Effect of gender on Akoya pearl quality
Introduction
The characteristics of cultured pearls are influenced by the two types of oysters used in this process: the donor, which provides a small piece of mantle to be transplanted, and the recipient, which receives the pearl nucleus and a small piece of mantle (Southgate and Lucas, 2008, Wada, 1999). Generally, the brightness, luster, and color of cultivated pearls are affected by the donor oyster, whereas the thickness of the nacre is affected by the recipient oyster (Wada and Komaru, 1996). The pearl nucleus and the piece of mantle are transplanted into the gonads of the recipient pearl oyster.
Pearl culture efficiency is affected by the state of the gonads, particularly that of the germ cell, and the sperm and egg in the gonads have been directly linked to the quality and efficiency of the pearls produced in this way (Wada, 1991). Nonetheless, it is unclear what effects gonads have on pearl formation. For these reasons, pearl farmers are forced to eliminate the germ cells from the gonads of pearl oysters prior to transplantation. In general, two methods have been used to achieve this: in one method, Akoya pearl oysters are cultured in narrow cages before the spawning season to suppress gonadal development; in the other method, Akoya pearl oysters are moved to a warmer sea area or treated with ozonated seawater to stimulate spermination and ovulation during the prespawning season. However, both of these methods are expensive and time consuming. Moreover, the Akoya pearl oyster is weakened by these treatments. For the pearl farmer, gonad management in the pearl oysters strongly impacts the quality and efficiency of pearl production, but the relationship between the pearl and gonad is still poorly understood. In our present study, we have analyzed the effect of Akoya pearl oyster sex on pearl production to better understand the relationship between pearl cultivation and oyster gonads.
Section snippets
Relationship between sex and pearl yield rate
To examine the relationship between the sex of the pearl oyster and pearl yield in culture, we assessed sex and pearl quality in individually cultured pearl oysters harvested from different pearl farms. To reduce the likely impact of data variation and complexity on result interpretation, we selected farms which utilize the single nucleus transplantation method and Akoya pearl oysters of Chinese and Japanese (for host oysters) origin. The practices used on each included farm are described in
Cultured pearl quality from male and female Akoya pearl oysters
We investigated pearl quality from eight oyster farms (Fig. 1A), among which differences in the rate of low-quality pearl production were evident. Interestingly, the proportion of low-quality pearls was observed to be lower from male oysters (30.8% on average compared with 47.7% in females). Thus, cultured pearl quality appeared to be clearly affected by the sex of the recipient Akoya pearl oyster.
Characteristics of the pearls cultivated from male and female Akoya pearl oysters
To examine the relationship between pearl quality and sex in more detail, the sex of the recipient
Discussion
To produce higher quality cultured pearls, various factors must be taken into account, such as the condition of the Akoya pearl oyster, the condition of the sea, and the transplantation technique (involving a pearl nucleus and a small piece of mantle). In particular, the state of the gonad of the recipient pearl oyster is crucial to the production of high-quality pearls (Wada, 1991). Accordingly, pearl farmers have expended a great deal of expense and time to optimize the gonadal state of their
Acknowledgments
This study was supported by a Grant-in-Aid from the Science and technology research promotion program for agriculture, forestry, fisheries and food industry (26019A).
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