edible and pearl oysters

EDIBLE OYSTER CULTURE TECHNIQUES NIDHIYA KOTTAYI III SEM MSC. MARINE BIOLOGY CUSAT

INTRODUCTION The molluscs are an important group of shellfish and are exploited along the Indian coasts The term "oyster" has been applied to more than 400 different mollusks world-wide, including such non-food species as the pearl oyster. Edible varieties include some 70 species. Only a small percentage of these are grown commercially. The edible oyster, popularly known as ' Aali ' in Tamil and ' Muringa ' in Malayalam, sedentary bivalve

The flesh is encased by two shell valves, The lower valve is cemented to the substratum and the upper valve acts as a lid. The hinge mechanism connecting both valves, allows the valve to open or close The animal feeds by filtering the microscopic organisms in the water which pass through the gap between the two valves. The flesh of oyster is highly nutritious containing 8-10% protein and 2% fat , in addition to minerals like calcium, phosphorus, zinc and iodine . Edible oysters occur attached to hard substrata in the intertidal areas, backwaters, muddy bays, lagoons, and creeks along the east and west coasts of India

As early as in the first century B. C, the Romans were first to culture the edible oysters. Then Japan developed various methods for farming the edible oysters.

TAXONOMY Kingdom : Animalia Phylum : Mollusca Class : Bivalvia Order : Ostreoida Family : Ostreidae

RESOURCES AND DISTRIBUTION Six species of oysters found in Indian waters The Indian backwater oyster Crassostrea madrasensis The Chinese oyster Crassostrea rivularis The West coast oyster Crassostrea gryphoides The Indian rock oyster Saccostrea cucullata The Bombay oyster Saxostrea cucullata The Giant oyster Hyastissa hyotis

Edible oysters found in other regions of the world Pacific oyster,= Crassoatrea gigas Portuguese oyster= Crassostrea angulata European oyster= Ostrea edulis American oyster= Crassostrea virginica Australian Rock oyster= Crassostrea commercialis New Zealand oyster = Crassostrea glomerata Olympian oyster= Ostrea lurida

Crassostrea madrasensis Outer surface of shell valves has numerous foliaceous laminae with sharp edges The left valve is deep,and the right one slightly concave Hinge is narrow and elongated Outer shell surface is grey, green or light purple depending on the area of collection due to presence of detritus, algae etc

This species grows to a size of 212 mm, the larger ones occurring in estuarine systems. It is a typical euryhaline species Temperature range 22.8 – 34.5 °C Salinity 7.2 – 34.1 ppt ( Kripa , 1998 It is found in Orissa , Andhra pradesh ,Kerala, Karnataka, Andaman&Nicobar , Tamilnadu. Also in Pakistan, Sri Lanka & Bangladesh

Crassostrea gryphoides ( Scholotheim ) The shell valves are elongate and thick. Left valves cup- like Hinge area is well developed and has median groove with lateral elevations Roundish and broad in shape Whitish o r greyish externaly

The species grows to a size of 170 mm Occurs in the intertidal zone and down upto a depth of 7 m C. gryphoides occurs along North Karnataka, Goa and Maharasthra

Crassostrea rivularis (Gould) Large, roughly rounded, flat Thick shell cavity with a shallow shell The left valve is thick and slightly concave , the right one is slightly larger Adductor muscle scar is oblong and white The species grows to a size of 150 mm Inhabits the intertidal zone of coastal waters and creeks. Salinity – 15- 30 ppt It occurs in the creeks of Kutch, Aratada - Creek, Poshetra Point, Port Okha , Dwarka and Porbundar in Gujarat and at Mahim , Ratnagiri and Jaytapur in Maharshtra . Outside India it is distributed along Pakistan coast

Saccostrea cucullata ( Born) The shell valves are trigonal or pear shaped The left valve is deep or moderately deep The right valve is flat or slightly convex and covers the left one like a lid The hinge is straight , moderate in size and devoid of teeth The margins of both valves have well developed angular folds Colour of outer shell– pale white, grey , light or dark brown, green or purplish

It grows up to 20cm It is distributed in the marine environment in shallow coastal waters and creeks.( up to 5m depth) Salinity -- 10.4 – 34.1 ppt The species enjoys a wide distribution and is found in East Africa, India, Pakistan, north western Australia and Philippines In India it occurs at various places along both the east and west coasts and around Andaman and Lakshadweep Islands

Crassostrea virginica Colour is white or brown and is grows up to75mm Hinge line without teeth in the adult. Shell margins smooth. temperatures between 15.5 and 20°C,salinity is 10-28 ppt. Larvae will not settle and metamorphose into spat when salinity is less than 6 ppt (Wilson et al. 2005). C . virginica is native to the eastern seaboard and Gulf of Mexico coast of North America.

Crassostrea gigas It is externally rough and sharp with radial folds. It’s shape varies with the environment, where it attatched The two valves of the shell are slightly different in size and shape, the right valve being moderately concave. Shell colour is variable, usually pale white or off-white. Mature specimens can vary from 80 mm to 400 mm . The optimum salinity 20 to 35 ppt , can tolerate salinities as high as 38 ppt ; It can withstand a range from -1.8 to 35°C.

biology Edible oyster is sedentary animal usually elongate with highly variable shape. The animal is attached to the substratum by lower left valve which is cup-shaped Food of oyster mainly consists of diatoms and detritus. Generally the oysters are filter feeders. The oysters obtain food by collecting particles suspended in the water column. They are also known to absorb dissolved organic matter from the water through the surface of gills, palps and mantle.

