Norton, John H., Lucas, John S., Turner, Ian, Mayer, Robert J., and Newnham, Raymond (2000) Approaches to improve cultured pearl formation in Pinctada margaritifera through use of relaxation, antiseptic application and incision closure during bead insertion. Aquaculture, 184 (1-2). pp. 1-17.

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Abstract

Three treatments were tested on blacklip pearl oysters, Pinctada margaritifera, during the bead insertion process as part of research to improve the efficiency of round pearl culture. The oysters (100 to 150 mm shell height) were maintained on a commercial farm at Manihiki atoll, Cook Islands. The three treatments were: (1) immersion of the oysters in 2 ml/l propylene phenoxetol for 15 min to relax them before operating; (2) disinfection of the operation site with an antiseptic (1:50 aqueous dilution of 10% povidone iodine solution); and (3) closure of the surgical incision with a flexible cyanoacrylate adhesive. The three treatments were applied or not applied to oysters in a 3-factor arrangement to give eight treatment combinations. The treatment combinations were applied to two groups of oysters: Group I, consisting of 96 oysters (12 sets×8 treatment combinations), and Group II, consisting of 768 oysters (12 sets×8 treatment combinations×8 oysters per set). After 6 weeks, all oysters were inspected for mortality and bead rejection, and Group I oysters were sacrificed. Group I oysters were necropsied and checked for the presence of pearl sacs. After 18 months, the pearls from Group II oysters were harvested and graded for quality; scoring for lustre, surface perfection, main colour, tint, shape, diameter, length and weight. Inserted beads ranged from 6.3–7.8 mm diameter and their size was not correlated significantly with shell size. Relaxant use was associated with significantly higher mortality in both Groups of oysters and total failures (deaths+bead rejections) in Group II oysters compared with controls. Relaxant use was also associated with pearls of significantly lighter weight, i.e., with lower nacre secretion, further suggesting that there was an adverse effect from this treatment. Adhesive use was associated with more deaths (significant in Group II) and with more bead rejections (significant in Group I). Adhesive use, however, had a positive effect on pearl quality through improved shape. Seventy-six of the 78 Group I oysters that retained beads for 6 weeks had formed pearl sacs, but 55 of the beads (71%) already had imperfections in the form of projecting calcified ‘tails' from the bead. Histological examination showed that the ‘tails' enclosed accumulations of inflammatory cells in the incision pathway. Adhesive use significantly reduced the prevalence of beads with ‘tails', apparently by better closing the incision pathway. The reduction in ‘tails' at 6 weeks was reflected in a significant reduction in the mean length/diameter ratio of pearls at harvest, i.e., a higher level of symmetry. This was the only significant, positive effect of the three treatments on pearl quality parameters. Antiseptic application to the operation site had no significant effect on mortality, bead rejection rate or ‘tail' formation. However, antiseptic was the only treatment that resulted in a lower percentage of total failures compared to controls. Inflammatory cells, arising from suspected bacterial infections, were associated with failures to produce a pearl sac and the ‘tails' imperfection. Better hygiene with beads, instruments, cloths, cutting boards and graft tissue should control infection levels. The additional times taken to apply the three treatments of this study were not excessive and encourage further innovative approaches to the bead insertion process

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