Pearl Culture

Pearl is a white, glossy, globular concretion that forms inside the shell of an oyster; India first cultured it commercially in 1973 at Thoothukudi, and pearl oysters — sedentary animals found along the coasts of Kanyakumari and the Gulf of Kutch — of the genus Pinctada give the highest-quality pearls, cultured in about 30 ppt salinity using racks, rafts or long-line methods (freshwater culture instead uses the bivalve Lamellidens). Pearl oysters mostly inhabit rock ridges or dead coral, forming extensive pearl banks that yield the finest grade, called 'Lingha Pearl'. A pearl forms naturally when a foreign particle accidentally lodges between the mantle and shell: the mantle epithelium encloses it like a sac and, as a defensive response, continuously secretes concentric layers of nacre around it — eventually building up enough calcium carbonate to form a hard, glossy pearl, though the oyster itself dies once the pearl grows large enough, and the shell must then be opened to remove and grade the pearls by hand. To produce pearls in commercial numbers, an artificial device deliberately inserts a nucleus (foreign particle) into the oyster's shell tissue, following a defined sequence: (i) collection — spat are caught on cages (84 x 54 x 20 cm) dipped in a sand-cement mix for a rough surface, suspended at about 6 m depth from July to November when spat are most available; (ii) rearing — collected oysters are stocked in protected rearing cages, safe from enemies such as octopus, eel and devil fish, and cleaned and settled for 10–20 days to recover from handling stress; (iii) insertion of nucleus — a piece of donor mantle tissue (the 'graft', whose outer, nacre-secreting surface must stay in contact with the nucleus) is cut and, together with a calcareous nucleus (chosen because it attracts the most nacre deposition), inserted into a healthy, stress-tolerant recipient oyster clamped with its right valve facing upward; (iv) post-operation care — nucleated oysters spend a roughly 6–7 day 'recovery period' in cages suspended in sea water, before 3,000–3,600 of them are moved to floating rafts at 2–3 m depth for 3–6 years, left largely undisturbed except during clearing and inspection. Pearls are typically harvested from December to February (varying with climate); after about three years from nucleus insertion …