Kishore, Pranesh, and Southgate, Paul C. (2015) Does the quality of cultured pearls from the black-lip pearl oyster, Pinctada margaritifera, improve after the second graft? Aquaculture, 446. pp. 97-102.

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Abstract

It is widely assumed that pearl oysters, Pinctada margaritifera, producing low quality pearls with concentric grooves or 'circles' are unlikely to produce pearls with improved quality if grafted again for pearl production. Such oysters are often discarded. However, if these oysters are capable of improved pearl quality when re-grafted, this would provide opportunities for improved income for pearl farmers. This study aimed to determine whether oysters producing circled pearls are able to produce pearls with improved quality after re-grafting. A total of 100 oysters that produced circled pearls and would have normally been discarded were re-grafted and the quality of successive pearls produced by individual oysters was compared in terms of shape, size, lustre, colour, surface perfection and overall quality. The proportion of pearls with circles decreased from 100% of first graft pearls to 48% after the second graft, and 18% of second graft pearls were classified as 'semi-round' and superior in shape to all first graft pearls. There was a significant improvement (p = 0.04) in the overall shape of second graft pearls compared to first graft pearls. The highest proportion of pearls (63%) from the first graft were 10-11 mm in size while the majority of second graft pearls (51%) were 11-12 mm in size, and the differences in pearl size between first and second graft were significant (p = 0.04). Second graft pearls had poorer lustre than first graft pearls with a higher proportion of dull pearls, a lower proportion of medium lustre pearls and no pearls with high lustre. Despite this, the number of pearls in different lustre categories after the first and second graft did not differ significantly (p = 0.07). For overall grading, most first graft pearls (83%) were assessed as 'C' grade with 17% categorised as 'D' grade. Similarly, most second graft pearls (78%) were assessed as 'C' grade and 20% as 'D' grade; however, 2% of pearls were assessed as 'B' grade which were not present in first graft pearls. Nonetheless, the number of pearls belonging to different grades was not significant (p = 0.08). Our data show for the first time that that production of circled pearls after second graft is not obligatory for P. margaritifera that produced circled pearls after first graft. They further show that marketable pearls can be produced from oysters that are normally discarded after the first pearl harvest and this has potential to generate increased revenue.

Statement of relevance:

This paper presents novel new information on whether P. margaritifera that produced circled pearls after first grafting will produce pearls with improved quality when re-grafted. It clears the confusion among pearl farmers on the decision of whether or not to re-graft P. margaritifera that produced low quality pearls with high number of circles. Overall, the study has a positive impact on pearl aquaculture.

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