Pearl culture involves placing a nucleus inside the shell of pearl oysters which are then suspended in cages in sea water at a depth of 2-3 meters for 3-6 years. The oysters are harvested after this time by removing them from the water and crushing the shells to extract the pearls formed around the nucleus. Each oyster can produce 3-4 pearls over its lifespan of 15 years. Pearls form as a defense mechanism when an irritant like a parasite gets inside the shell or the mantle tissue is injured, causing the oyster to secrete nacre to coat the irritant.