Based on its composition, pearls can be categorised as nacreous and non-nacreous (Kennedy, 1998). Nacreous pearls (mostly aragonite) are produced by species with nacreous lining to the inner surfaces of their shell(s) (MOP) but non-nacreous pearls can be produced either from non-nacreous shells or from nacreous shells that may secrete pearls with less aragonite platelets. Commercially cultured pearls, however, are mostly nacreous. The following description of pearl and nacre formation is based on studies with nacreous shells or pearls.
Cultured pearl formation begins with the development of a pearl-sac that is formed from proliferation of saibo tissue (Scoones, 1996). This is the tissue responsible for nacre secretion. Along with the development of the pearl-sac, mineral deposition occurs and continues after the mantle heals or forms a sac. The process of pearl-sac formation and mineral deposition may take up to six months after the implantation in P. margaritifera (Haws, 2002) and the complete healing of the pearl-sac in P. martensi can be within two weeks only (Strack, 2006).
Scoones (1996) studied pearl-sac formation and mineral/nacre deposition in P. maxima in detail. He reported that development of the pearl-sac took approximately 23 days from the implantation and that the first secretions from the pearl-sac onto the nucleus were evident about 30 days after the implantation. Mineral deposition within the pearl-sac begins with the secretion of periostracum and is followed by ostracum for non-nacreous pearls.
In nacreous pearl formation however, the layers of periostracum and ostracum are covered with a hypostracum (nacre) layer. The pearl formation mechanism follows the layering structure of the shell but in reversed order (Strack, 2006). In the shell, the periostracum forms the outer surface while it is the innermost layer at the interface between the nucleus and the pearl layers, in a cultured pearl.
The periostracum is a thin layer that contains mainly conchiolin. The other layers: ostracum or prismatic layer and the hypostracum or nacreous layers are two polymorphous layers of calcium carbonate. These two calcareous layers are composed of calcite (in the prismatic layer) and aragonite (in the nacreous layer). The building structure of the prismatic layer is columnar while the nacreous layer is composed of layers in a brick-mortar arrangement where the bricks are aragonite platelets and the mortar is composed with organic matrix (Addadi & Weiner, 1997; Barthelat & Espinosa, 2007; Checa & Rodriguez-Navarro, 2005; Fougerouse et al., 2008; Gre´goire, 1957; Katti & Katti, 2006; Rousseau et al., 2004) (Fig. 1.6).
A study with Pinctada maxima, reported that the uniformity of the nacre structure may contribute to the saturation of pearl colour; the more regular the structure the more saturated the colour will be (Snow et al., 2004). However, most of the studies on pearl layers have mainly focused on shell nacre (MOP) as a parallel comparison to the pearl. There is a difference between the structure of nacre in pearls and that in shells: nacre is concentric in pearls while it is layered in MOP (Strack, 2006). Recent detailed studies on pearl structure are few.
Fig. 1.6. Schematic drawings of (a) a three dimensional view of brick-mortar arrangement of aragonite platelets and conchiolin as a coating matrix, and (b) a cross-section view of the brick mortar arrangement of nacre in a pearl (Taylor & Strack, 2008).
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Cultured pearls are divided in two types: bead nucleated and tissue nucleated pearls (also called non-nucleated pearls) (Scarratt et al., 2000). Principally, bead nucleated pearls are pearls generated from nuclei and mantle tissue while tissue nucleated pearls are generated from mantle tissue only.
