Nacre tablet characteristics of pearls and shells of Pinctada fucata martensii and its influence on the pearl quality related to luster, interference color, and body color

概要

Color –consisting of body color and interference color– is an essential category in grading the quality of pearls. Nacre –CaCO3 deposit in the form of aragonite crystal– is also crucial in determining pearl quality. The present study focused on finding out the factors influencing the deposition of nacre on pearls and shells, which further affect the quality of pearls. Three experiments have been conducted: 1) the first experiment, saibo from red- and blue-line donors were implanted with a round nucleus into the red- and blue-line recipient oysters from the same batch in four treatments which were referred to as DRMR (red-donor, red-recipient), DRMB (red-donor, bluerecipient), DBMR (blue-donor, red-recipient), and DBMB (blue-donor, blue-recipient) oysters. 2) In the second experiment, 70 Japanese and 70 hybrids (between Japanese and Chinese P. fucata) oysters were selected as recipient oysters and submerged in an alizarin complexone (ALC) solution monthly to estimate the monthly nacre deposition rates. 3) In the third experiment, saibo from the Anterior, Middle, and Posterior sites of the pallial mantle portion of the donor oysters were excised and implanted into recipient oysters.

All oysters from the first, second, and third experiments were cultured for six months from July to December in Ago Bay, Mie Prefecture. Body yellowness index (YI) and interference color value categories, luster value, total nacre and tablet thickness, and surface microstructure for pearls from the first and the third experiments are reported. Moreover, the correlations of water temperature, food availability, and salinity with monthly nacre deposition rate, total nacre, and tablet thickness of pearls, and four positions of the shells –posterior, middle, anterior, and hinge– from the second experiment are also reported.

This study revealed 1) both grade H and grade L pearls deposit irregular layers; only the irregular layer of the grade L pearls showed a significant amount of organic structure and significantly thicker than grade H pearls; 2) pearl interference color to be significantly affected by nacre tablet microstructure. The saturation of pearl interference color is affected by the smoothness of the nacre surface, which is a function of the surface nacre tablet area. In addition, the hue of pearl interference color is affected by the changing pattern of the nacre tablet thickness through the nacreous layer, from surficial to basal regions; 3) P. fucata martensii oysters with red shell nacre interference color improve pearl quality, and for the purposes of pearl culture should be selected for; 4) Japanese strain pearl oysters exhibit better nacre deposition than hybrid oysters both in pearls and shells and would be more appropriate as recipient oysters in Ago Bay, Japan, should culture occur in environmental conditions comparable to those in this study; 5) Water temperature, food availability, and salinity influence shell and pearl nacre deposition and thickness; 6) the pallial mantle portion influences the shape, flaws, and the color categories on the pearl quality grading.