Sexes are separate Hermaphrodites also occur Oysters spawn throughout the year with 2 peak periods in April-May and August-September. The peak period varies from place to place, depending on environmental factors. In Kerala Coast, the peak spawning occurs during postmonsoon months (September-December) At spawning, eggs and sperms are discharged directly outside into the water where fertilization and subsequent development takes place. The larvae are pelagic. When the larvae attain pediveliger stage, they begin to crawl with the help of foot and settle on suitable substratum. This process is called spatting or setting and the young oyster is called spat or seed oyster.

LIFECYCLE

Nutritive value of One Oyster 40.8 calories. Carbohydrate -10.2 grams fat calories -10.4 grams protein calories -20.2 grams Vitamin B12 (120 percent of your daily recommended value) Rich in three minerals: zinc, copper and selenium (33-55% of daily requirement) Complete protein (having all the amino acids in the proper proportion)

Edible oyster culture Oyster farming has a long history and the Chinese practiced oyster culture before the Christian era Bardach et al. (1972) stated that ―Marine aquaculture may well have begun with oysters, which were cultivated in Europe during Roman times In India , work on oyster culture was taken up during 1970 at the Tuticorin Research Centre of CMFRI, Tuticorin

CRITERIONS FOR SITE SELECTION Wave/wind action. . Information regarding wave and wind pattern of occurrence and intensity is usually useful to determine whether a site is suitable or not. Salinity. Oysters thrive best in brackish- and full strength seawater. Optimum range is about 17–26 ppt . Areas which are prone to flooding or surface run-offs should be avoided. Natural food supply. There should be an abundant supply of phytoplankton. Plankton sampling and analysis may serve as a good guide in determining the productivity of the area.

Availability of broodstock /seeds. The best area, as far as this criterion is concerned, is one which has a natural population of the species to be cultured. Transportation of broodstock or seedling is difficult and costly, especially if the distance from the source to the farming area is more . Pollution. It is important that the culture area is free from any form of pollution. Areas which are endangered by chemical, industrial or domestic effluents should be avoided. Oysters are filter-feeders and have the capacity to absorb and accumulate heavy metals (such as zinc, copper and mercury) and pathogenic organisms. Water depth. Water depth should be sufficient for the selected culture method(about 2-5 m depth) Post-harvest and marketing facilities Oysters are highly perishable, and since local demand is largely for fresh or raw oysters, marketing facilities such as roads, transportation, ice plant and cold storage should be present.

The technology of oyster farming consist of two important phases 1)collection /production of seed oysters 2)growing the seed oysters to marketable size. Seed collection from wild The seed required for culture is met either from natural spat collection or through hatchery system. For collection of spat from nature, suitable spat collectors or cultch materials are provided at appropriate time. The spat collectors should be able to retain the oysters till they reach marketable size or up to the size at which they could be scrapped for further rearing.

SPAT COLLECTION The seed requirement for culture of oyster is met either from natural spat collection or through hatchery rearing For collection of spat from natural grounds, suitable spat collectors or cultch materials are provided at appropriate time which may be oyster shells, coconut shells, asbestos sheets, mussel shells or other materials. The choice of spat collectors depends on the culture method adopted, materials availability, economical and practical considerations. The spat collectors should be rough, free from slime, without any secretion of resins and strong to retain the oysters. Lime coated tiles and oyster shell are suitable for large scale spat collection. The tiles are arranged in trays and are placed on racks, where as oyster shell are arranged on a nylon rope of 1.50m,length,as strings and placed on racks.

SPAT COLLECTORS

For efficient spat collection the farmer should know the: (a) spat setting season (b) the sites to collect sufficient spat for stocking in the grow-out ponds.

SEED PRODUCTION THROUGH HATCHERY SYSTEM Selection and holding of broodstock Oysters of length ranging from 60-90 mm are ideal and 30% of them should be 60-75 mm in order to have assured availability of males. 25 oysters are selected, cleaned and placed on synthetic twine on a PVC frame in a 100 I fiberglass tank and precooled seawater (at 20°C - 22°C) is filled in the tank and aerated. Mixed algae with cell concentration of 1.5-2 million cells/ml is given as food daily .

Induced spawing The conditioned oysters are induced to spawn by trasferring them to seawater with temperature of 34-35°C. This sudden change of water temperature stimulates spawning in oysters. Once spawning is completed, the oysters are removed from the tray. Larval rearing The fertilized eggs undergo cleavage within 45 minutes. At the end of 4 hours, the eggs attain morula stage and begin to swim and at 20 hours, 'D' shelled larval stage is attained. Isochrysis galbana is provided as larval food. The rearing density and feeding protocol for the various larval stages are as follows:

stage Rearing density (larvae /ml) Algal cells/ day D shaped 5 3000-4000 Umbo 3 4000-5000 Eyed stage 2 5000-8000 Pediveliger 2 10,000-12,000 On 18-20th day the larvae settle down as spat. The cultch material provided is oyster shell valves for attached spat or polythene sheet for production of cultchless or free seed.

Spat settling and seed production For cultchless seed, oyster shell grit or polyethylene sheets are provided as spat collectors. The oyster shells are cleaned, hole is drilled centrally in the shell valves and are provided in the larval rearing FRP tanks for spat settlement. Once the larvae attain eyed stage, the spat collectors are uniformly spread in the bottom of the tank and the released larvae settle on the shells.