Bead nucleated pearls consist of blisters (mabè or half pearls), flat or coin pearls (not common) and round pearls (Fiske & Shepherd, 2007; Kennedy, 1998), while tissue nucleated pearls include several types of freshwater cultured pearls and keshi. (source: Wholesale Pearl)
1.4.1 Cultured blisters or mabè Cultured blisters or mabè are types of bead nucleated pearls. They are produced by gluing rounded or hemi-spherical nuclei (or beads) onto the inner surfaces of oyster shells (Strack, 2006). The nuclei are placed in the most lustrous area (Haws, 2002) and attached to either one or both shell valves (George, 1967). Nuclei used for mabè pearl production are manufactured from shells, plastics and paraffin (Strack, 2006). The shapes of the nuclei usually depend on operator preferences but hemispherical nuclei are commonly used. (source: Wholesale Pearl)
(source: Wholesale Pearl)
Blisters can be produced from all molluscs with nacreous-linings to their shells but only a few of these have been commercially cultivated. For example, the abalone, Haliotis iris, has been developed extensively in New Zealand for commercial blister pearl production (Strack, 2006). Most other molluscs used for culturing blister pearls are bivalves especially from the family Pteriidae. For example, the winged-pearl oyster, Pteria penguin, and the related Pt. sterna are commonly used for commercial blister pearl production (Gervis & Sims, 1992; Ruiz-Rubio et al., 2006; Southgate, 2007). Pt. penguin is mainly found and cultivated in the Indo-Pacific and Pt. sterna is in the Central America (Shirai, 1994). These types of pearl oysters are used primarily for producing mabè pearls due to their ability to produce lustrous nacre but their limited ability to be used for cultured round pearl production (George, 1967; Ruiz-Rubio et al., 2006; Shirai, 1994; Yu et al., 2004). Some mabè pearls are also developed from other members of family Pteriidae that are used primarily for round pearl production such as Pinctada maxima and P. margaritifera (Strack, 2006), however, mabè cultivation from these species is usually conducted once their use for round pearl cultivation has ceased. Cristaria plicata is the common species for blister production in freshwater (Webster, 1994). (source: Wholesale Pearl)
1.4.2 Cultured round pearl
The second type of bead-nucleated pearls is the cultured round pearl which has greater value. Production of round pearls requires a round nucleus to be implanted with a piece of mantle (nacre secreting) tissue from a donor oyster into the gonad of a recipient oyster. This process is known as ‘pearl implantation’ or ‘grafting’ or ‘seeding’. The mantle used in this process is known as ‘saibo’ (from the Japanese meaning ‘tiny penis’). This method is commonly applied to pearl oysters and is now being applied to freshwater mussels (Fiske & Shepherd, 2007; Strack, 2006). (source: Wholesale Pearl)
184.108.40.206 Pre operation phase
The oysters used for cultured pearl production are usually one to two years old (Haws, 2002). Prior to nucleus implantation, oysters selected for implantation undergo a conditioning or weakening phase for up to one month. They are usually held under crowded conditions which cause nutritional and or physiological stress that reduces their metabolic rate (Taylor & Strack, 2008; Taylor, 1999). They may also be induced to spawn or resorb material within the gonad to provide space within the gonad for nucleus implantation.
The conditioning phase may continue until 24 hours before implantation. With Pinctada maxima, the oysters are sometimes put into tanks overnight after the weakening phase in the sea (Taylor, 1999). Before the implantation, the water level is then lowered until the oysters are fully exposed to air. In this condition, the oysters are forced to open their shells and they are pegged open with wedges. This procedure reduces the potential for injury to the mantle (Joseph Taylor, Atlas South Sea Pearls, pers. comm). (source: Wholesale Pearl)
220.127.116.11 Operation phase
In the operation phase, the pegged oysters are brought to the operator for nucleus implantation or are selected for saibo preparation (donors). This technician is skilled for pearl implantation (Haws, 1998; Tun, 1994). The donor oysters are selected on the basis of their nacre colour and lustre because these characters may contribute to the quality of the resulting pearls (Taylor, 2002). Saibo tissue is usually prepared from the central-ventral region of the mantle where the pronounced colour and lustre exist. Following excision, the mantle tissue is cleaned to remove mucus and is cut into small sections (approximately 3 x 3 mm2) on a chopping board. (source: Wholesale Pearl)
(source: Wholesale Pearl)
For the implantation procedure, the oyster prepared for the implantation is placed in a stand and a shell-opening tool is used to hold both valves open while the peg is pulled out. The shell opener is then turned to the posterior part of the shell to allow other operating tools to access the oyster tissue. After that, a spatula is used to move mantle and gills aside to expose the gonad. An incision is then made into the gonad near the foot, or even sometimes on the foot (Fig. 1.5). A nucleus of particular size (selected by the technician on the basis of his/her observations) is inserted into the gonad and is followed with a single piece of saibo (Fig. 1.5). (source: Wholesale Pearl)
