Improving the quality of cultured round pearls produced by Pinctada margaritifera in Fiji (Linnaeus, 1758)

Kishore, Pranesh (2015) Improving the quality of cultured round pearls produced by Pinctada margaritifera in Fiji (Linnaeus, 1758). PhD thesis, James Cook University.

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Abstract

The pearl industry is recognised as one of the most profitable in the aquaculture sector. In many countries it makes up a large segment of the aquaculture sector and in French Polynesia for example, it is the second highest export earner (after tourism) and makes a substantial contribution to the country's economy and to the livelihoods of its people. The major products of the marine pearl industry are Akoya pearls, white South Sea Pearls, and 'black' South Sea Pearls pearls, otherwise known as Tahitian pearls that are produced almost exclusively in the Pacific. 'Black' South Sea Pearls produced by the black-lip pearl oyster Pinctada margaritifera are the focus of this study. At present, the proportion of high quality 'black' pearls in any pearl farm is very low making up to only around 3%-5% of the total harvest, yet earning around 95% of farm revenue. A major influence on the high proportion of low quality pearls is the presence of 'circles' or concentric depressions or grooves on surfaces that reduce pearl values considerably. Reducing the proportion of these low quality pearls by a small proportion would significantly increase the profit margin of a 'black' pearl farm. This study aimed to identify the causes of low quality 'black' pearls, particularly circles, and provide a basis for improved pearl grafting and oyster husbandry practices supporting increased production and revenue for pearl farmers.

In Chapter 2, pearl-sac development after grafting in P. margaritifera was studied in detail for the first time. A total of 110 P. margaritifera with a mean (± SE) anteroposterior measurement of 110.82 ± 0.41 mm and dorso-ventral measurement of 112.06 ± 0.45 mm were grafted to allow histological examination and chronological description of pearl-sac development in this species. Beginning two days after grafting, oysters were sacrificed regularly until the 48th day and the pearl-sacs of sampled oysters were sectioned and analysed. The graft tissue proliferated and developed into a complete pearl-sac within 14 days of grafting when the epithelial cells responsible for nacre secretion were fully developed. However, first nacre secretion onto the nucleus was not observed until 32 days after grafting. Furthermore, the presence of byssus in close proximity of developing pearl-sac was demonstrated in this study; a factor that has the potential to impact pearl-sac development affecting even nacre deposition and resulting pearl quality. Haemocytes were also present with clumps or aggregations noted in some pearl-sacs. The findings reported in this Chapter provide a more detailed understanding of pearl-sac development in P. margaritifera and a basis for future research towards developing improved pearl culture practices and pearl quality.

A detailed examination of haemocyte accumulation during pearl-sac formation provided the basis for Chapter 3. The level of haemocytes present in the pearl-sacs decreased overtime in many oysters with the samples from day two showing the highest levels. Such a trend generally supports the development of a spherical shaped pearl-sac that would form a regular shaped pearl. However, in some oysters, clumps of haemocytes persisted for a period longer than expected causing a bulge in the pearl-sacs. The pearl-sacs grew over the clumps that resulted in a deformity to what should have been spherical shaped pearl-sacs. Pearls produced from such misshapen pearl-sacs often have calcified "tails" or be of baroque shapes with much reduced values. The exact cause(s) of varying levels of haemocyte accumulation during pearl-sac development in P. margaritifera is not known. However, it is reasonable to assume that haemocyte production is positively related to the degree of damage caused to host oyster tissues during the grafting procedure. While haemocytes have an important wound healing role in pearl oysters, excessive haemocyte presence may be detrimental to maximizing pearl quality.

The feasibility of using regenerated graft tissue for pearl production in P. margaritifera was investigated in Chapter 4. Twelve days after grafting with regenerated graft tissue, there was complete encapsulation of the nucleus by the fully developed pearl-sac and the first layer of organic matrix had been secreted. Sixteen days after grafting, the pearl-sac was completely integrated with host tissue and could no longer be distinguished as foreign. The epithelial cells in the pearl-sac continued to secrete the organic matrix layer but there were no signs of nacre deposition at this stage. However, after three months of culture, nuclei in oysters grafted with regenerated mantle tissue were completely covered with nacre. The average nacre thickness on pearls produced from regenerated (0.55 ± 0.01 mm, n = 8) and normal (0.53 ± 0.01 mm, n = 8) mantle tissue did not differ significantly (p > 0.05). Nacre secretion rates, over the 80 day period subsequent to pearl-sac formation were 6.84 ± 0.1 μm day⁻¹ and 6.66 ± 0.1 μm day⁻¹ for oysters grafted with regenerated and normal mantle tissue, respectively. Again, these means were not significantly different (p = 0.258). These results clearly showed that regenerated mantle tissue can function successfully as saibo for pearl production in P. margaritifera. This finding could provide significant benefits to pearl farmers and provide a basis for further development of current pearl grafting practices.

