Correlation and Path Analysis Between the Quality of Nucleated Pearls from the Mantle of Hyriopsis cumingii and the Growth Traits of Host Mussels

The triangle-sail mussel (Hyriopsis cumingii) is the most important freshwater pearl-bearing mussel in China. Nucleated pearls from the mantle of H. cumingii are emerging pearl cultivation products. Compared to traditional non-nucleated and nucleated pearls from visceral mass, nucleated pearls from the mantle show significant cultivation advantages, such as high pearl quality and short nurturing cycles. These nucleated pearls are widely known as "freshwater Akoya" in the pearl market because they resemble Japanese saltwater Akoya pearls. However, techniques for cultivating nucleated pearls from the mantle of H. cumingii are still immature and lack a targeted quality evaluation system. Therefore, this study was conducted to identify growth traits related to the quality of nucleated pearls from the mantle of H. cumingii. Two-year-old H. cumingii were implanted with 10 nuclei, 5 mm each in size per mussel. These host mussels were cultured for 19 months to form commercial pearls. Correlation and path analyses of the key quality traits of pearls (pearl number/pearl formation percentage, maximum diameter, minimum diameter, roundness, nacre thickness) and the growth traits of host mussels (shell length, shell width, shell height, body weight, shell weight, soft tissue weight) and nuclei (maximum diameter, minimum diameter, roundness) were performed. The results showed that the pearl formation rate varied from 50% to 100%, with 43.33% of mussels exhibiting a pearl formation rate of 70% and only 3.33% of mussels exhibiting a pearl formation rate of 50% or 100%. The average pearl formation rate was (70.00±10.82)%. The variation in pearl size (minimum diameter) was 4.72‒9.19 mm with an average of (6.91±0.67) mm. The variation in pearl roundness (diameter variation) was 0‒45.60% with an average of (6.74±7.78)%. The highest percentage of pearls were round, followed by near-round pearls, and then by positive round pearls at 21.90%. The nacre thickness was thin in 1.43% of pearls and extra thick in 98.57%. Pearl quantity and roundness were not correlated with all quantitative traits of pearls, host mussels, or nuclei (P > 0.05). Pearl size and nacre thickness were highly significantly and positively correlated with shell width, body weight, and shell weight (P < 0.01) and were significantly and positively correlated with shell height and soft tissue weight of the host mussels (P < 0.05). Among pearl quantitative traits, path analysis of pearl size and nacre thickness showed (highly) significant correlations with host mussels and nuclei. Stepwise regression analysis was performed with the pearl minimum diameter (y1) and nacre thickness (y2) as dependent variables and the shell length (x1), shell width (x2), shell height (x3), body weight (x4), shell weight (x5), and soft tissue weight (x6) of host mussels, along with the maximum diameter (x7), minimum diameter (x8), and roundness (x9) of the nuclei, as independent variables. The regression equations were y1 = 3.464 + 0.078x2 − 0.055x9 (R2 = 0.75; P < 0.01) and y2 = −1.243 + 0.077x2 − 0.055x9 (R2 = 0.75; P < 0.01). The direct effect of shell width on pearl size was the highest (0.69). The direct effect of nucleus roundness on pearl size was −0.23. The indirect effect of shell width on pearl size through nucleus roundness (0.05) was significantly greater than that of nucleus roundness on shell width (−0.02). The direct and indirect effects of shell width and nucleus roundness on nacre thickness were similar to those on pearl size. The direct determinant coefficient of shell width on pearl size (0.50) was significantly higher than that of nucleus roundness (0.08) and that of shell width via nucleus roundness (−0.03). The direct and indirect determinant coefficients of nucleus thickness by shell width and nucleus roundness in pearl-bearing mussels showed the same trend. These findings suggest that the shell width of host mussels is a key trait for controlling pearl size and pearl nacre thickness in nucleated pearls from the mantle of H. cumingii. These results will contribute to the development of China´s pearl industry.