The region of the mantle which secretes minerals (outer surface) is placed facing the nucleus. This procedure (Fig. 1.5) is known as pearl implantation or seeding or grafting. However, this is just one of several techniques in pearl implantation. Other technique can be started with saibo before nucleus insertion (Taylor & Strack, 2008). After the implantation, oysters are placed back into seawater for further culture. Pearl nuclei used for the implantation are traditionally manufactured from the shells of freshwater mussels belong to family Unionidae (Roberts and Rose, 1989; Sonkar, 2004; Strack, 2006; Ward, 1995; Webster, 1994). (source: Wholesale Pearl)
The number of nuclei implanted into a recipient oyster varies among species. The Japanese pearl oyster, Pinctada fucata can be seeded with multiple nuclei in one implantation period (Alagarswami, 1976), but only one nucleus is seeded into both Pinctada maxima and P. margaritifera per implantation (Gervis & Sims, 1992; Strack, 2006). However, all species can be reseeded after pearl harvest and healthy oysters that produce good quality pearls can be used for a second (and sometimes third) implantation. (source: Wholesale Pearl)
18.104.22.168 Post operation phase and culture condition
After the implantation, oysters are placed into various positions in the sea; based on farm preferences. Some farms place the oysters onto the seabed while others put them into various types of nets or baskets that are hung from a long-line or raft. Farms that place oysters on the seabed are mostly in south Pacific countries that are surrounded with shallow atoll-reef, and several places in northern Australia. Japanese farms usually use baskets to hold oysters which are hung from rafts (Strack, 2006). The rafts are commonly set up in sheltered areas with low wind and wave actions (Southgate, 2008). However, nowadays most farms put the oysters in panel nets, which are hung from long-lines. The hanging method is an improvement of the Japanese system and makes it easier to maintain the oysters (O’Sullivan & Cropp, 1994; Ryan & O’Sullivan, 2001). (source: Wholesale Pearl)
Culture time for round pearl production (time between nucleus implantation and pearl harvest) varies between species. The longer the culture time the thicker the nacre coating on the nucleus will be. Akoya pearls used to be cultured for more than four years but in the mid 1990s the cultured time was reduced to 6 months only (Strack, 2006; Ward, 1995). South Sea pearls are usually harvested between eighteen months to two years after the implantation (Fong et al., 2004; O’Sullivan & Cropp, 1994; Pouvreau & Prasil, 2001; Strack, 2006). Time for culturing freshwater pearls varies between three and five years (Fiske & Shepherd, 2007). Initially the culture time for freshwater pearls is divided into three steps: firstly, a coin bead and a piece of mantle tissue are inserted to the mantle of a recipient mussel for one year; secondly, the resulting pearl is harvested and the mussel is placed back into the water to grow a keshi for another one year; and finally, the keshi is harvested and replaced by a round bead which produces a round pearl after a further one to two years (Fiske & Shepherd, 2007). From this method the farmer may have three types of pearls within five years period: flat (coin pearls), keshi and round pearls. (source: Wholesale Pearl)
(source: Wholesale Pearl)
1.4.3 Non-nucleated cultured pearls
A type of non-nucleated cultured pearl is called the ‘keshi’. The term keshi originated from Japanese language to describe something very small (Strack, 2006). In the pearl industry this term was adopted for small unplanned pearls that result accidentally from attempts at nucleated cultured pearl production (George, 1967; Strack, 2006). In this case, the nucleus is expelled by the recipient oyster, which retains the mantle tissue only. However, this term is also sometimes used for small pearls produced naturally by molluscs. Another type of non- nucleated cultured pearl is commonly produced from freshwater mussels when a piece of mantle tissue is the main source to produce pearl. The tissue is usually inserted into the mantle of a recipient mussel and goes on to generate a pearl. This is the traditional method for cultured freshwater pearl production which has now been modified by producing round pearls from re-operating the same recipient with nuclei (Fiske & Shepherd, 2007; Ward, 1995). Using the former method, one freshwater mussel can produce up to 50 pearls in one implantation period (Strack, 2006). (source: Wholesale Pearl)
Article source: Mamangkey, Noldy (2009) Improving the quality of pearls from Pinctada maxima. PhD thesis, James Cook University. (source: Wholesale Pearl) For Questions and answer you can contact & chat with us on:
Natural pearls were first discovered accidentally when human searched for food along the coastline and in lakes and rivers in prehistoric time (Dakin, 1913; Kunz & Stevenson, 1908; Strack, 2006; Ward, 1995). Pearls subsequently became parts of the rituals associated with cultural and religious activities (Strack, 2006).
