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The black-lipped pearl oyster, Pinctada margaritifera , produces the largest and most valuable coloured pearls in the world. Cultured pearl size remains one of the most important quality traits. Despite the great geographical area covered by pearl farms in the atolls of French Polynesia, little is known about the influence of grow-out site effects on pearl size attained. To explore the genetic and environmental impact on the size of pearls as well as the genetic × environment interaction, a uniform experimental graft was designed on two contrasting macro-geographical lagoons. Five biparental families of donor oysters were grafted and then reared at both sites. After 18 months of culture, phenotypic parameters corresponding to pearl size, i.e. nacre weight and thickness, were recorded among the harvested pearls. The expression of Pif-177 gene, a biomarker encoding protein in the aragonite nacreous layer, was analysed in the corresponding pearl sac. The results show a family effect for nacre weight and thickness on both sites, with family F058 producing the heaviest and thickest nacre, and F805 the lightest and thinnest. By contrast, inter-site comparison revealed no significant site effects for these two parameters. In addition, grow-out location did not modify the relative gene expression of Pif-177 in the pearl sac between donor families in either culture site. Both nacre weight and thickness were positively correlated with the level gene expression of Pif-177. These results suggest that pearl size parameters were not affected by the environment in the present study and this is supported by the relative gene expression of Pif-177 observed. This knowledge constitutes an initial step in the study of pearl size trait inheritance, which will be helpful in the near future for the diffusion of genetically selected donor oyster lines produced by hatchery systems throughout production sites.

Cultured pearls produced with Pinctada margaritifera, using the surgreffe method (implantation of a second nucleus following pearl harvest) were studied for the first time to: (1) examine family effect on nacre thickness, nacre weight and nacre deposition speed and (2) compare variation in these three traits with that obtained from the cultured pearls previously harvested after the corresponding initial grafts. A surgreffe experiment using 783 recipient oysters was realized in Rangiroa atoll (French Polynesia). After 24 months of culture, 389 cultured pearls were harvested. Significant donor family effect was found for the harvested pearl rate from surgreffe (P = 0.046). Highly significant donor family effect was recorded for nacre thickness (P = 0.004). Very highly significant donor family effects were recorded for nacre weight and nacre deposition speed (P

International audience; Environmental parameters, such as food level and water temperature, have been shown to be major factors influencing pearl oyster shell growth and molecular mechanisms involved in this biomineralization process. The present study investigates the effect of food level (i.e., microalgal concentration) and water temperature, in laboratory controlled conditions, on the last stages of pearl mineralization in order to assess their impact on pearl quality. To this end, grafted pearl oysters were fed at different levels of food and subjected to different water temperatures one month prior to harvest to evaluate the effect of these factors on 1) pearl and shell deposition rate, 2) expression of genes involved in biomineralization in pearl sacs, 3) nacre ultrastructure (tablet thickness and number of tablets deposited per day) and 4) pearl quality traits. Our results revealed that high water temperature stimulates both shell and pearl deposition rates. However, low water temperature led to thinner nacre tablets, a lower number of tablets deposited per day and impacted pearl quality with better luster and fewer defects. Conversely, the two tested food level had no significant effects on shell and pearl growth, pearl nacre ultrastructure or pearl quality. However, one gene, Aspein, was significantly downregulated in high food levels. These results will be helpful for the pearl industry. A wise strategy to increase pearl quality would be to rear pearl oysters at a high water temperature to increase pearl growth and consequently pearl size; and to harvest pearls after a period of low water temperature to enhance luster and to reduce the number of defects.

