Development of Cultured Pearl Circles and Shape after Initial Graft and Second Nucleus Insertion in the Black-Lipped Pearl OysterPinctada margaritifera

Production of larger, rounder, high-quality cultured pearls with fewer circles is one of themain challenges of Pinctada margaritifera aquaculture faced by every pearl farm in French Polynesia. Although bigger pearl sizes can be achieved through surgreffe operations (implantation of a second nucleus after pearl harvest), control of the development of pearl circles and shapes still remains unclear, as illustrated by grafter's empirical rules, where often the surgreffe process is only performed after production of uncircled and round pearl shapes. The present study was designed with a real pearl by pearl traceability to reveal for the first time the development of circles and shapes from graft and surgreffe, in relation to the size of the pearl sac. This was indirectly assessed by measuring the differences in diameter (DD) and weight (DW) between standardized surgreffe nuclei and the pearl that had been harvested after the initial graft.An experimental graft and surgreffe experimentwas designed using the same criteria: grafter, location, nuclei brand and size for graft and surgreffe, and donor oysters from 10 biparental families produced in a hatchery system.We studied the differences between pearls harvested after graft and surgreffe on the same recipient oysters (n=295 for both graft and surgreffe) in relation to three classes of DDand DWin which the surgreffe nuclei, were: (1) bigger/heavier, (2) equivalent to, or (3) smaller/lighter than the harvested pearl. Results revealed that to increase the rate of uncircled pearls after surgreffe, insertion of a nucleus larger than the harvested pearl may be advisable. Indeed, the formation of uncircled pearls after surgreffe was enhanced by inserting bigger/ heavier second nucleus, both in animals that had produced a uncircled pearl after the initial graft and in those that had produced a circled pearl. For pearl shape, significantly more round shape pearls were produced after surgreffe, after initial oval and baroque samples from graft, by inserting smaller/lighter and bigger/heavier second nucleus, respectively. Inserting a larger second nucleus will significantly increase the rate of both uncircled and round-shaped pearls. This finding has important implications for surgreffe practices, where recipient oysters with undesirable circle or baroque pearls could now be used in this second stage of production.