The shells of pearl bearing species or mother of pearl (MOP) has also been utilised for decoration throughout human history. MOP inlays were used around 4500 BC in Mesopotamia and around 4000 BC in Egypt (Strack, 2006; Ward, 1995). The use of pearls for decoration was assumed to have begun in the 5th Century BC during the Persian invasion (Kunz & Stevenson, 1908) where possibly the oldest necklace containing pearls was found in a sarcophagus at the Winter Palace of the Persian kings in Susa (Strack, 2006).
During the Roman Era pearls became the most valuable gems (Ward, 1995). The ‘Pearl Age’ began in the 16th century. Although there was a shift from pearls to diamonds in the 18th century, pearls regained their top position in the nineteenth century when new pearl oyster beds were discovered and cultured pearl production began (Strack, 2006).
Many believe that natural pearls are formed as a reaction to an irritant in the internal part of a mollusc (Kunz & Stevenson, 1908; Strack, 2006; Streeter, 1886; Ward, 1995). The irritant may be a trapped parasites, small particles, or mantle scratches due to friction or predator damage (Strack, 2006; Ward, 1995). However, pearls will not be formed without the existence of epithelial cells from the nacre secreting mantle tissue (Simkiss & Wada, 1980). Therefore, for a pearl to form the irritant (other than mantle epithelium) must be associated with some epithelial mantle (Strack, 2006). The epithelial cells begin to proliferate and form a ‘pearl-sac’ to cover the irritant (Taylor & Strack, 2008). The pearl-sac then begins to deposit minerals (nacre) as a kind of internal defence mechanism (Dakin, 1913; Kunz & Stevenson, 1908). Such deposition continues and the resulting pearl grows. The shape of the irritant is usually irregular and this irregularity causes pearls to grow asymmetrically in shape (baroque type). This type of pearl is common in natural pearls (Strack, 2006).
Other types of natural pearls may also be formed on the internal surface of the shell. They are called blisters (Taylor & Strack, 2008). The formation of natural blisters results from the reaction of the host to organisms that penetrate the shells or any material trapped between mantle and the shell (Kunz & Stevenson, 1908). The penetration is mainly caused by boring sponges, Cliona spp. (Fromont et al., 2005), boring polychaetes, Polydora spp. (Alagarswami and Chellam, 1976; Okoshi and Sato-Okoshi, 1996) and several lithopagan bivalves (Doroudi, 1994; Takemura and Okutani, 1956). Borers are usually categorised as pests in the cultured pearl industry because they may kill the oysters (Che et al., 1996; Humphrey, 2008; Humphrey & Norton, 2005; Jones, 2007). In response to shell penetration, the host begins to secrete nacreous material to cover the resulting damage or irritation on the inside of the shell. This process results eventually in the production of a blister (Taylor & Strack, 2008).