GROW- OUT CULTURE The Grow out culture technique is broadly divided into 1) on-bottom In the on-bottom culture, the seed oysters are grown on the ground. This method is substrata-specific and the area should be free from silting and predators. 2)off-bottom culture Off-bottom methods, the advantages lie in better growth and good condition of the meat. The methods involved in off-bottom culture are (1) rack & tray, (2) rack & string, (3) stake and (4) raft (5)long –line method

On-bottom culture A firm and stable bottom is required Is either intertidal or subtidal area directly on hard substratum. In intertidal ,minimum 16hrs submergence is suggested to ensure quality. Oyster seed attatched to the collectors are planted on the bottom. Farmers collect spat on concrete pipes, later transferring the juveniles to grow-out pipes, cementing them in place. Spat density is 2000-10,000 per ha. The average production is 45 oysters per pipe, or 75,000 per 0.16 ha A market-size oyster is 7 cm and a few can be harvested between four and five months of age.

Advantages Less labour to maintain the culture Lower capital and maintenance expenses as no nursery or grow-out structures are required Less labour in moving shellstock to new areas because structures are not involved Generally, fewer permits are required for bottom culture than off-bottom culture Disadvantages Higher rate of siltation Less rapid growth and less meat yield Growth takes place only one side of the mother shell Higher rate of predation

Off- bottom culture It is the culture of oysters in some type of mesh container (basket, bag, cage, etc.) that is held above the seafloor. Oysters grown this way are typically hatchery-reared single set oysters instead of clumps of oysters normally found in the wild. The container provides protection from predators and eliminates burial in sediment. Suspending oysters in the water column improves growth rates due to improved water flow.

This systems take advantage of the availability of food (single cell algae called phytoplankton) throughout the water column. They have the following potential advantages over other production methods like bottom culture: 1. Promote faster growth; 2. Increase survival 3. Allow control of fouling (e.g., barnacles, overset oysters, mud worms) 4.Improve shell shape and appearance 5. Increase product consistency.

Off- bottom culture methods 1.Rack and string ( ren ) method 2. Rack and tray method 3. Stake culture 4. Raft culture 5. Long line culture

Rack and string ( ren ) method This method can be practiced in areas having 1-2.5 m depth. The oyster spats collected on shells are made into strings having six shells using 5-6 mm synthetic rope. A string can hold six shell valves having 80-100 spat & 3-4 string are enclosed in a velon screen bag. The bag suspended from racks for nursery rearing. After 2 months, the velon screen bags are removed, the strings are suspended from the racks in the farm. Rack-and-string method is mainly advocated for oyster culture in shallow esturies , bays and backwaters. A series of vertical poles are driven into the bottom in rows and horizontal bars are connected on the top of the poles.

A rack covering 80 sq.m . area holds 90 strings and 125 racks in a ha. Oyster shell rens made of 5-6 empty cleaned oyster shells string in 5 mm thick nylon rope and positioned 10 cm apart are suspended from the racks.

At the end of 7-10 months, each string may weigh 7 to 7.5 kg. The production of oyster is estimated at 80 ton/ ha. The mortality is about 45 per cent. The meat yield is about 10%. The farmers of Kollam region have adopted it. The CMFRI is maintaining rack and ren oyster farms for Research and development for two decades in Tuticorin Bay and since 1993 in the Ashtamudi Lake .

Rack and tray method The nursery reared single spat (cultch-free) measuring about 25 mm are transferred to trays of size 40 x 40 x 10 cm Density is 150 to 200 spat / tray. The tray is knitted with 2 mm synthetic twine of appropriate mesh size and is suspended from the rack. Once the oyster reaches 50 mm length they are segregated and transferred to rectangular tray of size 90 x 60 x 15 cm

These trays are placed on the rack which occupies 25 sq.m area and holds 150-200 oysters The average growth rate of oyster is 7 mm/ month At the end of 12 months, the oyster attains an average length of 85 mm. The production estimated is 120 ton/ ha/ year which when compared to string method is higher, however the production cost is quite high.

Stake method Stake method is adopted if the substratum of the culture site is soft and muddy. In this culture method, stakes with one nail on the top end and two nails on the sides are driven into the substratum . These nails hold the shells with spat. The stakes are placed 60 cm apart In this method, the nursery rearing of spat is carried on the same stake. Initially for 2 months, the spat is covered with velon screen till a size of 25-30 mm. They reach marketable size in10 months The production is estimated to be 20 ton/ ha/ year

Casurina or eucalyptus poles act as stakes and level support to the spat, set on shells. The stake culture is suitable for shallow waters. Studies on stake culture is of oysters were conducted at Tuticorin and hatchery raised spat , set on oyster cultch , were used. Nursery and grow out culture are carried on the same stake Each stake holds 3-4 oyster shells with attached spat The number of oysters on the shell cultch vary from 15-20/stake(density )

Raft method In this system oysters are suspended from floating rafts. Raft is the most suitable farm structure in sheltered bays where the depth is 5m and more Rafts are constructed using bamboo or wooden poles and are floated with empty oil drums or wooden barrels. Once raft is positioned by anchors, shell strings with attached oyster spat are hung from the raft for further growth. An alternative to the raft is the long line method whereby a series of small floats are joined by synthetic ropes and the line is anchored at both ends. Spat set only on the under sides of the tiles& count varies from 200-1,600 per collector.

The trays or strings are suspended from the rope. The size of the raft varies however rafts of size 6m x 5 m are found to be quite suitable. PVC pipes instead of wooden poles and Styrofoam floats in place of barrels may be used. In India, raft culture method not been tried so far.

