A detailed description of pearl-sac development in the black-lip pearl oyster,Pinctada margaritifera(Linnaeus 1758)

Abstract

Appropriate development of the pearl-sac in pearl oysters is an important factor influencing the quality of cultured pearls. In this study, a total of 110 black-lip pearl oysters (Pinctada margaritifera) with a mean (AE SE) antero-posterior measurement of 110.82 AE 0.41 mm and dorsoventral measurement of 112.06 AE 0.45 mm were grafted to allow histological examination and chronological description of pearl-sac development in this species. Beginning 2 days after grafting, oysters were sacrificed regularly until the 48th day and the pearl-sacs of sampled oysters were sectioned and examined. 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. The presence and accumulation of haemocytes in the pearl pouch initially and in the pearl-sac thereafter is one of the primary factors potentially affecting pearl quality. Clumps of haemocytes present between the pearl-sac and nucleus caused distension of the pearl-sac from an ideally spherical shape. Furthermore, the presence of byssus in close proximity to the developing pearl-sac was demonstrated in this study. This has the potential to impact pearl-sac formation and resulting pearl quality. The findings reported in this paper 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.

Does the quality of cultured pearls from the black-lip pearl oyster, Pinctada margaritifera, improve after the second graft?

“…While no obvious structural anomalies in the pearl-sac were identified, close proximity of byssus threads to the pearl-sac in some samples indicated that perhaps pressure applied to the pearl-sac by byssus or associated muscles may be involved in circle formation (Kishore and Southgate, 2014). Accepting that the nucleus or developing pearl rotates within the pearl-sac (Cartwright et al, 2013), then pressure applied at a single point on the pearl-sac could result in a concentric anomaly on the pearl.…”

“…A second larger nucleus (generally of similar size to the harvested pearl) is then inserted into the pearl-sac through the incision that will subsequently heal. The pearl-sac that develops from the initial graft grows around the nucleus (Kishore and Southgate, 2014) and subsequent and continual deposition of nacre onto the nucleus is likely to result in increasing tension within the pearl-sac. There is no doubt that cutting into the pearl-sac to remove first graft pearls and subsequent insertion of a second nucleus will result in changes in tension within the pearl-sac.…”

“…But a number have hypothesised the cause(s) of circles which include the skills of grafting technicians, environment conditions at pearl culture sites and grafting season (Ky et al, 2015), interruption in the supply or changes in concentration of material(s) in the peal-sac (Cartwright et al, 2013) and the presence of byssus close to the pearlsac during pearl formation (Kishore and Southgate, 2014), as discussed above. Higher levels of suspended particulate matter present in island lagoons compared to atoll lagoons has been suggested as the cause of differences in the proportion of pearls with circles produced in these two environments (Ky et al, 2014b), and a higher proportion of circled pearls are produced by oysters held on chaplets compared to those held in panel (pocket) nets in Fiji (Kishore, unpublished data).…”

“…A number of factors have been suggested as a cause of circles in pearls. They include the skills of grafting technicians, environment conditions at pearl culture sites and grafting season (Ky et al, 2015), interruption in the supply or changes in concentration of material in the peal-sac (Cartwright et al, 2013) and the presence of byssus close to the pearl-sac during pearl formation that could disrupt even nacre deposition on developing pearls (Kishore and Southgate, 2014).…”

Development and function of pearl-sacs grown from regenerated mantle graft tissue in the black-lip pearl oyster, Pinctada margaritifera (Linnaeus, 1758)

“…Subsequent proliferation of the saibo forms a pearl-sac, which envelopes the nucleus, and epithelial cells within the pearl-sac secrete nacre onto the nucleus to produce a cultured round pearl [2e6]. The process of pearl-sac formation takes less than two weeks in the black-lip pearl oyster, Pinctada margaritifera (Linnaeus, 1758) [6] but around 18e24 months is required to produce pearls with a commercially acceptable nacre thickness [2,7]. Saibo has been shown to have a major influence on the quality of resulting pearls [8e10] with, for example, the colour of a cultured pearl being very similar to that of the nacre of the donor oyster shell [8,9].…”