It is widely assumed that P. margaritifera producing low quality pearls with circles are unlikely to produce pearls with improved quality if grafted again for pearl production. Such oysters are often discarded. However, if these oysters are capable of improved pearl quality when re-grafted, then this would provide opportunities for improved income for pearl farmers. Chapter 5 aimed to determine whether oysters producing circled pearls are able to produce pearls with improved quality after re-grafting. A total of 100 oysters that produced circled pearls and would have normally been discarded were re-grafted and the quality of successive pearls produced by individual oysters was compared in terms of shape, size, lustre, colour, surface perfection and overall quality. The proportion of pearls with circles decreased from 95% of first graft pearls to 48% after the second graft, and 18% of second graft pearls were classified as 'semi-round' and superior in shape to all first graft pearls. There was a significant improvement (p = 0.04) in the overall shape of second graft pearls compared to first graft pearls. The highest proportion of pearls (63%) from the first graft were 10-11 mm in size while the majority of second graft pearls (51%) were 11-12 mm in size, and the differences in pearl size between first and second graft were significant (p = 0.04). Second graft pearls had poorer lustre than first graft pearls with a higher proportion of dull pearls, a lower proportion of medium lustre pearls and no pearls with high lustre. Despite this, the number of pearls in different lustre categories after the first and second graft did not differ significantly (p = 0.07). For overall grading, most first graft pearls (83%) were assessed as 'C' grade with 17% categorised as 'D' grade. Similarly, most second graft pearls (78%) were assessed as 'C' grade and 20% as 'D' grade; however, 2% of pearls were assessed as 'B' grade which were not present in first graft pearls. Nonetheless, the number of pearls belonging to different grades was not significant (p = 0.08). The data in chapter show for the first time that that production of circled pearls after second graft is not obligatory for P. margaritifera that produced circled pearls after first graft. The data further show that marketable pearls can be produced from oysters that are normally discarded after the first pearl harvest and this has potential to generate increased revenue.

The potential effects of byssus production on the development or function of normal pearl-sacs was determined in Chapter 6. This was done after byssus was observed in close proximity to developing pearl-sacs in the experiment reported in Chapter 2. This Chapter investigated the impacts of relative current strength and different culture units on byssus secretion by P. margaritifera. Oysters were either 'ear–hung' or housed in panel nets before being transported to low (Nawi) and high (Raviravi) current sites. The quantity of new byssus produced by oysters in the two culture units at the two sites was counted 5, 10, 15 and 20 days after deployment. At the end of the experiment, the thicknesses and tensile strengths of randomly selected byssal threads from ear-hung oysters and oysters held in panel nets were determined. Ten days after deployment, there was no significant difference in the quantity of byssus produced by oysters in the two types of culture units at both sites. An average of around two threads per byssus secreting oyster was recorded by the tenth day. However, after 15 and 20 days, earhung oysters had produced significantly more byssus (p < 0.01) than those housed in panel nets at the high current site. On the twentieth day, ear-hung oysters had an average of six byssal threads while those housed in panel nets had an average of around three per oyster at the Raviravi site. In contrast, production of byssus by oysters in the two types culture units did not differ significantly for the same period at the low current site. Furthermore, ear-hung oysters produced significantly thicker byssus than those held in panel nets (p = 0.01) which had significantly high tensile strengths (p = 0.01). It is hypothesised that secretion of an increased number of byssal threads by earhung oysters is a response to a greater degree of agitation than those held in panel nets. This could be one of the reasons for anecdotal commentary relating to the production of a high proportion of pearl with inferior quality by oysters cultured using the 'earhanging' method.