Raft culture

Long line culture In this system long ropes or cabels are anchored at each end and are supported at each end. Long lines of 50-100m length are easy to manage. Double long line comprising one line on either side of the floats are also used. Oyster strings are hung from long lines. Long lines withstand rough sea conditions better than rafts Spat set from about 100 no.per line.

Long line method

Oyster culture in other countries CHINA Bivalve culture in China has a long history . Currently it occupies an important position in modern mariculture . Several species of edible oysters are found in Chinese coastal waters, however only Crassostrea plicatula and C. rivularis are cultured on a large commercial scale. Oyster production data are available from 1983 During 1997, China produced 23,28,568 t of oysters, ranked first, and far ahead of other countries.

CULTURE METHOD Bottom method The oyster C. rivularis is cultured mainly on the bottom of estuaries of low salinity. A number of cultches are typically used such as gravel, cement plates, cement bars and oyster shells. As many as 30,000–38,000 cement bars and 1,00,000– 1,40,000 cement plates are laid down per hectare. The growth period ranges between 7–12 months and yields vary from 30–80 t/ha.

Stake method . In the northern and eastern parts of Fujian, the bamboo stake is the typical culture method adopted . The coastal areas in these regions tend to be soft and muddy. Stakes 1.2 m long and 1.5 cm in diameter are laid down in bundles (4–5 stakes/bundle) in mid-tidal flats just before the spatfall peaks. The bundles are regularly spaced in rows. Usually 1,50,000– 1,80,000 stakes per/ha are used as collector. The spats settling in May reach marketable size in 11–15 months, and those in September, 16–18 months. Yields vary from 60–110 t/ha.

Stone-bridge method : This method is prefered for sandy muddy bottoms and is extensively practised in the southern part of Fujian. Bridges made of stone bars measuring 80×20×8 cm are used to collect spat on mid-tidal flats in May and June . Usually an average of 15,000 stone bars are laid per hectare Growth period ranges from 7–12 months and yields vary from 30–80 t/ha.

Raft method: The raft method has been recently introduced, especially in South China, where potential damage by typhoons has limited oyster culture to the traditional bottom method. Raft culture is becoming increasingly important, mainly due to the fact that higher yields in a shorter period are obtained when compared to bottom culture. An 84 m 2 raft produces the same yield in 2 years as 667 m 2 of bottom culture does in 4 years . In addition, well constructed rafts tend to withstand heavy action of waves and wind.

JAPAN At least 8 species are involved But by far most important is the Pacific oyster ( Crassostrea gigas ) Methods of culture Raft culture Stake culture Tetsuao SEKI (Japan Fisheries and Technology Association) reported a production of 662,000 tonnes in 2010. (FAP 2010 Yearbook B-53).

PHILIPPINES There are four species of oysters cultivated: Crassostrea iredalei , Crassostrea cucullata , Crassostrea malabonensis and Crassostrea palmipes . Of these four species , C. iredalei is the most commercially desirable because it grows at a faster rate to a larger size and has straight shell margins which make them easier to open Methods of culture Stake method Hanging method – the cultches installed in the oyster plots consists of oyster shells strung in polyethylene ropes of a certain length depending upon the height of the water column where they are to be installed. Holes are punched at the centre of the oyster shells by means of a nail. The holes are just big enough for the polyethylene rope to pass through

Other methods Broadcast method Lattice method Hanging method

Republic of Korea Crassostrea nippona , C. echinata , Ostrea denselamellosa and O. revularis are found along the coast of South Korea Pacific oyster ( Crassostrea gigas ) is of commercial importance 60% contribute oyster among mollusc Methods of culture Bottom sowing culture Stake culture Hanging culture .

FRANCE Important species cultivated are Crassostrea gigas and Ostrea edulis Methods of culture : on-bottom culture, off bottom culture suspended culture .

Suspended culture (5%), is done by hanging oysters fixed on ropes or in baskets from special frames (tables) in the Mediterranean lagoons or on lines in the open sea

OYSTER CULTURE IN INDIA Current status A number of oyster species occurs in Indian waters . S. cucullata , a purely marine form, is predominantly found in shallow areas with rocky substratum. The native Indian oyster ( C. madrasensis ) occurs throughout the coast of India, whereas the other two Crassostrea species are restricted to the northwest coast regions and have not been reported so far anywhere on the east coast . C. madrasensis dominates the entire east coast and Kerala. It is also found in various localities of Karnataka State. C. gryphoides is mainly present along the Maharashtra coast and in several localities of Goa State.

C. rivularis occurs along the coast of Gujarat State and to a lesser extent along the coast of Maharashtra State. S. cucullata occurs all along the Indian coast, however only the settlements along Maharashtra and Gujarat coasts are large enough for exploitation. Oyster fishing grounds along the west coast include Dahanu creek, Jaytapur , Versova , Satpuri , Alibag , Boiser , Marve , Ratnagiri , Cuff Parade, Mahd , Utsali , Navapur , Aramra creek, Gagwa creek, Gomati creek and Azad island. Oyster culture in India is not as well developed as in other Indo-Pacific countries, however it has received considerable interest within the last decade or so.

