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

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 (+/- SE) antero-posterior measurement of 110.82 +/- 0.41 mm and dorsoventral measurement of 112.06 +/- 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.