With oysters cultured using chaplets producing more byssus compared to oysters housed in panel nets, the experiment described in Chapter 7 was designed to determine if oysters held in panel nets produced higher quality pearls with fewer circles compared to oysters that were ear-hung on chaplets. Six hundred P. margaritifera were grafted for the first time and cultured using panel nets or chaplets at three commercial farm sites to determine if these different culture methods influence resulting pearl quality. The pearls produced were compared in terms of size, shape, lustre, colour, surface perfection and overall quality. The highest proportion of pearls produced in all treatments was in the 10-11 mm size category (37% to 54%) but culture method did not significantly (p = 0.211) influence the size of pearls produced. Oysters held on chaplets produced more pearls with concentric surface grooves or circles (47% to 60%) compared to oysters in held panel nets (43% to 45%) at all three culture sites. Oysters held in panel nets produced higher proportions of pearls in the more desirable 'round' and 'semi-round' shape categories (6% and 25%, respectively) than oysters held on chaplets (5% and 15%, respectively) at all three culture sites, and culture methods had a significant impact (p = 0.031) on pearl shape overall. Higher proportions of pearls in the 'very high' and 'high' lustre categories (8% and 40%, respectively) were produced by oysters held in panel nets compared to those on chaplets (3% and 16%, respectively) at each of the three culture sites. However, the overall impact of culture methods on pearl lustre was not significant (p = 0.100). At all three culture sites, higher proportions of pearls assigned to grades 'A' (6%) and 'B' (46%) were produced by oysters in panel nets compared to those held on chaplets where 3% and 29% of pearls were assigned to grade 'A' and grade 'B', respectively. Oysters held on chaplets produced higher proportions of grade 'C' (49%) and grade 'D' (19%) pearls than those in panel nets (39% and 9%, respectively) at all three culture sites. The grades of pearls were significantly influenced (p = 0.035) by culture method. The results of this experiment clearly demonstrated the benefits of pearl production using panel nets compared to the traditional chaplet-based system used by the majority of pearl farmers in Fiji and throughout the Pacific. Pearls production using panel nets will provide better returns with higher profit margins for pearl farmers but requires greater outlay for infrastructure and labour that may be beyond the scope of most pearl farmers in Fiji and the Pacific. A detailed cost-benefit analysis of the two husbandry options would be beneficial to pearl farmers.

This study addressed factors affecting the quality of cultured 'black' pearls through a number of experiments that assessed the impacts of both developmental and biological factors (e.g. pearl-sac development and function, oyster response to culture method and culture environment) as well as husbandry and culture conditions (e.g. culture method and current strength) on pearl production and pearl quality. The major applications of the results of this study are: (1) potential use of saibo donors producing high quality pearls for multiple saibo donations potentially improving the proportion of high quality pearls; (2) production of marketable pearls from oysters that are normally discarded after the first pearl harvest resulting in increased production and revenue; and (3) change to a panel net-based culture system resulted in higher pearl quality and a ~30% increase in the value of pearls produced. These findings provide a good basis for increased pearl production in Fiji and for future research in this field.

Item ID: 43780
Item Type: Thesis (PhD)
Keywords: aquaculture; black pearls; black-lip pearl oysters; cultured pearls; development; Fiji; formation; growth; low quality pearls; mother of pearl; nacre; pearl production; pearl-sacs; Pinctada margaritifera; quality
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Kishore, Pranesh, and Southgate, Paul C. (2016) A detailed description of pearl-sac development in the black-lip pearl oyster, Pinctada margaritifera (Linnaeus 1758). Aquaculture Research, 47 (7). pp. 2215-2226.

Chapter 3: Kishore, Pranesh, and Southgate, Paul C. (2015) Haemocyte persistence after grafting for pearl production in Pinctada margaritifera (Linnaeus, 1758). Fish and Shellfish Immunology, 42 (2). pp. 530-532.

Chapter 4: Kishore, Pranesh, and Southgate, Paul C. (2015) Development and function of pearl-sacs grown from regenerated mantle graft tissue in the black-lip pearl oyster, Pinctada margaritifera (Linnaeus, 1758). Fish and Shellfish Immunology, 45 (2). pp. 567-573.

Chapter 5: Kishore, Pranesh, and Southgate, Paul C. (2015) Does the quality of cultured pearls from the black-lip pearl oyster, Pinctada margaritifera, improve after the second graft? Aquaculture, 446. pp. 97-102.

Chapter 6: Kishore, Pranesh, Hunter, Justin, Zeng, Chaoshu, and Southgate, Paul C. (2014) The effects of different culture apparatuses and current velocities on byssus production by the black-lip pearl oyster, Pinctada margaritifera. Aquaculture, 434. pp. 74-77.

Chapter 7: Kishore, Pranesh, and Southgate, Paul C. (2016) The effect of different culture methods on the quality of round pearls produced by the black-lip pearl oyster Pinctada margaritifera (Linnaeus, 1758). Aquaculture, 451. pp. 65-71.

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Date Deposited: 18 May 2016 00:04
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 100%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830104 Aquaculture Oysters @ 100%
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