/1. Selected bibliography on seafarming species and production systems. 20 pp.
NACA-SF/BIB/88/2. Selected bibliography on seafarming species and production systems. 52 pp.
NACA-SF/BIB/89/1. Selected bibliography on seafarming species and production systems. 49 pp.
Training Manuals
Manual on seaweed farming: Eucheuma spp. (Training manual No. 1). 25 pp.
Culture of the Pacific oyster (Crassostrea gigas) in the Republic of Korea. (Training manual No. 2). 64 pp.
Culture of the seabass (Lates calcarifer) in Thailand. Training manual No. 3. 90 pp.
Training manual on marine finfish netcage culture in Singapore. (Training manual No. 4). 275 pp.
Culture of Kelp (Laminaria japonica) in China. (Training manual No. 5). 204 pp.
Training manual on Gracilaria culture and seaweed processing in China. Training manual No. 6. 155 pp.
Training manual on artificial breeding of abalone (Haliotis discus hannai) in Korea DPR. Training manual No. 7. 124 pp.
Meeting Reports
Report of the First National Coordinators' Meeting of the Regional Seafarming Development and Demonstration Project, 27–30 October 1987, Bangkok, Thailand. 71 pp.
Report of the Second National Coordinators' Meeting of the Regional Seafarming Development and Demonstration Project, 20–23 September 1988, Singapore. 102 pp.
Report of the Third National Coordinators' Meeting of the Regional Seafarming Development and Demonstration Project, 24–27 August 1989, Qingdao, China. 103 pp.
Workshop Reports
Report of the FAO Asian Regional Workshop on Geographical Information Systems: Applications in Aquaculture, 5–23 December 1988, Bangkok, Thailand. FAO Fisheries Report No. 414, FIRI/R414. 13 pp.
Report of the Workshop and Study Tour on Mollusc Sanitation and Marketing, 15–28 September 1989, France. FAO/UNDP Regional Seafarming Development and Demonstration Project RAS/86/024. 212 pp.
Report on the Regional Workshop on the Culture and Utilization of Seaweeds, 27–31 August 1990, Cebu City, Philippines. Volume I. FAO/UNDP Regional Seafarming Development and Demonstration Project RAS/90/002. 187 pp.
General Reports
Progress report on the 1988 Regional Training/Demonstration Courses organized under the Regional Seafarming Development and Demonstration Project (RAS/86/024). 26 pp.
Report of the Seafarming Resources Atlas Mission. Regional Seafarming Project RAS/86/024, July 1989. 74 pp.
Audio-visual Materials
Culture of the Pacific Oyster (Crassostrea gigas) in the Republic of Korea. 71 slides.
Culture of the seabass (Lates calcarifer) in Thailand. 40 slides.
Marine finfish netcage culture in Singapore. 37 slides.
Culture of Kelp (Laminaria japonica) in China. 30 minutes video.
Recommended