Cultured pearl nacre deposition rate in Pinctada margaritifera oysters was studied to examine its influence on Tahitian classification grade and colour and to further our understanding of using selected donor oysters produced in a hatchery system for the improvement of these traits. A large-scale grafting experiment using 150 farmed donor oysters was realised in Rangiroa atoll (French Polynesia). After 18 months of culture, 874 pearls were harvested and 7 pearls quality traits were recorded: (1) nacre deposition rate assessed by nacre thickness and weight, (2) classification grade, lustre and surface defects, and (3) colour assessed by pearl colour darkness level and visual colour categories. Nacre deposition rate significantly influence pearl grade, surface defects, darkness level and colour categories, but not the lustre. Grade A pearls had the thickest and heaviest nacre on average, compared with grade D pearls and rejects. In addition, pearls without any surface defect had thicker and heavier nacre. Pearls with low darkness were the thinnest and the lightest. Pearls of aubergine–peacock secondary colours were the thickest and the heaviest. These results highlight the importance of rapid nacre deposition rate for increasing the production of grade A pearls, with less surface defects and dominance of a black tone colour. Fast nacre deposition may, however, have a negative impact on other pearl quality traits, such as pearl lustre. This study demonstrates the importance of examining relationships between pearl quality traits in a way that provides solid information that can be directly used for breeding and genetic selection programmes.

International audience; n French Polynesia, the aquaculture of Pinctada margaritifera is carried out in numerous grow-out sites, located over three archipelagos (Gambier, Society and Tuamotu). To evaluate the impact of macro-geographical effects of these growing sites on pearl quality traits, five hatcheries produced families were used as homogeneous donor oysters in an experimental graft. The molluscs were then reared in two commercial locations: Tahaa Island (Society) and Rangiroa atoll (Tuamotu). At harvest, eight pearl quality traits were recorded and compared: surface defects, lustre, grade, circles, shape categories, darkness level, body and secondary colour and visual colour categories. Overall inter-site comparison revealed that: (1) all traits were affected by grow-out location except for lustre and round shape, and (2) a higher mean rate of valuable pearls was produced in Rangiroa. Indeed, for pearl grade, Rangiroa showed twice as many A-B and less reject samples than Tahaa. This was related to the number of surface defects (grade component): in Rangiroa, twice as many pearls had no defects and less pearls had up to 10 defects. Concerning pearl shape, more circled and baroque pearls were found in Tahaa (+10%). For colour variation, 10% more pearls have an attractive green overtone in Rangiroa than in Tahaa, where more grey body colour were harvested. Lustre does not seem to be affected by these two culture site (except at a family scale). This is the first time P. margaritifera donor family have been shown to vary in the quality of pearls they produce depending on their grow-out location.

International audience; The present study investigated the effect of electrolysis on the biomineralization capacities of juveniles of the mollusk Pinctada argaritifera for the first time. Size-selected individuals from two groups, “Medium” and “Large”, from a multi-parental family produced in a hatchery system were subjected to electrolysis under a low voltage current over a nine-week experimental period. The growth of the juveniles was individually monitored and assessed weekly by wet weight and shell height measurements. At the end of the experiment, mantle tissue was sampled for biomineralization-related gene expression analysis. Electrolysis significantly increased pearl oyster growth in terms of shell height and wet weight for Large juveniles from the 5th and the 2nd week, respectively, until the end of the experiment. However, differences were only significant for Medium individuals from the 7th week for shell height and from the 9th week for wet weight. Furthermore, transcriptional analysis of six known biomineralization genes coding for shell matrix proteins of calcitic prisms and/or nacreous shell structures revealed that five were significantly overexpressed in the mantle mineralizing tissue under electrolysis: three in common between the two size class groups and two that were expressed exclusively in one or the other group. Finally, we found no statistical difference of the shell thickness ratio between individuals undergoing electrolysis and control conditions. Taken together, our results indicate, for the first time in a calcifying marine organism, that electrolysis influences molecular mechanisms involved in biomineralization and may stimulate some parameters of pearl oyster growth rate.

