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.
18.104.22.168. 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).
22.214.171.124 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)
126.96.36.199 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 188.8.131.52). 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).
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