The Central Marine Fisheries Research Institute (CMFRI) under the Indian Council of Agricultural Research (ICAR) is the main institution involved with bivalve culture. Through applied research aimed at developing suitable and low-cost culture methods and through training and demonstration courses they are trying to popularize oyster farming. Small oyster beds exist in Kerala and Karnataka, but little exploitation occurs. Along the east coast, exploitation concentrates in the backwaters of Orissa, in Gokulapalli (Andhra Pradesh) and in Courtallayar and Adyar estuaries and Tuticorin (Tamil Nadu).

Small scale bottom culture of oysters by transplanting the spat from the natural beds has been practised for some time along the west coast. Oyster culture at Tuticorin and some experimental culture carried out at Athankarai estuary , Bheemunipatnam backwaters, Andhra Pradesh , Mulki estuary , Goa and Cochin backwaters indicated good prospects for oyster farming along the Indian coasts. Apart from bottom culture techniques the rack method has been developed and successfully practised at Tuticorin . Each rack can accommodate 20 rectangular trays which have a holding capacity of 3,000–4,000 oysters .

A variety of techniques and cultch materials have been tested in India. Cultches tested were oyster shell rens , coconut shells, lime coated asbestos sheets, mussel shells, Velon screen and polythene liner sheets, PVC tubes and lime coated tiles. Among the above coated materials, tiles are preferred due to the high setting rate, although more expensive than oyster and coconut shells. A new dimension has been given to oyster culture by the successful production of seed of the C. madrasensis at the Tuticorin Research Centre hatchery. Conditioning is carried out by supplementing the diet such as Chaetoceros affinis , Skeletonema costatum , Thalassiosira subtilis , Isochrysis galbana and Chlorella salina . Spawning has been accomplished by thermal stimulation of the conditioned oysters.

OYSTER CULTURE IN KERALA The Kerala state government promotes oyster farming among the coastal fishers by providing financial assistance and subsidies. The brackishwater Fish Farmers Development Agency (BFFDA) provides Rs.1,500 per a unit of 500 rens . The profit from a farm ranges Rs.2,500 to 25,000 depending mainly up on the density of spat fall on the rens .(Marine fisheries information service, CMFRI The oysters are heat shucked and the meat is purchased by the Aquaculture Development Agency in Kerala and marketed. The participation of women is high in several farming and postharvest activities.

GROWTH Growth of oyster is generally measured by volume, length or weight. Oyster growers measure volume by the number of oysters required to fill a standard box. Length of oyster is measured usually with a caliper The growth of oysters varies from place to place and depends on food availability and environmental conditions, particularly salinity and temperature

HARVESTING Oysters reach harvestable size (above 80 mm) within 10-12 months. They are harvested when the condition of meat reaches high value. Harvesting is done manually

DEPURATION During depuration the shellfish are placed in cleaning tanks under a flow of filtered seawater. In this system 10-20% of the seawater is continuously replaced in the tanks and the oysters are held for 24 hours. As a result the bacterial load of the shellfish is reduced. During depuration at the end of 12 hours the water in the tank is drained and the oysters are cleaned by hosing a strong jet of water to remove the accumulated faeces. Again the tanks are filled with filtered seawater and the flow is maintained for another 12 hours. At the end of this period, the tanks are drained again and flushed by hosing water. Finally the oysters are held for one hour in 3 ppm chlorine seawater. Then the oysters are washed once more in filtered seawater and kept ready for marketing.

Fig . Depuration centre

TRANSPORT AND STORAGE OF LIVE OYSTER Oysters can survive out of water for several days,if carefully handled and kept moist and cool. However, it is desirable that the oysters reach the consumer market within three days of harvest, if they are to be in prime condition. Experiments indicate that live marketable size oysters packed in wet gunny bags can safely be transported for 25 - 30 hours without mortality and in good condition. Small holding tanks having filtered seawater or artificial seawater and provided with adequate aeration, would keep the oysters alive for a few days at the whole salers premises

Shucked oysters require cooling and safe storage temperature to arrest bacterial and enzymatic decomposition. Shelf life is influenced by storage temperature as shown below: TEMPERATURE(⁰C) SHELFLIFE(DAYS) 12 3-5 8 7-8 1 16

SHUCKING Shucking is the process of removal of meat from depurated oysters . The oysters, after depuration are kept in boiling water for 2-3 minutes and transferred to shucking table. Using a stainless shucking knife between the valves, the valve is removed and meat is flipped into the container by cutting the base of adductor muscle. The meat thus collected is processed in salt and citric acid solution and packed-in 2 kg slabs for freezing. After thawing the frozen meat, the meat is canned and stored

DISEASES AND PARASITES In oyster farming system, pests, parasites and predators cause considerable loss of stock. ' Dermo ' a dreaded disease caused by a fungus Perkinsus marinus inflicted heavy mortality among East American oysters. Gill disease and shell disease attributed to pathogenic fungus, caused heavy mortalities among Japanese and European oysters. A disease caused by the haplosporidian parasite Minchinia nelsoni (formerly known as MSX) was responsible for heavy mortality in American oysters. Whereas predation by this gastropod occurred in the oysters cutlvated either by rack and string or stake method.

