Problems and potential factors for increasing pearl quality
Pearls are harvested from only about 30 to 35% of implanted oysters (Matlins, 2002). Of these, high quality pearls make up only 5 to 10% but generate around 95% of industry income (Haws, 2002).
On this basis, a relatively small increase in pearl quality could bring about major benefits in industry earnings. This has driven research into methods that can be used to improve the yield of high quality pearls. They have focused on factors such as:
- reducing oyster mortality after implantation
- increasing retention rate of grafted nuclei; and
- improving the proportion of high quality pearls (Haws, 2002).
Other factors that may have application in improving pearl quality are the use of anaesthetics, the excision of mantle from anaesthetised oysters and the use of regenerated mantle tissue from high quality donor oysters.
1.7.1 Use of anaesthetics
The pearl implantation procedure is extremely stressful for pearl oysters and may result in mortality. Some research has been conducted to determine whether the use of anaesthetics may minimise this stress and improve survival and nucleus retention.
Anaesthetics have also been used to assist internal assessment of pearl oyster tissues (Acosta-Salmón, 2004; Norton et al., 1996; Strack, 2006). Several basic studies on the effectiveness of particular anaesthetics have been conducted successfully with pearl oysters. Concentrations of propylene phenoxetol between 2 and 3 mL L-1 have been effectively use to relax Pinctada albina and P. imbricata (O’Connor & Lawler, 2002), P. margaritifera (Acosta-Salmon et al., 2005; Norton et al., 1996), and P. maxima (Mills et al., 1997). However, the effectiveness of various anaesthetics and the degree of success in using anaesthetics in the large-scale pearl production has not yet been determined. This may reflect satisfaction in the current implantation method and that the use of anaesthetics may increase production costs (labours and time) (Acosta-Salmón, 2004).
1.7.2 The potential of regenerated mantle
The ability to regenerate new tissue from wounds is essential for survival. This capability is a common phenomenon in metazoans (Alvarado, 2000; Bekkum, 2004), however, extensive studies on tissue regeneration in pearl oysters have only recently started. Recent studies by the Pearl Oyster Research Group, at James Cook University have shown that Pinctada fucata and P. margaritifera have the capacity to regenerate excised mantle tissues (Acosta-Salmon & Southgate, 2005; Acosta-Salmon & Southgate, 2006; Acosta-Salmon et al., 2004). Anaesthesia of oysters is required prior to mantle excision. They found that excised mantle tissue could heal within three days and complete regeneration was evident three months after excision. Since donor oysters are generally killed to obtain donor mantle tissue used in pearl implantation (Acosta-Salmon et al., 2004; Taylor & Strack, 2008), these studies indicated that pearl oysters used to provide donor mantle tissue for pearl implantation need not necessarily be killed. This offers considerable potential benefits to the pearling industry:
- donor oysters that produces high quality pearls can be used as future parent stock to
improve the quality of cultured oysters; and
- high quality oysters could potentially be used as donors on more than one occasion following mantle regeneration (Acosta-Salmon et al., 2005)
These potential benefits assume that the mantle tissue excised from anaesthetised pearl oysters or, from oysters which have regenerated previously excised mantle tissue, will perform in a similar fashion to ‘normal’ mantle tissue in the pearl process. This assumption has yet to be tested with any species of pearl oyster and it forms the basis of this study with P. maxima .
1.8 Major objectives of this study
The major objective of this study is to investigate the potential of using anaesthesia to facilitate mantle excision from P. maxima without mortality and to assess the use and potential anaesthetised and regenerated mantle as saibo for pearl production. It will attempt to achieve this by addressing the following questions:
- How do P. maxima respond to different anaesthetics and which anaesthetic is most
appropriate for use with this species? (Chapter 2)
- How does P. maxima respond to mantle excision? Does excised mantle tissue regenerate and, if so, how quickly and are the characteristics of regenerated mantle similar to normal mantle? (Chapter 3)
- Is the composition of shell and nacre secreted by regenerated mantle the same as the
normal shell? (Chapter 4)
- Can relaxed and or regenerated mantle be used for pearl implantation? (Chapter 5) If so, are pearls resulting from relaxed and or regenerated mantle similar to those produced by normal mantle? (Chapter 6)
- Do pearls from the same saibo donor have similar traits? (Chapter 7)
Several experiments were carried to address these questions and the specific aims were:
- to identify suitable relaxants for P. maxima and their concentrations for mantle
excision (Chapter 2);
- to assess mantle healing and the possibility of regeneration following mantle excision from anaesthetised P. maxima (Chapter 3);
- to compare the structure and composition of shells produced by normal and
regenerated mantle (Chapter 4);
- to observe the pearl-sac development between relaxed, regenerated and normal saibo
- to determine and to compare the quality pearls produced using both anaesthetised and
regenerated saibo inserted into recipient pearl oysters (Chapter 6); and
- to analyse pearl quality from the same donor oyster with non-destructive method
Article source: The Basic Methods of Pearl Farming, Author: A Layman’s ManualMaria Haws, Ph.D. (Director, Pearl Research and Training Program, Pacific Aquaculture and Coastal Resources Center, University of Hawaii at Hilo, Hilo, HI 96720 USA, Center for Tropical and Subtropical Aquaculture, Publication No. 127, March 2002)
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