Production of larger, rounder, high-quality cultured pearls with fewer circles is one of the main 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 following 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 following the initial graft. An experimental graft and surgreffe experiment was designed using the same criteria: grafter, location, nuclei brand and size for graft and surgreffe, and donor oysters from 10 bi-parental families produced in a hatchery system. We studied the differences between pearls harvested following graft and surgreffe on the same recipient oysters (N = 295 for both graft and surgreffe) in relation to three classes of DD and DW in 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 following surgreffe, insertion of a nucleus larger than the harvested pearl may be advisable. Indeed, the formation of uncircled pearls following 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, following 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.

The top aquaculture species in French Polynesia is Pinctada margaritifera , a mollusc grown for the production of a unique gem: the black pearl. One of the challenges facing the pearl farming industry is to “produce less but better pearls” through genetic improvement. An experimental hatchery system was used to generate full-sib families to be tested for their potential as donor “oysters”. A large-scale grafting experiment was done and seven cultured pearl quality traits: grade, surface defects, lustre, darkness level, visual colour categories, circles and shape categories were recorded. Our results revealed, for the first time, significant phenotypic relationships between these quality traits. The grade A cultured pearl class had the largest proportion of pearls with a green overtone (65%), the lowest number of circled pearls (15%) and the maximum of round-shaped pearls (45%). In contrast, the “reject” cultured pearl class had the largest proportion of pearls with grey body color (65%), the greatest number of circled pearls (35%) and the maximum with a baroque shape (nearly 60%). When grade components were studied separately, cultured pearls in the zero surface defect class exhibited the same tendencies as grade A pearls, contrasting with the class where there were more than ten defects on the surface of each pearl. When cultured pearls were classified according to the presence or absence of lustre, pearls with lustre mostly had a green overtone colour, whilst pearls without lustre did not. These findings have major implications for cultured pearl quality improvement, as modern genetic breeding methods can increase the proportion of high quality cultured pearls though selected lines of donor oysters capable of producing pearls with a green overtone. Selection of appropriate donor phenotypes, incorporation of pigmentation traits into a pearl oyster breeding programme and production of lines with desirable colours will be developed for oyster aquaculture in French Polynesia.

Individual Pinctada margaritifera molluscs were collected from the Takapoto atoll (Tuamotu Archipelago, French Polynesia) and used to produce ten first generation full-sib families in a hatchery system, following artifi- cial breeding protocols. After three years of culture, these progenies were transferred to Rangiroa atoll (Tuamotu Archipelago, French Polynesia) and tested for their potential as graft donors. A large-scale grafting experiment of 1500 grafts was conducted, in which a single professional grafter used ten individual donor oysters from each of the ten families, grafting 15 recipient oysters from each donor. The recipient oysters were all obtained from wild spat collec- tion in Ahe (Tuamotu Archipelago, French Polynesia). After 18 months of culture, 874 pearls were harvested. Highly significant donor family effects were found for nucleus retention, nacre thickness, nacre weight, pearl colour darkness and visually-perceived colour (bodycolor and overtone), pearl shape categories, surface defects and lustre, the last two of which are components of the Tahitian classification grade. No significant difference was recorded between the ten G1 families for the absence or presence of rings. The progenies could be ranked from "best" (i.e., the donor whose grafts produced the greatest number of grade A pearls) to the "worst". Some progenies had extreme characteristics: family B presented the greatest number of pearls with lustre (98%) and a high proportion of dark gray to black with green overtone pearls (70%). These results have important implications for the selective breeding of donor pearl oysters: it may be possible to reach a point where specific donor lines whose grafts produce pearls with specific quality traits could be identified and maintained as specific breeding lines.

International audience; In the Mediterranean Sea, Polylabris tubicirrus (Monogenea, Polyopisthocotylea) is found only on breams of the genus Diplodus : D. sargus, D. annularis, D. vulgaris. The discovery of this parasite on farmed gilt head sea breams (Sparus aurata) raises the problem of the enlargement of the host range of a parasite under artificial conditions of confinement, and of the potential danger this type of transfer can involve.