PREDATORS AND FOULERS Crabs, fishes, starfishes, polychaetes and gastropods are the predators that affect the oyster population Predation is generally size specific and young oysters are more vulnerable than grown-up oysters. However, fouling organisms such as barnacles, ascidians, other molluscs and algae are considered mainly a nuisance. Sponges , annelids, and bivalves are known to cause shell damage to oysters

FOULING IN OYSTER TRAYS BIOLOGICAL FOULING CONTROL; SEA URCHIN

CONTROL OF FOULERS Physical pressure pumps and special nozzles, and air drying in the sun. Chemical Exposure for 1-2 hr to freshwater , to copper sulphate (1-2%) solution and giving brine bath, are some of the chemical methods Biological Green sea urchins are used to remove foulers

Oyster Utalisation Shells The empty oyster shells contain 52-55% calcium oxide and are used in the manufacture of calcium carbide, lime and cement. The shells crushed to suitable size are used as poultry grit

AS FOOD…

EDIBLE OYSTER PRODUCTION IN INDIA IN 10 YEARS( Crassostrea madrasensis ) YEAR PRODUCTION (TON) 2003 800 2004 800 2005 900 2006 1500 2007 2150 2008 2400 2009 1450 2010 3200 2011 4058 2012 4202 SOURCE; FAO

PROBLEMS AND PROSPECTS OF OYSTER FARMING Sudden changes in hydrological parameters, mainly salinity cause much damage to the oyster population At a 50% drop from the ambient salinity level feeding is affected first followed by slow degeneration of tissues and cessation of the gametogenesis . Further drop in salinity totally prevents the oyster from feeding and eventually it dies.

Similarly higher saline condition of above 40 ppt affects the feeding of the oysters Pollution plays an important role in oyster beds Considerable quantity of industrial waste waters, agricultural run-off, dust from thermal power stations and oil enters the sea Oysters, particularly accumulate certain heavy metals and cause health hazards to those who consume them. Hence assessment of the level of pollution has to be made before starting culture work

FUTURE RESEARCH PRIORITIES In the culture of normal diploid oysters, time of harvest depends on the spawning season. The oysters could be harvested only during prespawning periods to maximise return. The triplold oysters with enhanced meat weight and condition factor are harvestable, irrespective of the spawning seasons. For better yield, production of triplold oyster seed and culturing the triplolds has vast potential.

PEARL OYSTER CULTURE TECHNIQUES

Introduction Pearls are classified by gemologists as cultured stones. They are not “inorganic” as most stones are but “organic” Fewer than 20 species of the 8,000 mollusks produce pearls

The world’s main pearling grounds were depleted in the 18 th century due to constant demand By the beginning of the twentieth century most of the world’s pearl-producing mollusks were in immediate danger of extinction Pearls are made of NACRE A natural substance produced by mollusks that also costs the inside of the animal’s shell (also called “mother of pearl”) Nacre is made up mostly of calcium carbonate in the form of the mineral aragonite and held together by crystal layers called conchiolin

Economic importance Billion dollar retail industry Sold all over the world Price depends on rarity and quality $50 Pair of freshwater pearl earings to $100,000 strand of South Sea pearls.

Taxonomy Phylum : Mollusca Class : Bivalvia Order : Pterioida Family : Pteriidae Genus : Pinctada ( Roding-1798)

MAJOR PEARL OYSTER SPECIES 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 .

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 .

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 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 .

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 .

Pinctada chemnitzii Pinctada sugillata

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 .

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 colour

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. 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.

Biology Food and feeding habits 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.

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.

Lifecycle

Pearl oyster farming 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.

Environmental conditions 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. 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.

Bottom Gravelly bottoms are suitable for pearl oyster farming, while sandy or muddy bottoms should be avoided. 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.

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. Silt load Pearl oysters generally prefer clear waters as high turbidity levels will affect their filtration efficiency.

Water current 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.

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.

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. 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.

Attempts have been made to collect the natural spat of P.fucata from the costal waters of Veppalodai,Tuticorin and Vizhinjam Bay using materials such as coconut and pearl oyster shells,tiles,spit bamboo reapers and frilled nylon ropes( Nayar et al .,1978;Victor et al .,1987;Achary ,1983;Appukuttan,1987 ) Achary and Thomson (1993) designed a double layered hapa of 1 m 3 size and covered by 2 mm nylon mosquito net.The hapa containing 25 P.fucata brood stock, was suspended in the Vizhinjam Bay during the spawning season and 3125 pearl oyster spat were collected in August 1988.

Silver lip oyster spat collected in rope collector Spat pearl oyster attached to spat collector (Southgate and Lucas, 2008 )

Alagarswami (1991) rightly stated that “it has been experienced that spat collection ,as done in japan,cannot be succesful in india to raise a pearl oyster resource of commercial value” However, India has recently succeeded in producing pearl oyster seed under hatchery conditions, there by providing the industry with a more dependable source of oysters.

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 pearl oyster hatchery of the CMFRI at Tuticorin has facilities for oyster conditioning, maturation and spawning, as well as larval and spat rearing

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. 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. (from 28.5 °C to 35.0 °C) Spawning of pearl oysters can also be effected by chemical stimulation(hydrogen peroxide in combination either with normal seawater or alkaline seawater (pH 9.1) is used in inducing spawning.) Fertilization takes place externally in the water medium

larvae duration trochophore 10 hour Veliger /D larva 20 hour umbo 10-12 days Eyed stage 15 th day pediveliger 18 th day Plantigrade stage 20 th day spat 24 th day(300 um size) Alagarswami et al (1987) observed that larval density of 2 nos /ml gaves faster growth and highest spat settlement

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. The oyster spat attain an average size of 40–45 mm in 12 months.

Oyster spat on shell Oyster spat on micro clutch

Rearing methods 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

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. 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.

A) Culture raft constructed with teak poles; (B) A FRP styrofoam buoy; (C) A mild steel buoy, and (D) Oyster long-line culture system.

