1.1 Pearl producing molluscs
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.
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