Natural pearls are very rare and occur in approximately one in a thousands oysters (Haws, 2002). However, the frequency with which natural pearl occurs varies according to species and the region in which they are found. In the nineteenth century one high-valued pearl could be found at a ratio of 500:1 in the Persian Gulf, 5000:1 in the Sulu Sea, 15.000:1 in French Polynesia, and 1.000.000:1 in the Gulf of Manaar, Ceylon (Strack, 2006). Obtaining such pearls is usually costly. Pearl divers are susceptible to accidents and shark attacks (Joyce & Addison, 1992; Kunz & Stevenson, 1908). Such conditions made pearls in 19th and 20th century among the most expensive gems which were restricted to the rich and to noblemen in particular (Dakin, 1913; Ward, 1995).
Before the early 20th Century, there were several places with large beds of oysters and mussels that supported a pearl fishery. In the marine environment, large pearl oyster beds stretched from Arabian waters to the Pacific and there were smaller, patchy distributions in Central America (Kunz & Stevenson, 1908). Pinctada radiata1 (synonym P. imbricata1) were abundant in the Persian Gulf and the Gulf of Manaar (Sri Lanka), P. fucata1(synonym P. martensii1) in Japanese waters, P. mazatlanica in Pacific Central America, P. margaritifera in the south Pacific and P. maxima in the tropical central Indo-Pacific region (Strack, 2006). Particular regions like the Persian Gulf, Gulf of Manaar and northern Australia were famous for their natural pearl fisheries.
In freshwater, smaller scale pearl fisheries were mainly distributed in the northern hemisphere with Margaritifera margaritifera being the main species. However, nowadays freshwater pearl mussel beds in Europe have been depleted and are being conserved (Bauer,1 There is some confusion over the taxonomic status of these taxa (Wada and Tëmkin, 2008). 1988; Beasley & Roberts, 1996; Cosgrove et al., 2000; Cosgrove & Hastie, 2001; Young, 1991).
1.3.2 Cultured pearls
Valuable pearls are usually produced from molluscs that have a nacreous lining (MOP) to the inner surface of their shells (Webster, 1994). These molluscs are selected for pearl production. For cultured marine pearls, the commonly cultured species are from the family Pteriidae: Pinctada maxima, P. margaritifera, P. fucata and Pteria penguin (Gervis & Sims, 1992; Southgate et al., 2008b). Marine pearls are cultivated mainly in the Indo-Pacific region; from the Red Sea to the Pacific Ocean (Bondad-Reantaso et al., 2007; Strack, 2006). Japan is famed for Akoya cultured pearls produced by Pinctada fucata; Indonesia and Australia lead ‘South Sea’ pearl production from P. maxima and the Pacific island countries produce ‘Tahitian’ cultured pearls from P. margaritifera (Southgate, 2007).
The first innovation towards cultured pearl production was introduced by the Chinese in the 5th Century who produced blister pearls in the shape of Buddha (Joyce & Addison, 1992). This was carried out using freshwater mussels. More than a thousand years later in Europe, Linneaus conducted experiments by creating a hole in the shells of the river mussels, Unio pictorum, into which he put a limestone nucleus attached to a wire in the shell (Strack, 2006). He then left the mussels in the water for five years, however, the resulting pearls were of very poor quality (Joyce & Addison, 1992). Several attempts to produce pearls were conducted by William Saville-Kent on Pinctada maxima in 1890, followed by Kokichi Mikimoto three years later on hemispherical pearls (George, 1996; Simkiss & Wada, 1980). In 1914, Mikimoto applied for a patent for producing round cultured pearls (George, 1967) and he received it two years later (Ward, 1995). This modern method of culturing round pearl production utilised a nucleus wrapped within a piece of mantle tissue, which was implanted into the gonad of a recipient oyster. This method was actually invented by Saville-Kent and adopted by Tatsuhei Mise and Tokichi Nishikawa, but Mikimoto claimed the patent (George, 1996; Matlins, 2002). The method was subsequently applied commercially in 1919 (Simkiss & Wada, 1980; Taylor & Strack, 2008).
Article source: Mamangkey, Noldy (2009) Improving the quality of pearls from Pinctada maxima. PhD thesis, James Cook University.
