WWW.WA S .ORG • WORLD AQUACULTURE • MARCH 2022 55 they produce calciferous substances with two different types of crystallization: calcite and aragonite. Often the secretions of shellfishes contain both types of crystallization in different proportions or ratios. The major minerals identified in the shell of the pearl oyster Pinctada margaritifera were more than 96 percent aragonite (Fig. 2), about 2.6 percent calcite (calcium carbonate) and the remaining 1.3 percent of an amorphous content. Aragonite is a polymorph of calcite, giving a pearly hue to the calciferous sacs, while calcite crystallization gives a non-shiny, bony appearance to the sacs. Experimental in vitro Pearl Culture In trials of in vitro pearl culture (Fig. 3), mantle tissues of Pinctada margaritifera have been successfully induced to produce granular cells with aragonite crystals in the culture medium. The study revealed that the black hue of pearls produced by mantle tissues of Pinctada margaritifera (Fig. 4) are influenced by environmental and genetic factors. The genetic constitution of the oyster produces the dark hue; however, the variety of color and luster is impacted by specific minerals and micronutrients added to the medium. These create specific epigenetic conditions, including temperature and alkalinity-like factors, which, during incubation in vitro, impact the ratio of aragonite to calcite crystallization that determines pearl value. In normal pearl culture operations, a live graft tissue fragment prepared from the mantle of a donor oyster is implanted into the gonad region of the same species of a recipient pearl oyster together with a spherical shell bead nucleus in contact with the graft piece. The grafting of mantle tissue induces a sudden immunological upheaval in the body of the recipient organism resulting in a reversal of differentiation and loss of specialized characteristics. The gonad region is actually not the normal place for the mantle tissue, so obviously the defense system of the body of the oyster tries to reject the unwelcome guest, but when it fails and can survive such upheaval, the graft mantle piece is accepted by the body. The mantle tissue then grows and envelops the shell bead and secretes concentric thin micro layers of pearl sac all around the nucleus (Fig. 1). Most researchers believe that secretions over the nucleus are the result of a reaction to a foreign body. However, my personal experience has been that, in several instances, even though the nucleus is rejected, the mantle tissue grafted into the gonad is accepted in the body. The graft tissue still produces pearl sac in an irregular shape, even in the absence of a foreign particle. This observation suggests that the phenomenon of pearl sac secretion is a natural function of the mantle tissue that is carried out to construct the shell when it is at its original location. This natural activity of the mantle occurs in every shellfish; In Vitro Pearl Culture with Pinctada margaritifera Ajai Kumar Sonkar ( C O N T I N U E D O N P A G E 5 6 ) FIGURE 2. Aragonite crystal formation on a culture plate. FIGURE 1. Gold-black and copper-gold pearls produced in the marine oysters of Andaman Islands. FIGURE 3. In vitro pearl culture.
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