Long-line culture In the long-line culture method, spherical or cylindrical floats which are connected by horizontal synthetic rope or chain are used. 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.

On-shore tank culture Large concrete tanks constructed on the shore with the holding capacity ranging from 75 to 150 tons of seawater can also be used for rearing the oysters. The preferred depth of the water is 3 m to get the normal growth in oysters. Under this system, feeding the oysters/juveniles with cultures of phytoplankters is needed. A stocking density of 125nos/ sqm is maintained.

Stocked oysters are supplemented with mixed culture of Chaetoceros spp. Cell concentration in the rearing medium is maintained at 75,000 cells/l in the tank. Daily 25% water exchange is required. By this method an average growth of 50 mm in 6-7 months from a stocking size of 5.0 mm is achieved.

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.

Rearing containers 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. 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.

Juvenile rearing Juvenile pearl oysters are reared in netcages . 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. (C) A netcage for rearing oyster spat of 3–10 mm in size and (D) Rearing netcage covered with velon screen.

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.

Biofouling organisms Barnacles , Ascidians , Bryozoans, Molluscs . Sponges Boring organisms Sponges , Polychaetes , Molluscs Predator organi s ms Perches , Rays , Starfish, Octopus

Small tunicates that compete with oyster spat.

Control measures 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.

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.

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.

Different Pearls….

Pearl culture Pearl culture is an art concerned with biological production of one of the finest gems, the pearl, nature's perfection of beauty and splendour . It is produced by 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 The credit for the development of modern pearl culture goes to Japan. They developed and perfected the techniques of pearl culture in the marine pearl oyster in the early part of the present century, starting from the initial success achieved by late Mr. Kokichi Mikimoto in 1893.

The Japanese marine cultured pearl production has dropped in the recent years from 127 tonnes (all-time) in 1966 to 34 tonnes in 1973 which has been stabilised at that level in subsequent years. The annual freshwater cultured pearl production has been around 7 tonnes recently. Pearl culture has spread to other countries, in all cases with the Japanese help perhaps with the exception of China. Australia has a sizable production but the other countries in South-east Asia such as Burma, Philippines, Malaysia, Thailand and Indonesia have very modest production limited by the availability of pearl oyster resource.

The major species of pearl culture in Japan is Pinctada fucata from the sea and Hyriopsis schlegelii from the lake. In Australia and South-east Asian countries it is the silver-lip pearl oyster Pinctada maxima . The black-lip P. margariiifera forms a small component at some centres as also black-winged pearl oyster Pteria penguin . Round pearls are produced in P. fucata , while both round and half-pearls are produced in P. maxima . Pearls produced in the freshwater mussels are largely baroque.

PEARL CULTURE IN INDIA 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.

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 . 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 .

CULTURE SYSTEM Culture operations 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.

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.

Surgical instruments

Surgical instruments 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 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.

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.

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.

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.

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.

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.

PEARL FORMATION 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. Pearls are not produced without the formation of the pearl-sac.

The pearl-sac is derived from the internal or external layer of the epithelium 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.

Process of pearl formation. (A) round and half-natural pearls; (B) half-cultured pearl; and (C) round cultured pearl with an artificially implanted nucleus.

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. 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 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.

Selection and preparation of host Healthy and adult oysters,preferably those above 45 mm size,are selected for the production of cultured pearls. Oysters collected either from natural pearl beds,or from culture systems. Gonad of the oyster is the ideal site for the insertion of the foreign particle ( nucleus) to induce pearl formation. The inactive or resting phase of the oyster,just after spawning,is the ideal time for the insertion of nuclei. Since this phase is short in duration,maturation must be delayed by keeping the oysters under low temperature. If only ripe specimens are available,their maturation is accelerated and they are induced to spawn by keeping them in warm temperature. Later, these oysters are used for nuclear insertion.

2. Preparation of graft tissue Graft is a piece of mantle epithelium, introduced in to culture oyster before the surgical implantation of the nucleus. It is used to wrap the nucleus before implantation. Graft is obtained from a healthy donor oyster. The oyster is opened carefully and its mantle is removed. The dirt and mucus,caught on the mantle,are removed using the blunt edge of a scalpel. A mantle strip is then cut away. It is a gain cut in to 2x3 mm squares. The mantle strips are kept in sterilized and filtered sea water. It is better to use the graft mantle tissue within 10-15 minutes after preparation.

3. Preparation of nucleus Nucleus is the foreign particle introduced in to recepient oysters for stimulating them to produce pearls. Deposition and binding of nacre take place most satisfactorily around calcareous particles. There for, the nacreous layer of molluscan shells is most ideal for the preparation of nucleus. The shell is made in to small spherical beads of 2 to 3 mm diameter using machines. These beads may be polished slightly, but never very smoothly.

4. Implantation of the nucleus Implantation is the insertion of nucleus into an ideal site of a recepient or host oyster. Many host oysters are made ready for this operation. They are placed in a plastic basin,with the hinge ( dorsal ) portion of their shell directed downwards. Sea water is then poured slowly into the basin for fully immersing the oysters. A little amount of menthol powder may be sprinkled over the water. The shell valves of the oysters start to open in a few minutes. A wooden plug or key is then placed between the valves to keep them open. The conditioned oyster is mounted on a stand and nucleus is inserted. There are three methods of nuclear implantation,namely mantle cavity insertion,mantle tissue insertion and gonadial insertion.

Mantle cavity insertion Nucleus is inserted between the mantle and the shell. Mantle epithelium secretes nacreous substance over around the nucleus and a pearl develops attached to the inner surface of the shell. The pearl formed by this method is a hemispherical blister pearl.

b . Mantle tissue insertion A small incision is made in the mantle of the recipient oyster and the nucleus is inserted into that incision. The nucleus is kept in piece of graft mantle before insertion,in order to stimulate nacre secretion. The graft tissue is made into a bag like structure with the help of a cotton thread. The thread is removed after insertion and the incised region is applied with antibiotics to prevent microbial infection. Nearly round pearls are produced by this method.

c. Gonadial insertion Gonad is the most ideal site for nucleus implantation. The success rate and the secretion of pearly material are very high in this method. Further,superior quality pearls are produced by gonadial insertion. A sharp incision is carefully made on the gonad. The graft mantle tissue from the donor is then inserted into the gonad. This is followed by nuclear insrtion .

The nucleus is placed in contact with the outer epithelium of the graft mantle tissue. The oyster is then taken out of the stand and the plug is removed. The number of nuclei to be inserted depends upon the size of the nucleus. Two to five nuclei are inserted,when they are 3mm or less in diameter; only one nucleus is inserted,when it is larger than 3mm.

ANTIBIOTICS Antbiotics have been used as a means of improving hygeine during nuclei insertion Reduce post operative mortality Antibiotics can be applied to the instruments and to the oyster tissue during operation or the nuclei can be coated with an antibiotic Neomycin (10% oxytetracycline )

POST-OPERATION CULTURE 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.

The farming methods described for the mother oyster culture are followed for the nucleus implanted oysters All cages containing the implanted oysters are stitched with velon screen of 1.5 mm mesh at the bottom to collect the rejected nuclei The number of oyster per cage is reduced by about 50% ie 50 nos /cage so as to provide better growing conditions Good quality pearls are produced when oysters are cultured in 5-10 m depth Strong sunlight result in poor quality pearls

Indian waters due to favourable water temparature , nacre secretion is continuous and the pearl grows fast They reach marketable size in 3-4 months with 2-3 diameter nuclei ,and 15 -18 months with nuclei of 6-7 mm diameter(Alagarswami,1991) PEARL HARVEST: the oysters are brought to the shore ,opened individually to collect the pearls and are killed in the process

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

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: 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. 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. 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 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. 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. Tool maintenance The tools must be sharp, rust-free and should have been either sterilized or suitably cleaned and sun-dried .

Color 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).

Variation in pearls

Status of pearl culture The world production of cultured pearls was estimated to vary between 226.7 to 276.6 t valued between 992.8 and 1029.8 million USD in 1993 ( fassler ,1994) Japan is the largest producer and also the largest importer of raw pearls Japan continues to dominate the production of marine cultured pearl with china,french polynesia,indonesia and australia fast emerging as important pearl producers. Cultured pearl production at 127.46 t peaked in japan in 1966 and declined to a low level of 34.43 t in 1973. China is world leader in the production of fresh water pearls.

references Algarswami,K & Meiyappan,M.M .(1989) Prospects and problems of Management and Development of Marine molluscan resources (Other than cephalopod) in India,CMFRI,Cochin,India,pp 250-259 Angell ,C .L. (1986). The biology and culture of tropical oysters. International centre for aquatic resource and management, pp 8 – 46 Appukkuttan,K.K.,Velayudhan,T.S.,Kuriyakose,P.S.,Laxmilatha,P.,Kripa,V. & Narasimham,K.A .(1993) Farming experiments and Transfer technology of Bivalve culture along the south-west coast of India,CMFRI,Cochin,India,pp 24-26

references Muthiah,P.,Ramadoss,K .& Appukkuttan,K.K . (1998) Edible oyster hatchery and culture, Central Marine Fisheries Research Institute , Cochin, India,pp 129-133 Narasimham,K.A . (2005) Molluscan Fisheries Of India , pp 40-80, B. R. Publishing Corporation,New Delhi Pillay ,T.V.R. (1990) Aquaculture:Principles and practices , pp 470-488 ,Marston book services Ltd.,oxford Pillai,V.N . & Menon,N.G .(2000) The edible oyster culture,CMFRI,Cochin,pp 786-797 Thangavelu,R .(1991) Technology of oyster farming in India,pp 533-539,Oxford and IBH publishing company.PVT.LTD,New Delhi http://www.fao.org/docrep/field/003/ab716e/AB716E01.htm

Alagarswami , K. (1987) Pearl culture,pp 72-76,Bulletin 39, Central Marine Fisheries Research Institute , Cochin, India Alagarswami , K., Chellam ,A.A.,& Victor,A.C.C . (1989) Potential for development of pearl oyster, pp. 93-96,Bulletin 43,Central Marine Fisheries Research Institute,Cochin,India John Bardach , E., John Ryther , H.& William McLarney , O.(1972) Aquaculture the farming and husbandry of fresh water and marine organism, John Willey and Sons, inc.,London , pp.633-648 references

references Narasimham,K.A . (2005) Molluscan Fisheries Of India, pp 40-110, B. R. Publishing Corporation,Delhi Pillay,T.V.R . (1990) Aquaculture:Principles and practices, pp 470-488,Marston book services Ltd.,oxford Victor, A.C.C., Chellam,A ., Dharmaraj,S .& Velayudhan,T.S .(1995) Manual on pearl oyster seed production, farming and pearl culture, special publication number 63 ,Central Marine Fisheries Research Institute,Cochin,India . Pp,1-60.

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