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1.2.1 Taxonomic position and distribution Pinctada maxima was recorded for the first time in northern Australian waters (Dakin, 1913). They are commonly called the silver-lipped or gold-lipped pearl oyster and produce the famous South Sea pearls (SSP)(O’Sullivan, 1992). It is the largest species in the genus Pinctada (Dakin, 1913; Hynd, 1955; Shirai, 1994; Xie, 1990) and it produces the largest and finest pearls (Kunz & Stevenson, 1908). Shell size may reach more than 30 centimetres and individuals may live for up to 40 years (Strack, 2006). Both Pinctada maxima (Jameson, 1901) and P. margaritifera (Linnaeus, 1758) are closely related species (He et al., 2005) and share the same position as the most primitive species within the genus Pinctada (Yu and Chu, 2006). P. maxima is distributed from the Indian Ocean to the Pacific and from the tropic of Cancer to the tropic of Capricorn (Wada & Tëmkin, 2008)(Fig. 1.1). They are found in depths of up to 90 metres although their optimal habitat is at about 35 metres (Strack, 2006).
Fig 1.1. Geographical distribution of Pinctada maxima (Wada & Tëmkin, 2008). The taxonomic position of Pinctada maxima is shown below:
Phyllum Mollusca >> Class Bivalvia >> Order Pterioida >> Family Pteriidae >> Genus Pinctada Röding, 1798 >> Species Pinctada maxima (Jameson, 1901)
1.2.2 Morphology and anatomy
The anatomy of Pinctada maxima described below relates to organs and structures that have importance to pearl production and therefore to the research conducted in this study. Like other members of genus Pinctada, P. maxima has nearly equivalve shells with less projecting posterior ‘wing’, compared to the genus Pteria, and concentric lines. Small projecting scales may also occur on the external surfaces of shells, particularly in young individuals (Lamprell & Healy, 1997). Colour bands of gold or silver occur in distal region of the nacreous part of the inner shell (Fig. 1.2).
Fig. 1.2. The inner surface of two valves of Pinctada maxima from different individuals representing gold lip pearl oyster (left) and silver-lip pearl oyster (right); arrows indicate lip colour.
Fig. 1.3. A pair of valves of Pinctada maxima showing shell morphology and orientation; ae, anterior ear (auricle); am, adductor muscle scar; bn, byssal notch; li, ligament; ms, pallial muscle scar; nb, nacre border; nl, nacre layer (mother of pearl=MOP); pl, prismatic layer, and um, umbo.
22.214.171.124. The shell
Like other bivalves, Pinctada maxima posses a pair of valves (Fig. 1.3). Both valves are attached with a ligament in the dorsal hinge region. There are no hinge teeth (Strack, 2006). The right valve is usually flatter than the left valve. Each shell valve is composed of three layers: (1) the outer layer is the periostracum or conchiolin layer; (2) the middle layer is the ostracum or prismatic layer; and (3) the inner layer is the hypostracum or nacre (mother of pearl) layer (Fougerouse et al., 2008). The periostracum may help reduce biofouling on the outer shell surface (De Nys & Ison, 2008; Guenther et al., 2006). Unlike the periostracum which is formed mainly from proteins, the prismatic and nacreous layers are composed of different forms of calcium carbonate. The prismatic layer is composed of calcite crystals, while the nacreous layer is built from aragonite. These structures are embedded within an organic matrix framework (Addadi et al., 2006; Bedouet et al., 2001; Checa and Rodriguez- Navarro, 2005; Matsushiro and Miyashita, 2004) composed mainly of protein (Matsushiro et al., 2003).
126.96.36.199 The mantle
The molluscan mantle (Fig. 1.4) has many functions. Besides protecting internal organs, the mantle has also roles in assimilation, respiration, locomotion and reproduction (Simkiss, 1988). In relation to shell formation, the mantle is responsible for producing ions and minerals used in the biomineralisation process (Blank et al., 2003). The bivalve mantle consists of two lobes of tissue that line the inner surfaces of both shell valves. As in bivalves, each mantle lobe in P. maxima can be divided into three zones: the marginal, pallial and central zones (Dix, 1973; Humphrey & Norton, 2005). The central zone covers the soft tissue, the pallial zone is composed primarily of muscular threads used in mantle retraction, while the outer marginal zone splits into three folds: the outer, middle and inner folds, each with specific roles (Fougerouse et al., 2008). Tissue from the pallial zone is used in the cultured pearl production process (Acosta-Salmón, 2004).
Mantle Gill , Adductor Muscle, Gonad, Auricle, Byssal gland Foot
Fig.1.4. Internal anatomy of Pinctada maxima (Jameson)
188.8.131.52 The gonad
The gonad (Fig. 1.4) has an important role in cultured pearl production as it is used as the organ that receives the nucleus and nacre secreting tissue implant (‘saibo’) required for pearl production (Taylor & Strack, 2008). The ripe gonad of male P. maxima is milky white but it is creamy yellow for females. When fully ripe the gonad may occupy one-third of the internal space of the oyster. However, pearl oysters with full gonads are not used for pearl production because space is required to house the nucleus and tissue implant. Because of this, the implantation for cultured pearl production takes place after the spawning period or following a conditioning period, which empties the gonad (see Section 184.108.40.206). Changes in water temperature are the main factor in inducing spawning of pearl oysters in nature (Behzadi et al., 1997; Hernandez-Olalde et al., 2007; Saucedo et al., 2002a; Saucedo & Southgate, 2008).
Article source: Mamangkey, Noldy (2009) Improving the quality of pearls from Pinctada maxima. PhD thesis, James Cook University. For Questions and answer you can contact & chat with us on:
Although logically all shell-bearing molluscs can produce pearls (Strack, 2006; Webster, 1994), they have only been recorded in several families and genera. In marine waters, pearls have been found in 17 genera of 11 families of bivalves, 11 genera of 8 families of gastropods and one genus of cephalopod (Nautilus) (Strack, 2006). Freshwater pearls have been found in 43 genera of two families only (Strack, 2006). Pearl producing molluscs can be divided in two groups; those producing nacreous and non-nacreous pearls.
1.1.1 Molluscs producing non-nacreous pearls There are at least five gastropod families (Strombidae, Cassidae, Muricidae, Fasciolariidae and Volutidae) and seven bivalve families (Arcidae, Pectinidae, Spondylidae, Placunidae, Ostreaide, Tridacnidae and Veneridae) that produce non-nacreous pearls (Strack, 2006). They are taxa without nacre-lined shells. Pearls from these species are generally natural pearls which lack colour and have low value. However, some of the pearls produced by these taxa are rare, colourful and coveted; for example, those produced by several species of conch (Family Strombidae) and volutes (Family Volutidae) (Matlins, 2002; Strack, 2006).
1.1.2 Molluscs producing nacreous pearls
In freshwater, nacreous pearls are produced from bivalves only. They are distributed in two families: Margaritiferidae and Unionidae (Strack, 2006). Both are from the superfamily Unionoidea and have nacre-lined shells. Three species from the family Unionidae are commonly cultivated for pearl production: Cristaria plicata, Hyriopsis cummingii and H. schlegeli (Strack, 2006; Wang & Wu, 1994).
In marine waters, there are at least three gastropod families (Haliotidae, Trochidae and Turbinidae) and four bivalve families (Mytilidae, Malleidae, Pinnidae and Pteriidae) that produce nacreous pearls (Strack, 2006). They have nacre-lined shells (Watabe, 1988). However, only two families are cultured for pearl production: the gastropod family Haliotidae and the bivalve family Pteriidae. Two genera are cultivated for pearl production in the family Pteriidae; Pinctada and Pteria (Strack, 2006).
By far the most important of these is the genus Pinctada (Southgate et al., 2008b). Of the taxa within the genus Pinctada, Pinctada maxima is the most important in terms of commercial cultured pearl production and as outlined above, it accounts for around 46% by value of global marine cultured pearl production (Southgate et al., 2008a). Pinctada maxima is the focus of this study.
Article source: Mamangkey, Noldy (2009) Improving the quality of pearls from Pinctada maxima. PhD thesis, James Cook University. For Questions and answer you can contact & chat with us on: